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{{Short description|The Sun and objects orbiting it}}
{{dablink|This article is about the Sun and its planetary system. For other systems, see [[Planetary system]] and [[Star system]].}}
{{Other uses}}
{{sprotect|small=yes}}
{{Pp|small=yes}}
[[Image:Planets2008.jpg|right|390px|thumb|Planets and some dwarf planets of the Solar System; while the sizes are to scale, the relative distances from the Sun are not.]]
{{Featured article}}
The '''Solar System'''{{Ref_label|A|a|none}} consists of the [[Sun]] and those [[Astronomical object|celestial objects]] bound to it by [[gravity]]. These objects are the eight [[planet]]s and their 166 known [[Natural satellite|moons]],<ref>{{cite web |title=The Jupiter Satellite Page |author=Scott S. Sheppard |work=Carnegie Institution for Science, Department of Terrestrial Magnetism |url=http://www.dtm.ciw.edu/sheppard/satellites/|accessdate=2008-04-02}}</ref> four [[dwarf planet]]s and billions of [[Small Solar System body|small bodies]], including [[asteroid]]s, [[volatiles|icy]] [[Kuiper belt]] objects, [[comet]]s, [[meteoroid]]s, and [[Interplanetary dust cloud|interplanetary dust]].
{{Use dmy dates|date=August 2024}}
{{Use American English|date=August 2024}}
{{Infobox planetary system|background=#efefef
| title = Solar System
| image = {{Switcher|[[File:Solar System true color.jpg|upright=1.3|frameless]]|without captions|[[File:Solar System true color (title and caption).jpg|upright=1.3|frameless]]|with captions|default=1}}
| image_alt = A true-color image of the Solar System with sizes, but not distances, to scale. The order of the planets are from right to left.
| image_size = 400px
| caption = {{Longitem|The [[Sun]], [[Planets of the solar system|planets, moons and dwarf planets]]{{Efn|The [[Asteroid Belt]], [[Kuiper Belt]], and [[Scattered Disc]] are not added because the individual asteroids are too small to be shown on the diagram.}}<br/>(true color, size to scale, distances not to scale)|style=padding:2px 0 4px 0;}}
| age = 4.568&nbsp;billion years{{refn|group=lower-alpha|name=AgeSolarSystem}}
| location = {{Longitem|{{unbulleted list
|{{nowrap|[[Local Interstellar Cloud]]}}
|[[Local Bubble]]<ref name="JPL interstellar">{{cite web |url=http://interstellar.jpl.nasa.gov/interstellar/probe/introduction/neighborhood.html |title=Our Local Galactic Neighborhood | website= interstellar.jpl.nasa.gov |publisher=NASA |series=Interstellar Probe Project |year= 2000 |access-date=8 August 2012 |archive-url=https://web.archive.org/web/20131121061128/http://interstellar.jpl.nasa.gov/interstellar/probe/introduction/neighborhood.html |archive-date=21 November 2013 |url-status=dead }}</ref>
|[[Orion–Cygnus Arm]]
|[[Milky Way]]<ref>{{Cite web |last=Hurt |first=R. |date=8 November 2017 |title=The Milky Way Galaxy |url=https://science.nasa.gov/resource/the-milky-way-galaxy/ |access-date=19 April 2024 |website=science.nasa.gov |language=en-US}}</ref>}}}}
| neareststar = {{Longitem|{{Ublist
|[[Proxima Centauri]]
|&nbsp;(4.2465&nbsp;[[Light-year|ly]])<ref name="lurie2014" group="D">{{cite journal |last1=Lurie |first1=John C. |last2=Henry |first2=Todd J. |last3=Jao |first3=Wei-Chun |last4=Quinn |first4=Samuel N. |last5=Winters |first5= Jennifer G. |last6=Ianna |first6=Philip A. |last7=Koerner |first7=David W. |last8= Riedel |first8=Adric R. |last9=Subasavage |first9=John P. | display-authors = 3| year= 2014 |title=The Solar neighborhood. XXXIV. A search for planets orbiting nearby M dwarfs using astrometry |journal=The Astronomical Journal |volume=148 |issue= 5 |pages=91 |arxiv=1407.4820 |bibcode= 2014AJ....148...91L |doi= 10.1088/0004-6256/148/5/91 |s2cid= 118492541 |issn = 0004-6256}}</ref>
|[[Alpha Centauri]]
|&nbsp;(4.36&nbsp;ly)<ref name="RECONS" group="D">{{cite web|work= astro.gsu.edu| publisher= Research Consortium On Nearby Stars, Georgia State University|date=7 September 2007|title=The One Hundred Nearest Star Systems |url=http://www.astro.gsu.edu/RECONS/TOP100.posted.htm|access-date=2 December 2014|archive-url=https://web.archive.org/web/20071112173559/http://www.chara.gsu.edu/RECONS/TOP100.posted.htm|archive-date=12 November 2007| url-status=live}}</ref>}}}}


| frostline = {{val|5|u=AU|p=~}}<ref name="Mumma">{{Cite journal |last1=Mumma |first1=M. J. |last2=Disanti |first2=M. A. |last3=Dello Russo |first3=N. |last4=Magee-Sauer |first4=K. |last5=Gibb |first5=E. |last6=Novak |first6=R. |display-authors = 3| year= 2003 |title=Remote infrared observations of parent volatiles in comets: A window on the early solar system |journal=Advances in Space Research |volume=31 |issue=12 |pages=2563–2575 |bibcode= 2003AdSpR..31.2563M |citeseerx= 10.1.1.575.5091 |doi=10.1016/S0273-1177(03)00578-7}}</ref>
Broadly, the charted regions of the Solar System are the Sun, four [[Terrestrial planet|terrestrial]] inner planets, the [[asteroid belt]], four [[gas giant]] outer planets, the Kuiper belt, the [[scattered disc]], and ultimately perhaps the hypothetical [[Oort cloud]].
| outerplanetname = [[Neptune]]
| semimajoraxis = 30.07 AU<ref name=Horizons group="D">{{cite web |first=Donald K. |last=Yeomans |url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%278%27&TABLE_TYPE=%27ELEMENTS%27&START_TIME=%272000-01-01%27&STOP_TIME=%272000-01-02%27&STEP_SIZE=%27200%20years%27&CENTER=%27@0%27&OUT_UNITS=%27AU-D%27 |title=HORIZONS Web-Interface for Neptune Barycenter (Major Body=8) |publisher=[[JPL Horizons On-Line Ephemeris System]] | website= jpl.nasa.gov|access-date=18 July 2014 |archive-date=7 September 2021 |archive-url=https://web.archive.org/web/20210907055935/https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%278%27&TABLE_TYPE=%27ELEMENTS%27&START_TIME=%272000-01-01%27&STOP_TIME=%272000-01-02%27&STEP_SIZE=%27200%20years%27&CENTER=%27%400%27&OUT_UNITS=%27AU-D%27 |url-status=live }}—Select "Ephemeris Type: Orbital Elements", "Time Span: 2000-01-01 12:00 to 2000-01-02". ("Target Body: Neptune Barycenter" and "Center: Solar System Barycenter (@0)".)</ref>
| Kuiper_cliff = 50–70 AU<ref name="twotino">{{cite journal | first1= E. I.| last1= Chiang |title=Resonance Occupation in the Kuiper Belt: Case Examples of the 5:2 and Trojan Resonances |journal=[[The Astronomical Journal]] |volume=126 |issue=1 |pages=430–443 |date=2003 |doi=10.1086/375207 |last2=Jordan |first2=A. B. |last3=Millis |first3=R. L. |last4=Buie |first4=M. W. |last5=Wasserman |first5=L. H. |last6=Elliot |first6=J. L. |last7=Kern |first7=S. D. |last8=Trilling |first8=D. E. |last9=Meech |first9=K. J. | display-authors = 3| bibcode= 2003AJ....126..430C |arxiv=astro-ph/0301458 |s2cid=54079935}}</ref><ref name= "KuiperGap">{{cite journal |first1=C. | last1=de la Fuente Marcos |first2=R. | last2= de la Fuente Marcos |title=Past the outer rim, into the unknown: structures beyond the Kuiper Cliff |journal=[[Monthly Notices of the Royal Astronomical Society Letters]] |volume=527 |issue=1 |pages= L110–L114 |url=https://academic.oup.com/mnrasl/article-abstract/527/1/L110/7280408 |publication-date=20 September 2023 |date=January 2024 |access-date=28 September 2023 |bibcode=2024MNRAS.527L.110D |arxiv=2309.03885 |doi=10.1093/mnrasl/slad132 | doi-access=free |s2cid= |archive-date=28 October 2023 |archive-url=https://web.archive.org/web/20231028132004/https://academic.oup.com/mnrasl/article-abstract/527/1/L110/7280408 |url-status=live}}</ref>
| heliopause = detected at 120 AU<ref name="heliopause">{{cite web|url=http://www.nasa.gov/mission_pages/voyager/voyager20130912.html#.UjJLPZKR86s|title=NASA Spacecraft Embarks on Historic Journey Into Interstellar Space|first=Tony|last=Greicius| website= nasa.gov| date=5 May 2015|access-date=19 April 2024|archive-date=11 June 2020|archive-url=https://web.archive.org/web/20200611233345/https://www.nasa.gov/mission_pages/voyager/voyager20130912.html#.UjJLPZKR86s|url-status=dead}}</ref>
| hillsphere = 1.1 pc (230,000 AU)<ref name="Chebotarev">{{cite journal |last1=Chebotarev |first1=G. A. |title=Gravitational Spheres of the Major Planets, Moon and Sun |journal=Astronomicheskii Zhurnal |date=1 January 1963 |volume=40 |pages=812 |bibcode=1964SvA.....7..618C |url=https://adsabs.harvard.edu/full/1964SvA.....7..618C |issn=0004-6299 |access-date=6 May 2024 |archive-date=7 May 2024 |archive-url=https://web.archive.org/web/20240507030847/https://adsabs.harvard.edu/full/1964SvA.....7..618C |url-status=live }}</ref> – 0.865 pc (178,419 AU)<ref>{{cite journal |last1=Souami |first1=D |last2= Cresson |first2=J |last3=Biernacki |first3=C |last4=Pierret |first4=F |title=On the local and global properties of gravitational spheres of influence |journal=[[Monthly Notices of the Royal Astronomical Society]] |date=21 August 2020 |volume= 496 |issue=4 |pages= 4287–4297 |doi= 10.1093/mnras/staa1520|doi-access=free | arxiv= 2005.13059 }}</ref>


| noknown_stars = yes
A flow of [[plasma (physics)|plasma]] from the Sun (the [[solar wind]]) permeates the Solar System. This creates a [[stellar wind bubble|bubble]] in the [[interstellar medium]] known as the [[heliosphere]], which extends out roughly to the middle of the scattered disc.
| noknown_planets = yes
| stars = [[Sun]]
| planets = {{Longitem|{{Plainlist|*[[Mercury (planet)|Mercury]]
*[[Venus]]
*[[Earth]]
*[[Mars]]
*[[Jupiter]]
*[[Saturn]]
*[[Uranus]]
*[[Neptune]]}}}}
| dwarfplanets = {{Longitem|{{Plainlist|
*{{Dp|Ceres}}
*{{Dp|Orcus}}
*[[Pluto]]
*[[Haumea]]
*{{Dp|Quaoar}}
*[[Makemake]]
*{{Dp|Gonggong}}
*[[Eris (dwarf planet)|Eris]]
*{{Dp|Sedna}}
*&nbsp;[[List of possible dwarf planets|''more candidates...'']]
}}}}
| satellites = 758<ref name="JPLbodies" group="D">{{Cite web |title=Solar System Objects |url=https://ssd.jpl.nasa.gov |url-status=live |archive-url=https://web.archive.org/web/20210707142304/https://ssd.jpl.nasa.gov |archive-date=7 July 2021 |access-date=14 August 2023 |publisher=NASA/JPL Solar System Dynamics}}</ref>
| minorplanets = 1,368,528<ref name="MPCSummary" group="D">{{Cite web |title=Latest Published Data |url=https://minorplanetcenter.net/mpc/summary |access-date=27 May 2024 |website=The International Astronomical Union Minor Planet Center |archive-date=5 March 2019 |archive-url=https://web.archive.org/web/20190305034947/https://minorplanetcenter.net/mpc/summary |url-status=live }}</ref>
| comets = 4,591<ref name=MPCSummary group="D"/>


| inclination = ~60°, to the ecliptic{{Refn |group=lower-alpha |name=angle}}<!-- If anyone can find a cited value for the inclination of the Solar System's invariable plane to the galactic plane, please replace this value -->
In order of their distances from the Sun, the eight planets are:
| galacticcenter = {{longitem|24,000–28,000 ly}}<ref name="francis14">{{cite journal |first1=Charles |last1=Francis |first2=Erik |last2=Anderson |s2cid= 119235554 |title=Two estimates of the distance to the Galactic Centre |journal=[[Monthly Notices of the Royal Astronomical Society]] |date=June 2014 |volume=441 |issue=2 |pages=1105–1114 |doi=10.1093/mnras/stu631 |doi-access=free |bibcode= 2014MNRAS.441.1105F |arxiv=1309.2629}}</ref>
* [[Mercury (planet)|Mercury]]
| orbitalspeed = {{longitem|720,000&nbsp;km/h (450,000&nbsp;mi/h)<ref name="roughfactsofthesun" />}}
* [[Venus]]
| orbitalperiod = ~230 [[million year]]s<ref name="roughfactsofthesun">{{Cite web |title=Sun: Facts |url=https://science.nasa.gov/sun/facts/ |access-date=19 April 2024 |website=science.nasa.gov |language=en-US |archive-date=19 April 2024 |archive-url=https://web.archive.org/web/20240419151126/https://science.nasa.gov/sun/facts/ |url-status=live }}</ref>
* [[Earth]]
| spectral = [[G-type main-sequence star|G2V]]
* [[Mars]]
}}
* [[Jupiter]]
* [[Saturn]]
* [[Uranus]]
* [[Neptune]]


The '''Solar System'''<ref group="lower-alpha">[[Capitalization]] of the name varies. The [[International Astronomical Union]], the authoritative body regarding [[astronomical nomenclature]], specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar System" and "solar system" structures in their [http://www.iau.org/public/themes/naming/ naming guidelines document] {{Webarchive|url=https://web.archive.org/web/20210725053113/https://www.iau.org/public/themes/naming |date=25 July 2021 }}. The name is commonly rendered in lower case ('solar system'), as, for example, in the ''[[Oxford English Dictionary]]'' and [http://www.m-w.com/dictionary/solar%20system ''Merriam-Webster's 11th Collegiate Dictionary''] {{Webarchive|url=https://web.archive.org/web/20080127201148/http://www.m-w.com/dictionary/solar%20system |date=27 January 2008 }}.</ref> is the [[gravitationally bound]] system of the [[Sun]] and the objects that [[orbit]] it.<ref name="IAU Office of Astronomy for Education y607">{{cite web | title=IAU Office of Astronomy for Education | website=astro4edu.org | publisher=IAU Office of Astronomy for Education | url=https://astro4edu.org/resources/glossary/term/314/ | access-date=11 December 2023 | archive-date=11 December 2023 | archive-url=https://web.archive.org/web/20231211093539/https://astro4edu.org/resources/glossary/term/314/ | url-status=live }}</ref> It [[Formation and evolution of the Solar System|formed about 4.6&nbsp;billion years ago]] when a dense region of a [[molecular cloud]] collapsed, forming the Sun and a [[protoplanetary disc]]. The Sun is a typical star that maintains a [[hydrostatic equilibrium|balanced equilibrium]] by the [[thermonuclear fusion|fusion]] of hydrogen into helium at its [[stellar core|core]], releasing this energy from its outer [[photosphere]]. Astronomers [[stellar classification|classify]] it as a [[G-type main-sequence star]].
As of mid-2008, four smaller objects are classified as ''dwarf planets'': [[Ceres (dwarf planet)|Ceres]] is in the asteroid belt, while the other three ([[Pluto]], [[Makemake (dwarf planet)|Makemake]] and [[Eris (dwarf planet)|Eris]]) all orbit the Sun beyond Neptune.


The largest objects that orbit the Sun are the eight [[planet]]s. In order from the Sun, they are four [[terrestrial planet]]s ([[Mercury (planet)|Mercury]], [[Venus]], [[Earth]] and [[Mars]]); two [[gas giant]]s ([[Jupiter]] and [[Saturn]]); and two [[ice giant]]s ([[Uranus]] and [[Neptune]]). All terrestrial planets have solid surfaces. Inversely, all [[giant planets]] do not have a definite surface, as they are mainly composed of gases and liquids. Over 99.86% of the Solar System's mass is in the Sun and nearly 90% of the remaining mass is in Jupiter and Saturn.
Six of the planets and two of the dwarf planets are in turn orbited by [[natural satellite]]s, usually termed "moons" after Earth's [[Moon]], and each of the outer planets is encircled by [[planetary ring]]s of dust and other particles.


There is a strong consensus among astronomers{{Efn|The [[International Astronomical Union]]'s Minor Planet Center has yet to officially list Orcus, Quaoar, Gonggong, and Sedna as dwarf planets as of 2024.}} that the Solar System has at least nine [[dwarf planet]]s: {{Dp|Ceres}}, {{Dp|Orcus}}, [[Pluto]], {{Dp|Haumea}}, {{Dp|Quaoar}}, {{Dp|Makemake}}, {{Dp|Gonggong}}, {{Dp|Eris}}, and {{Dp|Sedna}}. There are a vast number of [[Small Solar System body|small Solar System bodies]], such as [[asteroid]]s, [[comet]]s, [[Centaur (minor planet)|centaurs]], [[meteoroid]]s, and [[interplanetary dust cloud]]s. Some of these bodies are in the [[asteroid belt]] (between Mars's and Jupiter's orbit) and the [[Kuiper belt]] (just outside Neptune's orbit).{{efn|For more classifications of Solar System objects, see [[List of minor-planet groups]] and {{Section link|Comet|Classification}}.}} Six planets, seven dwarf planets, and other bodies have orbiting [[natural satellite]]s, which are commonly called 'moons'.


The Solar System is constantly flooded by the Sun's [[charged particle]]s, the [[solar wind]], forming the [[heliosphere]]. Around 75–90 [[astronomical unit]]s from the Sun,{{Efn|The scale of the Solar System is sufficiently large that astronomers use a custom unit to express distances. The [[astronomical unit]], abbreviated AU, is equal to {{Convert|1|AU|km mi|lk=off|disp=out|abbr=~}}. This is what the distance from the Earth to the Sun would be if the planet's orbit were perfectly circular.<ref>{{Cite journal |last=Standish |first=E. M. |date=April 2005 |title=The Astronomical Unit now |journal=Proceedings of the International Astronomical Union |volume=2004 |issue=IAUC196 |pages=163–179 |bibcode=2005tvnv.conf..163S |doi=10.1017/S1743921305001365 |s2cid=55944238 |doi-access=free}}</ref>}} the solar wind is halted, resulting in the [[Heliopause (astronomy)|heliopause]]. This is the boundary of the Solar System to [[interstellar space]]. The outermost region of the Solar System is the theorized [[Oort cloud]], the source for [[long-period comet]]s, extending to a radius of {{val|2000|–|200000|u=AU|fmt=commas}}. The closest star to the Solar System, [[Proxima Centauri]], is {{convert|4.25|ly|AU}} away. Both stars belong to the [[Milky Way]] galaxy.


==Discovery and exploration==
== Formation and evolution ==
{{main|Discovery and exploration of the Solar System}}
{{Main|Formation and evolution of the Solar System}}
For many thousands of years, humanity, with a few notable exceptions, did not recognise the existence of the Solar System. They believed the Earth to be stationary at the centre of the [[universe]] and categorically different from the divine or ethereal objects that moved through the sky. While the [[Indian astronomy|Indian]] mathematician-astronomer [[Aryabhata]] and the Greek philosopher [[Aristarchus of Samos]] had speculated on a heliocentric reordering of the cosmos, [[Nicolaus Copernicus]] first developed a mathematically predictive heliocentric system. His 17th-century successors [[Galileo Galilei]], [[Johannes Kepler]], and [[Isaac Newton]] developed an understanding of [[physics]] which led to the gradual acceptance of the idea that the Earth moves round the Sun and that the planets are governed by the same physical laws that governed the Earth. In more recent times, this led to the investigation of geological phenomena such as mountains and craters and seasonal meteorological phenomena such as clouds, dust storms and ice caps on the other planets.


== Layout and structure ==
=== Past ===
[[File:Soot-line1.jpg|thumb|upright=1.35|Diagram of the early Solar System's [[protoplanetary disk]], out of which Earth and other Solar System bodies formed]]
[[Image:Solarsys.svg|right|400px|thumb|The regions (or zones) of the Solar system: the ''inner solar system'', the ''asteroid belt'', the ''giant planets'' (Jovians) and the ''Kuiper belt''. Sizes and orbits not to scale.]]
The Solar System formed at least 4.568&nbsp;billion years ago from the gravitational collapse of a region within a large [[molecular cloud]].{{Refn|name=AgeSolarSystem|group=lower-alpha|The date is based on the oldest [[inclusion (mineral)|inclusions]] found to date in [[meteorite]]s, {{Val|4568.2|+0.2|-0.4}} million years, and is thought to be the date of the formation of the first solid material in the collapsing nebula.<ref>{{Cite journal |last1=Bouvier |first1=A. |last2=Wadhwa |first2=M. |author-link2=Meenakshi Wadhwa |year=2010 |title=The age of the Solar System redefined by the oldest Pb–Pb age of a meteoritic inclusion |journal=Nature Geoscience |volume=3 |issue=9 |pages= 637–641 |bibcode=2010NatGe...3..637B |doi=10.1038/NGEO941 |s2cid=56092512}}</ref>}} This initial cloud was likely several light-years across and probably birthed several stars.<ref name="Arizona">{{Cite web |last=Zabludoff |first=Ann |title= Lecture 13: The Nebular Theory of the origin of the Solar System |url=http://atropos.as.arizona.edu/aiz/teaching/nats102/mario/solar_system.html |url-status=live |archive-url=https://archive.today/20120710135114/http://atropos.as.arizona.edu/aiz/teaching/nats102/mario/solar_system.html |archive-date=10 July 2012 |access-date=27 December 2006 |website=NATS 102: The Physical Universe |publisher=University of Arizona}}</ref> As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elements [[Nuclear fusion|fused]] by previous generations of stars.<ref name=":3" />
[[Image:Oort cloud Sedna orbit.jpg|thumb|400px|The orbits of the bodies in the Solar System to scale (clockwise from top left)]]
The principal component of the Solar System is the Sun, a [[main sequence]] [[stellar classification|G2]] [[star]] that contains 99.86 percent of the system's known [[mass]] and dominates it [[gravitation]]ally.<ref>{{cite journal |author=M Woolfson |title=The origin and evolution of the solar system | doi= 10.1046/j.1468-4004.2000.00012.x |year=2000 |journal=Astronomy & Geophysics |volume=41 |pages=1.12 }}</ref> Jupiter and Saturn, the Sun's two largest orbiting bodies, account for more than 90 percent of the system's remaining mass.{{Ref_label|B|b|none}}


As the [[solar nebula|pre-solar nebula]]<ref name=":3">{{Cite conference |last=Irvine |first=W. M. |date=1983 |title= The chemical composition of the pre-solar nebula |volume=1 |pages=3 |bibcode= 1983coex....1....3I |book-title=Cometary exploration; Proceedings of the International Conference}}</ref> collapsed, [[conservation of angular momentum]] caused it to rotate faster. The center, where most of the mass collected, became increasingly hotter than the surroundings.<ref name="Arizona" /> As the contracting nebula spun faster, it began to flatten into a [[protoplanetary disc]] with a diameter of roughly {{val|200|u=AU}}<ref name="Arizona" /><ref>{{cite journal | title=Embedded Protostellar Disks Around (Sub-)Solar Stars. II. Disk Masses, Sizes, Densities, Temperatures, and the Planet Formation Perspective | last=Vorobyov | first=Eduard I. | journal=The Astrophysical Journal | volume=729 | issue=2 | at=id. 146 | date=March 2011 | doi=10.1088/0004-637X/729/2/146 | arxiv=1101.3090 | bibcode=2011ApJ...729..146V | quote=estimates of disk radii in the Taurus and Ophiuchus star forming regions lie in a wide range between 50 AU and 1000 AU, with a median value of 200 AU.}}</ref> and a hot, dense [[protostar]] at the center.<ref>{{Cite journal |last=Greaves |first=Jane S. |date=7 January 2005 |title=Disks Around Stars and the Growth of Planetary Systems |journal=[[Science (journal)|Science]] |volume=307 |issue=5706 |pages=68–71 |bibcode=2005Sci...307...68G |doi=10.1126/science.1101979 |pmid=15637266 |s2cid=27720602}}</ref><ref>{{Cite book |publisher= Space Studies Board, Committee on Planetary and Lunar Exploration, National Research Council, Division on Engineering and Physical Sciences, National Academies Press |title=Strategy for the Detection and Study of Other Planetary Systems and Extrasolar Planetary Materials: 1990–2000 |date=1990 |isbn=978-0309041935 |publication-place=Washington D.C. |pages=21–33 |chapter=3. Present Understanding of the Origin of Planetary Systems |access-date=9 April 2022 |chapter-url=https://books.google.com/books?id=y56pS7SJs_8C&pg=PT29 |archive-url=https://web.archive.org/web/20220409211803/https://books.google.com/books?id=y56pS7SJs_8C&pg=PT29&lpg=PT29 |archive-date=9 April 2022 |url-status=live}}</ref> The planets formed by [[accretion (astrophysics)|accretion]] from this disc,<ref>{{Cite journal |last1=Boss |first1=A. P. |last2=Durisen |first2=R. H. |date=2005 |title=Chondrule-forming Shock Fronts in the Solar Nebula: A Possible Unified Scenario for Planet and Chondrite Formation |journal=[[The Astrophysical Journal]] |volume=621 |issue=2 |page=L137 |arxiv=astro-ph/0501592 |bibcode=2005ApJ...621L.137B |doi=10.1086/429160 |s2cid=15244154}}</ref> in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies. Hundreds of protoplanets may have existed in the early Solar System, but they either merged or were destroyed or ejected, leaving the planets, dwarf planets, and leftover [[small Solar System body|minor bodies]].<ref name="bennett_8.2" /><ref>{{Cite book |last1=Nagasawa |first1=M. |title=Protostars and Planets V |last2=Thommes |first2=E. W. |last3=Kenyon |first3=S. J. |last4=Bromley |first4=B. C. |last5=Lin |first5=D. N. C. |date=2007 | display-authors = 3 |publisher=University of Arizona Press |editor-last=Reipurth |editor-first=B. |publication-place=Tucson |pages=639–654 |chapter=The Diverse Origins of Terrestrial-Planet Systems |bibcode= 2007prpl.conf..639N |access-date=10 April 2022 |editor-last2=Jewitt |editor-first2=D. |editor-last3=Keil |editor-first3=K. |chapter-url=https://jila.colorado.edu/~pja/astr5820/nagasawa.pdf |archive-url=https://web.archive.org/web/20220412010025/https://jila.colorado.edu/~pja/astr5820/nagasawa.pdf |archive-date=12 April 2022 |url-status=live}}</ref>
Informally, the Solar System is sometimes divided into separate regions. The inner Solar System includes the four [[terrestrial planet]]s and the main asteroid belt. The outer Solar System is beyond the asteroids, including the four [[gas giant]] planets.<ref>{{cite web |title=An Overview of the Solar System |author=nineplanets.org |url=http://www.nineplanets.org/overview.html |accessdate=2007-02-15}}</ref> Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.<ref>{{cite web |title=New Horizons Set to Launch on 9-Year Voyage to Pluto and the Kuiper Belt |author=Amir Alexander |work=The Planetary Society |year=2006 |url=http://www.planetary.org/news/2006/0116_New_Horizons_Set_to_Launch_on_9_Year.html |accessdate=2006-11-08}}</ref>
[[Image:Masses of the planets.png|thumb|The relative masses of the Solar planets. Jupiter at 71% of the total and Saturn at 21% dominate the system. Mercury and Mars, which together are less than 0.1%, are not visible at this scale.]]
[[Kepler's laws of planetary motion]] describe the orbits of objects about the Sun. According to Kepler's laws, each object travels along an [[ellipse]] with the Sun at one [[focus (geometry)|focus]]. Objects closer to the Sun (with smaller [[semi-major axis|semi-major axes]]) have shorter [[year]]s. On an elliptical orbit, a body's distance from the Sun varies over the course of its [[year]]. A body's closest approach to the Sun is called its ''[[perihelion]]'', while its most distant point from the Sun is called its ''[[aphelion]]''. Each body moves fastest at its perihelion and slowest at its aphelion. The orbits of the planets are nearly circular, but many [[comets]], [[asteroid]]s and [[Kuiper belt]] objects follow highly elliptical orbits.


Due to their higher boiling points, only metals and silicates could exist in solid form in the warm inner Solar System close to the Sun (within the [[Frost line (astrophysics)|frost line]]). They eventually formed the rocky planets of Mercury, Venus, Earth, and Mars. Because these [[Refractory (planetary science)|refractory]] materials only comprised a small fraction of the solar nebula, the terrestrial planets could not grow very large.<ref name="bennett_8.2" />
Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the [[ecliptic]]. The planets are very close to the ecliptic while comets and Kuiper belt objects are usually at significantly greater angles to it.


The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where material is cool enough for [[Volatile (astrogeology)|volatile]] icy compounds to remain solid. The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium, the lightest and most abundant elements.<ref name="bennett_8.2" /> Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud.<ref name="bennett_8.2">{{Cite book |last=Bennett |first=Jeffrey O. |title=The cosmic perspective |date=2020 |publisher=Pearson |isbn= 978-0-134-87436-4 |edition=9th |location=Hoboken, New Jersey |chapter= Chapter 8.2}}</ref>
All of the planets and most other objects also orbit with the Sun's rotation (counter-clockwise, as viewed from above the Sun's north pole). There are exceptions, such as [[Halley's Comet]].


Within 50&nbsp;million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin [[nuclear fusion|thermonuclear fusion]].<ref name="Yi2001">{{Cite journal |last1=Yi |first1=Sukyoung |last2=Demarque |first2=Pierre |last3=Kim |first3=Yong-Cheol |last4=Lee |first4=Young-Wook |last5=Ree |first5=Chang H. |last6=Lejeune |first6=Thibault |last7=Barnes |first7= Sydney | display-authors = 3| date=2001 |title=Toward Better Age Estimates for Stellar Populations: The ''Y''<sup>2</sup> Isochrones for Solar Mixture |journal= [[Astrophysical Journal Supplement]] |volume=136 |issue=2 |pages=417–437 |arxiv=astro-ph/0104292 |bibcode= 2001ApJS..136..417Y |doi=10.1086/321795 |s2cid=118940644}}</ref> As helium accumulates at its core, the Sun is growing brighter;<ref name=":4">{{Cite journal |last=Gough |first=D. O. |date=November 1981 |title=Solar Interior Structure and Luminosity Variations |journal=Solar Physics |volume=74 |issue=1 |pages=21–34 |bibcode=1981SoPh...74...21G |doi= 10.1007/BF00151270 |s2cid=120541081}}</ref> early in its main-sequence life its brightness was 70% that of what it is today.<ref>{{Cite journal |last=Shaviv |first=Nir J. |date=2003 |title=Towards a Solution to the Early Faint Sun Paradox: A Lower Cosmic Ray Flux from a Stronger Solar Wind |journal= [[Journal of Geophysical Research]] |volume=108 |issue=A12 |page=1437 |arxiv=astroph/0306477 |bibcode= 2003JGRA..108.1437S |doi= 10.1029/2003JA009997 |s2cid= 11148141}}</ref> The temperature, [[Nuclear reaction rate|reaction rate]], pressure, and density increased until [[hydrostatic equilibrium]] was achieved: the thermal pressure counterbalancing the force of gravity. At this point, the Sun became a [[main sequence|main-sequence]] star.<ref>{{Cite journal |last1= Chrysostomou |first1=A. |last2=Lucas |first2=P. W. |date=2005 |title=The Formation of Stars |journal= [[Contemporary Physics]] |volume=46 |issue=1 |pages=29–40 |bibcode= 2005ConPh..46...29C |doi= 10.1080/0010751042000275277 |s2cid= 120275197}}</ref> Solar wind from the Sun created the [[heliosphere]] and swept away the remaining gas and dust from the protoplanetary disc into interstellar space.<ref name=":4" />
To cope with the vast distances involved, many representations of the Solar System show orbits the same distance apart. In reality, with a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between it and the previous orbit. For example, Venus is approximately 0.33&nbsp;AU{{Ref_label|C|c|none}} farther out than Mercury, while Saturn is 4.3&nbsp;AU out from Jupiter, and Neptune lies 10.5&nbsp;AU out from Uranus. Attempts have been made to determine a correlation between these orbital distances (see [[Titius-Bode law]]), but no such theory has been accepted.


Following the dissipation of the [[protoplanetary disk]], the [[Nice model]] proposes that [[Gravity assist|gravitational encounters]] between planetisimals and the gas giants caused each to [[Planetary migration|migrate]] into different orbits. This led to dynamical instability of the entire system, which scattered the planetisimals and ultimately placed the gas giants in their current positions. During this period, the [[grand tack hypothesis]] suggests that a final inward migration of Jupiter dispersed much of the asteroid belt, leading to the [[Late Heavy Bombardment]] of the inner planets.<ref>{{cite journal | title=Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets | last1=Gomes | first1=R. | last2=Levison | first2=H. F. | last3=Tsiganis | first3=K. | last4=Morbidelli | first4=A. | journal=Nature | year=2005 | volume=435 | pages=466–469 | doi=10.1038/nature03676 | pmid=15917802 | issue=7041 | bibcode=2005Natur.435..466G | doi-access=free }}</ref><ref>{{cite book | last=Crida | first=A. | chapter=Solar System Formation | date=2009 | title=Reviews in Modern Astronomy: Formation and Evolution of Cosmic Structures | volume=21 | pages=215–227 | arxiv=0903.3008 | bibcode= 2009RvMA...21..215C | doi=10.1002/9783527629190.ch12 | isbn=9783527629190 | s2cid=118414100 }}</ref>
Dynamically and physically, objects orbiting the Sun are classed into three categories: ''planets'', ''dwarf planets'' and ''small Solar System bodies''. A [[planet]] is any body in orbit around the Sun that has enough [[mass]] to form itself into a [[sphere|spherical]] shape and has [[Cleared the neighbourhood|cleared its immediate neighbourhood]] of all smaller objects. By this definition, the Solar System has eight known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto was demoted from planetary status, as it has not cleared its orbit of surrounding [[Kuiper belt objects]].<ref name="FinalResolution"/> A [[dwarf planet]] is a [[celestial body]] [[orbit]]ing the [[Sun]] that is massive enough to be rounded by its own [[gravity]] but which has not cleared its neighbouring region of [[planetesimal]]s and is not a [[natural satellite|satellite]].<ref name="FinalResolution"/> By this definition, the Solar System has four known [[dwarf planet]]s: [[Ceres (dwarf planet)|Ceres]], [[Pluto]], [[Makemake (dwarf planet)|Makemake]], and [[Eris (dwarf planet)|Eris]].<ref name=name>{{cite web|date=2008-11-07 11:42:58|title=Dwarf Planets and their Systems|work= Working Group for Planetary System Nomenclature (WGPSN) |url=http://planetarynames.wr.usgs.gov/append7.html#DwarfPlanets| accessdate=2008-07-13 | publisher= U.S. Geological Survey }}</ref> Other objects that may become classified as dwarf planets are [[90377 Sedna|Sedna]], [[90482 Orcus|Orcus]], and [[50000 Quaoar|Quaoar]]. Dwarf planets that orbit in the trans-Neptunian region are called "[[plutoid]]s."<ref name="IAU0804">{{cite web
|date=Jun 11, 2008, Paris
|title=Plutoid chosen as name for Solar System objects like Pluto
|publisher=[[International Astronomical Union]] (News Release - IAU0804)
|url=http://www.iau.org/public_press/news/release/iau0804
|accessdate=2008-06-11}}</ref> The remainder of the objects in orbit around the Sun are [[small Solar System body|small Solar System bodies]].<ref name="FinalResolution">{{cite news |title=The Final IAU Resolution on the definition of "planet" ready for voting |publisher=IAU |date=2006-08-24 |url=http://www.iau.org/iau0602.423.0.html |accessdate=2007-03-02}}</ref>


=== Present and future ===
Most of the planets in the Solar System possess secondary systems of their own. Many are in turn orbited by planetary objects called [[natural satellites]], or moons, some of which are larger than planets. Most of the largest natural satellites are in [[synchronous orbit]], with one face permanently turned toward their parent. The four largest planets also possess [[planetary ring]]s, thin bands of tiny particles that orbit them in unison.
The Solar System remains in a relatively stable, slowly evolving state by following isolated, [[gravitationally bound]] orbits around the Sun.<ref>{{Cite journal |last1=Malhotra |first1=R. |last2=Holman |first2=Matthew |last3=Ito |first3=Takashi |date=October 2001 |title=Chaos and stability of the solar system |journal= Proceedings of the National Academy of Sciences |volume=98 |issue=22 |pages=12342–12343 |bibcode= 2001PNAS...9812342M |doi=10.1073/pnas.231384098 |pmc=60054 |pmid=11606772 |doi-access=free}}</ref> Although the Solar System has been fairly stable for billions of years, it is technically [[chaotic system|chaotic]], and may [[stability of the Solar System|eventually be disrupted]]. There is a small chance that another star will pass through the Solar System in the next few billion years. Although this could destabilize the system and eventually lead millions of years later to expulsion of planets, collisions of planets, or planets hitting the Sun, it would most likely leave the Solar System much as it is today.<ref>{{cite journal |first1=Sean |last1=Raymond |display-authors=etal |date=27 November 2023 |title=Future trajectories of the Solar System: dynamical simulations of stellar encounters within 100 au |url=https://academic.oup.com/mnras/article/527/3/6126/7452883?login=false |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=527 |issue=3 |pages=6126–6138 |arxiv=2311.12171 |bibcode=2024MNRAS.527.6126R |doi=10.1093/mnras/stad3604 |doi-access=free |access-date=10 December 2023 |archive-date=10 December 2023 |archive-url=https://web.archive.org/web/20231210152219/https://academic.oup.com/mnras/article/527/3/6126/7452883?login=false |url-status=live }}</ref>


[[File:Sun_red_giant.svg|thumb|The current Sun compared to its peak size in the red-giant phase]]
Planetary scientists use the terms ''gas'', ''ice'', and ''rock'' to describe the various classes of substances found throughout the Solar System. ''Rock'' is used to describe compounds with high melting points (greater than roughly 500&nbsp;K), such as [[silicate]]s. Rocky substances are prevalent in the inner Solar System, forming most of the [[terrestrial planet]]s and [[asteroid]]s. ''Gases'' are materials with low melting points such as [[atomic hydrogen]], [[helium]], and [[noble gases]]; they dominate the middle region, comprising most of Jupiter, Saturn, Uranus and Neptune. ''Ices'', like [[water]], [[methane]], [[ammonia]], and [[carbon dioxide]],<ref>{{cite journal | doi=10.1016/j.icarus.2007.04.009 | bibcode=2007Icar..190..345F | title=Asymmetries in the distribution of H2O and CO2 in the inner coma of Comet 9P/Tempel 1 as observed by Deep Impact | year=2007 | author=Feaga, L | journal=Icarus | volume=190 | pages=345 }}</ref> have melting points up to a few hundred [[Kelvin]]. Icy substances comprise the majority of the region beyond Nepune.<ref name=zeilik>{{cite book | pages=p. 240 | author= Michael Zellik| title=Astronomy: The Evolving Universe | edition= 9th ed. | year=2002 | publisher=Cambridge University Press | isbn=0521800900 }}</ref>
The Sun's main-sequence phase, from beginning to end, will last about 10&nbsp;billion years for the Sun compared to around two billion years for all other subsequent phases of the Sun's pre-[[stellar remnant|remnant]] life combined.<ref name= "mnras386_1">{{Cite journal |last1= Schröder |first1=K.-P. |last2=Connon Smith |first2=Robert |date=May 2008 |title= Distant future of the Sun and Earth revisited |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=386 |issue=1 |pages=155–163 |arxiv=0801.4031 |bibcode=2008MNRAS.386..155S |doi=10.1111/j.1365-2966.2008.13022.x |doi-access=free |s2cid=10073988}}</ref> The Solar System will remain roughly as it is known today until the hydrogen in the core of the Sun has been entirely converted to helium, which will occur roughly 5&nbsp;billion years from now. This will mark the end of the Sun's main-sequence life. At that time, the core of the Sun will contract with hydrogen fusion occurring along a shell surrounding the inert helium, and the energy output will be greater than at present. The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a [[red giant]]. Because of its increased surface area, the surface of the Sun will be cooler ({{Convert|2,600|K|F}} at its coolest) than it is on the main sequence.<ref name="mnras386_1" />
The term ''[[volatiles]]'' refers collectively to all materials with low boiling points (less than a few hundred [[Kelvin]]), including gases and ices; depending on the temperature, volatiles can be found as ices, liquids, or gases in various places in the Solar System.


The expanding Sun is expected to vaporize Mercury as well as Venus, and render Earth and Mars uninhabitable (possibly destroying Earth as well).<ref>{{cite web | url=https://science.nasa.gov/universe/exoplanets/giant-red-stars-may-heat-frozen-worlds-into-habitable-planets/ | title=Giant red stars may heat frozen worlds into habitable planets - NASA Science }}</ref><ref>{{cite journal |last1= Aungwerojwit |first1=Amornrat |last2= Gänsicke |first2=Boris T |last3=Dhillon |first3=Vikram S |last4=Drake |first4= Andrew |last5=Inight |first5=Keith |last6= Kaye |first6=Thomas G |last7=Marsh |first7=T R |last8=Mullen |first8=Ed |last9= Pelisoli |first9=Ingrid |last10=Swan |first10=Andrew | display-authors = 3 |title=Long-term variability in debris transiting white dwarfs |journal=[[Monthly Notices of the Royal Astronomical Society]] |date=2024 |volume=530 |issue=1 |pages=117–128 |doi=10.1093/mnras/stae750 |doi-access=free|arxiv=2404.04422 }}</ref> Eventually, the core will be hot enough for helium fusion; the Sun will burn helium for a fraction of the time it burned hydrogen in the core. The Sun is not massive enough to commence the fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will be ejected into space, leaving behind a dense [[white dwarf]], half the original mass of the Sun but only the size of Earth.<ref name="mnras386_1"/> The ejected outer layers may form a [[planetary nebula]], returning some of the material that formed the Sun—but now enriched with [[metallicity|heavier elements]] like carbon—to the [[interstellar medium]].<ref>{{Cite web|title=Planetary Nebulas|url=https://www.cfa.harvard.edu/research/topic/planetary-nebulas|access-date=6 April 2024|publisher=Harvard & Smithsonian Center for Astrophysics|website=cfa.harvard.edu|archive-date=6 April 2024|archive-url=https://web.archive.org/web/20240406205913/https://www.cfa.harvard.edu/research/topic/planetary-nebulas|url-status=live}}</ref><ref>{{Cite journal|url=https://www.nature.com/articles/s41550-018-0453-9.epdf?sharing_token=XozRTVzMDBR74HQBj2lbV9RgN0jAjWel9jnR3ZoTv0OzwWt8mLOdVW4Y_YiE39Le3Xp-8zVx5tUnLpAORu5j1mnJNZpxp_fWsbZgn60hEE3IHsu89UrtgD6uRRVi7jD74SBwEYsmB2RyB2RCfRqLbLr5EqTy1-rK2KrrLO-TxuHwLmapWXxYkuOn5Rgut4w4JuE1XKNeJeRNDNx_0juT0bPlXn9WB29_BzKx1pGlzEXtR677aZ3SUe5um8epWM4PgYT-VDXR6Jevm-M9SDszF4a2eWOeV0CdynDONJuE1n37sanK9itS1edHH_xrybrldJgWdACO4sxHnFn3DHdB0Q==|title=The mysterious age invariance of the planetary nebula luminosity function bright cut-off|first1=K.|last1=Gesicki|first2=A. A.|last2=Zijlstra|first3=M. M.|last3=Miller Bertolami|date=7 May 2018|journal=Nature Astronomy|volume=2|issue=7|pages=580–584|doi=10.1038/s41550-018-0453-9|arxiv=1805.02643|bibcode=2018NatAs...2..580G|hdl=11336/82487|s2cid=256708667|access-date=16 January 2024|archive-date=16 January 2024|archive-url=https://web.archive.org/web/20240116173409/https://www.nature.com/articles/s41550-018-0453-9.epdf?sharing_token=XozRTVzMDBR74HQBj2lbV9RgN0jAjWel9jnR3ZoTv0OzwWt8mLOdVW4Y_YiE39Le3Xp-8zVx5tUnLpAORu5j1mnJNZpxp_fWsbZgn60hEE3IHsu89UrtgD6uRRVi7jD74SBwEYsmB2RyB2RCfRqLbLr5EqTy1-rK2KrrLO-TxuHwLmapWXxYkuOn5Rgut4w4JuE1XKNeJeRNDNx_0juT0bPlXn9WB29_BzKx1pGlzEXtR677aZ3SUe5um8epWM4PgYT-VDXR6Jevm-M9SDszF4a2eWOeV0CdynDONJuE1n37sanK9itS1edHH_xrybrldJgWdACO4sxHnFn3DHdB0Q==|url-status=live}}</ref>
== Sun ==
{{main|Sun}}
[[Image:Sun in X-Ray.png|thumb|right|The Sun as seen in the [[x-ray]] region of the [[electromagnetic spectrum]]]]


==General characteristics==
The Sun is the Solar System's parent star, and far and away its chief component. Its large mass gives it an interior [[density]] high enough to sustain [[nuclear fusion]], which releases enormous amounts of [[energy]], mostly [[radiant energy|radiated]] into [[outer space|space]] as [[electromagnetic radiation]] such as [[visible spectrum|visible light]].
Astronomers sometimes divide the Solar System structure into separate regions. The [[inner Solar System]] includes Mercury, Venus, Earth, Mars, and the bodies in the [[asteroid belt]]. The [[outer Solar System]] includes Jupiter, Saturn, Uranus, Neptune, and the bodies in the [[Kuiper belt]].<ref>{{Cite web |title=The Planets |url=https://science.nasa.gov/solar-system/planets/ |access-date=6 April 2024 |publisher=NASA}}</ref> Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of [[Trans-Neptunian object|the objects beyond Neptune]].<ref>{{Cite web |title=Kuiper Belt: Facts |url=https://science.nasa.gov/solar-system/kuiper-belt/facts/ |access-date=6 April 2024 |publisher=NASA |archive-date=12 March 2024 |archive-url=https://web.archive.org/web/20240312024528/https://science.nasa.gov/solar-system/kuiper-belt/facts/ |url-status=live }}</ref>


=== Composition ===
The Sun is classified as a moderately large [[yellow dwarf]], but this name is misleading as, compared to stars in [[Milky Way|our galaxy]], the Sun is rather large and bright. Stars are classified by the [[Hertzsprung-Russell diagram]], a graph which plots the brightness of stars against their surface [[temperature]]s. Generally, hotter stars are brighter. Stars following this pattern are said to be on the [[main sequence]]; the Sun lies right in the middle of it. However, stars brighter and hotter than the Sun are rare, while stars dimmer and cooler are common.<ref>{{cite web |year=2001 |author=Smart, R. L.; Carollo, D.; Lattanzi, M. G.; McLean, B.; Spagna, A. |title=The Second Guide Star Catalogue and Cool Stars |work=Perkins Observatory |url=http://adsabs.harvard.edu/abs/2001udns.conf..119S |accessdate=2006-12-26}}</ref>
{{Further|List of Solar System objects|List of interstellar and circumstellar molecules}}


The principal component of the Solar System is the Sun, a [[G-type main-sequence star]] that contains 99.86% of the system's known mass and dominates it gravitationally.<ref>{{Cite journal |last=Woolfson |first=M. |date=2000 |title=The origin and evolution of the solar system |journal=[[Astronomy & Geophysics]] |volume=41 |issue=1 |pages=1.12–1.19 |bibcode=2000A&G....41a..12W |doi=10.1046/j.1468-4004.2000.00012.x |doi-access=free}}</ref> The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, [[asteroid]]s, and comets) together comprise less than 0.002% of the Solar System's total mass.{{Refn |The mass of the Solar System excluding the Sun, Jupiter and Saturn can be determined by adding together all the calculated masses for its largest objects and using rough calculations for the masses of the Oort cloud (estimated at roughly 3 Earth masses),<ref>{{Cite arXiv |eprint=astro-ph/0512256 |first=Alessandro |last=Morbidelli |title=Origin and dynamical evolution of comets and their reservoirs |date=2005}}</ref> the Kuiper belt (estimated at 0.1 Earth mass)<ref name="Delsanti-Beyond_The_Planets"/> and the asteroid belt (estimated to be 0.0005 Earth mass)<ref name="Krasinsky2002"/> for a total, rounded upwards, of ~37 Earth masses, or 8.1% of the mass in orbit around the Sun. With the combined masses of Uranus and Neptune (~31 Earth masses) subtracted, the remaining ~6 Earth masses of material comprise 1.3% of the total orbiting mass.|name=footnoteD|group=lower-alpha}}
It is believed that the Sun's position on the main sequence puts it in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion. The Sun is growing brighter; early in its history it was 75 percent as bright as it is today.<ref name="Kasting">{{cite journal |last=Kasting |first=J.F. |coauthors=Ackerman, T.P. |year=1986 |title=Climatic Consequences of Very High Carbon Dioxide Levels in the Earth's Early Atmosphere |journal=Science |volume=234 |pages=1383–1385 |doi=10.1126/science.11539665 |pmid=11539665}}</ref>


The Sun is composed of roughly 98% hydrogen and helium,<ref>{{Cite web |title=The Sun's Vital Statistics |url=http://solar-center.stanford.edu/vitalstats.html |url-status=live |archive-url=https://www.webcitation.org/6BOkQXma3?url=http://solar-center.stanford.edu/vitalstats.html |archive-date=14 October 2012 |access-date=29 July 2008 |publisher=Stanford Solar Center |postscript=,}} citing {{Cite book
The Sun is a [[metallicity|population I star]]; it was born in the later stages of the [[Universe#Evolution|universe's evolution]]. It contains more elements heavier than hydrogen and helium ("[[metallicity|metals]]" in astronomical parlance) than older population II stars.<ref>{{cite journal |author=T. S. van Albada, Norman Baker |title=On the Two Oosterhoff Groups of Globular Clusters |journal=Astrophysical Journal |volume=185 |year=1973 |pages=477–498 |doi=10.1086/152434}}</ref> Elements heavier than hydrogen and helium were formed in the [[solar core|cores]] of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, while stars born later have more. This high metallicity is thought to have been crucial to the Sun's developing a [[planetary system]], because planets form from accretion of metals.<ref> {{cite web |title=An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect |author=Charles H. Lineweaver |work=University of New South Wales |date=2001-03-09 |url=http://arxiv.org/abs/astro-ph/0012399 |accessdate=2006-07-23}}</ref>
|last=Eddy
|first=J.
|title=A New Sun: The Solar Results From Skylab
|url=https://history.nasa.gov/SP-402/contents.htm
|publisher=[[NASA]]
|date=1979
|page=37
|id=NASA SP-402
|access-date=12 July 2017
|archive-date=30 July 2021
|archive-url=https://web.archive.org/web/20210730024856/https://history.nasa.gov/SP-402/contents.htm
|url-status=live
}}</ref> as are Jupiter and Saturn.<ref>{{Cite web |last=Williams |first=David R. |date=7 September 2006 |title=Saturn Fact Sheet |url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturnfact.html |url-status=dead |archive-url=https://web.archive.org/web/20110804224236/http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturnfact.html |archive-date=4 August 2011 |access-date=31 July 2007 |publisher=NASA }}</ref><ref name="Williams-Jupiter" /> A composition gradient exists in the Solar System, created by heat and [[light pressure]] from the early Sun; those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points. Objects farther from the Sun are composed largely of materials with lower melting points.<ref>{{Cite book |last1=Weissman |first1=Paul Robert |url=https://archive.org/details/encyclopediaofso0000unse_u6d1/page/615 |title=Encyclopedia of the solar system |last2=Johnson |first2=Torrence V. |date=2007 |publisher=Academic Press |isbn=978-0-12-088589-3 |page=[https://archive.org/details/encyclopediaofso0000unse_u6d1/page/615 615]}}</ref> The boundary in the Solar System beyond which those volatile substances could coalesce is known as the [[Frost line (astrophysics)|frost line]], and it lies at roughly five times the Earth's distance from the Sun.<ref name="Mumma" />


=== Orbits ===
[[Image:Heliospheric-current-sheet.gif|left|thumb|The [[heliospheric current sheet]].]]
===Interplanetary medium===
{{main|Interplanetary medium}}


[[File:Solar system orrery inner planets.gif|thumb|Animations of the Solar System's [[inner planet]]s orbiting. Each frame represents 2 days of motion.]]
Along with [[light]], the Sun radiates a continuous stream of charged particles (a [[Plasma (physics)|plasma]]) known as the [[solar wind]]. This stream of particles spreads outwards at roughly 1.5 million kilometres per hour,<ref>{{cite web |title=Solar Physics: The Solar Wind |work=Marshall Space Flight Center |date=2006-07-16<!--Internet Archive estimate--> |url=http://solarscience.msfc.nasa.gov/SolarWind.shtml |accessdate=2006-10-03}}</ref> creating a tenuous atmosphere (the [[heliosphere]]) that permeates the Solar System out to at least 100&nbsp;AU (see [[#Heliopause|heliopause]]). This is known as the [[interplanetary medium]]. [[Geomagnetic storm]]s on the Sun's surface, such as [[solar flare]]s and [[coronal mass ejection]]s, disturb the heliosphere, creating [[space weather]].<ref name="SunFlip">{{cite web |url=http://science.nasa.gov/headlines/y2001/ast15feb_1.htm |title=The Sun Does a Flip |accessdate=2007-02-04 |last=Phillips |first=Tony |date=2001-02-15 |work=Science@NASA}}</ref> The Sun's rotating magnetic field acts on the interplanetary medium to create the [[heliospheric current sheet]], the largest structure in the Solar System.<ref>{{cite web |title=Artist's Conception of the Heliospheric Current Sheet |work=Wilcox Solar Observatory |url=http://quake.stanford.edu/~wso/gifs/HCS.html |accessdate=2006-06-22}}</ref>
[[File:Solar system orrery outer planets.gif|thumb|Animations of the Solar System's [[outer planet]]s orbiting. This animation is 100 times faster than the inner planet animation.]]


The planets and other large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the [[ecliptic]]. Smaller icy objects such as comets frequently orbit at significantly greater angles to this plane.<ref name="Levison2003">{{Cite journal |last1=Levison |first1=H.F. |author-link=Harold F. Levison |last2=Morbidelli |first2=A. |date=27 November 2003 |title=The formation of the Kuiper belt by the outward transport of bodies during Neptune's migration |journal=[[Nature (journal)|Nature]] |volume=426 |issue=6965 |pages=419–421 |bibcode=2003Natur.426..419L |doi=10.1038/nature02120 |pmid=14647375 |s2cid=4395099}}</ref><ref>{{Cite journal |last1=Levison |first1=Harold F. |last2=Duncan |first2=Martin J. |date=1997 |title=From the Kuiper Belt to Jupiter-Family Comets: The Spatial Distribution of Ecliptic Comets |journal=[[Icarus (journal)|Icarus]] |volume=127 |issue=1 |pages=13–32 |bibcode=1997Icar..127...13L |doi=10.1006/icar.1996.5637}}</ref> Most of the planets in the Solar System have secondary systems of their own, being orbited by natural satellites called moons. All of the largest natural satellites are in [[synchronous rotation]], with one face permanently turned toward their parent. The four giant planets have planetary rings, thin discs of tiny particles that orbit them in unison.<ref name="bennett_4.5">{{Cite book |last1=Bennett |first1=Jeffrey O. |title=The Cosmic Perspective |last2=Donahue |first2=Megan |last3=Schneider |first3=Nicholas |last4=Voit |first4=Mark |date=2020 |publisher=Pearson |isbn=978-0-134-87436-4 |edition=9th |location=Hoboken, NJ |chapter=4.5 Orbits, Tides, and the Acceleration of Gravity |oclc=1061866912 |author-link2=Megan Donahue}}</ref>
[[Image:Aurora-SpaceShuttle-EO.jpg|thumb|right|[[Aurora australis]] seen from orbit.]]


As a result of the [[Formation and evolution of the Solar System|formation of the Solar System]], planets and most other objects orbit the Sun in the same direction that the Sun is rotating. That is, counter-clockwise, as viewed from above Earth's north pole.<ref>{{Cite magazine |last=Grossman |first=Lisa |date=13 August 2009 |title=Planet found orbiting its star backwards for first time |url=https://www.newscientist.com/article/dn17603-planet-found-orbiting-its-star-backwards-for-first-time.html |url-status=live |magazine=New Scientist |archive-url=https://web.archive.org/web/20121017083955/http://www.newscientist.com/article/dn17603-planet-found-orbiting-its-star-backwards-for-first-time.html |archive-date=17 October 2012 |access-date=10 October 2009}}</ref> There are exceptions, such as [[Halley's Comet]].<ref>{{Cite web |last=Nakano |first=Syuichi |date=2001 |title=OAA computing section circular |url=http://www.oaa.gr.jp/~oaacs/nk/nk866.htm |url-status=live |archive-url=https://web.archive.org/web/20190921103057/http://www.oaa.gr.jp/~oaacs/nk/nk866.htm |archive-date=21 September 2019 |access-date=15 May 2007 |publisher=Oriental Astronomical Association}}</ref> Most of the larger moons orbit their planets in [[Retrograde and prograde motion|prograde]] direction, matching the direction of planetary rotation; Neptune's moon [[Triton (moon)|Triton]] is the largest to orbit in the opposite, retrograde manner.<ref>{{Cite journal |last1=Agnor |first1=Craig B. |last2=Hamilton |first2=Douglas P. |date=May 2006 |title=Neptune's capture of its moon Triton in a binary–planet gravitational encounter |url=https://www.nature.com/articles/nature04792 |url-status=live |journal=[[Nature (journal)|Nature]] |language=en |volume=441 |issue=7090 |pages=192–194 |bibcode=2006Natur.441..192A |doi=10.1038/nature04792 |issn=1476-4687 |pmid=16688170 |s2cid=4420518 |archive-url=https://web.archive.org/web/20220415081402/https://www.nature.com/articles/nature04792 |archive-date=15 April 2022 |access-date=28 March 2022}}</ref> Most larger objects rotate around their own axes in the prograde direction relative to their orbit, though the rotation of Venus is retrograde.<ref>{{Cite book |last=Gallant |first=Roy A. |url=https://www.worldcat.org/oclc/6533014 |title=National Geographic Picture Atlas of Our Universe |date=1980 |publisher=National Geographic Society |isbn=0-87044-356-9 |editor-last=Sedeen |editor-first=Margaret |location=Washington, D.C. |pages=82 |oclc=6533014 |access-date=28 March 2022 |archive-url=https://web.archive.org/web/20220420161217/https://www.worldcat.org/title/national-geographic-picture-atlas-of-our-universe/oclc/6533014 |archive-date=20 April 2022 |url-status=live}}</ref>
[[Earth's magnetic field]] protects [[Earth's atmosphere|its atmosphere]] from interacting with the solar wind. Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space.<ref>{{cite science |last=Lundin |first=Richard |date=2001-03-09 |title=Erosion by the Solar Wind |author=Rickard Lundin |journal=Science |volume=291 |issue=5510 |pages=1909 |doi=10.1126/science.1059763 |url=http://sciencemag.org/cgi/content/full/291/5510/1909 |accessdate=2006-12-26|abstract=http://sciencemag.org/cgi/content/summary/291/5510/1909}}</ref> The interaction of the solar wind with Earth's magnetic field creates the [[Aurora (astronomy)|aurorae]] seen near the [[Earth's magnetic field#Magnetic poles|magnetic poles]].


To a good first approximation, [[Kepler's laws of planetary motion]] describe the orbits of objects around the Sun.<ref name=":0">{{Cite book |last1=Frautschi |first1=Steven C. |title=The Mechanical Universe: Mechanics and Heat |title-link=The Mechanical Universe |last2=Olenick |first2=Richard P. |last3=Apostol |first3=Tom M. |last4=Goodstein |first4=David L. |date=2007 |publisher=Cambridge University Press |isbn=978-0-521-71590-4 |edition=Advanced |location=Cambridge [Cambridgeshire] |oclc=227002144 |author-link=Steven Frautschi |author-link3=Tom M. Apostol |author-link4=David L. Goodstein}}</ref>{{Rp|pages=433–437}} These laws stipulate that each object travels along an [[ellipse]] with the Sun at one [[focus (geometry)|focus]], which causes the body's distance from the Sun to vary over the course of its year. A body's closest approach to the Sun is called its ''[[perihelion]]'', whereas its most distant point from the Sun is called its ''[[aphelion]]''.<ref name=":8">{{Cite book |last1=Feynman |first1=Richard P. |title=The Feynman Lectures on Physics, Volume 1 |title-link=The Feynman Lectures on Physics |last2=Leighton |first2=Robert B. |last3=Sands |first3=Matthew L. |date=1989 |publisher=Addison-Wesley Pub. Co |isbn=0-201-02010-6 |location=Reading, Mass. |oclc=531535 |author-link=Richard Feynman |author-link2=Robert B. Leighton |author-link3=Matthew Sands |orig-date=1965}}</ref>{{Rp|location=9-6}} With the exception of Mercury, the orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits. Kepler's laws only account for the influence of the Sun's gravity upon an orbiting body, not the gravitational pulls of different bodies upon each other. On a human time scale, these perturbations can be accounted for using [[numerical model of the Solar System|numerical models]],<ref name=":8" />{{Rp|location=9-6}} but the planetary system can change chaotically over billions of years.<ref>{{Cite journal |last1=Lecar |first1=Myron |last2=Franklin |first2=Fred A. |last3=Holman |first3=Matthew J. |last4=Murray |first4=Norman J. |date=2001 |title=Chaos in the Solar System |journal=Annual Review of Astronomy and Astrophysics |volume=39 |issue=1 |pages=581–631 |arxiv=astro-ph/0111600 |bibcode=2001ARA&A..39..581L |doi=10.1146/annurev.astro.39.1.581 |s2cid=55949289}}</ref>
[[Cosmic ray]]s originate outside the Solar System. The heliosphere partially shields the Solar System, and planetary magnetic fields (for those planets that have them) also provide some protection. The density of cosmic rays in the [[interstellar medium]] and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic radiation in the Solar System varies, though by how much is unknown.<ref name="Langner_et_al_2005">{{cite journal |last=Langner |first=U. W. |coauthors=M.S. Potgieter |year=2005 |title=Effects of the position of the solar wind termination shock and the heliopause on the heliospheric modulation of cosmic rays |journal=Advances in Space Research |volume=35 |issue=12 |pages=2084–2090 |doi=10.1016/j.asr.2004.12.005 |url=http://adsabs.harvard.edu/abs/2005AdSpR..35.2084L |accessdate=2007-02-11}}</ref>


The [[angular momentum]] of the Solar System is a measure of the total amount of orbital and [[rotational momentum]] possessed by all its moving components.<ref>{{Cite book |last=Piccirillo |first=Lucio |url=https://books.google.com/books?id=W0jpDwAAQBAJ&pg=PA210 |title=Introduction to the Maths and Physics of the Solar System |date=2020 |publisher=CRC Press |isbn=978-0429682803 |page=210 |access-date=10 May 2022 |archive-url=https://web.archive.org/web/20220730084321/https://www.google.com/books/edition/Introduction_to_the_Maths_and_Physics_of/W0jpDwAAQBAJ?gbpv=1&pg=PA210 |archive-date=30 July 2022 |url-status=live}}</ref> Although the Sun dominates the system by mass, it accounts for only about 2% of the angular momentum.<ref name="Marochnik1995">{{Cite conference |last1=Marochnik |first1=L. |last2=Mukhin |first2=L. |date=1995 |title=Is Solar System Evolution Cometary Dominated? |series=Astronomical Society of the Pacific Conference Series |volume=74 |page=83 |bibcode=1995ASPC...74...83M |isbn=0-937707-93-7 |book-title=Progress in the Search for Extraterrestrial Life |editor=Shostak, G.S.}}</ref><ref>{{Cite journal |last1=Bi |first1=S. L. |last2=Li |first2=T. D. |last3=Li |first3=L. H. |last4=Yang |first4=W. M. |year=2011 |title=Solar Models with Revised Abundance |journal=[[The Astrophysical Journal]] |volume=731 |issue=2 |pages=L42 |arxiv=1104.1032 |bibcode=2011ApJ...731L..42B |doi=10.1088/2041-8205/731/2/L42 |s2cid=118681206}}</ref> The planets, dominated by Jupiter, account for most of the rest of the angular momentum due to the combination of their mass, orbit, and distance from the Sun, with a possibly significant contribution from comets.<ref name="Marochnik1995" />
The interplanetary medium is home to at least two disc-like regions of [[cosmic dust]]. The first, the [[interplanetary dust cloud|zodiacal dust cloud]], lies in the inner Solar System and causes [[zodiacal light]]. It was likely formed by collisions within the asteroid belt brought on by interactions with the planets.<ref>{{cite web |year=1998 |title=Long-term Evolution of the Zodiacal Cloud |url=http://astrobiology.arc.nasa.gov/workshops/1997/zodiac/backman/IIIc.html |accessdate=2007-02-03}}</ref> The second extends from about 10 AU to about 40 AU, and was probably created by similar collisions within the Kuiper belt.<ref>{{cite web |year=2003 |title=ESA scientist discovers a way to shortlist stars that might have planets |work=ESA Science and Technology |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=29471 |accessdate=2007-02-03}}</ref><ref>{{cite journal |last=Landgraf |first=M. |coauthors=Liou, J.-C.; Zook, H. A.; Grün, E. |month=May |year=2002 |title=Origins of Solar System Dust beyond Jupiter |journal=The Astronomical Journal |volume=123 |issue=5 |pages=2857–2861 |doi=10.1086/339704 |url=http://www.iop.org/EJ/article/1538-3881/123/5/2857/201502.html |accessdate=2007-02-09}}</ref>


== Inner Solar System ==
=== Distances and scales ===
[[File:Solar System distance to scale.svg|center|thumb|upright=2.5|To-scale diagram of distance between planets, with the white bar showing orbital variations. The size of the planets is not to scale.]]
The inner Solar System is the traditional name for the region comprising the terrestrial planets and asteroids. Composed mainly of [[silicate]]s and metals, the objects of the inner Solar System huddle very closely to the Sun; the radius of this entire region is shorter than the distance between Jupiter and Saturn.
===Inner planets=== <!--This heading linked from [[Extrasolar planet]]-->
{{main|Terrestrial planet}}
[[Image:Terrestrial planet size comparisons.jpg|thumb|The inner planets. From left to right: [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], and [[Mars]] (sizes to scale)]]


The radius of the Sun is {{Cvt|0.0047|AU|km mi|sigfig=1|abbr=unit}}.<ref name="arxiv1203_4898">{{Cite journal |last1=Emilio |first1=Marcelo |last2=Kuhn |first2=Jeff R. |last3=Bush |first3=Rock I. |last4=Scholl |first4=Isabelle F. |year=2012 |title=Measuring the Solar Radius from Space during the 2003 and 2006 Mercury Transits |journal=[[The Astrophysical Journal]] |volume=750 |issue=2 |page=135 |arxiv=1203.4898 |bibcode=2012ApJ...750..135E |doi=10.1088/0004-637X/750/2/135 |s2cid=119255559}}</ref> Thus, the Sun occupies 0.00001% (1 part in 10<sup>7</sup>) of the volume of a sphere with a radius the size of Earth's orbit, whereas Earth's volume is roughly 1&nbsp;millionth (10<sup>−6</sup>) that of the Sun. Jupiter, the largest planet, is {{val|5.2|u=AU}} from the Sun and has a radius of {{Convert|71000|km|AU mi|abbr=on|sigfig=2}}, whereas the most distant planet, Neptune, is {{val|30|u=AU}} from the Sun.<ref name="Williams-Jupiter">{{Cite web |last=Williams |first=David R. |date=23 December 2021 |title=Jupiter Fact Sheet |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html |url-status=live |archive-url=https://web.archive.org/web/20180122180353/https://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html |archive-date=22 January 2018 |access-date=28 March 2022 |website=NASA Goddard Space Flight Center}}</ref><ref>{{Cite web |last=Williams |first=David R. |date=23 December 2021 |title=Neptune Fact Sheet |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html |url-status=live |archive-url=https://web.archive.org/web/20161119045252/http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html |archive-date=19 November 2016 |access-date=28 March 2022 |website=NASA Goddard Space Flight Center}}</ref>
The four inner or [[terrestrial planet]]s have dense, [[rock (geology)|rocky]] compositions, few or no [[natural satellite|moons]], and no [[planetary ring|ring systems]]. They are composed largely of minerals with high melting points, such as the [[silicate]]s which form their [[crust (geology)|crusts]] and [[mantle (geology)|mantles]], and metals such as [[iron]] and [[nickel]], which form their [[planetary core|cores]]. Three of the four inner planets (Venus, Earth and Mars) have substantial [[atmosphere]]s; all have [[impact crater]]s and [[tectonics|tectonic]] surface features such as [[rift valley]]s and [[volcano]]es. The term ''inner planet'' should not be confused with ''[[inferior planet]]'', which designates those planets which are closer to the Sun than Earth is (i.e. Mercury and Venus).


With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and the orbit of the next nearest object to the Sun. For example, Venus is approximately 0.33&nbsp;AU farther out from the Sun than Mercury, whereas Saturn is 4.3&nbsp;AU out from Jupiter, and Neptune lies 10.5&nbsp;AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances, like the [[Titius–Bode law]]<ref>{{Cite journal |last=Jaki |first=Stanley L. |date=1 July 1972 |title=The Early History of the Titius-Bode Law |url=https://aapt.scitation.org/doi/abs/10.1119/1.1986734 |url-status=live |journal=American Journal of Physics |volume=40 |issue=7 |pages=1014–1023 |bibcode=1972AmJPh..40.1014J |doi=10.1119/1.1986734 |issn=0002-9505 |archive-url=https://web.archive.org/web/20220420161227/https://aapt.scitation.org/doi/abs/10.1119/1.1986734 |archive-date=20 April 2022 |access-date=2 April 2022}}</ref> and [[Mysterium Cosmographicum|Johannes Kepler's model]] based on the [[Platonic solid]]s,<ref>{{Cite journal |last=Phillips |first=J. P. |date=1965 |title=Kepler's Echinus |journal=Isis |volume=56 |issue=2 |pages=196–200 |doi=10.1086/349957 |issn=0021-1753 |jstor=227915 |s2cid=145268784}}</ref> but ongoing discoveries have invalidated these hypotheses.<ref name="Boss">{{Cite magazine |last=Boss |first=Alan |date=October 2006 |title=Is it a coincidence that most of the planets fall within the Titius-Bode law's boundaries? |url=https://astronomy.com/magazine/ask-astro/2006/10/is-it-a-coincidence-that-most-of-the-planets-fall-within-the-titius-bode-laws-boundaries |url-status=live |magazine=Astronomy |volume=30 |issue=10 |page=70 |archive-url=https://web.archive.org/web/20220316135255/https://astronomy.com/magazine/ask-astro/2006/10/is-it-a-coincidence-that-most-of-the-planets-fall-within-the-titius-bode-laws-boundaries |archive-date=16 March 2022 |access-date=9 April 2022 |series=Ask Astro}}</ref>
; Mercury
: [[Mercury (planet)|Mercury]] (0.4&nbsp;AU) is the closest planet to the Sun and the smallest planet (0.055 Earth masses). Mercury has no [[natural satellite]]s, and its only known geological features besides impact craters are lobed ridges or [[rupes]], probably produced by a period of contraction early in its history.<ref>Schenk P., Melosh H.J. (1994), ''Lobate Thrust Scarps and the Thickness of Mercury's Lithosphere'', Abstracts of the 25th Lunar and Planetary Science Conference, 1994LPI....25.1203S</ref> Mercury's almost negligible atmosphere consists of atoms blasted off its surface by the solar wind.<ref>{{cite web |year=2006 |author=Bill Arnett |title=Mercury |work=The Nine Planets |url=http://www.nineplanets.org/mercury.html |accessdate=2006-09-14}}</ref> Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the young Sun's energy.<ref>Benz, W., Slattery, W. L., Cameron, A. G. W. (1988), ''Collisional stripping of Mercury's mantle'', Icarus, v. 74, p. 516–528.</ref><ref>Cameron, A. G. W. (1985), ''The partial volatilization of Mercury'', Icarus, v. 64, p. 285–294.</ref>


Some [[Solar System model]]s attempt to convey the relative scales involved in the Solar System in human terms. Some are small in scale (and may be mechanical—called [[Orrery|orreries]])—whereas others extend across cities or regional areas.<ref>{{Cite web |last=Ottewell |first=Guy |date=1989 |title=The Thousand-Yard Model: or, Earth as a Peppercorn |url=http://www.noao.edu/education/peppercorn/pcmain.html |url-status=dead |archive-url=https://web.archive.org/web/20160710065429/http://www.noao.edu/education/peppercorn/pcmain.html |archive-date=10 July 2016 |access-date=10 May 2012 |website=NOAO Educational Outreach Office}}</ref> The largest such scale model, the [[Sweden Solar System]], uses the 110-meter (361-foot) [[Avicii Arena]] in [[Stockholm]] as its substitute Sun, and, following the scale, Jupiter is a 7.5-meter (25-foot) sphere at [[Stockholm Arlanda Airport]], 40&nbsp;km (25&nbsp;mi) away, whereas the farthest current object, [[90377 Sedna|Sedna]], is a 10&nbsp;cm (4&nbsp;in) sphere in [[Luleå]], 912&nbsp;km (567&nbsp;mi) away.<ref>{{Cite web |title=Tours of Model Solar Systems |url=http://internal.psychology.illinois.edu/~wbrewer/solarmodel.html |url-status=dead |archive-url=https://web.archive.org/web/20110412124455/http://internal.psychology.illinois.edu/~wbrewer/solarmodel.html |archive-date=12 April 2011 |access-date=10 May 2012 |publisher=University of Illinois}}</ref><ref name="Sedna">{{Cite web |title=Luleå är Sedna. I alla fall om vår sol motsvaras av Globen i Stockholm |url=http://www.kuriren.nu/arkiv/2005/11/17/Lokalt/1510647/Lule%C3%A5-%C3%A4r-Sedna.aspx |url-status=dead |archive-url=https://web.archive.org/web/20100715074955/http://www.kuriren.nu/arkiv/2005/11/17/Lokalt/1510647/Lule%C3%A5-%C3%A4r-Sedna.aspx |archive-date=15 July 2010 |access-date=10 May 2010 |publisher=Norrbotten Kuriren (in Swedish)}}</ref> At that scale, the distance to Proxima Centauri would be roughly 8 times further than the Moon is from Earth.
; Venus
: [[Venus]] (0.7&nbsp;AU) is close in size to Earth, (0.815 Earth masses) and like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere and evidence of internal geological activity. However, it is much drier than Earth and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 [[Celsius|°C]], most likely due to the amount of [[greenhouse gas]]es in the atmosphere.<ref>{{cite paper |author=Mark Alan Bullock |title=The Stability of Climate on Venus |publisher=Southwest Research Institute |date=1997 |url=http://www.boulder.swri.edu/~bullock/Homedocs/PhDThesis.pdf |format=[[Portable Document Format|PDF]] |accessdate=2006-12-26 }}</ref> No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is regularly replenished by volcanic eruptions.<ref>{{cite web |year=1999 |author=Paul Rincon |title=Climate Change as a Regulator of Tectonics on Venus |work=Johnson Space Center Houston, TX, Institute of Meteoritics, University of New Mexico, Albuquerque, NM |url=http://www.boulder.swri.edu/~bullock/Homedocs/Science2_1999.pdf |format=PDF |accessdate=2006-11-19}}</ref>


If the Sun–Neptune distance is scaled to {{Convert|100|m|ft|4=-1}}, then the Sun would be about {{Convert|3|cm|abbr=on}} in diameter (roughly two-thirds the diameter of a golf ball), the giant planets would be all smaller than about {{Convert|3|mm|abbr=on}}, and [[Earth's diameter]] along with that of the other terrestrial planets would be smaller than a [[flea]] ({{Convert|0.3|mm|abbr=on|disp=or}}) at this scale.<ref>See, for example, {{Cite web |last=Office of Space Science |date=9 July 2004 |title=Solar System Scale |url=http://www.nasa.gov/audience/foreducators/5-8/features/F_Solar_System_Scale.html |url-status=live |archive-url=https://web.archive.org/web/20160827184323/http://www.nasa.gov/audience/foreducators/5-8/features/F_Solar_System_Scale.html |archive-date=27 August 2016 |access-date=2 April 2013 |website=NASA Educator Features}}</ref>
; Earth
: [[Earth]] (1&nbsp;AU) is the largest and densest of the inner planets, the only one known to have current geological activity, and the only planet known to have [[life]]. Its liquid [[hydrosphere]] is unique among the terrestrial planets, and it is also the only planet where [[plate tectonics]] has been observed. Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free [[oxygen]].<ref>{{cite web |title=Earth's Atmosphere: Composition and Structure |author=Anne E. Egger, M.A./M.S. |work=VisionLearning.com |url=http://www.visionlearning.com/library/module_viewer.php?c3=&mid=107&l=|accessdate=2006-12-26}}</ref> It has one natural satellite, the [[Moon]] (Latin: Luna), the only large satellite of a terrestrial planet in the Solar System.


=== Habitability ===
; Mars
{{Main|Planetary habitability in the Solar System}}
: [[Mars]] (1.5&nbsp;AU) is smaller than Earth and Venus (0.107 Earth masses). It possesses a tenuous atmosphere of mostly [[carbon dioxide]]. Its surface, peppered with vast volcanoes such as [[Olympus Mons]] and rift valleys such as [[Valles Marineris]], shows geological activity that may have persisted until very recently. Its red color comes from rust in its iron-rich soil.<ref>{{cite web |year=2004 |title=Modern Martian Marvels: Volcanoes? |author=David Noever |work=NASA Astrobiology Magazine |url=http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=1360&mode=thread&order=0&thold=0 |accessdate=2006-07-23}}</ref> Mars has two tiny natural satellites ([[Deimos (moon)|Deimos]] and [[Phobos (moon)|Phobos]]) thought to be captured [[asteroid]]s.<ref>{{cite web |year=2004 |title=A Survey for Outer Satellites of Mars: Limits to Completeness |author=Scott S. Sheppard, David Jewitt, and Jan Kleyna |work=The Astronomical Journal |url=http://www.iop.org/EJ/article/1538-3881/128/5/2542/204263.html |accessdate=2006-12-26}}</ref>
{{Multiple image|perrow = 1|total_width = 350
| direction = vertical


<!--image 1-->
===Asteroid belt===
| image1 = PIA21424 - The TRAPPIST-1 Habitable Zone.jpg
{{main|Asteroid belt}}
| alt1 =
[[Image:InnerSolarSystem-en.png|300px|thumb|Image of the main asteroid belt and the Trojan asteroids]]
| caption1 = Comparison of the habitable zones of the Solar System and [[TRAPPIST-1]], an ultracool red dwarf star known to have seven terrestrial planets in stable orbits around the star.
<!--image 2-->
| image2 = Diagram of different habitable zone regions by Chester Harman.jpg
| alt2 =
| caption2 = Comparison of the [[habitable zone]]s for different stellar temperatures, with a sample of known exoplanets plus the Earth, Mars, and Venus. From top to bottom are an [[F-type main-sequence star]], a [[G-type main-sequence star|yellow dwarf]] (G-type main-sequence star), an [[orange dwarf]] (K-type main-sequence star), a typical [[red dwarf]], and an [[ultra-cool dwarf]].
}}
Besides solar energy, the primary characteristic of the Solar System enabling the presence of life is the heliosphere and planetary magnetic fields (for those planets that have them). These magnetic fields partially shield the Solar System from high-energy interstellar particles called [[cosmic ray]]s. The density of cosmic rays in the [[interstellar medium]] and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic-ray penetration in the Solar System varies, though by how much is unknown.<ref name="Langner_et_al_2005">{{Cite journal |last1=Langner |first1=U. W. |last2=Potgieter |first2=M. S. |date=2005 |title=Effects of the position of the solar wind termination shock and the heliopause on the heliospheric modulation of cosmic rays |journal=[[Advances in Space Research]] |volume=35 |issue=12 |pages=2084–2090 |bibcode=2005AdSpR..35.2084L |doi=10.1016/j.asr.2004.12.005}}</ref>


The [[Circumstellar habitable zone|zone of habitability]] of the Solar System is conventionally located in the inner Solar System, where planetary surface or atmospheric temperatures admit the possibility of [[liquid water]].<ref name="NASA-20150407">{{Cite web |last1=Dyches |first1=Preston |last2=Chou |first2=Felcia |date=7 April 2015 |title=The Solar System and Beyond is Awash in Water |url=http://www.nasa.gov/jpl/the-solar-system-and-beyond-is-awash-in-water |url-status=dead |archive-url=https://web.archive.org/web/20150410113514/http://www.nasa.gov/jpl/the-solar-system-and-beyond-is-awash-in-water/ |archive-date=10 April 2015 |access-date=8 April 2015 |website=[[NASA]]}}</ref> Habitability might be possible in [[subsurface ocean]]s of various outer Solar System moons.<ref>{{Cite book |last1=Robert T. Pappalardo |url=https://books.google.com/books?id=Jpcz2UoXejgC |title=Europa |last2=William B. McKinnon |last3=K. Khurana |publisher=University of Arizona Press |year=2009 |isbn=978-0-8165-2844-8 |page=658 |access-date=6 April 2023 |archive-date=6 April 2023 |archive-url=https://web.archive.org/web/20230406102731/https://books.google.com/books?id=Jpcz2UoXejgC |url-status=live }} [https://books.google.com/books?id=Jpcz2UoXejgC&pg=PA658 Extract of page 658] {{Webarchive|url=https://web.archive.org/web/20230415082720/https://books.google.com/books?id=Jpcz2UoXejgC&pg=PA658 |date=15 April 2023 }}</ref>
[[Asteroid]]s are mostly small Solar System bodies composed mainly of rocky and metallic non-volatile minerals.


=== Comparison with extrasolar systems ===
The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3&nbsp;AU from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.
Compared to many extrasolar systems, the Solar System stands out in lacking planets interior to the orbit of Mercury.<ref name="Martin082015">{{Cite journal |last1=Martin |first1=Rebecca G. |last2=Livio |first2=Mario |year=2015 |title=The Solar System as an Exoplanetary System |journal=[[The Astrophysical Journal]] |volume=810 |issue=2 |page=105 |arxiv=1508.00931 |bibcode=2015ApJ...810..105M |doi=10.1088/0004-637X/810/2/105 |s2cid=119119390}}</ref><ref>{{Cite journal |last=Kohler |first=Susanna |date=25 September 2015 |title=How Normal is Our Solar System? |url=https://aasnova.org/2015/09/25/how-normal-is-our-solar-system |url-status=live |journal=Aas Nova Highlights |publisher=American Astronomical Society |page=313 |bibcode=2015nova.pres..313K |archive-url=https://web.archive.org/web/20220407043952/https://aasnova.org/2015/09/25/how-normal-is-our-solar-system |archive-date=7 April 2022 |access-date=31 March 2022}}</ref> The known Solar System lacks [[super-Earth]]s, planets between one and ten times as massive as the Earth,<ref name="Martin082015" /> although the hypothetical [[Planet Nine]], if it does exist, could be a super-Earth orbiting in the edge of the Solar System.<ref>{{Cite journal |last1=Sheppard |first1=Scott S. |author-link=Scott S. Sheppard |last2=Trujillo |first2=Chadwick |author-link2=Chad Trujillo |date=7 December 2016 |title=New extreme trans-Neptunian objects: Toward a super-Earth in the outer solar system |journal=The Astronomical Journal |volume=152 |issue=6 |page=221 |arxiv=1608.08772 |bibcode=2016AJ....152..221S |doi=10.3847/1538-3881/152/6/221 |issn=1538-3881 |s2cid=119187392 |doi-access=free}}</ref>


Uncommonly, it has only small terrestrial and large gas giants; elsewhere planets of intermediate size are typical—both rocky and gas—so there is no "gap" as seen between the size of Earth and of Neptune (with a radius 3.8 times as large). As many of these super-Earths are closer to their respective stars than Mercury is to the Sun, a hypothesis has arisen that all planetary systems start with many close-in planets, and that typically a sequence of their collisions causes consolidation of mass into few larger planets, but in case of the Solar System the collisions caused their destruction and ejection.<ref name="Martin082015" /><ref>{{Cite journal |last1=Volk |first1=Kathryn |last2=Gladman |first2=Brett |year=2015 |title=Consolidating and Crushing Exoplanets: Did it happen here? |journal=[[The Astrophysical Journal Letters]] |volume=806 |page=L26 |arxiv=1502.06558 |bibcode=2015ApJ...806L..26V |doi=10.1088/2041-8205/806/2/L26 |s2cid=118052299 |number=2}}</ref>
Asteroids range in size from hundreds of kilometres across to microscopic. All asteroids save the largest, [[Ceres (dwarf planet)|Ceres]], are classified as small Solar System bodies, but some asteroids such as [[4 Vesta|Vesta]] and [[10 Hygiea|Hygieia]] may be reclassed as dwarf planets if they are shown to have achieved [[hydrostatic equilibrium]].


The orbits of Solar System planets are nearly circular. Compared to many other systems, they have smaller [[orbital eccentricity]].<ref name="Martin082015" /> Although there are attempts to explain it partly with a bias in the [[Doppler spectroscopy|radial-velocity detection method]] and partly with long interactions of a quite high number of planets, the exact causes remain undetermined.<ref name="Martin082015" /><ref>{{Cite journal |last1=Goldreich |first1=Peter |last2=Lithwick |first2=Yoram |last3=Sari |first3=Re'em |year=2004 |title=Final Stages of Planet Formation |journal=[[The Astrophysical Journal]] |volume=614 |issue=1 |pages=497–507 |arxiv=astro-ph/0404240 |bibcode=2004ApJ...614..497G |doi=10.1086/423612 |s2cid=16419857}}</ref>
The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter.<ref>{{cite web |year=2002 |title=New study reveals twice as many asteroids as previously believed |work=ESA |url=http://www.alphagalileo.org/index.cfm?fuseaction=readRelease&Releaseid=9162 |accessdate=2006-06-23}}</ref> Despite this, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth.<ref name=Krasinsky2002>{{cite journal |authorlink=Georgij A. Krasinsky |first=G. A. |last=Krasinsky |coauthors=[[Elena V. Pitjeva|Pitjeva, E. V.]]; Vasilyev, M. V.; Yagudina, E. I. |url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002Icar..158...98K&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=4326fb2cf906949 |title=Hidden Mass in the Asteroid Belt |journal=Icarus |volume=158 |issue=1 |pages=98–105 |month=July |year=2002 |doi=10.1006/icar.2002.6837}}</ref> The main belt is very sparsely populated; [[Space probe|spacecraft]] routinely pass through without incident. Asteroids with diameters between 10 and 10<sup>-4</sup>&nbsp;[[metre|m]] are called [[meteoroid]]s.<ref>{{cite journal |author=Beech, M. |coauthors=Duncan I. Steel |year=1995 |month=September |title=On the Definition of the Term Meteoroid |journal=Quarterly Journal of the Royal Astronomical Society |volume=36 |issue=3 |pages=281&ndash;284 |url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1995QJRAS..36..281B&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=44b52c369007834 |accessdate=2006-08-31}}</ref>


== Sun ==
[[Image:Ceres optimized.jpg|thumb|left|Ceres]]
{{Main|Sun}}
[[File:The Sun in white light.jpg|thumb|alt=White ball of plasma|The Sun in true white color]]


The Sun is the Solar System's star and by far its most massive component. Its large mass (332,900 [[Earth mass]]es),<ref>{{Cite web |title=Sun: Facts & Figures |url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts&System=Metric |url-status=dead |archive-url=https://web.archive.org/web/20080102034758/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts&System=Metric |archive-date=2 January 2008 |access-date=14 May 2009 |publisher=NASA}}</ref> which comprises 99.86% of all the mass in the Solar System,<ref name="Woolfson00">{{Cite journal |last=Woolfson |first=M. |date=2000 |title=The origin and evolution of the solar system |journal=[[Astronomy & Geophysics]] |volume=41 |issue=1 |page=12 |bibcode=2000A&G....41a..12W |doi=10.1046/j.1468-4004.2000.00012.x |doi-access=free}}</ref> produces temperatures and densities in its [[solar core|core]] high enough to sustain nuclear fusion of hydrogen into helium.<ref>{{Cite book |last=Zirker |first=Jack B. |url=https://archive.org/details/journeyfromcente0000zirk |title=Journey from the Center of the Sun |date=2002 |publisher=[[Princeton University Press]] |isbn=978-0-691-05781-1 |pages=[https://archive.org/details/journeyfromcente0000zirk/page/120 120–127] |url-access=registration}}</ref> This releases an enormous amount of [[energy]], mostly [[radiant energy|radiated]] into [[outer space|space]] as [[electromagnetic radiation]] peaking in [[visible light]].<ref>{{Cite web |title=What Color is the Sun? |work=NASA |url=https://eclipse2017.nasa.gov/what-color-sun |access-date=6 April 2024 |archive-date=26 April 2024 |archive-url=https://web.archive.org/web/20240426130849/https://eclipse2017.nasa.gov/what-color-sun |url-status=live }}</ref><ref>{{Cite web |title=What Color is the Sun? |url=http://solar-center.stanford.edu/SID/activities/GreenSun.html |url-status=live |archive-url=https://web.archive.org/web/20171030154449/http://solar-center.stanford.edu/SID/activities/GreenSun.html |archive-date=30 October 2017 |access-date=23 May 2016 |publisher=[[Stanford University|Stanford]] Solar Center}}</ref>
; Ceres
: [[Ceres (dwarf planet)|Ceres]] (2.77&nbsp;AU) is the largest body in the asteroid belt and is classified as a [[dwarf planet]]. It has a diameter of slightly under 1000&nbsp;km, large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in the 19th century, but was reclassified as an asteroid in the 1850s as further observation revealed additional asteroids.<ref>{{cite web |title=History and Discovery of Asteroids |format=DOC |work=NASA |url=http://dawn.jpl.nasa.gov/DawnClassrooms/1_hist_dawn/history_discovery/Development/a_modeling_scale.doc |accessdate=2006-08-29}}</ref> It was again reclassified in 2006 as a dwarf planet.


Because the Sun fuses hydrogen at its core, it is a main-sequence star. More specifically, it is a [[G-type main-sequence star|G2-type main-sequence star]], where the type designation refers to its [[effective temperature]]. Hotter main-sequence stars are more luminous but shorter lived. The Sun's temperature is intermediate between that of the [[O-type main-sequence star|hottest stars]] and that of the coolest stars. Stars brighter and hotter than the Sun are rare, whereas substantially dimmer and cooler stars, known as [[red dwarf]]s, make up about 75% of the [[Fusor (astronomy)|fusor]] stars in the [[Milky Way]].<ref>{{Cite journal |last1=Mejías |first1=Andrea |last2=Minniti |first2=Dante |last3=Alonso-García |first3=Javier |last4=Beamín |first4=Juan Carlos |last5=Saito |first5=Roberto K. |last6=Solano |first6=Enrique |year=2022 |title=VVVX near-IR photometry for 99 low-mass stars in the Gaia EDR3 Catalog of Nearby Stars |journal=Astronomy & Astrophysics |volume=660 |pages=A131 |arxiv=2203.00786 |bibcode=2022A&A...660A.131M |doi=10.1051/0004-6361/202141759 |s2cid=246842719}}</ref>
; Asteroid groups
: Asteroids in the main belt are divided into [[asteroid group]]s and [[:Category:Asteroid groups and families|families]] based on their orbital characteristics. [[Asteroid moon]]s are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains [[main-belt comet]]s which may have been the source of Earth's water.<ref>{{cite web |year=2006 |author=Phil Berardelli |title=Main-Belt Comets May Have Been Source Of Earths Water |work=SpaceDaily |url=http://www.spacedaily.com/reports/Main_Belt_Comets_May_Have_Been_Source_Of_Earths_Water.html |accessdate=2006-06-23}}</ref>


The Sun is a [[Population I stars|population I star]], having formed in the [[spiral arm]]s of the [[Milky Way]] galaxy. It has a higher abundance of elements heavier than hydrogen and helium ("[[metallicity|metals]]" in astronomical parlance) than the older population II stars in the [[galactic bulge]] and [[Galactic halo|halo]].<ref>{{Cite journal |last1=van Albada |first1=T.S. |last2=Baker |first2=Norman |date=1973 |title=On the Two Oosterhoff Groups of Globular Clusters |journal=[[The Astrophysical Journal]] |volume=185 |pages=477–498 |bibcode=1973ApJ...185..477V |doi=10.1086/152434 |doi-access=free}}</ref> Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, so the first generation of stars had to die before the [[universe]] could be enriched with these atoms. The oldest stars contain few metals, whereas stars born later have more. This higher metallicity is thought to have been crucial to the Sun's development of a [[planetary system]] because the planets formed from the accretion of "metals".<ref>{{Cite journal |last=Lineweaver |first=Charles H. |date=9 March 2001 |title=An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect |journal=[[Icarus (journal)|Icarus]] |volume=151 |issue=2 |pages=307–313 |arxiv=astro-ph/0012399 |bibcode=2001Icar..151..307L |citeseerx=10.1.1.254.7940 |doi=10.1006/icar.2001.6607 |s2cid=14077895}}</ref>
[[Trojan asteroid]]s are located in either of Jupiter's [[Lagrangian point|L<sub>4</sub> or L<sub>5</sub> points]] (gravitationally stable regions leading and trailing a planet in its orbit); the term "Trojan" is also used for small bodies in any other planetary or satellite Lagrange point. [[Hilda family|Hilda asteroids]] are in a 2:3 [[Orbital resonance|resonance]] with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.


{{Anchor|Interplanetary environment}}The region of space dominated by the Solar [[magnetosphere]] is the [[heliosphere]], which spans much of the Solar System. Along with [[Sunlight|light]], the Sun radiates a continuous stream of charged particles (a [[plasma (physics)|plasma]]) called the [[solar wind]]. This stream spreads outwards at speeds from {{Convert|900000|km/h|mph}} to {{Convert|2880000|km/h|mph}},<ref>{{Cite book |last=Kallenrode |first=May-Britt |url=https://www.worldcat.org/oclc/53443301 |title=Space Physics: An introduction to plasmas and particles in the heliosphere and magnetospheres |date=2004 |publisher=Springer |isbn=978-3-540-20617-0 |edition=3rd |location=Berlin |pages=150 |oclc=53443301 |access-date=1 April 2022 |archive-url=https://web.archive.org/web/20220420161223/https://www.worldcat.org/title/space-physics-an-introduction-to-plasmas-and-particles-in-the-heliosphere-and-magnetospheres/oclc/53443301 |archive-date=20 April 2022 |url-status=live}}</ref> filling the vacuum between the bodies of the Solar System. The result is a [[Vacuum|thin]], dusty atmosphere, called the [[interplanetary medium]], which extends to at least {{val|100|u=AU}}.<ref name="Voyager">{{Cite web |last=Steigerwald |first=Bill |date=24 May 2005 |title=Voyager Enters Solar System's Final Frontier |url=http://www.nasa.gov/vision/universe/solarsystem/voyager_agu.html |url-status=live |archive-url=https://web.archive.org/web/20200516082547/https://www.nasa.gov/vision/universe/solarsystem/voyager_agu.html |archive-date=16 May 2020 |access-date=2 April 2007 |website=NASA}}</ref>
The inner Solar System is also dusted with [[Near-Earth asteroid|rogue asteroids]], many of which cross the orbits of the inner planets.


Activity on the Sun's surface, such as [[solar flare]]s and [[coronal mass ejection]]s, disturbs the heliosphere, creating [[space weather]] and causing [[geomagnetic storm]]s.<ref name="SunFlip">{{Cite web |last=Phillips |first=Tony |date=15 February 2001 |title=The Sun Does a Flip |url=https://science.nasa.gov/science-news/science-at-nasa/2001/ast15feb_1 |url-status=live |archive-url=https://web.archive.org/web/20220401050813/https://science.nasa.gov/science-news/science-at-nasa/2001/ast15feb_1 |archive-date=1 April 2022 |access-date=1 April 2022 |website=NASA Science: Share the Science}}</ref> Coronal mass ejections and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. The interaction of this magnetic field and material with Earth's magnetic field funnels charged particles into Earth's upper atmosphere, where its interactions create [[Aurora (astronomy)|aurorae]] seen near the [[Earth's magnetic field#Magnetic poles|magnetic poles]].<ref>{{Cite book |last1=Fraknoi |first1=Andrew |url=https://www.worldcat.org/oclc/961476196 |title=Astronomy |last2=Morrison |first2=David |last3=Wolff |first3=Sidney C. |last4=Beck |first4=John |date=2022 |publisher=[[OpenStax]] |isbn=978-1-947-17224-1 |location=Houston, Texas |chapter=15.4 Space weather |oclc=961476196 |display-authors=3 |access-date=9 March 2022 |orig-date=2016 |chapter-url=https://openstax.org/books/astronomy/pages/15-4-space-weather |archive-url=https://web.archive.org/web/20200719090803/http://worldcat.org/oclc/961476196 |archive-date=19 July 2020 |url-status=live}}</ref> The largest stable structure within the heliosphere is the [[heliospheric current sheet]], a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium.<ref>{{Cite web |date=22 April 2003 |title=A Star with two North Poles |url=https://science.nasa.gov/science-news/science-at-nasa/2003/22apr_currentsheet |url-status=live |archive-url=https://web.archive.org/web/20220401192948/https://science.nasa.gov/science-news/science-at-nasa/2003/22apr_currentsheet |archive-date=1 April 2022 |access-date=1 April 2022 |website=NASA Science: Share the Science}}</ref><ref>{{Cite journal |last=Riley |first=Pete |date=2002 |title=Modeling the heliospheric current sheet: Solar cycle variations |journal=[[Journal of Geophysical Research]] |volume=107 |issue=A7 |page=1136 |bibcode=2002JGRA..107.1136R |doi=10.1029/2001JA000299 |doi-access=free}}</ref>
== Outer Solar System ==
The outer region of the Solar System is home to the gas giants and their planet-sized satellites. Many short period comets, including the [[Centaur (planetoid)|centaurs]], also orbit in this region. The solid objects in this region are composed of a higher proportion of [[volatiles]] (such as water, ammonia, methane, often called ''ices'' in planetary science) than the rocky denizens of the inner Solar System.


== Inner Solar System ==
===Outer planets===
The inner Solar System is the region comprising the [[#Terrestrial planets|terrestrial planets]] and the [[asteroid]]s.<ref>{{Cite web |title=Inner Solar System |url=https://science.nasa.gov/solar-system/focus-areas/inner-solar-system |url-status=live |archive-url=https://web.archive.org/web/20220410004501/https://science.nasa.gov/solar-system/focus-areas/inner-solar-system |archive-date=10 April 2022 |access-date=2 April 2022 |website=NASA Science: Share the Science}}</ref> Composed mainly of [[silicate]]s and metals,<ref>{{Cite book |last1=Del Genio |first1=Anthony D. |title=Planetary Astrobiology |last2=Brain |first2=David |last3=Noack |first3=Lena |last4=Schaefer |first4=Laura |date=2020 |publisher=University of Arizona Press |isbn=978-0816540655 |editor-last=Meadows |editor-first=Victoria S. |page=420 |chapter=The Inner Solar System's Habitability Through Time |bibcode=2018arXiv180704776D |author-link4=Laura K. Schaefer |editor-last2=Arney |editor-first2=Giada N. |editor-last3=Schmidt |editor-first3=Britney |editor-last4=Des Marais |editor-first4=David J. |arxiv=1807.04776}}</ref> the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn. This region is within the [[Frost line (astrophysics)|frost line]], which is a little less than {{val|5|u=AU}} from the Sun.<ref name="Levison2003"/>
{{main|Gas giant}}
[[Image:Gas giants in the solar system.jpg|thumb|From top to bottom: Neptune, Uranus, Saturn, and Jupiter (not to scale)]]


=== Inner planets<span class="anchor" id="Terrestrial planets"></span> ===
The four outer planets, or [[gas giant]]s (sometimes called Jovian planets), collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn consist overwhelmingly of hydrogen and helium; Uranus and Neptune possess a greater proportion of ices in their makeup. Some astronomers suggest they belong in their own category, “ice giants.”<ref>{{cite web |title=Formation of Giant Planets |author=Jack J. Lissauer, David J. Stevenson |work=NASA Ames Research Center; California Institute of Technology |year=2006 |url=http://caltech-era.org/faculty/stevenson/pdfs/lissauer&stevenson(PPV).pdf |format=PDF |accessdate=2006-01-16}}</ref> All four gas giants have [[Planetary ring|rings]], although only Saturn's ring system is easily observed from Earth. The term ''outer planet'' should not be confused with ''[[superior planet]]'', which designates planets outside Earth's orbit (the outer planets and Mars).
{{Main|Terrestrial planet}}
[[File:Terrestrial planet sizes 3.jpg|right|thumb|alt=Venus and Earth about the same size, Mars is about 0.55 times as big and Mercury is about 0.4 times as big|The four terrestrial planets [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]] and [[Mars]]]]
The four terrestrial or inner planets have dense, rocky compositions, few or no [[natural satellite|moons]], and no [[planetary ring|ring systems]]. They are composed largely of [[Refractory (planetary science)|refractory]] minerals such as [[silicates]]{{Mdash}}which form their [[crust (geology)|crusts]] and [[mantle (geology)|mantles]]{{Mdash}}and metals such as iron and nickel which form their [[planetary core|cores]]. Three of the four inner planets (Venus, Earth, and Mars) have [[atmosphere]]s substantial enough to generate weather; all have impact craters and [[tectonics|tectonic]] surface features, such as [[rift valley]]s and volcanoes.<ref name="Ryden">{{Cite journal |last=Ryden |first=Robert |date=December 1999 |title=Astronomical Math |url=https://pubs.nctm.org/view/journals/mt/92/9/article-p786.xml |url-status=live |journal=The Mathematics Teacher |volume=92 |issue=9 |pages=786–792 |doi=10.5951/MT.92.9.0786 |issn=0025-5769 |jstor=27971203 |archive-url=https://web.archive.org/web/20220412010049/https://pubs.nctm.org/view/journals/mt/92/9/article-p786.xml |archive-date=12 April 2022 |access-date=29 March 2022}}</ref>


* {{Visible anchor|Mercury|text=[[Mercury (planet)|Mercury]]}} (0.31–0.59&nbsp;AU from the Sun)<ref name="nasa-factsheet" group="D">{{Cite web |last=Williams |first=David |date=27 December 2021 |title=Planetary Fact Sheet - Metric |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet |access-date=11 December 2022 |publisher=[[Goddard Space Flight Center]] |archive-date=18 August 2011 |archive-url=https://web.archive.org/web/20110818181734/http://nssdc.gsfc.nasa.gov/planetary/factsheet/ |url-status=live }}</ref> is the smallest planet in the Solar System. Its surface is grayish, with an expansive [[rupes]] (cliff) system generated from [[thrust fault]]s and bright [[ray system]]s formed by [[Ejecta|impact event remnants]].<ref>{{Cite journal |last1=Watters |first1=Thomas R. |last2=Solomon |first2=Sean C. |last3=Robinson |first3=Mark S. |last4=Head |first4=James W. |last5=André |first5=Sarah L. |last6=Hauck |first6=Steven A. |last7=Murchie |first7=Scott L. |date=August 2009 |title=The tectonics of Mercury: The view after MESSENGER's first flyby |journal=Earth and Planetary Science Letters |language=en |volume=285 |issue=3–4 |pages=283–296 |bibcode=2009E&PSL.285..283W |doi=10.1016/j.epsl.2009.01.025}}</ref> The surface has widely varying temperature, with the [[equator]]ial regions ranging from {{convert|-170|C|F|sigfig=2}} at night to {{convert|420|C|F|sigfig=2}} during sunlight. In the past, Mercury was volcanically active, producing smooth [[basalt]]ic plains similar to the Moon.<ref name=Head_et_al_1981>{{cite journal |last1=Head |first1=James W. |author-link1=James W. Head |last2=Solomon |first2=Sean C. |author-link2=Sean Solomon |year=1981 |title=Tectonic Evolution of the Terrestrial Planets |url=http://www.planetary.brown.edu/pdfs/323.pdf |url-status=dead |journal=Science |volume=213 |issue=4503 |pages=62–76 |bibcode=1981Sci...213...62H |citeseerx=10.1.1.715.4402 |doi=10.1126/science.213.4503.62 |pmid=17741171 |archive-url=https://web.archive.org/web/20180721153426/http://www.planetary.brown.edu/pdfs/323.pdf |archive-date=21 July 2018 |access-date=25 October 2017 |hdl=2060/20020090713}}</ref> It is likely that Mercury has a silicate crust and a large iron core.<ref>{{cite web |date=21 March 2012 |editor-last=Talbert |editor-first=Tricia |title=MESSENGER Provides New Look at Mercury's Surprising Core and Landscape Curiosities |url=https://www.nasa.gov/mission_pages/messenger/media/PressConf20120321.html |url-status=dead |archive-url=https://web.archive.org/web/20190112170032/https://www.nasa.gov/mission_pages/messenger/media/PressConf20120321.html |archive-date=12 January 2019 |access-date=20 April 2018 |publisher=NASA}}</ref><ref name="Margot2012">{{cite journal |last1=Margot |first1=Jean-Luc |last2=Peale |first2=Stanton J. |last3=Solomon |first3=Sean C. |last4=Hauck |first4=Steven A. |last5=Ghigo |first5=Frank D. |last6=Jurgens |first6=Raymond F. |last7=Yseboodt |first7=Marie |last8=Giorgini |first8=Jon D. |last9=Padovan |first9=Sebastiano |last10=Campbell |first10=Donald B. |year=2012 |title=Mercury's moment of inertia from spin and gravity data |journal=Journal of Geophysical Research: Planets |volume=117 |issue=E12 |pages=n/a |bibcode=2012JGRE..117.0L09M |citeseerx=10.1.1.676.5383 |doi=10.1029/2012JE004161 |issn=0148-0227 |s2cid=22408219}}</ref> Mercury has a very tenuous atmosphere, consisting of [[Solar wind|solar-wind]] particles and ejected atoms.<ref>{{Cite journal |last1=Domingue |first1=Deborah L. |last2=Koehn |first2=Patrick L. |last3=Killen |first3=Rosemary M. |last4=Sprague |first4=Ann L. |last5=Sarantos |first5=Menelaos |last6=Cheng |first6=Andrew F. |last7=Bradley |first7=Eric T. |last8=McClintock |first8=William E. |display-authors=2 |date=2009 |title=Mercury's Atmosphere: A Surface-Bounded Exosphere |journal=Space Science Reviews |volume=131 |issue=1–4 |pages=161–186 |bibcode=2007SSRv..131..161D |doi=10.1007/s11214-007-9260-9 |s2cid=121301247 |quote=The composition of Mercury's exosphere, with its abundant H and He, clearly indicates a strong solar wind source. Once solar wind plasma and particles gain access to the magnetosphere, they predominantly precipitate to the surface, where solar wind species are neutralized, thermalized, and released again into the exosphere. Moreover, bombardment of the surface by solar wind particles, especially energetic ions, contributes to ejection of neutral species from the surface into the exosphere (via "sputtering") as well as other chemical and physical surface modification processes.}}</ref> Mercury has no natural satellites.<ref name="spaceplace.nasa.gov">{{Cite web |title=How Many Moons Does Each Planet Have? {{!}} NASA Space Place – NASA Science for Kids |url=https://spaceplace.nasa.gov/how-many-moons/en/ |access-date=21 April 2024 |website=spaceplace.nasa.gov |archive-date=21 April 2024 |archive-url=https://web.archive.org/web/20240421061913/https://spaceplace.nasa.gov/how-many-moons/en/ |url-status=live }}</ref>
; Jupiter
* {{Visible anchor|Venus|text=[[Venus]]}} (0.72–0.73&nbsp;AU)<ref name="nasa-factsheet" group="D" /> has a reflective, whitish atmosphere that is mainly composed of [[carbon dioxide]]. At the surface, the atmospheric pressure is ninety times as dense as on Earth's sea level.<ref name="u3r1a">{{cite journal |last1=Lebonnois |first1=Sebastien |last2=Schubert |first2=Gerald |date=26 June 2017 |title=The deep atmosphere of Venus and the possible role of density-driven separation of CO2 and N2 |url=https://hal.archives-ouvertes.fr/hal-01635402/file/deepatm_persp_rev2.pdf |url-status=live |journal=Nature Geoscience |publisher=Springer Science and Business Media LLC |volume=10 |issue=7 |pages=473–477 |bibcode=2017NatGe..10..473L |doi=10.1038/ngeo2971 |issn=1752-0894 |s2cid=133864520 |archive-url=https://web.archive.org/web/20190504081028/https://hal.archives-ouvertes.fr/hal-01635402/file/deepatm_persp_rev2.pdf |archive-date=4 May 2019 |access-date=11 August 2023}}</ref> Venus has a surface temperatures over {{Cvt|400|C|F}}, mainly due to the amount of [[greenhouse gas]]es in the atmosphere.<ref>{{Cite thesis |last=Bullock |first=Mark Alan |title=The Stability of Climate on Venus |date=1997 |degree=PhD |publisher=Southwest Research Institute |url=http://www.boulder.swri.edu/~bullock/Homedocs/PhDThesis.pdf |access-date=26 December 2006 |url-status=dead |archive-url=https://web.archive.org/web/20070614202751/http://www.boulder.swri.edu/~bullock/Homedocs/PhDThesis.pdf |archive-date=14 June 2007}}</ref> The planet lacks a protective magnetic field to protect against [[Atmospheric stripping|stripping]] by the solar wind, which suggests that its atmosphere is sustained by volcanic activity.<ref>{{Cite web |last=Rincon |first=Paul |date=1999 |title=Climate Change as a Regulator of Tectonics on Venus |url=http://www.boulder.swri.edu/~bullock/Homedocs/Science2_1999.pdf |url-status=dead |archive-url=https://web.archive.org/web/20070614202807/http://www.boulder.swri.edu/~bullock/Homedocs/Science2_1999.pdf |archive-date=14 June 2007 |access-date=19 November 2006 |website=Johnson Space Center Houston, TX, Institute of Meteoritics, University of New Mexico, Albuquerque, NM}}</ref> Its surface displays extensive evidence of volcanic activity with stagnant [[lid tectonics]].<ref>{{Cite journal |last1=Elkins-Tanton |first1=L. T. |last2=Smrekar |first2=S. E. |last3=Hess |first3=P. C. |last4=Parmentier |first4=E. M. |date=March 2007 |title=Volcanism and volatile recycling on a one-plate planet: Applications to Venus |journal=Journal of Geophysical Research |volume=112 |issue=E4 |bibcode=2007JGRE..112.4S06E |doi=10.1029/2006JE002793 |id=E04S06 |doi-access=free}}</ref> Venus has no natural satellites.<ref name="spaceplace.nasa.gov"/>
: [[Jupiter]] (5.2&nbsp;AU), at 318 Earth masses, masses 2.5 times all the other planets put together. It is composed largely of [[hydrogen]] and [[helium]]. Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the [[Great Red Spot]]. Jupiter has [[Moons of Jupiter|sixty-three known satellites]]. The four largest, [[Ganymede (moon)|Ganymede]], [[Callisto (moon)|Callisto]], [[Io (moon)|Io]], and [[Europa (moon)|Europa]], show similarities to the terrestrial planets, such as volcanism and internal heating.<ref>{{cite web |title=Geology of the Icy Galilean Satellites: A Framework for Compositional Studies |author=Pappalardo, R T |work=Brown University |year=1999 |url=http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=fm99&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Ffm99%2Ffm99&maxhits=200&=%22P11C-10%22 |accessdate=2006-01-16}}</ref> Ganymede, the largest satellite in the Solar System, is larger than Mercury.
* {{Visible anchor|Earth|text=[[Earth]]}} (0.98–1.02&nbsp;AU)<ref name="nasa-factsheet" group="D" /> is the only place in the universe where [[life]] and [[Water distribution on Earth|surface liquid water]] are known to exist.<ref name="life">{{Cite web |title=What are the characteristics of the Solar System that lead to the origins of life? |url=https://science.nasa.gov/planetary-science/big-questions/what-are-the-characteristics-of-the-solar-system-that-lead-to-the-origins-of-life-1 |url-status=dead |archive-url=https://web.archive.org/web/20100408055814/http://science.nasa.gov/planetary-science/big-questions/what-are-the-characteristics-of-the-solar-system-that-lead-to-the-origins-of-life-1 |archive-date=8 April 2010 |access-date=30 August 2011 |publisher=NASA Science (Big Questions)}}</ref> Earth's atmosphere contains 78% [[nitrogen]] and 21% [[oxygen]], which is the result of the presence of life.<ref name="handbook">{{Cite book |title=[[CRC Handbook of Chemistry and Physics]] |publisher=CRC Press |year=2016–2017 |isbn=978-1-4987-5428-6 |editor-last=Haynes |editor-first=H. M. |edition=97th |page=14{{Hyphen}}3<!-- the page ref itself is hyphenated -->}}</ref><ref name="NYT-20131003">{{Cite news |last=Zimmer |first=Carl |author-link=Carl Zimmer |date=3 October 2013 |title=Earth's Oxygen: A Mystery Easy to Take for Granted |work=[[The New York Times]] |url=https://www.nytimes.com/2013/10/03/science/earths-oxygen-a-mystery-easy-to-take-for-granted.html |url-access=limited |access-date=3 October 2013 |archive-url=https://web.archive.org/web/20131003121909/http://www.nytimes.com/2013/10/03/science/earths-oxygen-a-mystery-easy-to-take-for-granted.html |archive-date=3 October 2013}}</ref> The planet has a complex [[climate]] and [[weather]] system, with conditions differing drastically between [[climate region]]s.<ref name="climate_zones">{{cite web |author=Staff |title=Climate Zones |url=http://www.ace.mmu.ac.uk/eae/climate/older/Climate_Zones.html |archive-url=https://web.archive.org/web/20100808131632/http://www.ace.mmu.ac.uk/eae/climate/older/Climate_Zones.html |archive-date=8 August 2010 |access-date=24 March 2007 |publisher=UK Department for Environment, Food and Rural Affairs}}</ref> The solid surface of Earth is dominated by green [[vegetation]], [[Hot deserts|deserts]] and white [[ice sheet]]s.<ref name="Carlowicz Simmon 2019">{{cite web |last1=Carlowicz |first1=Michael |last2=Simmon |first2=Robert |date=15 July 2019 |title=Seeing Forests for the Trees and the Carbon: Mapping the World's Forests in Three Dimensions |url=https://earthobservatory.nasa.gov/features/ForestCarbon#:~:text=They%20cover%20about%2030%20percent,percent%20of%20the%20Earth's%20land. |access-date=31 December 2022 |website=NASA Earth Observatory |archive-date=31 December 2022 |archive-url=https://web.archive.org/web/20221231005400/https://earthobservatory.nasa.gov/features/ForestCarbon#:~:text=They%20cover%20about%2030%20percent,percent%20of%20the%20Earth's%20land. |url-status=live }}</ref><ref name="Cain 2010">{{cite web |last=Cain |first=Fraser |date=1 June 2010 |title=What Percentage of the Earth's Land Surface is Desert? |url=https://www.universetoday.com/65639/what-percentage-of-the-earths-land-surface-is-desert/ |access-date=3 January 2023 |website=Universe Today |archive-date=3 January 2023 |archive-url=https://web.archive.org/web/20230103153344/https://www.universetoday.com/65639/what-percentage-of-the-earths-land-surface-is-desert/ |url-status=live }}</ref><ref name="National Geographic Society 2006">{{cite web |date=6 August 2006 |title=Ice Sheet |url=https://education.nationalgeographic.org/resource/ice-sheet/ |access-date=3 January 2023 |website=National Geographic Society |archive-date=27 November 2023 |archive-url=https://web.archive.org/web/20231127174259/https://education.nationalgeographic.org/resource/ice-sheet/ |url-status=live }}</ref> Earth's surface is shaped by [[plate tectonics]] that formed the continental masses.<ref name=Head_et_al_1981/> Earth's planetary [[magnetosphere]] shields the surface from radiation, limiting [[atmospheric stripping]] and maintaining life habitability.<ref>{{Cite book |last=Pentreath |first=R. J. |url=https://books.google.com/books?id=avRVEAAAQBAJ&pg=PA94 |title=Radioecology: Sources and Consequences of Ionising Radiation in the Environment |date=2021 |publisher=Cambridge University Press |isbn=978-1009040334 |pages=94–97 |access-date=12 April 2022 |archive-url=https://web.archive.org/web/20220420161217/https://www.google.com/books/edition/Radioecology/avRVEAAAQBAJ?hl=en&gbpv=1&pg=PA94 |archive-date=20 April 2022 |url-status=live}}</ref>
** The [[Moon]] is Earth's only natural satellite.<ref>{{Cite web |title=Facts About Earth - NASA Science |url=https://science.nasa.gov/earth/facts/ |access-date=11 January 2024 |website=NASA Science |language=en}}</ref> Its diameter is one-quarter the size of Earth's.<ref name="Metzger2021">{{Citation |last1=Metzger |first1=Philip |title=Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science |date=2021 |journal=[[Icarus (journal)|Icarus]] |volume=374 |page=114768 |arxiv=2110.15285 |bibcode=2022Icar..37414768M |doi=10.1016/j.icarus.2021.114768 |s2cid=240071005 |last2=Grundy |first2=Will |last3=Sykes |first3=Mark |last4=Stern |first4=Alan |last5=Bell |first5=James |last6=Detelich |first6=Charlene |last7=Runyon |first7=Kirby |last8=Summers |first8=Michael |author-link1=Philip T. Metzger}}</ref> Its surface is covered in [[Lunar soil|very fine regolith]] and dominated by [[impact crater]]s.<ref>{{cite web |date=30 January 2006 |title=The Smell of Moondust |url=https://science.nasa.gov/headlines/y2006/30jan_smellofmoondust.htm |url-status=dead |archive-url=https://web.archive.org/web/20100308112332/http://science.nasa.gov/headlines/y2006/30jan_smellofmoondust.htm |archive-date=8 March 2010 |access-date=15 March 2010 |publisher=NASA}}</ref><ref>{{cite book |last=Melosh |first=H. J. |title=Impact cratering: A geologic process |date=1989 |publisher=[[Oxford University Press]] |isbn=978-0-19-504284-9}}</ref> Large dark patches on the Moon, [[Lunar mare|maria]], are formed from past volcanic activity.<ref>{{cite web |last=Norman |first=M. |date=21 April 2004 |title=The Oldest Moon Rocks |url=http://www.psrd.hawaii.edu/April04/lunarAnorthosites.html |url-status=live |archive-url=https://web.archive.org/web/20070418152325/http://www.psrd.hawaii.edu/April04/lunarAnorthosites.html |archive-date=18 April 2007 |access-date=12 April 2007 |work=Planetary Science Research Discoveries |publisher=Hawai'i Institute of Geophysics and Planetology}}</ref> The Moon's atmosphere is extremely thin, consisting of a [[partial vacuum]] with particle densities of under 10<sup>7</sup> per cm<sup>−3</sup>.<ref>{{cite book |last=Globus |first=Ruth |title=Space Settlements: A Design Study |date=1977 |publisher=NASA |editor=Richard D. Johnson & Charles Holbrow |chapter=Chapter 5, Appendix J: Impact Upon Lunar Atmosphere |access-date=17 March 2010 |chapter-url=http://settlement.arc.nasa.gov/75SummerStudy/5appendJ.html |archive-url=https://web.archive.org/web/20100531205037/http://settlement.arc.nasa.gov/75SummerStudy/5appendJ.html |archive-date=31 May 2010 |url-status=dead}}</ref>
* {{Visible anchor|Mars|text=[[Mars]]}} (1.38–1.67&nbsp;AU)<ref name="nasa-factsheet" group="D" /> has a radius about half of that of Earth.<ref name="Seidelmann2007">{{cite journal |last1=Seidelmann |first1=P. Kenneth |last2=Archinal |first2=Brent A. |last3=A'Hearn<!-- written A'hearn here, mostly A'Hearn elsewhere --> |first3=Michael F. |last4=Conrad |first4=Albert R. |last5=Consolmagno |first5=Guy J. |last6=Hestroffer |first6=Daniel |last7=Hilton |first7=James L. |last8=Krasinsky |first8=Georgij A. |last9=Neumann |first9=Gregory A. |last10=Oberst |first10=Jürgen |last11=Stooke |first11=Philip J. |last12=Tedesco |first12=Edward F. |last13=Tholen |first13=David J. |last14=Thomas |first14=Peter C. |last15=Williams |first15=Iwan P. |year=2007 |title=Report of the IAU/IAG Working Group on cartographic coordinates and rotational elements: 2006 |journal=Celestial Mechanics and Dynamical Astronomy |volume=98 |issue=3 |pages=155–180 |bibcode=2007CeMDA..98..155S |doi=10.1007/s10569-007-9072-y |ref={{sfnRef|Seidelmann Archinal A'hearn et al.|2007}} |doi-access=free}}</ref> Most of the planet is red due to [[iron(III) oxide|iron oxide]] in Martian soil,<ref>{{Cite journal |last=Peplow |first=Mark |date=6 May 2004 |title=How Mars got its rust |url=http://www.nature.com/articles/news040503-6 |url-status=live |journal=Nature |language=en |pages=news040503–6 |doi=10.1038/news040503-6 |issn=0028-0836 |archive-url=https://web.archive.org/web/20220407105832/https://www.nature.com/articles/news040503-6 |archive-date=7 April 2022 |access-date=9 April 2022}}</ref> and the polar regions are covered in [[Martian polar ice caps|white ice caps]] made of water and [[carbon dioxide]].<ref>{{Cite web |title=Polar Caps |url=https://marsed.asu.edu/mep/ice/polar-caps |access-date=6 January 2022 |website=Mars Education at Arizona State University |archive-date=28 May 2021 |archive-url=https://web.archive.org/web/20210528133135/https://marsed.asu.edu/mep/ice/polar-caps |url-status=live }}</ref> Mars has an atmosphere composed mostly of carbon dioxide, with surface pressure 0.6% of that of Earth, which is sufficient to support some weather phenomena.<ref>{{Cite book |last1=Gatling |first1=David C. |title=Encyclopaedia of the Solar System |last2=Leovy |first2=Conway |date=2007 |editor-last=Lucy-Ann McFadden |pages=301–314 |chapter=Mars Atmosphere: History and Surface Interactions |display-editors=etal}}</ref> During the Mars year (687 Earth days), there are large surface temperature swings on the surface between {{Cvt|-78.5|C|F}} to {{Cvt|5.7|C|F}}. The surface is peppered with volcanoes and [[rift valley]]s, and has a rich collection of [[mineral]]s.<ref>{{Cite web |last=Noever |first=David |date=2004 |title=Modern Martian Marvels: Volcanoes? |url=https://www.astrobio.net/mars/modern-martian-marvels-volcanoes |url-status=dead |archive-url=https://web.archive.org/web/20200314112555/https://www.astrobio.net/mars/modern-martian-marvels-volcanoes |archive-date=14 March 2020 |access-date=23 July 2006 |website=NASA Astrobiology Magazine}}</ref><ref name="ismars">[http://mars.jpl.nasa.gov/msl/multimedia/videos/index.cfm?v=29&a=2 NASA – ''Mars in a Minute: Is Mars Really Red?''] {{Webarchive|url=https://web.archive.org/web/20140720135450/http://mars.jpl.nasa.gov/msl/multimedia/videos/index.cfm?v=29&a=2|date=20 July 2014}} ([http://mars.jpl.nasa.gov/multimedia/videos/movies/miam20111110/miam20111110.pdf Transcript] {{Webarchive|url=https://web.archive.org/web/20151106174558/http://mars.jpl.nasa.gov/multimedia/videos/movies/miam20111110/miam20111110.pdf|date=6 November 2015}}) {{PD-notice}}</ref> Mars has a highly [[Planetary differentiation|differentiated]] internal structure, and lost its magnetosphere 4&nbsp;billion years ago.<ref name="Nimmo 2005">{{cite journal |last1=Nimmo |first1=Francis |last2=Tanaka |first2=Ken |year=2005 |title=Early Crustal Evolution of Mars |journal=Annual Review of Earth and Planetary Sciences |volume=33 |issue=1 |pages=133–161 |bibcode=2005AREPS..33..133N |doi=10.1146/annurev.earth.33.092203.122637 |s2cid=45843366}}</ref><ref name="swind">{{cite web |last=Philips |first=Tony |date=31 January 2001 |title=The Solar Wind at Mars |url=https://science.nasa.gov/science-news/science-at-nasa/2001/ast31jan_1/ |url-status=dead |archive-url=https://web.archive.org/web/20110818180040/https://science.nasa.gov/science-news/science-at-nasa/2001/ast31jan_1/ |archive-date=18 August 2011 |access-date=22 April 2022 |website=Science@NASA}} {{PD-notice}}</ref> [[Moons of Mars|Mars has two tiny moons]]:<ref name="NYT-20200725">{{cite news |last=Andrews |first=Robin George |date=25 July 2020 |title=Why the 'Super Weird' Moons of Mars Fascinate Scientists - What's the big deal about little Phobos and tinier Deimos? |url=https://www.nytimes.com/2020/07/25/science/mars-moons-phobos-deimos.html |url-status=live |archive-url=https://web.archive.org/web/20200725094039/https://www.nytimes.com/2020/07/25/science/mars-moons-phobos-deimos.html |archive-date=25 July 2020 |access-date=25 July 2020 |work=[[The New York Times]]}}</ref>
** [[Phobos (moon)|Phobos]] is Mars's inner moon. It is a small, irregularly shaped object with a mean radius of {{convert|11|km|abbr=on|sigfig=1}}. Its surface is very unreflective and dominated by impact craters.<ref name="jplssd" group="D">{{cite web |date=13 July 2006 |title=Planetary Satellite Physical Parameters |url=http://ssd.jpl.nasa.gov/?sat_phys_par |access-date=29 January 2008 |publisher=[[JPL]] (Solar System Dynamics) |archive-date=1 November 2013 |archive-url=https://web.archive.org/web/20131101144111/http://ssd.jpl.nasa.gov/?sat_phys_par |url-status=live }}</ref><ref>{{cite web |date=12 January 2004 |title=Phobos |url=http://www.bbc.co.uk/science/space/solarsystem/mars/phobos.shtml |url-status=dead |archive-url=https://web.archive.org/web/20090422160500/http://www.bbc.co.uk/science/space/solarsystem/mars/phobos.shtml |archive-date=22 April 2009 |access-date=19 July 2021 |work=[[BBC Online]]}}</ref> In particular, Phobos's surface has a very large [[Stickney (crater)|Stickney impact crater]] that is roughly {{convert|4.5|km|mi|abbr=on}} in radius.<ref>{{cite web |title=Stickney Crater-Phobos |url=http://www.solarviews.com/cap/mars/phobos2.htm |quote=One of the most striking features of Phobos, aside from its irregular shape, is its giant crater Stickney. Because Phobos is only {{Convert|28|by|20|km||sp=us}}, it must have been nearly shattered from the force of the impact that caused the giant crater. Grooves that extend across the surface from Stickney appear to be surface fractures caused by the impact. |access-date=21 April 2024 |archive-date=3 November 2011 |archive-url=https://web.archive.org/web/20111103010644/http://www.solarviews.com/cap/mars/phobos2.htm |url-status=live }}</ref>
** [[Deimos (moon)|Deimos]] is Mars's outer moon. Like Phobos, it is irregularly shaped, with a mean radius of {{convert|6|km|mi|abbr=on|sigfig=1}} and its surface reflects little light.<ref name="Horizons-Deimos" group="D">{{cite web |date=21 September 2013 |title=HORIZONS Web-Interface |url=http://ssd.jpl.nasa.gov/?horizons |access-date=4 December 2013 |publisher=NASA |archive-date=28 March 2007 |archive-url=https://web.archive.org/web/20070328180634/http://ssd.jpl.nasa.gov/?horizons |url-status=live }}</ref><ref name="JPLSSD" group="D">{{cite web |date=13 July 2006 |title=Planetary Satellite Physical Parameters |url=http://ssd.jpl.nasa.gov/?sat_phys_par |access-date=29 January 2008 |publisher=[[Jet Propulsion Laboratory]] (Solar System Dynamics) |archive-date=1 November 2013 |archive-url=https://web.archive.org/web/20131101144111/http://ssd.jpl.nasa.gov/?sat_phys_par |url-status=live }}</ref> However, the surface of Deimos is noticeably smoother than Phobos because the regolith partially covers the impact craters.<ref>{{Cite web |date=6 June 2023 |title=Deimos |url=https://www.britannica.com/place/Deimos-moon-of-Mars |access-date=21 April 2024 |website=Britannica |language=en |quote=It thus appears smoother than Phobos because its craters lie partially buried under this loose material. |archive-date=12 November 2018 |archive-url=https://web.archive.org/web/20181112023547/https://www.britannica.com/place/Deimos-moon-of-Mars |url-status=live }}</ref>


===Asteroids ===
; Saturn
{{Main|Asteroid}}
: [[Saturn]] (9.5&nbsp;AU), distinguished by its extensive [[Rings of Saturn|ring system]], has similarities to Jupiter, such as its atmospheric composition. Saturn is far less massive, being only 95 Earth masses. Saturn has [[Moons of Saturn|sixty known satellites]] (and three unconfirmed); two of which, [[Titan (moon)|Titan]] and [[Enceladus (moon)|Enceladus]], show signs of geological activity, though they are largely [[Cryovolcano|made of ice]].<ref>{{cite web |title=Cryovolcanism on the icy satellites |author=J. S. Kargel |work=U.S. Geological Survey |year=1994 |url=http://www.springerlink.com/content/n7435h4506788p22/ |accessdate=2006-01-16}}</ref> Titan is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.
[[File:Inner solar system objects top view for wiki.png|right|thumb|alt=Asteroid populations depicted: near-Earth asteroids, Earth trojans, Mars trojans, main asteroid belt, Jupiter trojans, Jupiter Greeks, Jupiter Hilda's triangle|Overview of the inner Solar System up to Jupiter's orbit]]Asteroids except for the largest, Ceres, are classified as [[small Solar System bodies]] and are composed mainly of [[carbon]]aceous, refractory rocky and metallic minerals, with some ice.<ref>{{Cite web |date=2006 |title=IAU Planet Definition Committee |url=http://www.iau.org/public_press/news/release/iau0601/newspaper |url-status=dead |archive-url=https://web.archive.org/web/20090603001603/http://www.iau.org/public_press/news/release/iau0601/newspaper |archive-date=3 June 2009 |access-date=1 March 2009 |publisher=International Astronomical Union}}</ref><ref>{{Cite web |title=Are Kuiper Belt Objects asteroids? Are large Kuiper Belt Objects planets? |url=http://curious.astro.cornell.edu/question.php?number=601 |url-status=dead |archive-url=https://web.archive.org/web/20090103110110/http://curious.astro.cornell.edu/question.php?number=601 |archive-date=3 January 2009 |access-date=1 March 2009 |publisher=[[Cornell University]]}}</ref> They range from a few meters to hundreds of kilometers in size. {{Visible anchor|Asteroid groups|text=Many asteroids are divided into [[asteroid group]]s and [[Asteroid family|families]]}} based on their orbital characteristics. Some [[Minor-planet moon|asteroids have natural satellites that orbit them]], that is, asteroids that orbit larger asteroids.<ref>{{Cite journal |last1=Snodgrass |first1=Colin |last2=Agarwal |first2=Jessica |last3=Combi |first3=Michael |last4=Fitzsimmons |first4=Alan |last5=Guilbert-Lepoutre |first5=Aurelie |last6=Hsieh |first6=Henry H. |last7=Hui |first7=Man-To |last8=Jehin |first8=Emmanuel |last9=Kelley |first9=Michael S. P. |last10=Knight |first10=Matthew M. |last11=Opitom |first11=Cyrielle |date=November 2017 |title=The Main Belt Comets and ice in the Solar System |url=http://link.springer.com/10.1007/s00159-017-0104-7 |url-status=live |journal=The Astronomy and Astrophysics Review |language=en |volume=25 |issue=1 |page=5 |arxiv=1709.05549 |bibcode=2017A&ARv..25....5S |doi=10.1007/s00159-017-0104-7 |issn=0935-4956 |s2cid=7683815 |archive-url=https://web.archive.org/web/20220420161227/https://idp.springer.com/favicon.ico |archive-date=20 April 2022 |access-date=9 March 2022}}</ref>


* [[List of Mercury-crossing minor planets|Mercury-crossing asteroids]] are those with [[perihelia]] within the orbit of Mercury. At least 362 are known to date, and include the closest objects to the Sun known in the Solar System.<ref name="JPLcrosserlist">[http://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=all;obj_kind=ast;obj_numbered=all;OBJ_field=0;ORB_field=0;c1_group=ORB;c1_item=Bi;c1_op=%3C;c1_value=0.3075;table_format=HTML;max_rows=200;format_option=comp;c_fields=BgBhBiBjBnBsChAcCq;.cgifields=format_option;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=com_orbit_class&query=1&c_sort=BiA List of asteroids with q<0.3075 AU generated by the JPL Small-Body Database Search Engine] {{Webarchive|url=https://web.archive.org/web/20160303213657/http://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=all&obj_kind=ast&obj_numbered=all&OBJ_field=0&ORB_field=0&c1_group=ORB&c1_item=Bi&c1_op=%3C&c1_value=0.3075&table_format=HTML&max_rows=200&format_option=comp&c_fields=BgBhBiBjBnBsChAcCq&.cgifields=format_option&.cgifields=ast_orbit_class&.cgifields=table_format&.cgifields=obj_kind&.cgifields=obj_group&.cgifields=obj_numbered&.cgifields=com_orbit_class&query=1&c_sort=BiA |date=3 March 2016 }} Retrieved 30 May 2012</ref> No [[vulcanoid]]s, asteroids between the orbit of Mercury and the Sun, have been discovered.<ref>{{Cite journal |last1=Durda |first1=D .D. |last2=Stern |first2=S. A. |last3=Colwell |first3=W. B. |last4=Parker |first4=J. W. |last5=Levison |first5=H. F. |last6=Hassler |first6=D. M. |date=2004 |title=A New Observational Search for Vulcanoids in SOHO/LASCO Coronagraph Images |journal=[[Icarus (journal)|Icarus]] |volume=148 |issue=1 |pages=312–315 |bibcode=2000Icar..148..312D |doi=10.1006/icar.2000.6520}}</ref><ref name="Steffl2013">{{Cite journal |last1=Steffl |first1=A. J. |last2=Cunningham |first2=N. J. |last3=Shinn |first3=A. B. |last4=Stern |first4=S. A. |date=2013 |title=A Search for Vulcanoids with the STEREO Heliospheric Imager |journal=Icarus |volume=233 |issue=1 |pages=48–56 |arxiv=1301.3804 |bibcode=2013Icar..223...48S |doi=10.1016/j.icarus.2012.11.031 |s2cid=118612132}}</ref> As of 2024, one asteroid has been discovered to orbit completely within Venus's orbit, [[594913 ꞌAylóꞌchaxnim]].<ref>{{cite journal |last1=Bolin |first1=Bryce T. |last2=Ahumada |first2=T. |last3=van Dokkum |first3=P. |last4=Fremling |first4=C. |last5=Granvik |first5=M. |last6=Hardegree-Ullmann |first6=K. K. |last7=Harikane |first7=Y. |last8=Purdum |first8=J. N. |last9=Serabyn |first9=E. |last10=Southworth |first10=J. |last11=Zhai |first11=C. |date=November 2022 |title=The discovery and characterization of (594913) 'Ayló'chaxnim, a kilometre sized asteroid inside the orbit of Venus |url=https://academic.oup.com/mnrasl/article/517/1/L49/6665933 |journal=[[Monthly Notices of the Royal Astronomical Society Letters]] |volume=517 |issue=1 |pages=L49–L54 |arxiv=2208.07253 |bibcode=2022MNRAS.517L..49B |doi=10.1093/mnrasl/slac089 |access-date=1 October 2022 |doi-access=free |archive-date=1 October 2022 |archive-url=https://web.archive.org/web/20221001070557/https://academic.oup.com/mnrasl/article/517/1/L49/6665933 |url-status=live }}</ref>
; Uranus
* [[Venus-crossing asteroid]]s are those that cross the orbit of Venus. There are 2,809 as of 2015.<ref name=jpello/>
: [[Uranus]] (19.6&nbsp;AU), at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its [[axial tilt]] is over ninety degrees to the [[ecliptic]]. It has a much colder core than the other gas giants, and radiates very little heat into space.<ref>{{cite web |title=10 Mysteries of the Solar System |author=Hawksett, David; Longstaff, Alan; Cooper, Keith; Clark, Stuart |work=Astronomy Now |year=2005 |url=http://adsabs.harvard.edu/abs/2005AsNow..19h..65H |accessdate=2006-01-16}}</ref> Uranus has [[Moons of Uranus|twenty-seven known satellites]], the largest ones being [[Titania (moon)|Titania]], [[Oberon (moon)|Oberon]], [[Umbriel (moon)|Umbriel]], [[Ariel (moon)|Ariel]] and [[Miranda (moon)|Miranda]].
* [[near-Earth object|Near-Earth asteroids]] have orbits that approach relatively close to Earth's orbit,<ref name="MorbidelliAstIII">{{Cite book |last1=Morbidelli |first1=A. |last2=Bottke |first2=W.F. |last3=Froeschlé |first3=Ch. |last4=Michel |first4=P. |date=January 2002 |editor2-last=A. Cellino |editor3-last=P. Paolicchi |editor4-last=R.P. Binzel |title=Origin and Evolution of Near-Earth Objects |url=http://www.boulder.swri.edu/~bottke/Reprints/Morbidelli-etal_2002_AstIII_NEOs.pdf |url-status=live |journal=Asteroids III |pages=409–422 |publisher=University of Arizona Press |bibcode=2002aste.book..409M |doi=10.2307/j.ctv1v7zdn4.33 |isbn=978-0-8165-2281-1 |archive-url=https://web.archive.org/web/20170809014123/http://www.boulder.swri.edu/~bottke/Reprints/Morbidelli-etal_2002_AstIII_NEOs.pdf |archive-date=9 August 2017 |access-date=30 August 2009 |editor=W.F. Bottke Jr.}}</ref> and some of them are [[potentially hazardous object]]s because they might collide with Earth in the future.<ref name="CNEOS-Basics">{{Cite web |title=NEO Basics – Potentially Hazardous Asteroids (PHAs) |url=https://cneos.jpl.nasa.gov/about/neo_groups.html |url-status=live |archive-url=https://web.archive.org/web/20211111141623/https://cneos.jpl.nasa.gov/about/neo_groups.html |archive-date=11 November 2021 |access-date=10 March 2022 |publisher=CNEOS NASA/JPL}}</ref><ref name="NEO-groups">{{cite web |last1=Baalke |first1=Ron |title=Near-Earth Object Groups |url=http://neo.jpl.nasa.gov/neo/groups.html |url-status=dead |archive-url=https://web.archive.org/web/20020202160655/http://neo.jpl.nasa.gov/neo/groups.html |archive-date=2 February 2002 |access-date=11 November 2016 |website=[[Jet Propulsion Laboratory]] |publisher=[[NASA]]}}</ref>
* [[Mars-crossing asteroids]] are those with perhihelia above 1.3&nbsp;AU which cross the orbit of Mars.<ref>{{cite journal|journal=Astronomy & Astrophysics|volume=391|pages=757–765|year=2002|doi=10.1051/0004-6361:20020834|title=Spectral properties of Mars-crossers and near-Earth objects|author=C. A. Angeli - D. Lazzaro|issue=2 }}</ref> As of 2024, NASA lists 26,182 confirmed Mars-crossing asteroids.<ref name=jpello>{{cite web|title=Small-Body Database Query|url=https://ssd.jpl.nasa.gov/tools/sbdb_query.html#!|work=NASA|accessdate=3 June 2024|archive-date=27 September 2021|archive-url=https://web.archive.org/web/20210927184129/https://ssd.jpl.nasa.gov/tools/sbdb_query.html#!|url-status=live}}</ref>


====Asteroid belt====
; Neptune
The [[asteroid belt]] occupies a torus-shaped region between 2.3 and {{val|3.3|u=AU}} from the Sun, which lies between the orbits of Mars and Jupiter. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.<ref>{{Cite journal |last1=Petit |first1=J.-M. |last2=Morbidelli |first2=A. |last3=Chambers |first3=J. |date=2001 |title=The Primordial Excitation and Clearing of the Asteroid Belt |url=http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf |url-status=dead |journal=[[Icarus (journal)|Icarus]] |volume=153 |issue=2 |pages=338–347 |bibcode=2001Icar..153..338P |doi=10.1006/icar.2001.6702 |archive-url=https://web.archive.org/web/20070221085835/http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf |archive-date=21 February 2007 |access-date=22 March 2007}}</ref> The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometer in diameter.<ref>{{Cite journal |last1=Tedesco |first1=Edward F. |last2=Cellino |first2=Alberto |last3=Zappalá |first3=Vincenzo |date=June 2005 |title=The Statistical Asteroid Model. I. The Main-Belt Population for Diameters Greater than 1 Kilometer |journal=The Astronomical Journal |language=en |volume=129 |issue=6 |pages=2869–2886 |bibcode=2005AJ....129.2869T |doi=10.1086/429734 |issn=0004-6256 |s2cid=119906696 |doi-access=free}}</ref> Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth.<ref name="Krasinsky2002">{{Cite journal |last1=Krasinsky |first1=G. A. |author-link=Georgij A. Krasinsky |last2=Pitjeva |first2=E. V. |author-link2=Elena V. Pitjeva |last3=Vasilyev |first3=M. V. |last4=Yagudina |first4=E. I. |date=July 2002 |title=Hidden Mass in the Asteroid Belt |journal=[[Icarus (journal)|Icarus]] |volume=158 |issue=1 |pages=98–105 |bibcode=2002Icar..158...98K |doi=10.1006/icar.2002.6837}}</ref> The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident.<ref>{{Cite web |date=14 April 2000 |title=Cassini Passes Through Asteroid Belt |url=https://solarsystem.nasa.gov/news/12195/cassini-passes-through-asteroid-belt |url-status=live |archive-url=https://web.archive.org/web/20210125180703/https://solarsystem.nasa.gov/news/12195/cassini-passes-through-asteroid-belt |archive-date=25 January 2021 |access-date=1 March 2021 |website=NASA}}</ref>
: [[Neptune]] (30&nbsp;AU), though slightly smaller than Uranus, is more [[mass]]ive (equivalent to 17 Earths) and therefore more [[Density|dense]]. It radiates more internal heat, but not as much as Jupiter or Saturn.<ref>{{cite web |title=Post Voyager comparisons of the interiors of Uranus and Neptune |author=Podolak, M.; Reynolds, R. T.; Young, R. |work=NASA, Ames Research Center |year=1990 |url=http://adsabs.harvard.edu/abs/1990GeoRL..17.1737P |accessdate=2006-01-16}}</ref> Neptune has [[Moons of Neptune|thirteen known satellites]]. The largest, [[Triton (moon)|Triton]], is geologically active, with [[geyser]]s of [[liquid nitrogen]].<ref>{{cite web |title=The Plausibility of Boiling Geysers on Triton |author=Duxbury, N.S., Brown, R.H. |work=Beacon eSpace |year=1995 |url=http://trs-new.jpl.nasa.gov/dspace/handle/2014/28034?mode=full |accessdate=2006-01-16 }}</ref> Triton is the only large satellite with a [[retrograde and direct motion|retrograde]] orbit. Neptune is accompanied in its orbit by a number of [[minor planet]]s, termed [[Neptune Trojan]]s, that are in 1:1 [[Orbital resonance|resonance]] with it.
[[File:The Four Largest Asteroids.jpg|thumb|The four largest asteroids: [[Ceres (dwarf planet)|Ceres]], [[4 Vesta|Vesta]], [[2 Pallas|Pallas]], [[10 Hygiea|Hygiea]]. Only Ceres and Vesta have been visited by a spacecraft and thus have a detailed picture.]]
Below are the descriptions of the three largest bodies in the asteroid belt. They are all considered to be relatively intact [[protoplanet]]s, a precursor stage before becoming a fully-formed planet (see [[List of exceptional asteroids]]):<ref>{{Cite journal |last1=McCord |first1=Thomas B. |last2=McFadden |first2=Lucy A. |last3=Russell |first3=Christopher T. |last4=Sotin |first4=Christophe |last5=Thomas |first5=Peter C. |date=7 March 2006 |title=Ceres, Vesta, and Pallas: Protoplanets, Not Asteroids |url=https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006EO100002 |url-status=live |journal=Eos |volume=87 |page=105 |bibcode=2006EOSTr..87..105M |doi=10.1029/2006EO100002 |archive-url=https://web.archive.org/web/20210928160233/https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006EO100002 |archive-date=28 September 2021 |access-date=12 September 2021 |number=10}}</ref><ref name="nasa-dawn20110329">{{cite web |author=Cook, Jia-Rui C. |date=29 March 2011 |title=When Is an Asteroid Not an Asteroid? |url=http://www.nasa.gov/mission_pages/dawn/news/dawn20110329.html |url-status=live |archive-url=https://web.archive.org/web/20110629163004/http://www.nasa.gov/mission_pages/dawn/news/dawn20110329.html |archive-date=29 June 2011 <!--DASHBot--> |access-date=30 July 2011 |publisher=NASA/JPL}}</ref><ref name="Marsset2020">{{Cite journal |last1=Marsset |first1=M. |last2=Brož |first2=M. |last3=Vernazza |first3=P. |last4=Drouard |first4=A. |display-authors=3 |date=2020 |title=The violent collisional history of aqueously evolved (2) Pallas |url=https://orbi.uliege.be/bitstream/2268/246670/1/Pallas_Marsset.pdf |journal=Nature Astronomy |volume=4 |issue=6 |pages=569–576 |bibcode=2020NatAs...4..569M |doi=10.1038/s41550-019-1007-5 |s2cid=212927521 |hdl-access=free |hdl=10261/237549 |access-date=4 January 2023 |archive-date=7 January 2023 |archive-url=https://web.archive.org/web/20230107085352/https://orbi.uliege.be/bitstream/2268/246670/1/Pallas_Marsset.pdf |url-status=live }}</ref>


* {{Visible anchor|Ceres|text=[[Ceres (dwarf planet)|Ceres]]}} (2.55–2.98&nbsp;AU) is the only dwarf planet in the asteroid belt.<ref name="IAU-QA">{{Cite web |title=Question and answers 2 |url=https://www.iau.org/public/themes/pluto/ |url-status=live |archive-url=https://web.archive.org/web/20160130022141/http://www.iau.org/public/themes/pluto/ |archive-date=30 January 2016 |access-date=31 January 2008 |publisher=IAU |quote=Ceres is (or now we can say it was) the largest asteroid{{spaces}}... There are many other asteroids that can come close to the orbital path of Ceres.}}</ref> It is the largest object in the belt, with a diameter of {{Convert|940|km|abbr=on}}.<ref name="Ermakov2017">{{cite journal |last1=Ermakov |first1=A. I. |last2=Fu |first2=R. R. |last3=Castillo-Rogez |first3=J. C. |last4=Raymond |first4=C. A. |last5=Park |first5=R. S. |last6=Preusker |first6=F. |last7=Russell |first7=C. T. |last8=Smith |first8=D. E. |last9=Zuber |first9=M. T. |date=November 2017 |title=Constraints on Ceres' Internal Structure and Evolution From Its Shape and Gravity Measured by the Dawn Spacecraft |journal=Journal of Geophysical Research: Planets |volume=122 |issue=11 |pages=2267–2293 |bibcode=2017JGRE..122.2267E |doi=10.1002/2017JE005302 |s2cid=133739176 |doi-access=free}}</ref> Its surface contains a mixture of [[carbon]],<ref name="Nature 12 2018">{{Cite journal |last1=Marchi |first1=S. |last2=Raponi |first2=A. |last3=Prettyman |first3=T. H. |last4=De Sanctis |first4=M. C. |last5=Castillo-Rogez |first5=J. |last6=Raymond |first6=C. A. |last7=Ammannito |first7=E. |last8=Bowling |first8=T. |last9=Ciarniello |first9=M. |last10=Kaplan |first10=H. |last11=Palomba |first11=E. |last12=Russell |first12=C. T. |last13=Vinogradoff |first13=V. |last14=Yamashita |first14=N. |year=2018 |title=An aqueously altered carbon-rich Ceres |journal=[[Nature Astronomy]] |volume=3 |issue=2 |pages=140–145 |doi=10.1038/s41550-018-0656-0 |s2cid=135013590}}</ref> frozen water and [[hydrate]]d minerals.<ref name="EPSC2">{{Cite book |last1=Raymond |first1=C. |title=European Planetary Science Congress |last2=Castillo-Rogez |first2=J. C. |last3=Park |first3=R. S. |last4=Ermakov |first4=A. |last5=Bland |first5=M. T. |last6=Marchi |first6=S. |last7=Prettyman |first7=T. |last8=Ammannito |first8=E. |last9=De Sanctis |first9=M. C. |date=September 2018 |volume=12 |chapter=Dawn Data Reveal Ceres' Complex Crustal Evolution |display-authors=4 |access-date=19 July 2020 |chapter-url=https://meetingorganizer.copernicus.org/EPSC2018/EPSC2018-645-1.pdf |archive-url=https://web.archive.org/web/20200130111631/https://meetingorganizer.copernicus.org/EPSC2018/EPSC2018-645-1.pdf |archive-date=30 January 2020 |url-status=live |author10=Russell, C.T.}}</ref> There are signs of past [[cryovolcano|cryovolcanic]] activity, where [[volatile (astrogeology)|volatile]] material such as water are erupted onto the surface, as seen in [[Bright spots on Ceres|surface bright spots]].<ref>{{cite web |last1=Krummheuer |first1=Birgit |date=6 March 2017 |title=Cryovolcanism on Dwarf Planet Ceres |url=http://www.mps.mpg.de/Cryovolcanism-on-Dwarf-Planet-Ceres |website=Max Planck Institute for Solar System Research |access-date=22 April 2024 |archive-date=2 February 2024 |archive-url=https://web.archive.org/web/20240202180118/https://www.mps.mpg.de/Cryovolcanism-on-Dwarf-Planet-Ceres |url-status=live }}</ref> Ceres has a very thin water vapor atmosphere, but practically speaking it is indistinguishable from a vacuum.<ref>{{Cite news |date=6 April 2017 |title=Confirmed: Ceres Has a Transient Atmosphere |url=https://www.universetoday.com/134922/confirmed-ceres-transient-atmosphere/ |url-status=live |archive-url=https://web.archive.org/web/20170415103956/https://www.universetoday.com/134922/confirmed-ceres-transient-atmosphere/ |archive-date=15 April 2017 |access-date=14 April 2017 |work=Universe Today |language=en}}</ref>
===Comets===
* {{Visible anchor|Pallas and Vesta|text=[[4 Vesta|Vesta]]}} (2.13–3.41&nbsp;AU) is the second-largest object in the asteroid belt.<ref name="Astronomy & Astrophysics">{{Cite report |url=https://meetingorganizer.copernicus.org/EPSC2022/EPSC2022-103.html |title=VLT/SPHERE imaging survey of D>100 km asteroids: Final results and synthesis |last1=Vernazza |first1=Pierre |last2=Ferrais |first2=Marin |date=6 July 2022 |publisher=Astronomy & Astrophysics |doi=10.5194/epsc2022-103 |page=A56 |last3=Jorda |first3=Laurent |last4=Hanus |first4=Josef |last5=Carry |first5=Benoit |last6=Marsset |first6=Michael |last7=Brož |first7=Miroslav |last8=Fetick |first8=Roman |last9=HARISSA team |doi-access=free |access-date=22 April 2024 |archive-date=22 April 2024 |archive-url=https://web.archive.org/web/20240422145344/https://meetingorganizer.copernicus.org/EPSC2022/EPSC2022-103.html |url-status=live }}</ref> Its fragments survive as the [[Vesta family|Vesta asteroid family]]<ref name="planetarysociety" /> and numerous [[HED meteorite]]s found on Earth.<ref name="Vestainterior">{{cite web |date=6 January 2011 |title=A look into Vesta's interior |url=https://www.mpg.de/877913/Vesta_asteroid |work=Max-Planck-Gesellschaft |access-date=22 April 2024 |archive-date=5 March 2023 |archive-url=https://web.archive.org/web/20230305200352/https://www.mpg.de/877913/Vesta_asteroid |url-status=live }}</ref> Vesta's surface, dominated by [[basalt]]ic and [[Metamorphic rock|metamorphic]] material, has a denser composition than Ceres's.<ref name="Takeda1997">{{cite journal |author=Takeda, H. |date=1997 |title=Mineralogical records of early planetary processes on the HED parent body with reference to Vesta |journal=Meteoritics & Planetary Science |volume=32 |issue=6 |pages=841–853 |bibcode=1997M&PS...32..841T |doi=10.1111/j.1945-5100.1997.tb01574.x |doi-access=free}}</ref> Its surface is marked by two giant craters: [[Rheasilvia]] and [[Veneneia (crater)|Veneneia]].<ref name="Schenk2012">{{cite journal |author=Schenk, P. |display-authors=etal |date=2012 |title=The Geologically Recent Giant Impact Basins at Vesta's South Pole |journal=Science |volume=336 |issue=6082 |pages=694–697 |bibcode=2012Sci...336..694S |doi=10.1126/science.1223272 |pmid=22582256 |s2cid=206541950}}</ref>
{{Main|Comet}}
* [[2 Pallas|Pallas]] (2.15–2.57&nbsp;AU) is the third-largest object in the asteroid belt.<ref name="Astronomy & Astrophysics"/> It has its own [[Pallas family|Pallas]] [[Vesta family|asteroid family]].<ref name="planetarysociety">{{Cite web |last=Lakdawalla |first=Emily |author-link=Emily Lakdawalla |display-authors=etal |date=21 April 2020 |title=What Is A Planet? |url=https://www.planetary.org/worlds/what-is-a-planet |url-status=live |archive-url=https://web.archive.org/web/20220122142140/https://www.planetary.org/worlds/what-is-a-planet |archive-date=22 January 2022 |access-date=3 April 2022 |website=The Planetary Society}}</ref> Not much is known about Pallas because it has never been visited by a spacecraft,<ref>{{cite web |title=Athena: A SmallSat Mission to (2) Pallas |url=https://josephgorourke.com/research |url-status=dead |archive-url=https://web.archive.org/web/20211121181742/https://josephgorourke.com/research |archive-date=21 November 2021 |access-date=7 October 2020}}</ref> though its surface is predicted to be composed of silicates.<ref name="AutoCB-19">{{cite journal |author=Feierberg, M. A. |author2=Larson, H. P. |author3=Lebofsky, L. A. |date=1982 |title=The 3 Micron Spectrum of Asteroid 2 Pallas |journal=Bulletin of the American Astronomical Society |volume=14 |page=719 |bibcode=1982BAAS...14..719F}}</ref>
[[Image:Comet c1995o1.jpg|right|thumb|Comet Hale-Bopp]]


[[Hilda asteroid]]s are in a 3:2 resonance with Jupiter; that is, they go around the Sun three times for every two Jovian orbits.<ref name="Barucci">{{Cite book |last1=Barucci |first1=M. A. |title=Asteroids III |last2=Kruikshank |first2=D. P. |last3=Mottola |first3=S. |last4=Lazzarin |first4=M. |date=2002 |publisher=University of Arizona Press |location=Tucson, Arizona |pages=273–287 |chapter=Physical Properties of Trojan and Centaur Asteroids}}</ref> They lie in three linked clusters between Jupiter and the main asteroid belt.
Comets are small Solar System bodies, usually only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a [[perihelion]] within the orbits of the inner planets and an [[aphelion]] far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to [[Sublimation (chemistry)|sublimate]] and [[ion]]ise, creating a [[Coma (cometary)|coma]]: a long tail of gas and dust often visible to the naked eye.


[[Trojan (celestial body)|Trojan]]s are bodies located within another body's gravitationally stable [[Lagrangian point|Lagrange points]]: {{L4}}, 60° ahead in its orbit, or {{L5}}, 60° behind in its orbit.<ref name="spies">{{cite web |title=Trojan Asteroids |url=http://astronomy.swin.edu.au/cosmos/T/Trojan+Asteroids |url-status=live |archive-url=https://web.archive.org/web/20170623182748/http://astronomy.swin.edu.au/cosmos/T/Trojan+Asteroids |archive-date=23 June 2017 |access-date=13 June 2017 |website=Cosmos |publisher=Swinburne University of Technology}}</ref> Every planet except Mercury and Saturn is known to possess at least 1 trojan.<ref name="Connors">{{cite journal |last1= Connors|first1= Martin|last2= Wiegert|first2= Paul|last3= Veillet|first3= Christian|title= Earth's Trojan asteroid|date= 27 July 2011|journal= [[Nature (journal)|Nature]]|volume= 475|pages= 481–483|doi= 10.1038/nature10233|issue= 7357|bibcode= 2011Natur.475..481C|pmid= 21796207|s2cid= 205225571}}</ref><ref name=secondUranus>{{cite journal
Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are believed to originate in the [[Kuiper belt]], while long-period comets, such as [[Comet Hale-Bopp|Hale-Bopp]], are believed to originate in the [[Oort cloud]]. Many comet groups, such as the [[Kreutz Sungrazers]], formed from the breakup of a single parent.<ref>{{cite journal |author=Sekanina, Zdenek |year=2001 |title=Kreutz sungrazers: the ultimate case of cometary fragmentation and disintegration? |journal=Publications of the Astronomical Institute of the Academy of Sciences of the Czech Republic |volume=89 p.78–93}}</ref> Some comets with [[Comet#Orbital characteristics|hyperbolic]] orbits may originate outside the Solar System, but determining their precise orbits is difficult.<ref name="hyperbolic">{{cite journal |last=Królikowska |first=M. |year=2001 |title=A study of the original orbits of ''hyperbolic'' comets |journal=Astronomy & Astrophysics |volume=376 |issue=1 |pages=316–324 |doi=10.1051/0004-6361:20010945 |url=http://www.aanda.org/index.php?option=com_base_ora&url=articles/aa/full/2001/34/aa1250/aa1250.right.html&access=standard&Itemid=81 |accessdate=2007-01-02}}</ref> Old comets that have had most of their volatiles driven out by solar warming are often categorised as asteroids.<ref>{{cite web |url=http://www.springerlink.com/content/x0358l71h463w246/ |title=The activities of comets related to their aging and origin |author=Fred L. Whipple |accessdate=2006-12-26|date=1992-04}}</ref>
|last2=de la Fuente Marcos |first2=Raúl
|last1=de la Fuente Marcos |first1=Carlos
|title=Asteroid 2014 YX<sub>49</sub>: a large transient Trojan of Uranus
|journal=Monthly Notices of the Royal Astronomical Society
|date=21 May 2017
|volume=467 |issue=2
|arxiv=1701.05541
|doi=10.1093/mnras/stx197 |pages=1561–1568|doi-access=free
|bibcode=2017MNRAS.467.1561D}}</ref><ref name=CeresVestatrojans>{{cite journal |first1=Apostolos A. |last1=Christou |first2=Paul |last2=Wiegert |title=A population of main belt asteroids co-orbiting with Ceres and Vesta |journal=Icarus |volume=217 |issue=1 |date=January 2012 |pages=27–42 |arxiv=1110.4810 |doi=10.1016/j.icarus.2011.10.016|bibcode=2012Icar..217...27C |s2cid=59474402 }}</ref> The [[Jupiter trojan]] population is roughly equal to that of the asteroid belt.<ref name="Yoshida2005">{{cite journal |last1=Yoshida |first1=Fumi |last2=Nakamura |first2=Tsuko |title=Size distribution of faint L4 Trojan asteroids |year=2005 |journal=The Astronomical Journal |volume=130 |issue=6 |pages=2900–11 |doi=10.1086/497571 |bibcode=2005AJ....130.2900Y|doi-access=free }}</ref> After Jupiter, Neptune possesses the most confirmed trojans, at 28.<ref>{{cite web |title=List of Neptune Trojans |work=Minor Planet Center |date=28 October 2018 |access-date=28 December 2018 |url=http://www.minorplanetcenter.org/iau/lists/NeptuneTrojans.html |archive-date=25 May 2012 |archive-url=https://archive.today/20120525133119/http://www.minorplanetcenter.org/iau/lists/NeptuneTrojans.html |url-status=live }}</ref>


== Outer Solar System ==
; Centaurs
The outer region of the Solar System is home to the [[giant planet]]s and their large moons. The [[Centaur (minor planet)|centaurs]] and many [[short-period comet]]s orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles such as water, ammonia, and methane, than planets of the inner Solar System because their lower temperatures allow these compounds to remain solid, without significant [[Sublimation (phase transition)|sublimation]].<ref name="bennett_8.2"/>
:The [[Centaur (planetoid)|centaur]]s, which extend from 9 to 30&nbsp;AU, are icy comet-like bodies that orbit in the region between Jupiter and Neptune. The largest known centaur, [[10199 Chariklo]], has a diameter of between 200 and 250&nbsp;km.<ref>{{cite web |url=http://www.johnstonsarchive.net/astro/tnodiam.html |title=TNO/Centaur diameters and albedos |accessdate=2006-11-08 |last=Stansberry |date=2006-04-14}}</ref> The first centaur discovered, [[2060 Chiron]], has been called a comet since it develops a coma just as comets do when they approach the Sun.<ref>{{cite web |year=1995 |author=Patrick Vanouplines |title=Chiron biography |work=Vrije Universitiet Brussel |url=http://www.vub.ac.be/STER/www.astro/chibio.htm |accessdate=2006-06-23}}</ref> Some astronomers classify centaurs as inward-scattered [[#Kuiper belt|Kuiper belt]] objects along with the outward-scattered residents of the [[#Scattered disc|scattered disc]].<ref>{{cite web |url=http://cfa-www.harvard.edu/iau/lists/Centaurs.html |title=List Of Centaurs and Scattered-Disk Objects |work=IAU: Minor Planet Center |accessdate=2007-04-02}}</ref>

=== Outer planets ===
{{Main|Giant planet}}
[[File:Planet collage to scale (captioned).jpg|right|thumb|alt=Jupiter and Saturn is about 2 times bigger than Uranus and Neptune, 10 times bigger than Venus and Earth, 20 times bigger than Mars and 25 times bigger than Mercury|The outer planets [[Jupiter]], [[Saturn]], [[Uranus]] and [[Neptune]], compared to the inner planets Earth, Venus, Mars, and Mercury at the bottom right]]
The four outer planets, called giant planets or Jovian planets, collectively make up 99% of the mass orbiting the Sun.<ref group="lower-alpha" name="footnoteD" /> All four giant planets have multiple moons and a ring system, although only Saturn's rings are easily observed from Earth.<ref name="Ryden" /> Jupiter and Saturn are composed mainly of gases with extremely low melting points, such as hydrogen, helium, and [[neon]],<ref name="Podolak Podolak et al. 2000">{{Cite journal |last1=Podolak |first1=M. |last2=Podolak |first2=J. I. |last3=Marley |first3=M. S. |date=February 2000 |title=Further investigations of random models of Uranus and Neptune |url=https://zenodo.org/record/998024 |url-status=live |journal=Planetary and Space Science |volume=48 |issue=2–3 |pages=143–151 |bibcode=2000P&SS...48..143P |doi=10.1016/S0032-0633(99)00088-4 |archive-url=https://web.archive.org/web/20191221231229/https://zenodo.org/record/998024 |archive-date=21 December 2019 |access-date=25 August 2019 |ref={{SfnRef|Podolak Podolak et al.|2000}}}}</ref> hence their designation as [[gas giant]]s.<ref>{{Cite web |title=Gas Giant {{!}} Planet Types |url=https://exoplanets.nasa.gov/what-is-an-exoplanet/planet-types/gas-giant |url-status=live |archive-url=https://web.archive.org/web/20201128232046/https://exoplanets.nasa.gov/what-is-an-exoplanet/planet-types/gas-giant |archive-date=28 November 2020 |access-date=22 December 2020 |website=Exoplanet Exploration: Planets Beyond our Solar System}}</ref> Uranus and Neptune are [[ice giants]],<ref>{{Cite web |last1=Lissauer |first1=Jack J. |last2=Stevenson |first2=David J. |date=2006 |title=Formation of Giant Planets |url=http://www.gps.caltech.edu/uploads/File/People/djs/lissauer&stevenson(PPV).pdf |url-status=dead |archive-url=https://web.archive.org/web/20090326060004/http://www.gps.caltech.edu/uploads/File/People/djs/lissauer%26stevenson%28PPV%29.pdf |archive-date=26 March 2009 |access-date=16 January 2006 |website=NASA Ames Research Center; California Institute of Technology}}</ref> meaning they are largely composed of [[Volatile (astrogeology)|'ice' in the astronomical sense]] (chemical compounds with melting points of up to a few hundred [[kelvin]]s<ref name="Podolak Podolak et al. 2000" /> such as water, methane, ammonia, [[hydrogen sulfide]], and [[carbon dioxide]].<ref name="Podolak Weizman et al. 1995">{{Cite journal |last1=Podolak |first1=M. |last2=Weizman |first2=A. |last3=Marley |first3=M. |date=December 1995 |title=Comparative models of Uranus and Neptune |journal=Planetary and Space Science |volume=43 |issue=12 |pages=1517–1522 |bibcode=1995P&SS...43.1517P |doi=10.1016/0032-0633(95)00061-5 |ref={{SfnRef|Podolak Weizman et al.|1995}}}}</ref>) Icy substances comprise the majority of the satellites of the giant planets and small objects that lie beyond Neptune's orbit.<ref name="Podolak Weizman et al. 1995" /><ref name="zeilik">{{Cite book |last=Zellik |first=Michael |title=Astronomy: The Evolving Universe |date=2002 |publisher=[[Cambridge University Press]] |isbn=978-0-521-80090-7 |edition=9th |page=240 |oclc=223304585}}</ref>

* {{Visible anchor|Jupiter|text=[[Jupiter]]}} (4.95–5.46&nbsp;AU)<ref name="nasa-factsheet" group="D" /> is the biggest and most massive planet in the Solar System. On its surface, there are orange-brown and white cloud bands moving via the principles of [[atmospheric circulation]], with giant storms swirling on the surface such as the [[Great Red Spot]] and [[Oval BA|white 'ovals']]. [[Magnetosphere of Jupiter|Jupiter possesses a strong enough magnetosphere]] to redirect [[ionizing radiation]] and cause [[aurora]]s on its poles.<ref>{{Cite book |last=Rogers |first=John H. |url=https://books.google.com/books?id=SO48AAAAIAAJ&pg=PA293 |title=The giant planet Jupiter |date=1995 |publisher=Cambridge University Press |isbn=978-0521410083 |page=293 |access-date=13 April 2022 |archive-url=https://web.archive.org/web/20220420161219/https://www.google.com/books/edition/The_Giant_Planet_Jupiter/SO48AAAAIAAJ?gbpv=1&pg=PA293 |archive-date=20 April 2022 |url-status=live}}</ref> As of 2024, Jupiter has [[Moons of Jupiter|95 confirmed satellites]], which can roughly be sorted into three groups:
** The Amalthea group, consisting of [[Metis (moon)|Metis]], [[Adrastea (moon)|Adrastea]], [[Amalthea (moon)|Amalthea]], and [[Thebe (moon)|Thebe]]. They orbit substantially closer to Jupiter than other satellites.<ref>{{cite journal |last=Anderson |first=J.D. |author2=Johnson, T.V. |author3=Shubert, G. |display-authors=etal |date=2005 |title=Amalthea's Density Is Less Than That of Water |journal=Science |volume=308 |issue=5726 |pages=1291–1293 |bibcode=2005Sci...308.1291A |doi=10.1126/science.1110422 |pmid=15919987 |s2cid=924257}}</ref> Materials from these natural satellites are the source of Jupiter's faint ring.<ref>{{cite journal |author=Burns, J. A. |author2=Showalter, M. R. |author3=Hamilton, D. P. |display-authors=etal |date=1999 |title=The Formation of Jupiter's Faint Rings |journal=Science |volume=284 |issue=5417 |pages=1146–1150 |bibcode=1999Sci...284.1146B |doi=10.1126/science.284.5417.1146 |pmid=10325220 |s2cid=21272762}}</ref>
** The [[Galilean moons]], consisting of [[Ganymede (moon)|Ganymede]], [[Callisto (moon)|Callisto]], [[Io (moon)|Io]], and [[Europa (moon)|Europa]]. They are the largest moons of Jupiter and exhibit planetary properties.<ref>{{Cite web |last=Pappalardo |first=R. T. |date=1999 |title=Geology of the Icy Galilean Satellites: A Framework for Compositional Studies |url=http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=fm99&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Ffm99%2Ffm99&maxhits=200&=%22P11C-10%22 |url-status=dead |archive-url=https://web.archive.org/web/20070930165551/http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=fm99&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Ffm99%2Ffm99&maxhits=200&=%22P11C-10%22 |archive-date=30 September 2007 |access-date=16 January 2006 |website=Brown University}}</ref>
** Irregular satellites, consisting of substantially smaller natural satellites. They have more distant orbits than the other objects.<ref name="list">{{cite book |author=Sheppard, Scott S. |title=Jupiter. The planet, satellites and magnetosphere |author2=Jewitt, David C. |author3=Porco, Carolyn |date=2004 |publisher=Cambridge University Press |isbn=0-521-81808-7 |editor=Fran Bagenal |volume=1 |location=Cambridge, UK |pages=263–280 |chapter=Jupiter's outer satellites and Trojans |editor2=Timothy E. Dowling |editor3=William B. McKinnon |chapter-url=http://www.ifa.hawaii.edu/~jewitt/papers/JUPITER/JSP.2003.pdf |archive-url=https://web.archive.org/web/20090326065151/http://www.ifa.hawaii.edu/~jewitt/papers/JUPITER/JSP.2003.pdf |archive-date=26 March 2009 |url-status=dead}}</ref>
* {{Visible anchor|Saturn|text=[[Saturn]]}} (9.08–10.12&nbsp;AU)<ref name="nasa-factsheet" group="D" /> has a distinctive visible [[Rings of Saturn|ring system]] orbiting around its equator composed of small ice and rock particles. Like Jupiter, it is mostly made of hydrogen and helium.<ref>{{Cite web |date=18 August 2021 |title=In Depth: Saturn |url=https://solarsystem.nasa.gov/planets/saturn/in-depth |url-status=live |archive-url=https://web.archive.org/web/20180224210031/https://solarsystem.nasa.gov/planets/saturn/in-depth |archive-date=24 February 2018 |access-date=31 March 2022 |website=NASA Science: Solar System Exploration}}</ref> At its north and south poles, Saturn has peculiar [[Saturn's hexagon|hexagon-shaped storms]] larger than the diameter of Earth. [[Magnetosphere of Saturn|Saturn has a magnetosphere]] capable of producing weak auroras. As of 2024, Saturn has [[Moons of Saturn|146 confirmed satellites]], grouped into:
** Ring [[moonlet]]s and [[Shepherd moon|shepherds]], which orbit inside or close to Saturn's rings. A moonlet can only partially clear out dust in its orbit,<ref name="Sremcevic2007">{{cite journal |last1=Sremčević |first1=Miodrag |last2=Schmidt |first2=Jürgen |last3=Salo |first3=Heikki |last4=Seiß |first4=Martin |last5=Spahn |first5=Frank |last6=Albers |first6=Nicole |date=2007 |title=A belt of moonlets in Saturn's A ring |journal=[[Nature (journal)|Nature]] |volume=449 |issue=7165 |pages=1019–21 |bibcode=2007Natur.449.1019S |doi=10.1038/nature06224 |pmid=17960236 |s2cid=4330204}}</ref> while the ring shepherds are able to completely clear out dust, forming visible gaps in the rings.<ref name="Porco2005">{{cite journal |last1=Porco |first1=C. C. |last2=Baker |first2=E. |last3=Barbara |first3=J. |display-authors=etal |date=2005 |title=Cassini Imaging Science: Initial Results on Saturn's Rings and Small Satellites |url=http://ciclops.org/sci/docs/RingsSatsPaper.pdf |journal=Science |volume=307 |issue=5713 |pages=1234 |bibcode=2005Sci...307.1226P |doi=10.1126/science.1108056 |pmid=15731439 |s2cid=1058405 |access-date=21 April 2024 |archive-date=25 July 2011 |archive-url=https://web.archive.org/web/20110725171940/http://ciclops.org/sci/docs/RingsSatsPaper.pdf |url-status=live }}</ref>
** Inner large satellites [[Mimas]], [[Enceladus]], [[Tethys (moon)|Tethys]], and [[Dione (moon)|Dione]]. These satellites orbit within [[Rings of Saturn#E Ring|Saturn's E ring]]. They are composed mostly of water ice and are believed to have differentiated internal structures.<ref name=":2">{{Cite web |last=Williams |first=Matt |date=7 August 2015 |title=The moons of Saturn |url=https://phys.org/news/2015-08-moons-saturn.html |access-date=21 April 2024 |website=phys.org |language=en |archive-date=21 April 2024 |archive-url=https://web.archive.org/web/20240421075712/https://phys.org/news/2015-08-moons-saturn.html |url-status=live }}</ref>
** Trojan moons [[Calypso (moon)|Calypso]] and [[Telesto (moon)|Telesto]] (trojans of Tethys), and [[Helene (moon)|Helene]] and [[Polydeuces (moon)|Polydeuces]] (trojans of Dione). These small moons share their orbits with Tethys and Dione, leading or trailing either.<ref name="Calypso">{{cite web |publisher=NASA |url=https://science.nasa.gov/saturn/moons/calypso/ |title=Calypso |date=January 2024 |access-date=16 May 2024 |archive-date=17 May 2024 |archive-url=https://web.archive.org/web/20240517022857/https://science.nasa.gov/saturn/moons/calypso/ |url-status=live }}</ref><ref name="Polydeuces">{{cite web |publisher=NASA |url=https://science.nasa.gov/saturn/moons/polydeuces/ |title=Polydeuces |date=January 2024 |access-date=16 May 2024 }}</ref>
** Outer large satellites [[Rhea (moon)|Rhea]], [[Titan (moon)|Titan]], [[Hyperion (moon)|Hyperion]], and [[Iapetus (moon)|Iapetus]].<ref name=":2" /> Titan is the only satellite in the Solar System to have a substantial atmosphere.<ref name="Forget2017">{{cite journal |last1=Forget |first1=F. |last2=Bertrand |first2=T. |last3=Vangvichith |first3=M. |last4=Leconte |first4=J. |last5=Millour |first5=E. |last6=Lellouch |first6=E. |title=A post-New Horizons Global climate model of Pluto including the N 2, CH 4 and CO cycles |date=May 2017 |journal=Icarus |volume=287 |pages=54–71 |doi=10.1016/j.icarus.2016.11.038 |bibcode=2017Icar..287...54F|url=https://hal.sorbonne-universite.fr/hal-01427123/file/Forget_A_post-New_Horizons.pdf }}</ref>
** Irregular satellites, consisting of substantially smaller natural satellites. They have more distant orbits than the other objects. [[Phoebe (moon)|Phoebe]] is the largest irregular satellite of Saturn.<ref name="Jewitt2007"/>
* {{Visible anchor|Uranus|text=[[Uranus]]}} (18.3–20.1&nbsp;AU),<ref name="nasa-factsheet" group="D" /> uniquely among the planets, orbits the Sun on its side with an [[axial tilt]] >90°. This gives the planet extreme seasonal variation as each pole points alternately toward and then away from the Sun.<ref>{{Cite web |last=Devitt |first=Terry |date=14 October 2008 |title=New images yield clues to seasons of Uranus |publisher=University of Wisconsin–Madison |url=https://news.wisc.edu/new-images-yield-clues-to-seasons-of-uranus/ |access-date=6 April 2024 |archive-date=6 April 2024 |archive-url=https://web.archive.org/web/20240406210615/https://news.wisc.edu/new-images-yield-clues-to-seasons-of-uranus/ |url-status=live }}</ref> Uranus' outer layer has a muted [[cyan]] color, but underneath these clouds are [[Climate of Uranus|many mysteries about its climate]], such as unusually low [[internal heat]] and erratic cloud formation. As of 2024, Uranus has [[Moons of Uranus|28 confirmed satellites]], divided into three groups:
** Inner satellites, which orbit inside Uranus' ring system.<ref name="Esposito2002">{{cite journal |last=Esposito |first=L. W. |author-link=Larry W. Esposito |year=2002 |title=Planetary rings |journal=Reports on Progress in Physics |volume=65 |issue=12 |pages=1741–1783 |bibcode=2002RPPh...65.1741E |doi=10.1088/0034-4885/65/12/201 |s2cid=250909885}}</ref> They are very close to each other, which suggests that their orbits are [[Chaotic system|chaotic]].<ref name="Duncan Lissauer 1997">{{cite journal |last1=Duncan |first1=Martin J. |last2=Lissauer |first2=Jack J. |year=1997 |title=Orbital Stability of the Uranian Satellite System |journal=Icarus |volume=125 |issue=1 |pages=1–12 |bibcode=1997Icar..125....1D |doi=10.1006/icar.1996.5568}}</ref>
** Large satellites, consisting of [[Titania (moon)|Titania]], [[Oberon (moon)|Oberon]], [[Umbriel]], [[Ariel (moon)|Ariel]], and [[Miranda (moon)|Miranda]].<ref>{{Cite journal |last1=Sheppard |first1=S. S. |last2=Jewitt |first2=D. |last3=Kleyna |first3=J. |year=2005 |title=An Ultradeep Survey for Irregular Satellites of Uranus: Limits to Completeness |journal=The Astronomical Journal |volume=129 |issue=1 |page=518 |arxiv=astro-ph/0410059 |bibcode=2005AJ....129..518S |doi=10.1086/426329 |s2cid=18688556}}</ref> Most of them have roughly equal amounts of rock and ice, except Miranda, which is made primarily of ice.<ref name="Hussmann Sohl et al. 2006">{{cite journal |last1=Hussmann |first1=Hauke |last2=Sohl |first2=Frank |last3=Spohn |first3=Tilman |date=November 2006 |title=Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects |journal=[[Icarus (journal)|Icarus]] |volume=185 |issue=1 |pages=258–273 |bibcode=2006Icar..185..258H |doi=10.1016/j.icarus.2006.06.005 |ref={{sfnRef|Hussmann Sohl et al.|2006}}}}</ref>
** Irregular satellites, having more distant and eccentric orbits than the other objects.<ref name="Sheppardmoons2024">{{cite web |date=23 February 2024 |title=New Uranus and Neptune Moons |url=https://sites.google.com/carnegiescience.edu/sheppard/home/newuranusneptunemoons |accessdate=23 February 2024 |work=Earth & Planetary Laboratory |publisher=Carnegie Institution for Science |archive-date=23 February 2024 |archive-url=https://web.archive.org/web/20240223160326/https://sites.google.com/carnegiescience.edu/sheppard/home/newuranusneptunemoons |url-status=live }}</ref>
* {{Visible anchor|Neptune|text=[[Neptune]]}} (29.9–30.5&nbsp;AU)<ref name="nasa-factsheet" group="D" /> is the furthest planet known in the Solar System. Its outer atmosphere has a slightly muted cyan color, with occasional storms on the surface that look like dark spots. Like Uranus, many atmospheric phenomena of Neptune are unexplained, such as the [[thermosphere]]'s abnormally high temperature or the strong tilt (47°) of its magnetosphere. As of 2024, Neptune has [[Moons of Neptune|16 confirmed satellites]], divided into two groups:
** Regular satellites, which have circular orbits that lie near Neptune's equator.<ref name="Jewitt2007">{{cite journal |last=Jewitt |first=David |author2=Haghighipour, Nader |date=2007 |title=Irregular Satellites of the Planets: Products of Capture in the Early Solar System |url=http://www2.ess.ucla.edu/~jewitt/papers/2007/JH07.pdf |journal=Annual Review of Astronomy and Astrophysics |volume=45 |issue=1 |pages=261–95 |arxiv=astro-ph/0703059 |bibcode=2007ARA&A..45..261J |doi=10.1146/annurev.astro.44.051905.092459 |s2cid=13282788 |access-date=21 April 2024 |archive-date=25 February 2014 |archive-url=https://web.archive.org/web/20140225204338/http://www2.ess.ucla.edu/~jewitt/papers/2007/JH07.pdf |url-status=live }}</ref>
** Irregular satellites, which as the name implies, have less regular orbits. One of them, [[Triton (moon)|Triton]], is Neptune's largest moon. It is geologically active, with erupting [[geyser]]s of nitrogen gas, and possesses a thin, cloudy nitrogen atmosphere.<ref name="Soderblom2">{{Cite journal |last1=Soderblom |first1=L. A. |last2=Kieffer |first2=S. W. |last3=Becker |first3=T. L. |last4=Brown |first4=R. H. |last5=Cook |first5=A. F. II |last6=Hansen |first6=C. J. |last7=Johnson |first7=T. V. |last8=Kirk |first8=R. L. |last9=Shoemaker |first9=E. M. |author-link9=Eugene Merle Shoemaker |date=19 October 1990 |title=Triton's Geyser-Like Plumes: Discovery and Basic Characterization |url=https://www.geology.illinois.edu/~skieffer/papers/Truiton_Science_1990.pdf |url-status=live |journal=[[Science (journal)|Science]] |volume=250 |issue=4979 |pages=410–415 |bibcode=1990Sci...250..410S |doi=10.1126/science.250.4979.410 |pmid=17793016 |s2cid=1948948 |archive-url=https://web.archive.org/web/20210831121844/https://geology.illinois.edu/~skieffer/papers/Truiton_Science_1990.pdf |archive-date=31 August 2021 |access-date=31 March 2022}}</ref><ref name="Forget2017"/>

=== {{Anchor|Centaurs}}Centaurs ===
{{Main|Centaur (small Solar System body)|l1 = Centaur}}

The centaurs are icy, comet-like bodies whose [[Semi-major and semi-minor axes|semi-major axes]] are longer than Jupiter's and shorter than Neptune's (between 5.5 and 30&nbsp;AU). These are former Kuiper belt and [[Scattered disc|scattered disc objects]] (SDOs) that were gravitationally [[perturbation (astronomy)|perturbed]] closer to the Sun by the outer planets, and are expected to become comets or be ejected out of the Solar System.<ref name="Delsanti-Beyond_The_Planets" /> While most centaurs are inactive and asteroid-like, some exhibit cometary activity, such as the first centaur discovered, [[2060 Chiron]], which has been classified as a comet (95P) because it develops a coma just as comets do when they approach the Sun.<ref>{{Cite web |last=Vanouplines |first=Patrick |date=1995 |title=Chiron biography |url=http://www.vub.ac.be/STER/www.astro/chibio.htm |url-status=dead |archive-url=https://web.archive.org/web/20090502122306/http://www.vub.ac.be/STER/www.astro/chibio.htm |archive-date=2 May 2009 |access-date=23 June 2006 |website=Vrije Universitiet Brussel}}</ref> The largest known centaur, [[10199 Chariklo]], has a diameter of about {{Convert|250|km|abbr=on}} and is one of the few minor planets possessing a ring system.<ref name="spitzer">{{Cite conference |last1=Stansberry |first1=John |last2=Grundy |first2=Will |last3=Brown |first3=Mike |last4=Cruikshank |first4=Dale |last5=Spencer |first5=John |last6=Trilling |first6=David |last7=Margot |first7=Jean-Luc |date=2007 |title=Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope |page=161 |arxiv=astro-ph/0702538 |bibcode=2008ssbn.book..161S |book-title=The Solar System Beyond Neptune}}</ref><ref name="Braga-Ribas-2014">{{Cite journal |last=Braga-Ribas |first=F. |display-authors=etal |date=April 2014 |title=A ring system detected around the Centaur (10199) Chariklo |journal=[[Nature (journal)|Nature]] |volume=508 |issue=7494 |pages=72–75 |arxiv=1409.7259 |bibcode=2014Natur.508...72B |doi=10.1038/nature13155 |issn=0028-0836 |pmid=24670644 |s2cid=4467484}}</ref>


== Trans-Neptunian region ==
== Trans-Neptunian region ==
The area beyond Neptune, or the "[[trans-Neptunian object|trans-Neptunian region]]", is still [[Timeline of Solar System exploration|largely unexplored]]. It appears to consist overwhelmingly of small worlds (the largest having a diameter only a fifth that of the Earth and a mass far smaller than that of the Moon) composed mainly of rock and ice. This region is sometimes known as the "outer Solar System", though others use that term to mean the region beyond the [[asteroid belt]].
Beyond the orbit of Neptune lies the area of the "[[trans-Neptunian object|trans-Neptunian region]]", with the doughnut-shaped Kuiper belt, home of Pluto and several other dwarf planets, and an overlapping disc of scattered objects, which is [[Orbital inclination|tilted toward the plane]] of the Solar System and reaches much further out than the Kuiper belt. The entire region is still [[Timeline of Solar System exploration|largely unexplored]]. It appears to consist overwhelmingly of many thousands of small worlds—the largest having a diameter only a fifth that of Earth and a mass far smaller than that of the Moon—composed mainly of rock and ice. This region is sometimes described as the "third zone of the Solar System", enclosing the inner and the outer Solar System.<ref>{{Cite web |last=Stern |first=Alan |author-link=Alan Stern |date=February 2015 |title=Journey to the Solar System's Third Zone |url=https://www.americanscientist.org/article/journey-to-the-solar-systems-third-zone |url-status=live |archive-url=https://web.archive.org/web/20181026222414/https://www.americanscientist.org/article/journey-to-the-solar-systems-third-zone |archive-date=26 October 2018 |access-date=26 October 2018 |website=American Scientist}}</ref>


===Kuiper belt===
=== Kuiper belt ===
{{main|Kuiper belt}}
{{Main|Kuiper belt}}
[[Image:Outersolarsystem objectpositions labels comp.png|thumb|300 px|Plot of all known Kuiper belt objects, set against the four outer planets]]
[[File:Kuiper belt plot objects of outer solar system.png|right|thumb|Plot of objects around the [[Kuiper belt]] and other asteroid populations. J, S, U and N denotes Jupiter, Saturn, Uranus and Neptune.]]


[[File:TheKuiperBelt_classes-en.svg|thumb|Orbit classification of Kuiper belt objects. Some clusters that is subjected to [[orbital resonance]] are marked.]]
The Kuiper belt, the region's first formation, is a great ring of debris similar to the asteroid belt, but composed mainly of ice. It extends between 30 and 50&nbsp;AU from the Sun. It is composed mainly of small Solar System bodies, but many of the largest Kuiper belt objects, such as [[50000 Quaoar|Quaoar]], [[20000 Varuna|Varuna]], {{mpl|(136108) 2003 EL|61}} and [[90482 Orcus|Orcus]], may be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50&nbsp;km, but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of the Earth.<ref name="Delsanti-Beyond_The_Planets">{{cite web |year=2006 |author=Audrey Delsanti and David Jewitt |title=The Solar System Beyond The Planets |work=Institute for Astronomy, University of Hawaii |url=http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf |format=PDF |accessdate=2007-01-03}}</ref> Many Kuiper belt objects have multiple satellites, and most have orbits that take them outside the plane of the ecliptic.
The Kuiper belt is a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice.<ref name="physical">{{Cite book |last=Tegler |first=Stephen C. |url=https://archive.org/details/encyclopediasola00mcfa_702 |title=Encyclopedia of the Solar System |date=2007 |isbn=978-0120885893 |editor-last=Lucy-Ann McFadden |page=[https://archive.org/details/encyclopediasola00mcfa_702/page/n622 605]–620 |chapter=Kuiper Belt Objects: Physical Studies |display-editors=etal |url-access=limited}}</ref> It extends between 30 and 50&nbsp;AU from the Sun. It is composed mainly of small Solar System bodies, although the largest few are probably large enough to be dwarf planets.<ref name="Grundy2019">{{Cite journal |last1=Grundy |first1=W. M. |last2=Noll |first2=K. S. |last3=Buie |first3=M. W. |last4=Benecchi |first4=S. D. |last5=Ragozzine |first5=D. |last6=Roe |first6=H. G. |date=December 2018 |title=The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁʼhòmdímà ({{Mp|(229762) 2007 UK|126}}) |url=http://www2.lowell.edu/~grundy/abstracts/2019.G-G.html |volume=334 |pages=30–38 |doi=10.1016/j.icarus.2018.12.037 |s2cid=126574999 |archive-url=https://web.archive.org/web/20190407045339/http://www2.lowell.edu/~grundy/abstracts/preprints/2019.G-G.pdf |archive-date=7 April 2019 |journal=Icarus|bibcode=2019Icar..334...30G }}</ref> There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than {{Convert|50|km|abbr=on|sigfig=1}}, but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of Earth.<ref name="Delsanti-Beyond_The_Planets">{{Cite web |last1=Delsanti |first1=Audrey |last2=Jewitt |first2=David |date=2006 |title=The Solar System Beyond The Planets |url=http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf |url-status=dead |archive-url=https://web.archive.org/web/20070129151907/http://www.ifa.hawaii.edu/faculty/jewitt/papers/2006/DJ06.pdf |archive-date=29 January 2007 |access-date=3 January 2007 |website=Institute for Astronomy, University of Hawaii}}</ref> Many Kuiper belt objects have satellites,<ref>{{Cite journal |last1=Brown |first1=M.E. |author-link=Michael E. Brown |last2=Van Dam |first2=M.A. |last3=Bouchez |first3=A.H. |last4=Le Mignant |first4=D. |last5=Campbell |first5=R.D. |last6=Chin |first6=J.C.Y. |last7=Conrad |first7=A. |last8=Hartman |first8=S.K. |last9=Johansson |first9=E.M. |last10=Lafon |first10=R.E. |last11=Rabinowitz |first11=D.L. Rabinowitz |last12=Stomski |first12=P.J. Jr. |last13=Summers |first13=D.M. |last14=Trujillo |first14=C.A. |last15=Wizinowich |first15=P.L. |year=2006 |title=Satellites of the Largest Kuiper Belt Objects |url=http://web.gps.caltech.edu/~mbrown/papers/ps/gab.pdf |url-status=live |journal=[[The Astrophysical Journal]] |volume=639 |issue=1 |pages=L43–L46 |arxiv=astro-ph/0510029 |bibcode=2006ApJ...639L..43B |doi=10.1086/501524 |s2cid=2578831 |archive-url=https://web.archive.org/web/20180928185647/http://web.gps.caltech.edu/~mbrown/papers/ps/gab.pdf |archive-date=28 September 2018 |access-date=19 October 2011 |ref={{SfnRef|Brown Van Dam et al.|2006}}}}</ref> and most have orbits that are substantially inclined (~10°) to the plane of the ecliptic.<ref name="trojan">{{Cite journal |last1=Chiang |first1=E.I. |last2=Jordan |first2=A.B. |last3=Millis |first3=R.L. |last4=Buie |first4=M.W. |last5=Wasserman |first5=L.H. |last6=Elliot |first6=J.L. |last7=Kern |first7=S.D. |last8=Trilling |first8=D.E. |last9=Meech |first9=K.J. |last10=Wagner |first10=R.M. |display-authors=9 |date=2003 |title=Resonance Occupation in the Kuiper Belt: Case Examples of the 5:2 and Trojan Resonances |url=http://www.boulder.swri.edu/~buie/biblio/pub047.pdf |url-status=live |journal=[[The Astronomical Journal]] |volume=126 |issue=1 |pages=430–443 |arxiv=astro-ph/0301458 |bibcode=2003AJ....126..430C |doi=10.1086/375207 |s2cid=54079935 |archive-url=https://web.archive.org/web/20160315175243/http://www.boulder.swri.edu//~buie/biblio/pub047.pdf |archive-date=15 March 2016 |access-date=15 August 2009}}</ref>


The Kuiper belt can be roughly divided into the "[[Classical Kuiper belt object|classical]]" belt and the [[resonant trans-Neptunian object]]s.<ref name="physical" /> The latter have orbits whose periods are in a simple ratio to that of Neptune: for example, going around the Sun twice for every three times that Neptune does, or once for every two. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 to 47.7&nbsp;AU.<ref>{{Cite journal |last1=Buie |first1=M. W. |last2=Millis |first2=R. L. |last3=Wasserman |first3=L. H. |last4=Elliot |first4=J. L. |last5=Kern |first5=S. D. |last6=Clancy |first6=K. B. |last7=Chiang |first7=E. I. |last8=Jordan |first8=A. B. |last9=Meech |first9=K. J. |last10=Wagner |first10=R. M. |last11=Trilling |first11=D. E. |date=2005 |title=Procedures, Resources and Selected Results of the Deep Ecliptic Survey |journal=[[Earth, Moon, and Planets]] |volume=92 |issue=1 |pages=113–124 |arxiv=astro-ph/0309251 |bibcode=2003EM&P...92..113B |doi=10.1023/B:MOON.0000031930.13823.be |s2cid=14820512}}</ref> Members of the classical Kuiper belt are sometimes called "cubewanos", after the first of their kind to be discovered, originally designated [[15760 Albion|1992 ''QB<sub>1</sub>'']], (and has since been named Albion); they are still in near primordial, low-eccentricity orbits.<ref>{{Cite journal |last1=Dotto |first1=E. |last2=Barucci |first2=M. A. |last3=Fulchignoni |first3=M. |date=1 January 2003 |title=Beyond Neptune, the new frontier of the Solar System |url=http://sait.oat.ts.astro.it/MSAIS/3/PDF/20.pdf |url-status=live |journal=Memorie della Societa Astronomica Italiana Supplementi |volume=3 |page=20 |bibcode=2003MSAIS...3...20D |issn=0037-8720 |archive-url=https://web.archive.org/web/20140825122005/http://sait.oat.ts.astro.it/MSAIS/3/PDF/20.pdf |archive-date=25 August 2014 |access-date=26 December 2006}}</ref>
[[Image:TheKuiperBelt Projections 55AU Classical Plutinos.svg|left|thumb|Diagram showing the resonant and classical Kuiper belt divisions]]


There is strong consensus among astronomers that five members of the Kuiper belt are {{Visible anchor|Others|text=dwarf planets}}.<ref name="Grundy2019" /><ref name="JWST">{{cite journal |arxiv=2309.15230 |first1=J. P. |last1=Emery |first2=I. |last2=Wong |author-link= |title=A Tale of 3 Dwarf Planets: Ices and Organics on Sedna, Gonggong, and Quaoar from JWST Spectroscopy |date=2024 |first3=R. |last3=Brunetto |first4=J. C. |last4=Cook |first5=N. |last5=Pinilla-Alonso |first6=J. A. |last6=Stansberry |first7=B. J. |last7=Holler |first8=W. M. |last8=Grundy |first9=S. |last9=Protopapa |first10=A. C. |last10=Souza-Feliciano |first11=E. |last11=Fernández-Valenzuela |first12=J. I. |last12=Lunine |first13=D. C. |last13=Hines|journal=Icarus |volume=414 |doi=10.1016/j.icarus.2024.116017 |bibcode=2024Icar..41416017E }}</ref> Many dwarf planet candidates are being considered, pending further data for verification.<ref name="Tancredi2008">{{Cite journal |last1=Tancredi |first1=G. |last2=Favre |first2=S. A. |year=2008 |title=Which are the dwarfs in the Solar System? |journal=Icarus |volume=195 |issue=2 |pages=851–862 |bibcode=2008Icar..195..851T |doi=10.1016/j.icarus.2007.12.020}}</ref>
The Kuiper belt can be roughly divided into the "[[Classical Kuiper belt object|classical]]" belt and the [[Resonant trans-Neptunian object|resonances]]. Resonances are orbits linked to that of Neptune (e.g. twice for every three Neptune orbits, or once for every two). The first resonance actually begins within the orbit of Neptune itself. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4&nbsp;AU to 47.7&nbsp;AU.<ref>{{cite web |year=2005 |author=M. W. Buie, R. L. Millis, L. H. Wasserman, J. L. Elliot, S. D. Kern, K. B. Clancy, E. I. Chiang, A. B. Jordan, K. J. Meech, R. M. Wagner, D. E. Trilling |work=Lowell Observatory, University of Pennsylvania, Large Binocular Telescope Observatory, Massachusetts Institute of Technology, University of Hawaii, University of California at Berkeley |title=Procedures, Resources and Selected Results of the Deep Ecliptic Survey |url=http://www.citebase.org/fulltext?format=application%2Fpdf&identifier=oai%3AarXiv.org%3Aastro-ph%2F0309251 |accessdate=2006-09-07}}</ref> Members of the classical Kuiper belt are classified as [[Classical Kuiper belt object|cubewanos]], after the first of their kind to be discovered, {{mpl|(15760) 1992 QB|1}}.<ref>{{cite web |url=http://sait.oat.ts.astro.it/MSAIS/3/PDF/20.pdf |format=PDF |title=Beyond Neptune, the new frontier of the Solar System |author=E. Dotto1, M.A. Barucci2, and M. Fulchignoni |accessdate=2006-12-26 |date=2006-08-24}}</ref>


* {{Visible anchor|Pluto and Charon|text=[[Pluto]]}} (29.7–49.3&nbsp;AU) is the largest known object in the Kuiper belt. Pluto has a relatively eccentric orbit, inclined 17 degrees to the [[ecliptic plane]]. Pluto has a [[Orbital resonance|2:3 resonance]] with Neptune, meaning that Pluto orbits twice around the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called [[plutino]]s.<ref name="Fajans_et_al_2001">{{Cite journal |last1=Fajans |first1=J. |last2=Frièdland |first2=L. |date=October 2001 |title=Autoresonant (nonstationary) excitation of pendulums, Plutinos, plasmas, and other nonlinear oscillators |url=http://ist-socrates.berkeley.edu/~fajans/pub/pdffiles/AutoPendAJP.pdf |url-status=dead |journal=[[American Journal of Physics]] |volume=69 |issue=10 |pages=1096–1102 |bibcode=2001AmJPh..69.1096F |doi=10.1119/1.1389278 |archive-url=https://web.archive.org/web/20110607210435/http://ist-socrates.berkeley.edu/~fajans/pub/pdffiles/AutoPendAJP.pdf |archive-date=7 June 2011 |access-date=26 December 2006}}</ref> [[Moons of Pluto|Pluto has five moons]]: Charon, [[Styx (moon)|Styx]], [[Nix (moon)|Nix]], [[Kerberos (moon)|Kerberos]], and [[Hydra (moon)|Hydra]].<ref>{{Cite web |date=6 August 2021 |title=In Depth: Pluto |url=https://solarsystem.nasa.gov/planets/dwarf-planets/pluto/in-depth |url-status=live |archive-url=https://web.archive.org/web/20220331112026/https://solarsystem.nasa.gov/planets/dwarf-planets/pluto/in-depth |archive-date=31 March 2022 |access-date=31 March 2022 |website=NASA Science: Solar System Exploration}}</ref>
; Pluto and Charon
** [[Charon (moon)|Charon]], the largest of Pluto's moons, is sometimes described as part of a [[binary system (astronomy)|binary system]] with Pluto, as the two bodies orbit a [[barycenter]] of gravity above their surfaces (i.e. they appear to "orbit each other").
: [[Pluto]] (39&nbsp;AU average), a dwarf planet, is the largest known object in the Kuiper belt. When discovered in 1930, it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal [[definition of planet]]. Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7&nbsp;AU from the Sun at perihelion (within the orbit of Neptune) to 49.5&nbsp;AU at aphelion.
* {{Dp|Orcus}} (30.3–48.1&nbsp;AU), is in the same 2:3 orbital resonance with Neptune as Pluto, and is the largest such object after Pluto itself.<ref name="brownlargest" /> Its eccentricity and inclination are similar to Pluto's, but its perihelion lies about 120° from that of Pluto. Thus, the [[Phase (waves)#Phase difference|phase]] of Orcus's orbit is opposite to Pluto's: Orcus is at aphelion (most recently in 2019) around when Pluto is at perihelion (most recently in 1989) and vice versa.<ref name="MPC2004-D15">{{Cite web |date=20 February 2004 |title=MPEC 2004-D15 : 2004 DW |url=http://www.minorplanetcenter.net/mpec/K04/K04D15.html |url-status=live |archive-url=https://web.archive.org/web/20160303232947/http://www.minorplanetcenter.net/mpec/K04/K04D15.html |archive-date=3 March 2016 |access-date=5 July 2011 |publisher=Minor Planet Center}}</ref> For this reason, it has been called the ''anti-Pluto''.<ref name="MBP">{{Cite web |last=Michael E. Brown |author-link=Michael E. Brown |date=23 March 2009 |title=S/2005 (90482) 1 needs your help |url=http://www.mikebrownsplanets.com/2009/03/s1-90482-2005-needs-your-help.html |url-status=live |archive-url=https://web.archive.org/web/20090328012339/http://www.mikebrownsplanets.com/2009/03/s1-90482-2005-needs-your-help.html |archive-date=28 March 2009 |access-date=25 March 2009 |publisher=Mike Brown's Planets (blog)}}</ref><ref>{{Cite book |last=Moltenbrey |first=Michael |url=https://www.worldcat.org/oclc/926914921 |title=Dawn of Small Worlds: Dwarf planets, asteroids, comets |date=2016 |publisher=Springer |isbn=978-3-319-23003-0 |location=Cham |page=171 |oclc=926914921 |access-date=9 April 2022 |archive-url=https://web.archive.org/web/20220420161222/https://www.worldcat.org/title/dawn-of-small-worlds-dwarf-planets-asteroids-comets/oclc/926914921 |archive-date=20 April 2022 |url-status=live}}</ref> It has one known moon, [[Vanth (moon)|Vanth]].<ref name="IAUC8812">{{Cite web |last=Green |first=Daniel W. E. |date=22 February 2007 |title=IAUC 8812: Sats OF 2003 AZ_84, (50000), (55637), (90482) |url=http://www.cbat.eps.harvard.edu/iauc/08800/08812.html |url-status=live |archive-url=https://web.archive.org/web/20120314060043/http://cbat.eps.harvard.edu/iauc/08800/08812.html |archive-date=14 March 2012 |access-date=4 July 2011 |publisher=International Astronomical Union Circular}}</ref>
* [[Haumea]] (34.6–51.6&nbsp;AU) was discovered in 2005.<ref>{{Cite web |date=17 September 2008 |title=IAU names fifth dwarf planet Haumea |url=https://www.iau.org/news/pressreleases/detail/iau0807 |url-status=live |archive-url=https://web.archive.org/web/20140425065601/http://iau.org/public_press/news/detail/iau0807 |archive-date=25 April 2014 |access-date=9 April 2022 |website=International Astronomical Union}}</ref> It is in a temporary 7:12 orbital resonance with Neptune.<ref name="brownlargest">{{Cite book |last=Brown |first=Mike |title=The Solar System Beyond Neptune |publisher=University of Arizona Press |year=2008 |isbn=978-0-816-52755-7 |editor-last=Barucci |editor-first=M. Antonietta |pages=335–344 |chapter=The largest Kuiper belt objects |oclc=1063456240 |author-link=Mike Brown (astronomer) |access-date=9 April 2022 |chapter-url=http://www.gps.caltech.edu/~mbrown/papers/ps/kbochap.pdf |archive-url=https://web.archive.org/web/20121113114533/http://www.gps.caltech.edu/~mbrown/papers/ps/kbochap.pdf |archive-date=13 November 2012 |url-status=live}}</ref> Haumea possesses a ring system, two known moons named [[Hiʻiaka (moon)|Hiʻiaka]] and [[Namaka (moon)|Namaka]], and rotates so quickly (once every 3.9 hours) that it is stretched into an [[ellipsoid]]. It is part of a [[collisional family]] of Kuiper belt objects that share similar orbits, which suggests a giant impact on Haumea ejected fragments into space billions of years ago.<ref name="Noviello2022">{{Cite journal |last1=Noviello |first1=Jessica L. |last2=Desch |first2=Stephen J. |last3=Neveu |first3=Marc |last4=Proudfoot |first4=Benjamin C. N. |last5=Sonnett |first5=Sarah |date=September 2022 |title=Let It Go: Geophysically Driven Ejection of the Haumea Family Members |journal=The Planetary Science Journal |volume=3 |issue=9 |page=19 |bibcode=2022PSJ.....3..225N |doi=10.3847/PSJ/ac8e03 |s2cid=252620869 |id=225 |doi-access=free}}</ref>
* [[Makemake]] (38.1–52.8&nbsp;AU), although smaller than Pluto, is the largest known object in the ''classical'' Kuiper belt (that is, a Kuiper belt object not in a confirmed resonance with Neptune). Makemake is the brightest object in the Kuiper belt after Pluto. Discovered in 2005, it was officially named in 2009.<ref>{{Cite web |date=19 July 2009 |title=Fourth dwarf planet named Makemake |url=https://www.iau.org/news/pressreleases/detail/iau0806 |url-status=live |archive-url=https://web.archive.org/web/20170730222925/https://www.iau.org/news/pressreleases/detail/iau0806 |archive-date=30 July 2017 |access-date=9 April 2022 |website=International Astronomical Union}}</ref> Its orbit is far more inclined than Pluto's, at 29°.<ref name="Buie136472">{{Cite web |last=Buie |first=Marc W. |author-link=Marc W. Buie |date=5 April 2008 |title=Orbit Fit and Astrometric record for 136472 |url=http://www.boulder.swri.edu/~buie/kbo/astrom/136472.html |url-status=live |archive-url=https://web.archive.org/web/20200527191044/https://www.boulder.swri.edu/~buie/kbo/astrom/136472.html |archive-date=27 May 2020 |access-date=15 July 2012 |publisher=SwRI (Space Science Department)}}</ref> It has one known moon, [[S/2015 (136472) 1]].<ref name="ParkerA2016">{{Cite journal |last1=Parker |first1=A. H. |last2=Buie |first2=M. W. |last3=Grundy |first3=W. M. |last4=Noll |first4=K. S. |date=25 April 2016 |title=Discovery of a Makemakean Moon |journal=[[The Astrophysical Journal]] |volume=825 |issue=1 |page=L9 |arxiv=1604.07461 |bibcode=2016ApJ...825L...9P |doi=10.3847/2041-8205/825/1/L9 |s2cid=119270442 |doi-access=free}}</ref>
* {{Dp|Quaoar}} (41.9–45.5&nbsp;AU) is the second-largest known object in the classical Kuiper belt, after Makemake. Its orbit is significantly less eccentric and inclined than those of Makemake or Haumea.<ref name="brownlargest" /> It possesses a ring system and one known moon, [[Weywot (moon)|Weywot]].<ref name="Morgado2023">{{Cite Q|Q116754015|display-authors=1}}</ref>


=== Scattered disc ===
[[Image:Pluto system 2006.jpg|right|thumb|Pluto and its three known moons]]
{{Main|Scattered disc}}


[[File:TheKuiperBelt Projections 100AU Classical SDO.svg|thumb|The orbital eccentricities and inclinations of the scattered disc population compared to the classical and resonant Kuiper belt objects]]
: It is unclear whether [[Charon (moon)|Charon]], Pluto's largest moon, will continue to be classified as such or as a dwarf planet itself. Both Pluto and Charon orbit a [[Center of mass#Barycenter in astronomy|barycenter]] of gravity above their surfaces, making Pluto-Charon a [[binary system (astronomy)|binary system]]. Two much smaller moons, [[Nix (moon)|Nix]] and [[Hydra (moon)|Hydra]], orbit Pluto and Charon.
The scattered disc, which overlaps the Kuiper belt but extends out to near 500&nbsp;AU, is thought to be the source of short-period comets. Scattered-disc objects are believed to have been perturbed into erratic orbits by the gravitational influence of [[Formation and evolution of the Solar System#Planetary migration|Neptune's early outward migration]]. Most scattered disc objects have perihelia within the Kuiper belt but aphelia far beyond it (some more than 150&nbsp;AU from the Sun). SDOs' orbits can be inclined up to 46.8° from the ecliptic plane.<ref>{{Cite book |last1=Gomes |first1=R. S. |url=https://www.lpi.usra.edu/books/ssbn2008/7003.pdf |title=The Solar System Beyond Neptune |last2=Fernández |first2=J. A. |last3=Gallardo |first3=T. |last4=Brunini |first4=A. |date=2008 |publisher=University of Arizona Press |isbn=978-0816527557 |pages=259–273 |chapter=The Scattered Disk: Origins, Dynamics, and End States |access-date=12 May 2022 |archive-url=https://web.archive.org/web/20220121172507/https://www.lpi.usra.edu/books/ssbn2008/7003.pdf |archive-date=21 January 2022 |url-status=live}}</ref> Some astronomers consider the scattered disc to be merely another region of the Kuiper belt and describe scattered-disc objects as "scattered Kuiper belt objects".<ref>{{Cite web |last=Jewitt |first=David |date=2005 |title=The 1,000 km Scale KBOs |url=http://www2.ess.ucla.edu/~jewitt/kb/big_kbo.html |url-status=live |archive-url=https://web.archive.org/web/20140609134900/http://www2.ess.ucla.edu/~jewitt/kb/big_kbo.html |archive-date=9 June 2014 |access-date=16 July 2006 |website=University of Hawaii}}</ref> Some astronomers classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.<ref>{{Cite web |title=List of Centaurs and Scattered-Disk Objects |url=http://www.minorplanetcenter.org/iau/lists/Centaurs.html |url-status=live |archive-url=https://web.archive.org/web/20170629210646/http://www.minorplanetcenter.org/iau/lists/Centaurs.html |archive-date=29 June 2017 |access-date=2 April 2007 |website=IAU: Minor Planet Center}}</ref>
: Pluto lies in the resonant belt and has a 3:2 [[orbital resonance|resonance]] with Neptune, meaning that Pluto orbits twice round the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called [[plutino]]s.<ref name="Fajans_et_al_2001">{{cite science |last=Fajans |first=J. |coauthors=L. Frièdland |month=October |year=2001 |title=Autoresonant (nonstationary) excitation of pendulums, Plutinos, plasmas, and other nonlinear oscillators |journal=American Journal of Physics |volume=69 |issue=10 |pages=1096–1102 |doi=10.1119/1.1389278 |url=http://scitation.aip.org/journals/doc/AJPIAS-ft/vol_69/iss_10/1096_1.html |accessdate=2006-12-26 |abstract=http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=AJPIAS000069000010001096000001&idtype=cvips&gifs=yes}} </ref>


Currently, there is strong consensus among astronomers that two of the bodies in the scattered disc are {{Visible anchor|Gonggong and Eris|text=dwarf planets}}:
;Haumea
{{main|Haumea (dwarf planet)}}
{{sect-stub}}


* {{Dp|Eris}} (38.3–97.5&nbsp;AU) is the largest known scattered disc object and the most massive known dwarf planet. Eris's discovery contributed to a debate about the definition of a planet because it is 25% more massive than Pluto<ref name="Brown Schaller 2007">{{Cite journal |last1=Brown |first1=Michael E. |author-link=Michael E. Brown |last2=Schaller |first2=Emily L. |date=15 June 2007 |title=The Mass of Dwarf Planet Eris |journal=Science |volume=316 |issue=5831 |page=1585 |bibcode=2007Sci...316.1585B |doi=10.1126/science.1139415 |pmid=17569855 |s2cid=21468196|url=https://resolver.caltech.edu/CaltechAUTHORS:20121001-135149660 }}</ref> and about the same diameter. It has one known moon, [[Dysnomia (moon)|Dysnomia]]. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2&nbsp;AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane at an angle of 44°.<ref>{{Cite journal |last1=Dumas |first1=C. |last2=Merlin |first2=F. |last3=Barucci |first3=M. A. |last4=de Bergh |first4=C. |last5=Hainault |first5=O. |last6=Guilbert |first6=A. |last7=Vernazza |first7=P. |last8=Doressoundiram |first8=A. |date=August 2007 |title=Surface composition of the largest dwarf planet 136199 Eris (2003 UB{313}) |journal=Astronomy and Astrophysics |volume=471 |issue=1 |pages=331–334 |bibcode=2007A&A...471..331D |doi=10.1051/0004-6361:20066665 |doi-access=free}}</ref>
; Makemake
* {{Dp|Gonggong}} (33.8–101.2&nbsp;AU) is a dwarf planet in a comparable orbit to Eris, except that it is in a 3:10 resonance with Neptune.<ref name="jpldata" group="D">{{Cite web |date=10 April 2017 |title=JPL Small-Body Database Browser: 225088 Gonggong (2007 OR10) |url=https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2225088 |url-status=live |archive-url=https://web.archive.org/web/20200610013703/https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2225088 |archive-date=10 June 2020 |access-date=20 February 2020 |publisher=[[Jet Propulsion Laboratory]] |type=20 September 2015 last obs.}}</ref> It has one known moon, [[Xiangliu (moon)|Xiangliu]].<ref name="Kissetal2017">{{Cite journal |last1=Kiss |first1=Csaba |last2=Marton |first2=Gábor |last3=Farkas-Takács |first3=Anikó |last4=Stansberry |first4=John |last5=Müller |first5=Thomas |last6=Vinkó |first6=József |last7=Balog |first7=Zoltán |last8=Ortiz |first8=Jose-Luis |last9=Pál |first9=András |date=16 March 2017 |title=Discovery of a Satellite of the Large Trans-Neptunian Object (225088) 2007 OR<sub>10</sub> |journal=[[The Astrophysical Journal Letters]] |volume=838 |page=5 |arxiv=1703.01407 |bibcode=2017ApJ...838L...1K |doi=10.3847/2041-8213/aa6484 |s2cid=46766640 |id=L1 |doi-access=free |number=1}}</ref>
: [[Makemake (dwarf planet)|Makemake]] (45.79 AU average) is the largest classical KBO, is the brightest object in the Kuiper belt after Pluto. Originally designated '''2005 FY<sub>9</sub>''', it was granted a name (and the status of dwarf planet) on July 11, 2008.<ref name=name/> Makemake's orbit is far more inclined than Pluto's (29°)<ref name=Buie>{{cite web
|author=[[Marc W. Buie]]
|date=2008/04/05
|title=Orbit Fit and Astrometric record for 136472
|publisher=SwRI (Space Science Department)
|url=http://www.boulder.swri.edu/~buie/kbo/astrom/136472.html
|accessdate=2008-07-13}}</ref> and, unlike Pluto, as a classical KBO, its orbit is not affected by Neptune.


=== Extreme trans-Neptunian objects<span class="anchor" id="Detached objects"></span>===
===Scattered disc===
{{Main|Extreme trans-Neptunian object}}
{{main|Scattered disc}}
[[File:Distant object orbits + Planet Nine.png|thumb|upright=1.3|The current orbits of [[90377 Sedna|Sedna]], [[2012 VP113]], [[541132 Leleākūhonua|Leleākūhonua]] (pink), and other very [[ETNO|distant objects]] (red, brown and cyan) along with the predicted orbit of the hypothetical [[Planet Nine]] (dark blue)]]
[[Image:TheKuiperBelt Projections 100AU Classical SDO.svg|thumb|right|Black: scattered; blue: classical; green: resonant]]


Some objects in the Solar System have a very large orbit, and therefore are much less affected by the known giant planets than other minor planet populations. These bodies are called extreme trans-Neptunian objects, or ETNOs for short.<ref name="Sheppard-2018">{{cite journal |last1=Sheppard |first1=Scott S. |last2=Trujillo |first2=Chadwick A. |last3=Tholen |first3=David J. |last4=Kaib |first4=Nathan |year=2019 |title=A New High Perihelion Trans-Plutonian Inner Oort Cloud Object: 2015 TG387 |journal=The Astronomical Journal |volume=157 |issue=4 |page=139 |arxiv=1810.00013 |bibcode=2019AJ....157..139S |doi=10.3847/1538-3881/ab0895 |s2cid=119071596 |doi-access=free}}</ref> Generally, ETNOs' [[semi-major axis|semi-major axes]] are at least 150–250&nbsp;AU wide.<ref name="Sheppard-2018" /><ref name="Caju_outlier">{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |date=12 September 2018 |title=A Fruit of a Different Kind: 2015 BP<sub>519</sub> as an Outlier among the Extreme Trans-Neptunian Objects |journal=[[Research Notes of the AAS]] |volume=2 |issue=3 |pages=167 |arxiv=1809.02571 |bibcode=2018RNAAS...2..167D |doi=10.3847/2515-5172/aadfec |s2cid=119433944 |doi-access=free}}</ref> For example, [[541132 Leleākūhonua]] orbits the Sun once every ~32,000&nbsp;years, with a distance of 65–2000&nbsp;AU from the Sun.<ref name="jpldata2" group="D">{{cite web |title=JPL Small-Body Database Browser: (2015 TG387) |url=https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=3830896 |access-date=13 December 2018 |publisher=[[Jet Propulsion Laboratory]] |type=2018-10-17 last obs. |archive-date=14 April 2020 |archive-url=https://web.archive.org/web/20200414180200/https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=3830896 |url-status=live }}</ref>
The scattered disc overlaps the Kuiper belt but extends much further outwards. This region is thought to be the source of short-period comets. Scattered disc objects are believed to have been ejected into erratic orbits by the gravitational influence of [[Formation and evolution of the Solar System#Planetary migration|Neptune's early outward migration]]. Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia as far as 150&nbsp;AU from the Sun. SDOs' orbits are also highly inclined to the ecliptic plane, and are often almost perpendicular to it. Some astronomers consider the scattered disc to be merely another region of the Kuiper belt, and describe scattered disc objects as "scattered Kuiper belt objects."<ref>{{cite web |year=2005 |author=David Jewitt |title=The 1000 km Scale KBOs |work=University of Hawaii |url=http://www.ifa.hawaii.edu/faculty/jewitt/kb/big_kbo.html |accessdate=2006-07-16}}</ref>


This population is divided into three subgroups by astronomers. The [[Scattered disc object|scattered]] ETNOs have [[perihelia]] around 38–45&nbsp;AU and an exceptionally high [[Orbital eccentricity|eccentricity]] of more than 0.85. As with the regular scattered disc objects, they were likely formed as result of [[Planetary migration#Gravitational scattering|gravitational scattering]] by Neptune and still interact with the giant planets. The [[Detached object|detached]] ETNOs, with perihelia approximately between 40–45 and 50–60&nbsp;AU, are less affected by Neptune than the scattered ETNOs, but are still relatively close to Neptune. The [[sednoid]]s or [[Hills Cloud|inner Oort cloud]] objects, with perihelia beyond 50–60&nbsp;AU, are too far from Neptune to be strongly influenced by it.<ref name="Sheppard-2018" />
[[Image:Eris and dysnomia2.jpg|thumb|left|Eris and its moon [[Dysnomia (moon)|Dysnomia]]]]


Currently, there is one ETNO that is classified as a dwarf planet:
; Eris
: [[Eris (dwarf planet)|Eris]] (68&nbsp;AU average) is the largest known scattered disc object, and caused a debate about [[Definition of planet|what constitutes a planet]], since it is at least 5% larger than Pluto with an estimated diameter of 2400&nbsp;km (1500&nbsp;mi). It is the largest of the known dwarf planets.<ref>{{cite web |year=2005 |author=Mike Brown |title=The discovery of <s>2003 UB313</s> Eris, the <s>10th planet</s> largest known dwarf planet. |work=CalTech |url=http://www.gps.caltech.edu/~mbrown/planetlila/ |accessdate=2006-09-15}}</ref> It has one moon, [[Dysnomia (moon)|Dysnomia]]. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2&nbsp;AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6&nbsp;AU, and steeply inclined to the ecliptic plane.


* {{Dp|Sedna}} (76.2–937&nbsp;AU) was the first extreme trans-Neptunian object to be discovered. It is a large, reddish object, and it takes ~11,400&nbsp;years for Sedna to complete one orbit. [[Michael E. Brown|Mike Brown]], who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper belt because its perihelion is too distant to have been affected by Neptune's migration.<ref>{{Cite web |last=Jewitt |first=David |date=2004 |title=Sedna – 2003 VB<sub>12</sub> |url=http://www2.ess.ucla.edu/~jewitt/kb/sedna.html |url-status=live |archive-url=https://web.archive.org/web/20110716032018/http://www2.ess.ucla.edu/~jewitt/kb/sedna.html |archive-date=16 July 2011 |access-date=23 June 2006 |website=University of Hawaii}}</ref> The [[Sednoid|sednoid population]] is named after Sedna.<ref name="Sheppard-2018" />
== Farthest regions ==
The point at which the Solar System ends and interstellar space begins is not precisely defined, since its outer boundaries are shaped by two separate forces: the solar wind and the Sun's gravity. The outer limit of the solar wind's influence is roughly four times Pluto's distance from the Sun; this ''[[heliopause]]'' is considered the beginning of the [[interstellar medium]].<ref name="Voyager" /> However, the Sun's [[Hill sphere|Roche sphere]], the effective range of its gravitational influence, is believed to extend up to a thousand times farther.


=== Edge of the heliosphere ===
===Heliopause===
[[File:Magnetosphere Levels.jpg|thumb|Diagram of the Sun's magnetosphere and helioshealth]]
[[Image:Voyager 1 entering heliosheath region.jpg|left|thumb|300px|The [[Voyager program|Voyagers]] entering the [[heliosheath]].]]


{{Anchor|Heliopause}}The Sun's [[stellar-wind bubble]], the [[heliosphere]], a region of space dominated by the Sun, has its boundary at the ''termination shock''. Based on the Sun's [[peculiar motion]] relative to the [[local standard of rest]], this boundary is roughly 80–100&nbsp;AU from the Sun upwind of the interstellar medium and roughly 200&nbsp;AU from the Sun downwind.<ref name="fahr">{{Cite journal |last1=Fahr |first1=H. J. |last2=Kausch |first2=T. |last3=Scherer |first3=H. |date=2000 |title=A 5-fluid hydrodynamic approach to model the Solar System-interstellar medium interaction |url=http://aa.springer.de/papers/0357001/2300268.pdf |url-status=dead |journal=[[Astronomy & Astrophysics]] |volume=357 |page=268 |bibcode=2000A&A...357..268F |archive-url=https://web.archive.org/web/20170808135422/http://aa.springer.de/papers/0357001/2300268.pdf |archive-date=8 August 2017 |access-date=24 August 2008}} See Figures 1 and 2.</ref> Here the solar wind collides with the interstellar medium<ref>{{Cite web |last=Hatfield |first=Miles |date=3 June 2021 |title=The Heliopedia |url=http://www.nasa.gov/mission_pages/sunearth/the-heliopedia |url-status=live |archive-url=https://web.archive.org/web/20220325142928/https://www.nasa.gov/mission_pages/sunearth/the-heliopedia |archive-date=25 March 2022 |access-date=29 March 2022 |website=NASA}}</ref> and dramatically slows, condenses and becomes more turbulent, forming a great oval structure known as the [[heliosheath]].<ref name="fahr" />
The [[heliosphere]] is divided into two separate regions. The solar wind travels at roughly 40,000&nbsp;km/s until it collides with flows of plasma in the [[interstellar medium]]. The collision occurs at the [[termination shock]], which is roughly 80&nbsp;100&nbsp;AU from the Sun in the upwind direction and roughly 200&nbsp;AU from the Sun downwind.<ref name=fahr /> Here the wind slows dramatically, condenses and becomes more turbulent,<ref name=fahr /> forming a great oval structure known as the [[heliosheath]] that looks and behaves very much like a comet's tail, extending outward for a further 40&nbsp;AU on the upwind side but tailing many times that distance in the opposite direction. Both ''[[Voyager 1]]'' and ''[[Voyager 2]]'' are reported to have passed the termination shock and entered the heliosheath, at 94 and 84&nbsp;AU from the Sun, respectively.<ref>{{cite journal | doi=10.1126/science.1117684 | year=2005 | month=Sep | author=Stone, E. C.; Cummings, A. C.; Mcdonald, F. B.; Heikkila, B. C.; Lal, N.; Webber, W. R. | title=Voyager 1 explores the termination shock region and the heliosheath beyond | volume=309 | issue=5743 | pages=2017–20 | issn=0036-8075 | pmid=16179468 | journal=Science (New York, N.Y.)}}</ref><ref>{{cite journal | doi=10.1038/nature07022 | year=2008 | month=Jul | author=Stone, E. C.; Cummings, A. C.; Mcdonald, F. B.; Heikkila, B. C.; Lal, N.; Webber, W. R. | title=An asymmetric solar wind termination shock | volume=454 | issue=7200 | pages=71–4 | issn=0028-0836 | pmid=18596802 | journal=Nature }}</ref> The outer boundary of the heliosphere, the [[heliopause]], is the point at which the solar wind finally terminates and is the beginning of interstellar space.<ref name="Voyager">{{cite web |url=http://www.nasa.gov/vision/universe/solarsystem/voyager_agu.html |title=Voyager Enters Solar System's Final Frontier |work=NASA |accessdate=2007-04-02}}</ref>


The heliosheath has been theorized to look and behave very much like a comet's tail, extending outward for a further 40&nbsp;AU on the upwind side but tailing many times that distance downwind to possibly several thousands of AU.<ref name="n092">{{cite journal | last1=Brandt | first1=P. C. | last2=Provornikova | first2=E. | last3=Bale | first3=S. D. | last4=Cocoros | first4=A. | last5=DeMajistre | first5=R. | last6=Dialynas | first6=K. | last7=Elliott | first7=H. A. | last8=Eriksson | first8=S. | last9=Fields | first9=B. | last10=Galli | first10=A. | last11=Hill | first11=M. E. | last12=Horanyi | first12=M. | last13=Horbury | first13=T. | last14=Hunziker | first14=S. | last15=Kollmann | first15=P. | last16=Kinnison | first16=J. | last17=Fountain | first17=G. | last18=Krimigis | first18=S. M. | last19=Kurth | first19=W. S. | last20=Linsky | first20=J. | last21=Lisse | first21=C. M. | last22=Mandt | first22=K. E. | last23=Magnes | first23=W. | last24=McNutt | first24=R. L. | last25=Miller | first25=J. | last26=Moebius | first26=E. | last27=Mostafavi | first27=P. | last28=Opher | first28=M. | last29=Paxton | first29=L. | last30=Plaschke | first30=F. | last31=Poppe | first31=A. R. | last32=Roelof | first32=E. C. | last33=Runyon | first33=K. | last34=Redfield | first34=S. | last35=Schwadron | first35=N. | last36=Sterken | first36=V. | last37=Swaczyna | first37=P. | last38=Szalay | first38=J. | last39=Turner | first39=D. | last40=Vannier | first40=H. | last41=Wimmer-Schweingruber | first41=R. | last42=Wurz | first42=P. | last43=Zirnstein | first43=E. J. | title=Future Exploration of the Outer Heliosphere and Very Local Interstellar Medium by Interstellar Probe | journal=Space Science Reviews | volume=219 | issue=2 | date=2023 | issn=0038-6308 | pmid=36874191 | pmc=9974711 | doi=10.1007/s11214-022-00943-x | page=18| bibcode=2023SSRv..219...18B }}</ref><ref>{{Cite journal |last1=Baranov |first1=V. B. |last2=Malama |first2=Yu. G. |date=1993 |title=Model of the solar wind interaction with the local interstellar medium: Numerical solution of self-consistent problem |url=http://doi.wiley.com/10.1029/93JA01171 |journal=Journal of Geophysical Research |language=en |volume=98 |issue=A9 |page=15157 |bibcode=1993JGR....9815157B |doi=10.1029/93JA01171 |issn=0148-0227 |access-date=9 April 2022 |archive-date=20 April 2022 |archive-url=https://web.archive.org/web/20220420161220/https://onlinelibrary.wiley.com/resolve/doi?DOI=10.1029%2F93JA01171 |url-status=live }}</ref> Evidence from the ''[[Cassini (spacecraft)|Cassini]]'' and [[Interstellar Boundary Explorer]] spacecraft has suggested that it is forced into a bubble shape by the constraining action of the interstellar magnetic field,<ref>{{Cite web |date=19 November 2009 |title=Cassini's Big Sky: The View from the Center of Our Solar System |url=https://www.jpl.nasa.gov/news/cassinis-big-sky-the-view-from-the-center-of-our-solar-system |url-status=live |archive-url=https://web.archive.org/web/20220409213721/https://www.jpl.nasa.gov/news/cassinis-big-sky-the-view-from-the-center-of-our-solar-system |archive-date=9 April 2022 |access-date=9 April 2022 |website=Jet Propulsion Laboratory}}</ref><ref>{{Cite journal |last1=Kornbleuth |first1=M. |last2=Opher |first2=M. |last3=Baliukin |first3=I. |last4=Gkioulidou |first4=M. |last5=Richardson |first5=J. D. |last6=Zank |first6=G. P. |last7=Michael |first7=A. T. |last8=Tóth |first8=G. |last9=Tenishev |first9=V. |last10=Izmodenov |first10=V. |last11=Alexashov |first11=D. |date=1 December 2021 |title=The Development of a Split-tail Heliosphere and the Role of Non-ideal Processes: A Comparison of the BU and Moscow Models |journal=[[The Astrophysical Journal]] |volume=923 |issue=2 |page=179 |arxiv=2110.13962 |bibcode=2021ApJ...923..179K |doi=10.3847/1538-4357/ac2fa6 |issn=0004-637X |s2cid=239998560 |doi-access=free}}</ref> but the actual shape remains unknown.<ref>{{Cite journal |last1=Reisenfeld |first1=Daniel B. |last2=Bzowski |first2=Maciej |last3=Funsten |first3=Herbert O. |last4=Heerikhuisen |first4=Jacob |last5=Janzen |first5=Paul H. |last6=Kubiak |first6=Marzena A. |last7=McComas |first7=David J. |last8=Schwadron |first8=Nathan A. |last9=Sokół |first9=Justyna M. |last10=Zimorino |first10=Alex |last11=Zirnstein |first11=Eric J. |date=1 June 2021 |title=A Three-dimensional Map of the Heliosphere from IBEX |journal=[[The Astrophysical Journal Supplement Series]] |volume=254 |issue=2 |page=40 |bibcode=2021ApJS..254...40R |doi=10.3847/1538-4365/abf658 |issn=0067-0049 |osti=1890983 |s2cid=235400678 |doi-access=free}}</ref>
The shape and form of the outer edge of the heliosphere is likely affected by the [[fluid dynamics]] of interactions with the interstellar medium<ref name="fahr">{{cite journal |year=2000 |author=Fahr, H. J.; Kausch, T.; Scherer, H. |title=A 5-fluid hydrodynamic approach to model the Solar System-interstellar medium interaction |journal=Astronomy & Astrophysics | volume=357 | pages=268 |url=http://aa.springer.de/papers/0357001/2300268.pdf | format=PDF | bibcode=2000A&A...357..268F }} See Figures 1 and 2.</ref> as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU (roughly 900 million miles) farther than the southern hemisphere. Beyond the heliopause, at around 230&nbsp;AU, lies the [[bow shock]], a plasma "wake" left by the Sun as it travels through the [[Milky Way]].<ref>{{cite web | date=June 24, 2002 |author=P. C. Frisch (University of Chicago) |title=The Sun's Heliosphere & Heliopause | work=[[Astronomy Picture of the Day]] | url=http://antwrp.gsfc.nasa.gov/apod/ap020624.html |accessdate=2006-06-23}}</ref>


The shape and form of the outer edge of the heliosphere is likely affected by the [[fluid dynamics]] of interactions with the interstellar medium as well as [[solar magnetic field]]s prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9&nbsp;AU farther than the southern hemisphere.<ref name="fahr" /> The heliopause is considered the beginning of the interstellar medium.<ref name="Voyager" /> Beyond the heliopause, at around 230&nbsp;AU, lies the [[bow shock]]: a plasma "wake" left by the Sun as it travels through the Milky Way.<ref>{{Cite APOD|date=24 June 2002|title=The Sun's Heliosphere & Heliopause|access-date=23 June 2006}}</ref> Large objects outside the heliopause remain gravitationally bound to the Sun, but the flow of matter in the interstellar medium homogenizes the distribution of micro-scale objects.<ref name="Voyager" />
No spacecraft have yet passed beyond the heliopause, so it is impossible to know for certain the conditions in local interstellar space. It is expected that [[NASA]]'s [[Voyager program|Voyager spacecraft]] will pass the heliopause some time in the next decade and transmit valuable data on radiation levels and solar wind back to the Earth.<ref>{{cite web | year=2007 | title=Voyager: Interstellar Mission | work=NASA Jet Propulsion Laboratory | url=http://voyager.jpl.nasa.gov/mission/interstellar.html |accessdate=2008-05-08}}</ref> How well the heliosphere shields the Solar System from cosmic rays is poorly understood. A NASA-funded team has developed a concept of a "Vision Mission" dedicated to sending a probe to the heliosphere.<ref>{{cite conference |title=Innovative Interstellar Explorer |author=R. L. McNutt, Jr. et al. | booktitle= Physics of the Inner Heliosheath: Voyager Observations, Theory, and Future Prospects |publisher=AIP Conference Proceedings |volume=858 |pages=341–347 |year=2006 |url=http://adsabs.harvard.edu/abs/2006AIPC..858..341M |doi=10.1063/1.2359348}}</ref><ref>{{cite web |year=2007 |title=Interstellar space, and step on it! |work=New Scientist |url=http://space.newscientist.com/article/mg19325850.900-interstellar-space-and-step-on-it.html |date=2007-01-05 |accessdate=2007-02-05 | author=Anderson, Mark}}</ref>


== Miscellaneous populations ==
===Oort cloud===
=== Comets ===
{{main|Oort cloud}}
{{Main|Comet}}
[[Image:Kuiper oort.jpg|thumb|Artist's rendering of the Kuiper Belt and hypothetical [[Oort cloud]].]]


[[File:Comet_Hale-Bopp_1995O1.jpg|thumb|[[Comet Hale–Bopp]] seen in 1997]]
The hypothetical Oort cloud is a great mass of up to a trillion icy objects that is believed to be the source for all long-period comets and to surround the Solar System at roughly 50,000&nbsp;AU (around 1&nbsp;[[light-year]] (LY)), and possibly to as far as 100,000&nbsp;AU (1.87&nbsp;LY). It is believed to be composed of comets which were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events such as collisions, the gravitational effects of a passing star, or the [[galactic tide]], the [[tidal force]] exerted by the [[Milky Way]].<ref>{{cite web |year=2001 |author= Stern SA, Weissman PR. |title=Rapid collisional evolution of comets during the formation of the Oort cloud. |work=Space Studies Department, Southwest Research Institute, Boulder, Colorado| url=http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11214311&dopt=Citation |accessdate=2006-11-19}}</ref><ref>{{cite web |year=2006 |author=Bill Arnett |title=The Kuiper Belt and the Oort Cloud |work=nineplanets.org |url=http://www.nineplanets.org/kboc.html |accessdate=2006-06-23}}</ref>
Comets are [[small Solar System bodies]], typically only a few kilometers across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to [[sublimation (chemistry)|sublimate]] and [[ion]]ise, creating a [[coma (cometary)|coma]]: a long tail of gas and dust often visible to the naked eye.<ref>{{Cite web |date=19 December 2019 |title=In Depth: Comets |url=https://solarsystem.nasa.gov/asteroids-comets-and-meteors/comets/in-depth |url-status=live |archive-url=https://web.archive.org/web/20220331200633/https://solarsystem.nasa.gov/asteroids-comets-and-meteors/comets/in-depth |archive-date=31 March 2022 |access-date=31 March 2022 |website=NASA Science: Solar System Exploration}}</ref>


Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are thought to originate in the Kuiper belt, whereas long-period comets, such as [[Comet Hale–Bopp|Hale–Bopp]], are thought to originate in the Oort cloud. Many comet groups, such as the [[Kreutz sungrazer]]s, formed from the breakup of a single parent.<ref>{{Cite journal |last=Sekanina |first=Zdeněk |date=2001 |title=Kreutz sungrazers: the ultimate case of cometary fragmentation and disintegration? |journal=Publications of the Astronomical Institute of the Academy of Sciences of the Czech Republic |volume=89 |pages=78–93 |bibcode=2001PAICz..89...78S}}</ref> Some comets with [[hyperbolic trajectory|hyperbolic]] orbits may originate outside the Solar System, but determining their precise orbits is difficult.<ref name="hyperbolic">{{Cite journal |last=Królikowska |first=M. |date=2001 |title=A study of the original orbits of ''hyperbolic'' comets |journal=[[Astronomy & Astrophysics]] |volume=376 |issue=1 |pages=316–324 |bibcode=2001A&A...376..316K |doi=10.1051/0004-6361:20010945 |doi-access=free}}</ref> Old comets whose volatiles have mostly been driven out by solar warming are often categorized as asteroids.<ref>{{Cite journal |last=Whipple |first=Fred L. |date=1992 |title=The activities of comets related to their aging and origin |journal=[[Celestial Mechanics and Dynamical Astronomy]] |volume=54 |issue=1–3 |pages=1–11 |bibcode=1992CeMDA..54....1W |doi=10.1007/BF00049540 |s2cid=189827311}}</ref>
[[Image:Sedna-NASA.JPG|right|thumb|Telescopic image of Sedna]]


=== Meteoroids, meteors and dust ===
;Sedna and the inner Oort cloud
{{Main|Meteoroid|Interplanetary dust cloud|Cosmic dust}}
:[[90377 Sedna]] (525.86 AU average) is a large, reddish Pluto-like object with a gigantic, highly elliptical orbit that takes it from about 76&nbsp;AU at perihelion to 928&nbsp;AU at aphelion and takes 12,050 years to complete. [[Michael E. Brown|Mike Brown]], who discovered the object in 2003, asserts that it cannot be part of the [[scattered disc]] or the [[Kuiper belt]] as its perihelion is too distant to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, which also may include the object {{mpl|2000 CR|105}}, which has a perihelion of 45&nbsp;AU, an aphelion of 415&nbsp;AU, and an orbital period of 3420 years.<ref>{{cite web |year=2004 |author=David Jewitt |title=Sedna – 2003 VB<sub>12</sub> |work=University of Hawaii |url=http://www.ifa.hawaii.edu/~jewitt/kb/sedna.html |accessdate=2006-06-23}}</ref> Brown terms this population the "Inner Oort cloud," as it may have formed through a similar process, although it is far closer to the Sun.<ref>{{cite web |title=Sedna |author=Mike Brown |url=http://www.gps.caltech.edu/~mbrown/sedna/ |work=CalTech |accessdate=2007-05-02}}</ref> Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty.
[[File:Meteor shower in the Chilean Desert (annotated and cropped).jpg|thumb|The planets, zodiacal light and meteor shower (top left of image)]]
Solid objects smaller than one meter are usually called meteoroids and micrometeoroids (grain-sized), with the exact division between the two categories being debated over the years.<ref>{{Cite journal |last1=Rubin |first1=Alan E. |last2=Grossman |first2=Jeffrey N. |date=February 2010 |title=Meteorite and meteoroid: new comprehensive definitions |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2009.01009.x |url-status=live |journal=Meteoritics and Planetary Science |language=en |volume=45 |issue=1 |page=114 |bibcode=2010M&PS...45..114R |doi=10.1111/j.1945-5100.2009.01009.x |s2cid=129972426 |archive-url=https://web.archive.org/web/20220325111938/https://onlinelibrary.wiley.com/doi/10.1111/j.1945-5100.2009.01009.x |archive-date=25 March 2022 |access-date=10 April 2022}}</ref> By 2017, the IAU designated any solid object having a diameter between ~30&nbsp;[[Micrometre|micrometers]] and 1&nbsp;meter as meteoroids, and depreciated the micrometeoroid categorization, instead terms smaller particles simply as 'dust particles'.<ref>{{cite web |last1= |date=30 April 2017 |title=Definition of terms in meteor astronomy |url=https://www.iau.org/enwiki/static/science/scientific_bodies/commissions/f1/meteordefinitions_approved.pdf |access-date=25 July 2020 |website=International Astronomical Union |publisher=IAU Commission F1 |page=2 |archive-date=22 December 2021 |archive-url=https://web.archive.org/web/20211222205136/https://www.iau.org/enwiki/static/science/scientific_bodies/commissions/f1/meteordefinitions_approved.pdf |url-status=live }}</ref>


Some meteoroids formed via disintegration of comets and asteroids, while a few formed via impact debris ejected from planetary bodies. Most meteoroids are made of silicates and heavier metals like [[nickel]] and [[iron]].<ref>{{cite web |title=Meteoroid |url=http://education.nationalgeographic.co.uk/encyclopedia/meteoroid/ |archive-url=https://web.archive.org/web/20151007141358/https://education.nationalgeographic.co.uk/encyclopedia/meteoroid/ |archive-date=7 October 2015 |access-date=24 August 2015 |work=National Geographic}}</ref> When passing through the Solar System, comets produce a trail of meteoroids; it is hypothesized that this is caused either by vaporization of the comet's material or by simple breakup of dormant comets. When crossing an atmosphere, these meteoroids will produce bright streaks in the sky due to [[atmospheric entry]], called [[meteor]]s. If a stream of meteoroids enter the atmosphere on parallel trajectories, the meteors will seemingly 'radiate' from a point in the sky, hence the phenomenon's name: [[meteor shower]].<ref>{{cite book | chapter=The Evolution of Meteoroid Streams | first=Iwan P. | last=Williams | title=Meteors in the Earth's Atmosphere: Meteoroids and Cosmic Dust and Their Interactions with the Earth's Upper Atmosphere | pages=13–32 | year=2002 | isbn=9780521804318 | publisher=Cambridge University Press | editor1-first=Edmond | editor1-last=Murad | editor2-first=Iwan P. | editor2-last=Williams | chapter-url=https://books.google.com/books?id=eqd4e34uE-MC&pg=PA13 }}</ref>
===Boundaries===
{{see also|Vulcanoid asteroid|Planets beyond Neptune|Nemesis (star)}}


The inner Solar System is home to the [[interplanetary dust cloud|zodiacal dust cloud]], which is visible as the hazy [[zodiacal light]] in dark, unpolluted skies. It may be generated by collisions within the asteroid belt brought on by gravitational interactions with the planets; a more recent proposed origin is materials from planet Mars.<ref>{{Cite journal |last1=Jorgensen |first1=J. L. |last2=Benn |first2=M. |last3=Connerney |first3=J. E. P. |last4=Denver |first4=T. |last5=Jorgensen |first5=P. S. |last6=Andersen |first6=A. C. |last7=Bolton |first7=S. J. |date=March 2021 |title=Distribution of Interplanetary Dust Detected by the Juno Spacecraft and Its Contribution to the Zodiacal Light |journal=Journal of Geophysical Research: Planets |language=en |volume=126 |issue=3 |bibcode=2021JGRE..12606509J |doi=10.1029/2020JE006509 |issn=2169-9097 |s2cid=228840132 |doi-access=free}}</ref> The outer Solar System hosts a cosmic dust cloud. It extends from about {{val|10|u=AU}} to about {{val|40|u=AU}}, and was probably created by collisions within the Kuiper belt.<ref>{{Cite web |date=2003 |title=ESA scientist discovers a way to shortlist stars that might have planets |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=29471 |url-status=live |archive-url=https://web.archive.org/web/20130502033116/http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=29471 |archive-date=2 May 2013 |access-date=3 February 2007 |website=ESA Science and Technology}}</ref><ref>{{Cite journal |last1=Landgraf |first1=M. |last2=Liou |first2=J.-C. |last3=Zook |first3=H. A. |last4=Grün |first4=E. |date=May 2002 |title=Origins of Solar System Dust beyond Jupiter |url=http://astron.berkeley.edu/~kalas/disksite/library/ladgraf02.pdf |url-status=live |journal=[[The Astronomical Journal]] |volume=123 |issue=5 |pages=2857–2861 |arxiv=astro-ph/0201291 |bibcode=2002AJ....123.2857L |doi=10.1086/339704 |s2cid=38710056 |archive-url=http://arquivo.pt/wayback/20160515115002/http://astron.berkeley.edu/~kalas/disksite/library/ladgraf02.pdf |archive-date=15 May 2016 |access-date=9 February 2007}}</ref>
Much of our Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of [[List of nearest stars|surrounding stars]] out to about two light years (125,000&nbsp;AU). The outer extent of the Oort cloud, by contrast, may not extend farther than 50,000&nbsp;AU.<ref>{{cite book |title=The Solar System: Third edition |author=T. Encrenaz, JP. Bibring, M. Blanc, MA. Barucci, F. Roques, PH. Zarka |publisher=Springer |year=2004 |page=1}}</ref> Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. There are also ongoing studies of the region between Mercury and the Sun.<ref>{{cite web |year=2004 |author=Durda D.D.; Stern S.A.; Colwell W.B.; Parker J.W.; Levison H.F.; Hassler D.M. |title=A New Observational Search for Vulcanoids in SOHO/LASCO Coronagraph Images |url=http://www.ingentaconnect.com/search/expand?pub=infobike://ap/is/2000/00000148/00000001/art06520&unc=ml |accessdate=2006-07-23}}</ref> Objects may yet be discovered in the Solar System's uncharted regions.


== Boundary region and uncertainties ==
== Galactic context ==
[[Image:Milky Way Spiral Arm.svg|left|thumb|Location of the Solar System within our galaxy]]
{{See also|Planets beyond Neptune|Planet Nine|List of Solar System objects by greatest aphelion}}


[[File:Kuiper oort-en.svg|thumb|An [[artist's impression]] of the [[Oort cloud]], a region still well within the [[Sphere of influence (astrodynamics)|sphere of influence]] of the Solar System, including a depiction of the much further inside [[Kuiper belt]] (inset); the sizes of objects are over-scaled for visibility.]]
The Solar System is located in the [[Milky Way]] [[galaxy]], a [[barred spiral galaxy]] with a diameter of about 100,000 [[light-year]]s containing about 200 billion stars.<ref>{{cite web |title=Magnetic fields in cosmology |author=A.D. Dolgov |year=2003 |url=http://arxiv.org/abs/astro-ph/0306443 |accessdate=2006-07-23}}</ref> Our Sun resides in one of the Milky Way's outer spiral arms, known as the [[Orion Arm]] or Local Spur.<ref>{{cite web |title=Three Dimensional Structure of the Milky Way Disk |author=R. Drimmel, D. N. Spergel |year=2001 |url=http://arxiv.org/abs/astro-ph/0101259 |accessdate=2006-07-23}}</ref> The Sun lies between 25,000 and 28,000 light years from the [[Galactic Centre]], and its speed within the galaxy is about 220 [[metre per second|kilometres per second]], so that it completes one revolution every 225–250 million years. This revolution is known as the Solar System's [[galactic year]].<ref>{{cite web |title=Period of the Sun's Orbit around the Galaxy (Cosmic Year |first=Stacy |last=Leong |url=http://hypertextbook.com/facts/2002/StacyLeong.shtml |year=2002 |work=The Physics Factbook |accessdate=2007-04-02}}</ref>


{{Anchor|Farthest regions|Boundaries}}Much of the Solar System is still unknown. Regions beyond thousands of AU away are still virtually unmapped and learning about this region of space is difficult. Study in this region depends upon inferences from those few objects whose orbits happen to be perturbed such that they fall closer to the Sun, and even then, detecting these objects has often been possible only when they happened to become bright enough to register as comets.<ref>{{Cite journal |last1=Bernardinelli |first1=Pedro H. |last2=Bernstein |first2=Gary M. |last3=Montet |first3=Benjamin T. |last4=Weryk |first4=Robert |last5=Wainscoat |first5=Richard |last6=Aguena |first6=M. |last7=Allam |first7=S. |last8=Andrade-Oliveira |first8=F. |last9=Annis |first9=J. |last10=Avila |first10=S. |last11=Bertin |first11=E. |display-authors=3 |date=1 November 2021 |title=C/2014 UN 271 (Bernardinelli-Bernstein): The Nearly Spherical Cow of Comets |journal=[[The Astrophysical Journal Letters]] |volume=921 |issue=2 |page=L37 |arxiv=2109.09852 |bibcode=2021ApJ...921L..37B |doi=10.3847/2041-8213/ac32d3 |issn=2041-8205 |s2cid=237581632 |doi-access=free}}</ref> Many objects may yet be discovered in the Solar System's uncharted regions.<ref>{{Cite web |last=Loeffler |first=John |date=1 October 2021 |title=Our solar system may have a hidden planet beyond Neptune – no, not that one |url=https://www.msn.com/en-us/news/technology/our-solar-system-may-have-a-hidden-planet-beyond-neptune-no-not-that-one/ar-AAP3b0l?ocid=msedgntp |url-status=live |archive-url=https://web.archive.org/web/20211001203656/https://www.msn.com/en-us/news/technology/our-solar-system-may-have-a-hidden-planet-beyond-neptune-no-not-that-one/ar-AAP3b0l?ocid=msedgntp |archive-date=1 October 2021 |access-date=7 April 2022 |website=MSN}}</ref>
The Solar System's location in the galaxy is very likely a factor in the [[evolution]] of [[life]] on Earth. Its orbit is close to being circular and is at roughly the same speed as that of the spiral arms, which means it passes through them only rarely. Since spiral arms are home to a far larger concentration of potentially dangerous [[supernova]]e, this has given Earth long periods of interstellar stability for life to evolve.<ref name="astrobiology">{{cite web |year=2001 |author=Leslie Mullen |title=Galactic Habitable Zones |work=Astrobiology Magazine |url=http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=139 |accessdate=2006-06-23}}</ref> The Solar System also lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort Cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the galactic centre could also interfere with the development of complex life.<ref name=astrobiology/> Even at the Solar System's current location, some scientists have hypothesised that recent supernovae may have adversely affected life in the last 35,000 years by flinging pieces of expelled stellar core towards the Sun in the form of radioactive dust grains and larger, comet-like bodies.<ref>{{cite web |year=2005 |author=|title=Supernova Explosion May Have Caused Mammoth Extinction |work=Physorg.com |url=http://www.physorg.com/news6734.html |accessdate=2007-02-02}}</ref>


{{Anchor|Oort cloud}}The [[Oort cloud]] is a theorized spherical shell of up to a trillion icy objects that is thought to be the source for all long-period comets.<ref name=":5">{{Cite journal |vauthors=Stern SA, Weissman PR |date=2001 |title=Rapid collisional evolution of comets during the formation of the Oort cloud |journal=Nature |volume=409 |issue=6820 |pages=589–591 |bibcode=2001Natur.409..589S |doi=10.1038/35054508 |pmid=11214311 |s2cid=205013399}}</ref><ref name=":6">{{Cite web |last=Arnett |first=Bill |date=2006 |title=The Kuiper Belt and the Oort Cloud |url=http://www.nineplanets.org/kboc.html |url-status=live |archive-url=https://web.archive.org/web/20190807064224/http://nineplanets.org/kboc.html |archive-date=7 August 2019 |access-date=23 June 2006 |website=Nine Planets}}</ref> No direct observation of the Oort cloud is possible with present imaging technology.<ref>{{Cite web |title=Oort Cloud |url=https://solarsystem.nasa.gov/solar-system/oort-cloud/overview |url-status=live |archive-url=https://web.archive.org/web/20230630162050/https://solarsystem.nasa.gov/solar-system/oort-cloud/overview/ |archive-date=30 June 2023 |access-date=1 July 2023 |website=NASA Solar System Exploration}}</ref> It is theorized to surround the Solar System at roughly 50,000&nbsp;AU (~0.9&nbsp;[[Light-year|ly]]) from the Sun and possibly to as far as 100,000&nbsp;AU (~1.8&nbsp;ly). The Oort cloud is thought to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events, such as collisions, the gravitational effects of a passing star, or the [[galactic tide]], the [[tidal force]] exerted by the Milky Way.<ref name=":5" /><ref name=":6" />
===Neighbourhood===
[[Image:local bubble.jpg|thumb|Artist's conception of the [[Local Bubble]]]]


As of the 2020s, a few astronomers have hypothesized that [[Planet Nine]] (a planet beyond Neptune) might exist, based on statistical variance in the orbit of [[extreme trans-Neptunian object]]s.<ref name="P9H2019">{{cite journal |last1=Batygin |first1=Konstantin |last2=Adams |first2=Fred C. |last3=Brown |first3=Michael E. |last4=Becker |first4=Juliette C. |date=2019 |title=The Planet Nine Hypothesis |journal=[[Physics Reports]] |volume=805 |pages=1–53 |arxiv=1902.10103 |bibcode=2019PhR...805....1B |doi=10.1016/j.physrep.2019.01.009 |s2cid=119248548}}</ref> Their closest approaches to the Sun are mostly clustered around one sector and their orbits are similarly tilted, suggesting that a large planet might be influencing their orbit over millions of years.<ref name="Sheppard2014">{{cite journal |last1=Trujillo |first1=Chadwick A. |author-link=Chad Trujillo |last2=Sheppard |first2=Scott S. |author-link2=Scott S. Sheppard |date=2014 |title=A Sedna-like Body with a Perihelion of 80 Astronomical Units |url=http://home.dtm.ciw.edu/users/sheppard/pub/TrujilloSheppard2014.pdf |url-status=dead |journal=[[Nature (journal)|Nature]] |volume=507 |issue=7493 |pages=471–474 |bibcode=2014Natur.507..471T |doi=10.1038/nature13156 |pmid=24670765 |s2cid=4393431 |url-access=subscription |archive-url=https://web.archive.org/web/20141216183818/http://home.dtm.ciw.edu/users/sheppard/pub/TrujilloSheppard2014.pdf |archive-date=16 December 2014 |access-date=20 January 2016}}</ref><ref name="nodes-2021">{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |date=1 September 2021 |title=Peculiar orbits and asymmetries in extreme trans-Neptunian space |url=https://academic.oup.com/mnras/article-abstract/506/1/633/6307523 |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=506 |issue=1 |pages=633–649 |arxiv=2106.08369 |bibcode=2021MNRAS.506..633D |doi=10.1093/mnras/stab1756 |doi-access=free |access-date=20 April 2024 |archive-date=19 October 2021 |archive-url=https://web.archive.org/web/20211019195919/https://academic.oup.com/mnras/article-abstract/506/1/633/6307523 |url-status=live }}</ref><ref name="nodes-2022">{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |date=1 May 2022 |title=Twisted extreme trans-Neptunian orbital parameter space: statistically significant asymmetries confirmed |url=https://academic.oup.com/mnrasl/article-abstract/512/1/L6/6524836 |journal=[[Monthly Notices of the Royal Astronomical Society Letters]] |volume=512 |issue=1 |pages=L6–L10 |arxiv=2202.01693 |bibcode=2022MNRAS.512L...6D |doi=10.1093/mnrasl/slac012 |doi-access=free |access-date=20 April 2024 |archive-date=9 April 2023 |archive-url=https://web.archive.org/web/20230409170426/https://academic.oup.com/mnrasl/article-abstract/512/1/L6/6524836 |url-status=live }}</ref> However, some astronomers said that this observation might be credited to observational biases or just sheer coincidence.<ref>{{cite journal |last=Napier |first=K. J. |date=2021 |title=No Evidence for Orbital Clustering in the Extreme Trans-Neptunian Objects |journal=The Planetary Science Journal |volume=2 |issue=2 |page=59 |arxiv=2102.05601 |bibcode=2021PSJ.....2...59N |doi=10.3847/PSJ/abe53e |doi-access=free}}</ref> An alternative hypothesis has a close flyby of another star disrupting the outer Solar System.<ref>{{cite journal | title=Trajectory of the stellar flyby that shaped the outer Solar System | first1=Susanne | last1=Pfalzner | first2=Amith | last2=Govind | first3=Simon Portegies | last3=Zwart | journal=Nature Astronomy | date=September 2024 | volume=8 | issue=11 | pages=1380–1386 | doi=10.1038/s41550-024-02349-x | arxiv=2409.03342 | bibcode=2024NatAs...8.1380P }}</ref>
The immediate galactic neighbourhood of the Solar System is known as the [[Local Interstellar Cloud]] or Local Fluff, an area of dense cloud in an otherwise sparse region known as the [[Local Bubble]], an hourglass-shaped cavity in the [[interstellar medium]] roughly 300 light years across. The bubble is suffused with high-temperature [[Plasma (physics)|plasma]] that suggests it is the product of several recent supernovae.<ref>{{cite web |title=Near-Earth Supernovas |work=NASA |url=http://science.nasa.gov/headlines/y2003/06jan_bubble.htm |accessdate=2006-07-23}}</ref>


The Sun's gravitational field is estimated to [[Sphere of influence (astrodynamics)|dominate the gravitational forces of surrounding stars]] out to about two light-years ({{val|125,000|fmt=commas|u=AU}}). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than {{val|50,000|fmt=commas|u=AU}}.<ref name="Encrenaz_et_al_2004">{{Cite book |last1=Encrenaz |first1=T. |author-link=Thérèse Encrenaz |title=The Solar System |last2=Bibring |first2=J. P. |last3=Blanc |first3=M. |last4=Barucci |first4=M. A. |last5=Roques |first5=F. |last6=Zarka |first6=P. H. |date=2004 |publisher=Springer |edition=3rd |page=1}}</ref> Most of the mass is orbiting in the region between 3,000 and {{val|100,000|fmt=commas|u=AU}}.<ref>{{Cite journal |last1=Torres |first1=S. |last2=Cai |first2=M. X. |last3=Brown |first3=A. G. A. |last4=Portegies Zwart |first4=S. |date=September 2019 |title=Galactic tide and local stellar perturbations on the Oort cloud: creation of interstellar comets |journal=Astronomy & Astrophysics |volume=629 |page=13 |arxiv=1906.10617 |bibcode=2019A&A...629A.139T |doi=10.1051/0004-6361/201935330 |s2cid=195584070 |id=A139}}</ref> The furthest known objects, such as [[Comet West]], have aphelia around {{val|70,000|fmt=commas|u=AU}} from the Sun.<ref>{{Cite web |last=Norman |first=Neil |date=May 2020 |title=10 great comets of recent times |url=https://www.skyatnightmagazine.com/space-science/greatest-comets-of-recent-times |url-status=live |archive-url=https://web.archive.org/web/20220125042109/https://www.skyatnightmagazine.com/space-science/greatest-comets-of-recent-times |archive-date=25 January 2022 |access-date=10 April 2022 |website=[[BBC Sky at Night|BBC Sky at Night Magazine]] |language=en}}</ref> The Sun's [[Hill sphere]] with respect to the galactic nucleus, the effective range of its gravitational influence, is thought to extend up to a thousand times farther and encompasses the hypothetical Oort cloud.<ref name="Littmann">{{Cite book |last=Littmann |first=Mark |url=https://archive.org/details/planetsbeyonddis00mlit |title=Planets Beyond: Discovering the Outer Solar System |date=2004 |publisher=Courier Dover Publications |isbn=978-0-486-43602-9 |pages=[https://archive.org/details/planetsbeyonddis00mlit/page/n92 162]–163 |url-access=limited}}</ref> It was calculated by [[Gleb Alexandrovich Chebotarev|G. A. Chebotarev]] to be 230,000&nbsp;AU.<ref name=Chebotarev />
The [[solar apex]], the direction of the Sun's path through interstellar space, is near the constellation of [[Hercules (constellation)|Hercules]] in the direction of the current location of the bright star [[Vega]].<ref>{{cite web |year=2003 |author=C. Barbieri |title=Elementi di Astronomia e Astrofisica per il Corso di Ingegneria Aerospaziale V settimana |work=IdealStars.com |url=http://www.google.com/search?q=cache:yKkhLXIaAvoJ:debora.pd.astro.it/planets/barbieri/IngAeroAnnoA2004-05/5_LecturesAstroAstrofIng04_05QuintaSettimana.ppt+Elementi+di+Astronomia+e+Astrofisica+per+il+Corso+di+Ingegneria+Aerospaziale+V+settimana&hl=en&ct=clnk&cd=1&gl=us |accessdate=2007-02-12}}</ref>


[[File:Interstellar medium annotated.jpg|thumb|center|upright=2.5|The Solar System (left) within the [[interstellar medium]], with the different regions and their distances on a [[logarithmic scale]]]]
There are relatively few [[List of nearest stars|stars within ten light years]] (95 trillion&nbsp;km) of the Sun. The closest is the triple star system [[Alpha Centauri]], which is about 4.4 light years away. Alpha Centauri A and B are a closely tied pair of Sun-like stars, while the small [[red dwarf]] Alpha Centauri C (also known as [[Proxima Centauri]]) orbits the pair at a distance of 0.2 light years. The stars next closest to the Sun are the red dwarfs [[Barnard's Star]] (at 5.9 light years), [[Wolf 359]] (7.8 light years) and [[Lalande 21185]] (8.3 light years). The largest star within ten light years is [[Sirius]], a bright [[main sequence]] star roughly twice the Sun's mass and orbited by a [[white dwarf]] called Sirius B. It lies 8.6 light years away. The remaining systems within ten light years are the binary red dwarf system [[Luyten 726-8]] (8.7 light years) and the solitary red dwarf [[Ross 154]] (9.7 light years).<ref>{{cite web |title=Stars within 10 light years |url=http://www.solstation.com/stars/s10ly.htm|work=SolStation |accessdate=2007-04-02}}</ref> Our closest solitary sunlike star is [[Tau Ceti]], which lies 11.9 light years away. It has roughly 80 percent the Sun's mass, but only 60 percent its luminosity.<ref>{{cite web |title=Tau Ceti |url=http://www.solstation.com/stars/tau-ceti.htm |work=SolStation |accessdate=2007-04-02}}</ref> The closest known [[extrasolar planet]] to the Sun lies around the star [[Epsilon Eridani]], a star slightly dimmer and redder than the Sun, which lies 10.5 light years away. Its one confirmed planet, [[Epsilon Eridani b]], is roughly 1.5 times Jupiter's mass and orbits its star every 6.9 years.<ref>{{cite web |title=HUBBLE ZEROES IN ON NEAREST KNOWN EXOPLANET |work=Hubblesite |year=2006 |url=http://hubblesite.org/newscenter/archive/releases/2006/32/text/ |accessdate-2008-01-13}}</ref>


== Celestial neighborhood ==
== Formation and evolution == <!--This heading linked by {{Earth}}-->
{{Main|List of nearest stars and brown dwarfs|List of nearest exoplanets|List of nearby stellar associations and moving groups}}
{{main|Formation and evolution of the Solar System}}
[[File:The Local Interstellar Cloud and neighboring G-cloud complex.svg|thumb|Diagram of the [[Local Interstellar Cloud]], the [[G-Cloud]] and surrounding stars. As of 2022, the precise location of the Solar System in the clouds is an open question in astronomy.<ref>{{Cite journal |last1=Swaczyna |first1=Paweł |last2=Schwadron |first2=Nathan A. |last3=Möbius |first3=Eberhard |last4=Bzowski |first4=Maciej |last5=Frisch |first5=Priscilla C. |last6=Linsky |first6=Jeffrey L. |last7=McComas |first7=David J. |last8=Rahmanifard |first8=Fatemeh |last9=Redfield |first9=Seth |last10=Winslow |first10=Réka M. |last11=Wood |first11=Brian E. |last12=Zank |first12=Gary P. |date=1 October 2022 |title=Mixing Interstellar Clouds Surrounding the Sun |journal=[[The Astrophysical Journal Letters]] |volume=937 |issue=2 |pages=L32:1–2 |arxiv=2209.09927 |bibcode=2022ApJ...937L..32S |doi=10.3847/2041-8213/ac9120 |issn=2041-8205 |doi-access=free}}</ref>]]
[[Image:M42proplyds.jpg|left|thumb|[[Hubble Space Telescope|Hubble]] image of protoplanetary disks in the [[Orion Nebula]], a light-years-wide "stellar nursery" likely very similar to the primordial nebula from which our Sun formed.]]


Within 10&nbsp;light-years of the Sun there are relatively few stars, the closest being the triple star system [[Alpha Centauri]], which is about 4.4&nbsp;light-years away and may be in the Local Bubble's [[G-Cloud]].<ref>{{Cite journal |last1=Linsky |first1=Jeffrey L. |last2=Redfield |first2=Seth |last3=Tilipman |first3=Dennis |date=November 2019 |title=The Interface between the Outer Heliosphere and the Inner Local ISM: Morphology of the Local Interstellar Cloud, Its Hydrogen Hole, Strömgren Shells, and 60Fe Accretion |journal=[[The Astrophysical Journal]] |volume=886 |issue=1 |page=19 |arxiv=1910.01243 |bibcode=2019ApJ...886...41L |doi=10.3847/1538-4357/ab498a |s2cid=203642080 |id=41 |doi-access=free}}</ref> Alpha Centauri A and B are a closely tied pair of [[Solar analog|Sun-like stars]], whereas the closest star to the Sun, the small [[red dwarf]] [[Proxima Centauri]], orbits the pair at a distance of 0.2&nbsp;light-years. In 2016, a potentially habitable [[exoplanet]] was found to be orbiting Proxima Centauri, called [[Proxima Centauri b]], the closest confirmed exoplanet to the Sun.<ref name="proxima b discovery paper">{{Cite journal |last1=Anglada-Escudé |first1=Guillem |last2=Amado |first2=Pedro J. |last3=Barnes |first3=John |last4=Berdiñas |first4=Zaira M. |last5=Butler |first5=R. Paul |last6=Coleman |first6=Gavin A. L. |last7=de la Cueva |first7=Ignacio |last8=Dreizler |first8=Stefan |last9=Endl |first9=Michael |last10=Giesers |first10=Benjamin |last11=Jeffers |first11=Sandra V. |last12=Jenkins |first12=James S. |last13=Jones |first13=Hugh R. A. |last14=Kiraga |first14=Marcin |last15=Kürster |first15=Martin |display-authors=3 |year=2016 |title=A terrestrial planet candidate in a temperate orbit around Proxima Centauri |url=https://www.nature.com/articles/nature19106 |journal=[[Nature (journal)|Nature]] |volume=536 |issue=7617 |pages=437–440 |arxiv=1609.03449 |bibcode=2016Natur.536..437A |doi=10.1038/nature19106 |pmid=27558064 |s2cid=4451513 |last16=López-González |first16=María J. |last17=Marvin |first17=Christopher J. |last18=Morales |first18=Nicolás |last19=Morin |first19=Julien |last20=Nelson |first20=Richard P. |last21=Ortiz |first21=José L. |last22=Ofir |first22=Aviv |last23=Paardekooper |first23=Sijme-Jan |last24=Reiners |first24=Ansgar |last25=Rodríguez |first25=Eloy |last26=Rodríguez-López |first26=Cristina |last27=Sarmiento |first27=Luis F. |last28=Strachan |first28=John P. |last29=Tsapras |first29=Yiannis |last30=Tuomi |first30=Mikko |first31=Mathias |last31=Zechmeister}}</ref>
The Solar System is believed to have formed according to the [[nebular hypothesis]], which holds that it emerged from the gravitational collapse of a giant [[molecular cloud]] 4.6 billion years ago. This initial cloud was likely several light-years across and probably birthed several stars.<ref name="Arizona">{{cite web |title=Lecture 13: The Nebular Theory of the origin of the Solar System |url=http://atropos.as.arizona.edu/aiz/teaching/nats102/mario/solar_system.html |work=University of Arizona |accessdate=2006-12-27}}</ref> Studies of ancient [[meteorite]]s reveal traces of [[Chemical element|elements]] only formed in the hearts of very large exploding stars, indicating that the Sun formed within a [[star cluster]], and in range of a number of nearby [[supernova]]e explosions. The [[shock wave]] from these supernovae may have triggered the formation of the Sun by creating regions of overdensity in the surrounding nebula, allowing gravitational forces to overcome internal [[gas]] pressures and cause collapse.<ref>{{cite web |year=2004 |title=New Theory Proposed for Solar System Formation |author=Jeff Hester |work=Arizona State University |url=http://www.universetoday.com/am/publish/new_theory_solar_system_formation.html |accessdate=2007-01-11}}</ref>


The Solar System is surrounded by the [[Local Interstellar Cloud]], although it is not clear if it is embedded in the Local Interstellar Cloud or if it lies just outside the cloud's edge.<ref name=":1">{{Cite journal |last1=Linsky |first1=Jeffrey L. |last2=Redfield |first2=Seth |last3=Tilipman |first3=Dennis |date=20 November 2019 |title=The Interface between the Outer Heliosphere and the Inner Local ISM: Morphology of the Local Interstellar Cloud, Its Hydrogen Hole, Strömgren Shells, and 60 Fe Accretion* |journal=[[The Astrophysical Journal]] |volume=886 |issue=1 |page=41 |arxiv=1910.01243 |bibcode=2019ApJ...886...41L |doi=10.3847/1538-4357/ab498a |issn=0004-637X |s2cid=203642080 |doi-access=free}}</ref> Multiple other [[interstellar cloud]]s exist in the region within 300&nbsp;light-years of the Sun, known as the [[Local Bubble]].<ref name=":1" /> The latter feature is an hourglass-shaped cavity or [[superbubble]] in the interstellar medium roughly 300&nbsp;light-years across. The bubble is suffused with high-temperature plasma, suggesting that it may be the product of several recent supernovae.<ref>{{Cite journal |last1=Zucker |first1=Catherine |last2=Goodman |first2=Alyssa A. |author-link2=Alyssa A. Goodman |last3=Alves |first3=João |last4=Bialy |first4=Shmuel |last5=Foley |first5=Michael |last6=Speagle |first6=Joshua S. |last7=Großschedl |first7=Josefa |last8=Finkbeiner |first8=Douglas P. |last9=Burkert |first9=Andreas |last10=Khimey |first10=Diana |last11=Swiggum |first11=Cameren |display-authors=3 |date=January 2022 |title=Star formation near the Sun is driven by expansion of the Local Bubble |url=https://www.nature.com/articles/s41586-021-04286-5 |journal=[[Nature (journal)|Nature]] |language=en |volume=601 |issue=7893 |pages=334–337 |arxiv=2201.05124 |bibcode=2022Natur.601..334Z |doi=10.1038/s41586-021-04286-5 |issn=1476-4687 |pmid=35022612 |s2cid=245906333}}</ref>
{| class="wikitable" style="float:right;"
|+ Solar System's Most<br />Abundant Isotopes<ref>{{cite book |first=David |last=Arnett |year=1996 |title=Supernovae and Nucleosynthesis |edition=First edition |publisher=Princeton University Press |location=Princeton, New Jersey |id=ISBN 0-691-01147-8}}</ref>
![[Isotope]]
![[Atomic nucleus|Nuclei]]&nbsp;per<br />Million
|-
| [[Hydrogen-1]] ||style="text-align:right"| 705,700
|-
| [[Helium-4]] ||style="text-align:right"| 275,200
|-
| [[Oxygen-16]] ||style="text-align:right"| 5,920
|-
| [[Carbon-12]] ||style="text-align:right"| 3,032
|-
| [[Neon-20]] ||style="text-align:right"| 1,548
|-
| [[Iron-56]] ||style="text-align:right"| 1,169
|-
| [[Nitrogen-14]] ||style="text-align:right"| 1,105
|-
| [[Silicon-28]] ||style="text-align:right"| 653
|-
| [[Magnesium-24]] ||style="text-align:right"| 513
|-
| [[Sulfur-32]] ||style="text-align:right"| 396
|-
| [[Neon-22]] ||style="text-align:right"| 208
|-
| [[Magnesium-26]] ||style="text-align:right"| 79
|-
| [[Argon-36]] ||style="text-align:right"| 77
|-
| [[Iron-54]] ||style="text-align:right"| 72
|-
| [[Magnesium-25]] ||style="text-align:right"| 69
|-
| [[Calcium-40]] ||style="text-align:right"| 60
|-
| [[Aluminum-27]] ||style="text-align:right"| 58
|-
| [[Nickel-58]] ||style="text-align:right"| 49
|-
| [[Carbon-13]] ||style="text-align:right"| 37
|-
| [[Helium-3]] ||style="text-align:right"| 35
|-
| [[Silicon-29]] ||style="text-align:right"| 34
|-
| [[Sodium-23]] ||style="text-align:right"| 33
|-
| [[Iron-57]] ||style="text-align:right"| 28
|-
| [[Hydrogen-2]] ||style="text-align:right"| 23
|-
| [[Silicon-30]] ||style="text-align:right"| 23
|}


The Local Bubble is a small superbubble compared to the neighboring wider [[Radcliffe Wave]] and ''Split'' linear structures (formerly [[Gould Belt]]), each of which are some thousands of light-years in length.<ref name="Alves Zucker Goodman Speagle 2020">{{Cite journal |last1=Alves |first1=João |last2=Zucker |first2=Catherine |last3=Goodman |first3=Alyssa A. |last4=Speagle |first4=Joshua S. |last5=Meingast |first5=Stefan |last6=Robitaille |first6=Thomas |last7=Finkbeiner |first7=Douglas P. |last8=Schlafly |first8=Edward F. |last9=Green |first9=Gregory M. |date=23 January 2020 |title=A Galactic-scale gas wave in the Solar Neighborhood |journal=[[Nature (journal)|Nature]] |volume=578 |issue=7794 |pages=237–239 |arxiv=2001.08748v1 |bibcode=2020Natur.578..237A |doi=10.1038/s41586-019-1874-z |pmid=31910431 |s2cid=210086520}}</ref> All these structures are part of the [[Orion Arm]], which contains most of the stars in the Milky Way that are visible to the unaided eye.<ref>{{Cite journal |last1=McKee |first1=Christopher F. |last2=Parravano |first2=Antonio |last3=Hollenbach |first3=David J. |date=November 2015 |title=Stars, Gas, and Dark Matter in the Solar Neighborhood |journal=[[The Astrophysical Journal]] |volume=814 |issue=1 |pages=24 |arxiv=1509.05334 |bibcode=2015ApJ...814...13M |doi=10.1088/0004-637X/814/1/13 |s2cid=54224451 |id=13}}</ref>
The region that would become the Solar System, known as the [[solar nebula|pre-solar nebula]],<ref>{{cite web |title=The chemical composition of the pre-solar nebula |author=Irvine, W. M. |work=Amherst College, Massachusetts |url=http://adsabs.harvard.edu/abs/1983coex....1....3I |accessdate=2007-02-15}}</ref> had a diameter of between 7000 and 20,000&nbsp;AU<ref name="Arizona" /><ref name="Rawal_1985">{{cite science |last=Rawal |first=J. J. |month=January |year=1985 |title=Further Considerations on Contracting Solar Nebula |journal=Physics and Astronomy |volume=34 |issue=1 |pages=93–100 |doi=10.1007/BF00054038 |url=http://www.springerlink.com/content/r5825j48k66n8284/fulltext.pdf |format=[[Portable Document Format|PDF]] |abstract=http://www.springerlink.com/content/r5825j48k66n8284/ |accessdate=2006-12-27}}</ref> and a mass just over that of the Sun (by between 0.1 and 0.001 solar masses).<ref name="Kitamara">{{cite journal |author=Yoshimi Kitamura |coauthors=Munetake Momose; Sozo Yokogawa; Ryohei Kawabe; Shigeru Ida; Motohide Tamura | year=2002 | title=Investigation of the Physical Properties of Protoplanetary Disks around T Tauri Stars by a 1 Arcsecond Imaging Survey: Evolution and Diversity of the Disks in Their Accretion Stage | journal=The Astrophysical Journal |volume=581 |issue=1 | pages=357–380 | doi=10.1086/344223 }}</ref> As the nebula collapsed, conservation of [[angular momentum]] made it rotate faster. As the material within the nebula [[condensation|condensed]], the [[atom]]s within it began to collide with increasing frequency. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc.<ref name="Arizona"/> As gravity, gas pressure, [[magnetic field]]s, and rotation acted on the contracting nebula, it began to flatten into a spinning [[protoplanetary disc]] with a diameter of roughly 200&nbsp;AU<ref name="Arizona"/> and a hot, dense [[protostar]] at the centre.<ref>{{cite science |last=Greaves |first=Jane S. |date=2005-01-07 |title=Disks Around Stars and the Growth of Planetary Systems |journal=Science |volume=307 |issue=5706 |pages=68–71 |doi=10.1126/science.1101979 |url=http://www.sciencemag.org/cgi/content/full/307/5706/68 |abstract=http://www.sciencemag.org/cgi/content/abstract/sci;307/5706/68
|accessdate=2006-11-16}}</ref><ref>{{cite web |date=2000-04-05 |url=http://www7.nationalacademies.org/ssb/detectionch3.html |title=Present Understanding of the Origin of Planetary Systems |publisher=National Academy of Sciences |accessdate=2007-01-19 }}</ref>


Groups of stars form together in [[star cluster]]s, before dissolving into co-moving associations. A prominent grouping that is visible to the naked eye is the [[Ursa Major moving group]], which is around 80 light-years away within the Local Bubble. The nearest star cluster is [[Hyades (star cluster)|Hyades]], which lies at the edge of the Local Bubble. The closest star-forming regions are the [[Corona Australis Molecular Cloud]], the [[Rho Ophiuchi cloud complex]] and the [[Taurus molecular cloud]]; the latter lies just beyond the Local Bubble and is part of the Radcliffe wave.<ref>{{Cite journal |last1=Alves |first1=João |last2=Zucker |first2=Catherine |last3=Goodman |first3=Alyssa A. |author-link3=Alyssa A. Goodman |last4=Speagle |first4=Joshua S. |last5=Meingast |first5=Stefan |last6=Robitaille |first6=Thomas |last7=Finkbeiner |first7=Douglas P. |last8=Schlafly |first8=Edward F. |last9=Green |first9=Gregory M. |display-authors=3 |year=2020 |title=A Galactic-scale gas wave in the solar neighborhood |journal=[[Nature (journal)|Nature]] |volume=578 |issue=7794 |pages=237–239 |arxiv=2001.08748 |bibcode=2020Natur.578..237A |doi=10.1038/s41586-019-1874-z |pmid=31910431 |s2cid=210086520}}</ref>
Within 50 million years, the pressure and density of [[hydrogen]] in the centre of the collapsing nebula became great enough for the protosun to begin [[nuclear fusion|thermonuclear fusion]].<ref name=Yi2001>{{cite journal | author= Sukyoung Yi; Pierre Demarque; Yong-Cheol Kim; Young-Wook Lee; Chang H. Ree; Thibault Lejeune; Sydney Barnes | title=Toward Better Age Estimates for Stellar Populations: The <math>Y^{2}</math> Isochrones for Solar Mixture | journal=Astrophysical Journal Supplement | id={{arXiv|astro-ph|0104292}} | year=2001 | volume=136 | pages=417 | doi=10.1086/321795 | url=http://adsabs.harvard.edu/abs/2001ApJS..136..417Y}}</ref> The temperature, reaction rate, pressure, and density increased until [[hydrostatic equilibrium]] was achieved, with the thermal energy countering the force of gravitational contraction. At this point the Sun became a full-fledged [[main sequence]] star.<ref>{{cite journal | author=A. Chrysostomou, P. W. Lucas | title=The Formation of Stars | journal=Contemporary Physics | year=2005 | volume=46 | pages=29 | url=http://adsabs.harvard.edu/abs/2005ConPh..46...29C | doi=10.1080/0010751042000275277}}</ref>


Stellar flybys that pass within {{Convert|0.25|pc|ly|1|abbr=off|disp=out}} of the Sun occur roughly once every 100,000&nbsp;years. The [[List of nearest stars and brown dwarfs#Distant future and past encounters|closest well-measured approach]] was [[Scholz's Star]], which approached to ~{{val|50,000|fmt=commas|u=AU}} of the Sun some ~70&nbsp;thousands years ago, likely passing through the outer Oort cloud.<ref>{{Cite journal |last1=Mamajek |first1=Eric E. |last2=Barenfeld |first2=Scott A. |last3=Ivanov |first3=Valentin D. |last4=Kniazev |first4=Alexei Y. |last5=Väisänen |first5=Petri |last6=Beletsky |first6=Yuri |last7=Boffin |first7=Henri M. J. |date=February 2015 |title=The Closest Known Flyby of a Star to the Solar System |journal=[[The Astrophysical Journal Letters]] |volume=800 |issue=1 |page=4 |arxiv=1502.04655 |bibcode=2015ApJ...800L..17M |doi=10.1088/2041-8205/800/1/L17 |s2cid=40618530 |id=L17}}</ref> There is a 1% chance every billion years that a star will pass within {{val|100|u=AU}} of the Sun, potentially disrupting the Solar System.<ref name="Raymond_et_al_2024">{{cite journal |last1=Raymond |first1=Sean N. |last2=Kaib |first2=Nathan A. |last3=Selsis |first3=Franck |last4=Bouy |first4=Herve |display-authors=1 |date=January 2024 |title=Future trajectories of the Solar System: dynamical simulations of stellar encounters within 100 au |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=527 |issue=3 |pages=6126–6138 |arxiv=2311.12171 |bibcode=2024MNRAS.527.6126R |doi=10.1093/mnras/stad3604|doi-access=free }}</ref>
From the remaining cloud of gas and dust (the "[[solar nebula]]"), the various planets formed. They are believed to have formed by [[accretion (astrophysics)|accretion]]: the planets began as dust grains in orbit around the central protostar; then gathered by direct contact into clumps between one and ten metres in diameter; then collided to form larger bodies ([[planetesimal]]s) of roughly 5&nbsp;km in size; then gradually increased by further collisions at roughly 15&nbsp;[[centimetre|cm]] per year over the course of the next few million years.<ref>{{cite web |year=1973 |author=Peter Goldreich and William R. Ward |title=The Formation of Planetesimals |work=The American Astronomical Society|url=http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v589n1/56674/56674.text.html
|accessdate=2006-11-16}}</ref>


== Galactic position ==
The inner Solar System was too warm for volatile [[molecule]]s like [[water]] and [[methane]] to condense, and so the planetesimals which formed there were relatively small (comprising only 0.6% the mass of the disc)<ref name="Arizona" /> and composed largely of [[chemical compound|compounds]] with high [[melting point]]s, such as [[silicate]]s and [[metal]]s. These rocky bodies eventually became the [[terrestrial planet]]s. Farther out, the gravitational effects of Jupiter made it impossible for the protoplanetary objects present to come together, leaving behind the [[asteroid belt]].<ref>{{cite web |year=2001 |author=Jean-Marc Petit and Alessandro Morbidelli |title=The Primordial Excitation and Clearing of the Asteroid Belt |work=Centre National de la Recherche Scientifique, Observatoire de Nice |url=http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf |format=PDF |accessdate=2006-11-19}}</ref>
{{See also|Location of Earth|Galactic year|Orbit of the Sun}}
[[File:Milky Way side view.png|thumb|Diagram of the Milky Way, with galactic features and the relative position of the Solar System labeled.]]


The Solar System is located in the [[Milky Way]], a [[barred spiral galaxy]] with a diameter of about 100,000&nbsp;[[light-year]]s containing more than 100&nbsp;billion stars.<ref name="Lang2013">{{Cite book |last=Lang |first=Kenneth R. |url=https://books.google.com/books?id=MN-UCkUK9pcC&pg=PA264 |title=The Life and Death of Stars |date=2013 |publisher=Cambridge University Press |isbn=978-1107016385 |page=264 |access-date=8 April 2022 |archive-url=https://web.archive.org/web/20220420161220/https://www.google.com/books/edition/The_Life_and_Death_of_Stars/MN-UCkUK9pcC?gbpv=1&pg=PA264 |archive-date=20 April 2022 |url-status=live}}</ref> The Sun is part of one of the Milky Way's outer spiral arms, known as the [[Orion–Cygnus Arm]] or Local Spur.<ref>{{Cite journal |last1=Drimmel |first1=R. |last2=Spergel |first2=D. N. |date=2001 |title=Three Dimensional Structure of the Milky Way Disk |journal=[[The Astrophysical Journal]] |volume=556 |issue=1 |pages=181–202 |arxiv=astro-ph/0101259 |bibcode=2001ApJ...556..181D |doi=10.1086/321556 |s2cid=15757160}}</ref><ref>{{Cite journal |last=Gerhard |first=O. |date=2011 |title=Pattern speeds in the Milky Way |journal=Memorie della Societa Astronomica Italiana, Supplementi |volume=18 |page=185 |arxiv=1003.2489 |bibcode=2011MSAIS..18..185G}}</ref> It is a member of the [[thin disk]] population of stars orbiting close to the galactic plane.<ref>{{cite journal | title=The formation of the Oort cloud in open cluster environments | last1=Kaib | first1=Nathan A. | last2=Quinn | first2=Thomas | journal=Icarus | date=September 2008 | volume=197 | issue=1 | pages=221–238 | doi=10.1016/j.icarus.2008.03.020 | arxiv=0707.4515 | bibcode=2008Icar..197..221K }}</ref>
Farther out still, beyond the [[frost line (astrophysics)|frost line]], where more volatile icy compounds could remain solid, Jupiter and Saturn became the [[gas giant]]s. Uranus and Neptune captured much less material and are known as ice giants because their cores are believed to be made mostly of ices (hydrogen compounds).<ref>{{cite journal |last=Mummma |first=M. J. |coauthors=M. A. DiSanti, N. Dello Russo, K. Magee-Sauer, E. Gibb, and R. Novak |month=June |year=2003 |title=Remote infrared observations of parent volatiles in comets: A window on the early solar system |journal=Advances in Space Research |volume=31 |issue=12 |pages=2563–2575 |doi=10.1016/S0273-1177(03)00578-7 |url=http://www.ifa.hawaii.edu/~meech/a740/papers/mumma03.pdf |format=PDF|accessdate=2006-11-16}}</ref><ref>{{cite web |title=The formation of Uranus and Neptune in the Jupiter–Saturn region of the Solar System |author=Edward W. Thommes, Martin J. Duncan and Harold F. Levison |work=Department of Physics, Queen's University, Kingston, Ontario; Space Studies Department, Southwest Research Institute, Boulder, Colorado |url=http://www.nature.com/nature/journal/v402/n6762/abs/402635a0.htmlyear=1999 |accessdate=2007-04-02}}</ref>


Its speed around the center of the Milky Way is about 220&nbsp;km/s, so that it completes one revolution every 240&nbsp;million years.<ref name="Lang2013"/> This revolution is known as the Solar System's [[galactic year]].<ref>{{Cite web |last=Leong |first=Stacy |date=2002 |title=Period of the Sun's Orbit around the Galaxy (Cosmic Year) |url=http://hypertextbook.com/facts/2002/StacyLeong.shtml |url-status=live |archive-url=https://web.archive.org/web/20190107010909/https://hypertextbook.com/facts/2002/StacyLeong.shtml |archive-date=7 January 2019 |access-date=2 April 2007 |website=The Physics Factbook}}</ref> The [[solar apex]], the direction of the Sun's path through interstellar space, is near the constellation [[Hercules (constellation)|Hercules]] in the direction of the current location of the bright star [[Vega]].<ref>{{Cite book |last=Greiner |first=Walter |url=https://www.worldcat.org/oclc/56727455 |title=Classical Mechanics: Point particles and relativity |date=2004 |publisher=Springer |isbn=978-0-387-21851-9 |location=New York |page=323 |oclc=56727455 |access-date=29 March 2022 |archive-url=https://web.archive.org/web/20220420161222/https://www.worldcat.org/title/classical-mechanics-point-particles-and-relativity/oclc/56727455 |archive-date=20 April 2022 |url-status=live}}</ref> The plane of the ecliptic lies at an angle of about 60° to the [[galactic plane]].{{Refn |group=lower-alpha |name=angle |1=If <math>\psi</math> is the angle between the [[Ecliptic pole|north pole of the ecliptic]] and the north [[galactic pole]] then:
[[Image:redgiantsun.gif|thumb|left|Artist's conception of the future evolution of our Sun. Left: main sequence; middle: red giant; right: white dwarf]]
<br/>{{Big|1=<math>\cos\psi=\cos(\beta_g)\cos(\beta_e)\cos(\alpha_g-\alpha_e)+\sin(\beta_g)\sin(\beta_e)</math>}}<br/>
The Solar System as we know it today will last until the Sun begins its journey off of the main sequence. As the Sun burns through its supply of hydrogen fuel, it gets hotter in order to be able to burn the remaining fuel, and so burns it even faster. As a result, the Sun is growing brighter at a rate of roughly ten percent every 1.1 billion years.<ref>{{cite web|title=Science: Fiery future for planet Earth |author=Jeff Hecht |work=NewScientist |url=http://www.newscientist.com/article/mg14219191.900.html |year=1994 |accessdate=2007-10-29}}</ref>
where <math>\beta_g</math> = 27° 07′ 42.01″ and <math>\alpha_g</math> = 12h 51m 26.282s are the declination and right ascension of the north galactic pole,<ref>{{Cite journal |last1=Reid |first1=M. J. |last2=Brunthaler |first2=A. |date=2004 |title=The Proper Motion of Sagittarius A* |journal=[[The Astrophysical Journal]] |volume=616 |issue=2 |pages=872–884 |arxiv=astro-ph/0408107 |bibcode=2004ApJ...616..872R |doi=10.1086/424960 |s2cid=16568545}}</ref> whereas <math>\beta_e</math> = 66° 33′ 38.6″ and <math>\alpha_e</math> = 18h 0m 00s are those for the north pole of the ecliptic. (Both pairs of coordinates are for [[J2000]] epoch.) The result of the calculation is 60.19°.}}


The Sun follows a nearly circular orbit around the [[Galactic Center]] (where the [[supermassive black hole]] [[Sagittarius A*]] resides) at a distance of 26,660&nbsp;light-years,<ref>{{Cite journal |last1=Abuter |first1=R. |last2=Amorim |first2=A. |last3=Bauböck |first3=M. |last4=Berger |first4=J. P. |last5=Bonnet |first5=H. |last6=Brandner |first6=W. |last7=Clénet |first7=Y. |last8=Coudé du Foresto |first8=V. |last9=de Zeeuw |first9=P. T. |last10=Dexter |first10=J. |display-authors=6 |date=May 2019 |title=A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty |url=https://www.aanda.org/10.1051/0004-6361/201935656 |url-status=live |journal=Astronomy & Astrophysics |volume=625 |page=L10 |arxiv=1904.05721 |bibcode=2019A&A...625L..10G |doi=10.1051/0004-6361/201935656 |issn=0004-6361 |s2cid=119190574 |archive-url=https://web.archive.org/web/20220420161243/https://www.aanda.org/articles/aa/full_html/2019/05/aa35656-19/aa35656-19.html |archive-date=20 April 2022 |access-date=1 April 2022}}</ref> orbiting at roughly the same speed as that of the spiral arms.<ref name="astrobiology">{{Cite web |last=Mullen |first=Leslie |date=18 May 2001 |title=Galactic Habitable Zones |url=http://www.astrobio.net/news-exclusive/galactic-habitable-zones |url-status=live |archive-url=https://web.archive.org/web/20110807024530/http://www.astrobio.net/exclusive/139/galactic-habitable-zones |archive-date=7 August 2011 |access-date=1 June 2020 |website=Astrobiology Magazine}}</ref> If it orbited close to the center, gravitational tugs from nearby stars could perturb bodies in the [[#Oort cloud|Oort cloud]] and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. In this scenario, the intense radiation of the Galactic Center could interfere with the development of complex life.<ref name="astrobiology" />
Around 7.6 billion years from now, the Sun's core will become hot enough to cause hydrogen fusion to occur in its less dense upper layers. This will cause the Sun to expand to roughly up to 260 times its current diameter, and become a [[red giant]].<ref>{{cite journal|author= K. P. Schroder, Robert Cannon Smith|title= Distant future of the Sun and Earth revisited|journal=Monthly Notices of the Royal Astronomical Society | volume=386 |pages=155&ndash;163 | year=2008 |doi=10.1111/j.1365-2966.2008.13022.x |url=http://adsabs.harvard.edu/abs/2008MNRAS.386..155S }}</ref> At this point, the sun will have cooled and dulled, because of its vastly increased surface area.


The Solar System's location in the Milky Way is a factor in the [[evolutionary history of life]] on Earth. Spiral arms are home to a far larger concentration of [[supernova]]e, gravitational instabilities, and radiation that could disrupt the Solar System, but since Earth stays in the Local Spur and therefore does not pass frequently through spiral arms, this has given Earth long periods of stability for life to evolve.<ref name="astrobiology" /> However, according to the controversial [[Shiva hypothesis]], the changing position of the Solar System relative to other parts of the Milky Way could explain periodic [[extinction events]] on Earth.<ref>{{Cite journal |last=Bailer-Jones |first=C. A. L. |date=1 July 2009 |title=The evidence for and against astronomical impacts on climate change and mass extinctions: a review |url=https://ui.adsabs.harvard.edu/abs/2009IJAsB...8..213B |url-status=live |journal=International Journal of Astrobiology |volume=8 |issue=3 |pages=213–219 |arxiv=0905.3919 |bibcode=2009IJAsB...8..213B |doi=10.1017/S147355040999005X |s2cid=2028999 |archive-url=https://web.archive.org/web/20220401231355/https://ui.adsabs.harvard.edu/abs/2009IJAsB...8..213B |archive-date=1 April 2022 |access-date=1 April 2022}}</ref><ref>{{Cite journal |last=Racki |first=Grzegorz |date=December 2012 |title=The Alvarez Impact Theory of Mass Extinction; Limits to its Applicability and the "Great Expectations Syndrome" |url=https://www.app.pan.pl/article/item/app20110058.html |url-status=live |journal=Acta Palaeontologica Polonica |language=en |volume=57 |issue=4 |pages=681–702 |doi=10.4202/app.2011.0058 |issn=0567-7920 |s2cid=54021858 |archive-url=https://web.archive.org/web/20220401214314/https://www.app.pan.pl/article/item/app20110058.html |archive-date=1 April 2022 |access-date=1 April 2022 |doi-access=free |hdl-access=free |hdl=20.500.12128/534}}</ref>
Eventually, the Sun's outer layers will fall away, leaving a [[white dwarf]], an extraordinarily dense object, half its original mass but only the size of the Earth.<ref>{{cite web|author=Pogge, Richard W.|year=1997|url=http://www-astronomy.mps.ohio-state.edu/~pogge/Lectures/vistas97.html|title=The Once & Future Sun|format=lecture notes|work=[http://www-astronomy.mps.ohio-state.edu/Vistas/ New Vistas in Astronomy]|accessdate=2005-12-07}}</ref>


== Discovery and exploration ==
{{Main|Discovery and exploration of the Solar System}}
[[File:Apparent_retrograde_motion_of_Mars_in_2003.gif|thumb|The motion of 'lights' moving across the sky is the basis of the classical definition of planets: wandering stars.]]
Humanity's knowledge of the Solar System has grown incrementally over the centuries. Up to the [[Late Middle Ages]]–[[Renaissance]], astronomers from Europe to India believed Earth to [[Geocentric model|be stationary at the center]] of the universe<ref>{{Cite book |last=Orrell |first=David |url=https://books.google.com/books?id=mNMsa18vTpsC&pg=PA25 |title=Truth Or Beauty: Science and the Quest for Order |date=2012 |publisher=Yale University Press |isbn=978-0300186611 |pages=25–27 |access-date=13 May 2022 |archive-url=https://web.archive.org/web/20220730084322/https://www.google.com/books/edition/Truth_Or_Beauty/mNMsa18vTpsC?gbpv=1&pg=PA25 |archive-date=30 July 2022 |url-status=live}}</ref> and categorically different from the divine or ethereal objects that moved through the sky. Although the [[Ancient Greece|Greek]] philosopher [[Aristarchus of Samos]] had speculated on a [[heliocentric]] reordering of the cosmos, [[Nicolaus Copernicus]] was the first person known to have developed [[Copernican heliocentrism|a mathematically predictive heliocentric system]].<ref>{{Cite magazine |last=Rufus |first=W. C. |date=1923 |title=The astronomical system of Copernicus |magazine=[[Popular Astronomy (US magazine)|Popular Astronomy]] |volume=31 |page=510 |bibcode=1923PA.....31..510R}}</ref><ref>{{Cite book |last=Weinert |first=Friedel |url=https://archive.org/details/copernicusdarwin00wein |title=Copernicus, Darwin, & Freud: revolutions in the history and philosophy of science |date=2009 |publisher=[[Wiley-Blackwell]] |isbn=978-1-4051-8183-9 |page=[https://archive.org/details/copernicusdarwin00wein/page/n29 21] |url-access=limited}}</ref>


Heliocentrism did not triumph immediately over geocentrism, but the work of Copernicus had its champions, notably [[Johannes Kepler]]. Using a heliocentric model that improved upon Copernicus by allowing orbits to be elliptical, and the precise observational data of [[Tycho Brahe]], Kepler produced the ''[[Rudolphine Tables]]'', which enabled accurate computations of the positions of the then-known planets. [[Pierre Gassendi]] used them to predict a [[transit of Mercury]] in 1631, and [[Jeremiah Horrocks]] did the same for a [[transit of Venus]] in 1639. This provided a strong vindication of heliocentrism and Kepler's elliptical orbits.<ref>{{Cite book |last=LoLordo |first=Antonia |url=https://www.worldcat.org/oclc/182818133 |title=Pierre Gassendi and the Birth of Early Modern Philosophy |date=2007 |publisher=Cambridge University Press |isbn=978-0-511-34982-9 |location=New York |pages=12, 27 |oclc=182818133 |access-date=1 April 2022 |archive-url=https://web.archive.org/web/20220420161223/https://www.worldcat.org/title/pierre-gassendi-and-the-birth-of-early-modern-philosophy/oclc/182818133 |archive-date=20 April 2022 |url-status=live}}</ref><ref>{{Cite journal |last1=Athreya |first1=A. |last2=Gingerich |first2=O. |date=December 1996 |title=An Analysis of Kepler's Rudolphine Tables and Implications for the Reception of His Physical Astronomy |journal=Bulletin of the American Astronomical Society |volume=28 |issue=4 |page=1305 |bibcode=1996AAS...189.2404A}}<!--|accessdate=26 December 2013--></ref>

In the 17th century, [[Galileo Galilei|Galileo]] publicized the use of the telescope in astronomy; he and [[Simon Marius]] independently discovered that Jupiter had four satellites in orbit around it.<ref>{{Cite journal |last=Pasachoff |first=Jay M. |date=May 2015 |title=Simon Marius's Mundus Iovialis: 400th Anniversary in Galileo's Shadow |url=http://journals.sagepub.com/doi/10.1177/0021828615585493 |url-status=live |journal=Journal for the History of Astronomy |language=en |volume=46 |issue=2 |pages=218–234 |bibcode=2015JHA....46..218P |doi=10.1177/0021828615585493 |issn=0021-8286 |s2cid=120470649 |archive-url=https://web.archive.org/web/20211127213209/https://journals.sagepub.com/doi/10.1177/0021828615585493 |archive-date=27 November 2021 |access-date=1 April 2022}}</ref> [[Christiaan Huygens]] followed on from these observations by discovering Saturn's moon [[Titan (moon)|Titan]] and the shape of the [[rings of Saturn]].<ref>{{Cite web |date=8 December 2012 |title=Christiaan Huygens: Discoverer of Titan |url=https://www.esa.int/About_Us/ESA_history/Christiaan_Huygens_Discoverer_of_Titan |url-status=live |archive-url=https://web.archive.org/web/20191206001920/http://www.esa.int/About_Us/ESA_history/Christiaan_Huygens_Discoverer_of_Titan |archive-date=6 December 2019 |access-date=27 October 2010 |website=ESA Space Science |publisher=The European Space Agency}}</ref> In 1677, [[Edmond Halley]] observed a transit of Mercury across the Sun, leading him to realize that observations of the [[solar parallax]] of a planet (more ideally using the transit of Venus) could be used to [[Trigonometry|trigonometrically]] determine the distances between Earth, [[Venus]], and the Sun.<ref>{{Cite conference |last=Chapman |first=Allan |date=April 2005 |editor-last=Kurtz |editor-first=D. W. |title=Jeremiah Horrocks, William Crabtree, and the Lancashire observations of the transit of Venus of 1639 |conference=Transits of Venus: New Views of the Solar System and Galaxy, Proceedings of IAU Colloquium #196, held 7–11 June 2004 in Preston, U.K. |publisher=Cambridge University Press |publication-place=Cambridge |volume=2004 |pages=3–26 |bibcode=2005tvnv.conf....3C |doi=10.1017/S1743921305001225 |doi-access=free |journal=Proceedings of the International Astronomical Union}}</ref> Halley's friend [[Isaac Newton]], in his magisterial ''[[Philosophiæ Naturalis Principia Mathematica|Principia Mathematica]]'' of 1687, demonstrated that celestial bodies are not quintessentially different from Earthly ones: the same [[Newton's laws of motion|laws of motion]] and of [[Newton's law of universal gravitation|gravity]] apply on Earth and in the skies.<ref name=":0"/>{{Rp|page=142}}
[[File:The Solar System, with the orbits of 5 remarkable comets. LOC 2013593161 (cropped).jpg|thumb|Solar System diagram made by [[Emanuel Bowen]] in 1747. At that time, Uranus, Neptune, nor the asteroid belts have been discovered yet. Orbits of planets are drawn to scale, but the orbits of moons and the size of bodies are not.]]
The term "Solar System" entered the English language by 1704, when [[John Locke]] used it to refer to the Sun, planets, and comets.<ref>See, for example:

*{{Cite web |title=solar |url=https://www.etymonline.com/word/solar |url-status=live |archive-url=https://web.archive.org/web/20220318002833/https://www.etymonline.com/word/solar |archive-date=18 March 2022 |access-date=17 March 2022 |website=[[Online Etymology Dictionary]]}}
*{{Cite OED|solar system}}
*{{Cite book |last=Locke |first=John |url=https://books.google.com/books?id=Ni9bAAAAcAAJ |title=Elements of Natural Philosophy ... To which are added. Some Thoughts concerning Reading and Study for a Gentleman. By the same author. With prefatory remarks by P. Des Maizeaux |date=1754 |publisher=R. Taylor |page=8 |language=en |author-link=John Locke |orig-date=1720 |access-date=18 March 2022 |archive-date=18 March 2022 |archive-url=https://web.archive.org/web/20220318005707/https://books.google.com/books?id=Ni9bAAAAcAAJ&newbks=0 |url-status=live }} Posthumous publication.</ref> In 1705, Halley realized that repeated sightings of [[Halley's Comet|a comet]] were of the same object, returning regularly once every 75–76&nbsp;years. This was the first evidence that anything other than the planets repeatedly orbited the Sun,<ref>{{Cite book |last1=Festou |first1=M. C. |title=Comets II |last2=Keller |first2=H. U. |last3=Weaver |first3=H. A. |date=2004 |publisher=University of Arizona Press |isbn=978-0816524501 |publication-place=Tucson |pages=3–16 |chapter=A brief conceptual history of cometary science |bibcode=2004come.book....3F |access-date=7 April 2022 |chapter-url=https://books.google.com/books?id=ehA8EAAAQBAJ&pg=PA4 |archive-url=https://web.archive.org/web/20220420161222/https://www.google.com/books/edition/Comets_II/ehA8EAAAQBAJ?gbpv=1&pg=PA4 |archive-date=20 April 2022 |url-status=live}}</ref> though [[Seneca the Younger|Seneca]] had theorized this about comets in the 1st century.<ref>{{Cite book |last1=Sagan |first1=Carl |url=https://books.google.com/books?id=LhkoowKFaTsC |title=Comet |last2=Druyan |first2=Ann |publisher=Random House |year=1997 |isbn=978-0-3078-0105-0 |location=New York |pages=26–27, 37–38 |author-link=Carl Sagan |author-link2=Ann Druyan |access-date=28 June 2021 |archive-url=https://web.archive.org/web/20210615020250/https://books.google.com/books?id=LhkoowKFaTsC |archive-date=15 June 2021 |url-status=live}}</ref> Careful observations of the 1769 transit of Venus allowed astronomers to calculate the average Earth–Sun distance as {{Convert|93726900|mi|km}}, only 0.8% greater than the modern value.<ref>{{Cite journal |last=Teets |first=Donald |date=December 2003 |title=Transits of Venus and the Astronomical Unit |url=http://www.maa.org/sites/default/files/pdf/pubs/mm_dec03-Venus.pdf |url-status=live |journal=Mathematics Magazine |volume=76 |pages=335–348 |doi=10.1080/0025570X.2003.11953207 |jstor=3654879 |s2cid=54867823 |archive-url=https://web.archive.org/web/20220203080207/https://www.maa.org/sites/default/files/pdf/pubs/mm_dec03-Venus.pdf |archive-date=3 February 2022 |access-date=3 April 2022 |number=5}}</ref>

[[Uranus]], having occasionally been observed since 1690 and possibly from antiquity, was recognized to be a planet orbiting beyond Saturn by 1783.<ref>{{Cite journal |last=Bourtembourg |first=René |date=2013 |title=Was Uranus Observed by Hipparchos? |journal=Journal for the History of Astronomy |volume=44 |issue=4 |pages=377–387 |bibcode=2013JHA....44..377B |doi=10.1177/002182861304400401 |s2cid=122482074}}</ref> In 1838, [[Friedrich Bessel]] successfully measured a [[stellar parallax]], an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism.<ref>{{Cite book |last=Di Bari |first=Pasquale |url=https://books.google.com/books?id=hPm7DwAAQBAJ&pg=PA4 |title=Cosmology and the Early Universe |date=2018 |publisher=CRC Press |isbn=978-1351020138 |pages=3–4}}</ref> [[Neptune]] was identified as a planet some years later, in 1846, thanks to its gravitational pull causing a slight but detectable variation in the orbit of Uranus.<ref>{{Cite journal |last1=Bhatnagar |first1=Siddharth |last2=Vyasanakere |first2=Jayanth P. |last3=Murthy |first3=Jayant |date=May 2021 |title=A geometric method to locate Neptune |url=https://aapt.scitation.org/doi/10.1119/10.0003349 |url-status=live |journal=American Journal of Physics |language=en |volume=89 |issue=5 |pages=454–458 |arxiv=2102.04248 |bibcode=2021AmJPh..89..454B |doi=10.1119/10.0003349 |issn=0002-9505 |s2cid=231846880 |archive-url=https://web.archive.org/web/20211129125826/https://aapt.scitation.org/doi/10.1119/10.0003349 |archive-date=29 November 2021 |access-date=1 April 2022}}</ref> [[Perihelion precession of Mercury|Mercury's orbital anomaly]] observations led to searches for [[Vulcan (hypothetical planet)|Vulcan]], a planet interior of Mercury, but these attempts were quashed with [[Albert Einstein]]'s theory of [[general relativity]] in 1915.<ref name="Clemence">{{cite journal |last=Clemence |first=G. M. |date=1947 |title=The Relativity Effect in Planetary Motions |journal=Reviews of Modern Physics |volume=19 |issue=4 |pages=361–364 |bibcode=1947RvMP...19..361C |doi=10.1103/RevModPhys.19.361}} [http://www.mathpages.com/rr/s6-02/6-02.htm (math)]</ref>

In the 20th century, humans began their space exploration around the Solar System, starting with placing [[Space telescope|telescopes in space]] since the 1960s.<ref>{{Cite web |last=Garner |first=Rob |date=10 December 2018 |title=50th Anniversary of OAO 2: NASA's 1st Successful Stellar Observatory |url=http://www.nasa.gov/feature/goddard/2018/nasa-s-first-stellar-observatory-oao-2-turns-50 |url-status=live |archive-url=https://web.archive.org/web/20211229231948/https://www.nasa.gov/feature/goddard/2018/nasa-s-first-stellar-observatory-oao-2-turns-50 |archive-date=29 December 2021 |access-date=20 April 2022 |website=NASA}}</ref> By 1989, all eight planets have been visited by space probes.<ref name="FactSheet">{{cite web |title=Fact Sheet |url=https://voyager.jpl.nasa.gov/news/factsheet.html |url-status=live |archive-url=https://web.archive.org/web/20161129230752/http://voyager.jpl.nasa.gov/news/factsheet.html |archive-date=29 November 2016 |access-date=3 March 2016 |publisher=JPL}}</ref> Probes have returned samples from comets<ref>{{Cite magazine |last=Woo |first=Marcus |date=20 November 2014 |title=This Is What It Sounded Like When We Landed on a Comet |url=https://www.wired.com/2014/11/sounded-like-landed-comet |archive-url=https://web.archive.org/web/20141123021050/https://www.wired.com/2014/11/sounded-like-landed-comet |archive-date=23 November 2014 |access-date=20 April 2022 |magazine=Wired}}</ref> and asteroids,<ref>{{Cite web |last=Marks |first=Paul |date=3 December 2014 |title=Hayabusa 2 probe begins journey to land on an asteroid |url=https://www.newscientist.com/article/dn26650-hayabusa-2-probe-begins-journey-to-land-on-an-asteroid |url-status=live |archive-url=https://web.archive.org/web/20220211062123/https://www.newscientist.com/article/dn26650-hayabusa-2-probe-begins-journey-to-land-on-an-asteroid |archive-date=11 February 2022 |access-date=20 April 2022 |website=New Scientist |language=en-US}}</ref> as well as flown through the [[Sun's corona]]<ref>{{Cite web |date=14 December 2021 |title=NASA's Parker Solar Probe becomes first spacecraft to 'touch' the sun |url=https://www.cnn.com/2021/12/14/world/nasa-parker-solar-probe-sun-scn/index.html |url-status=live |archive-url=https://web.archive.org/web/20211214235239/https://www.cnn.com/2021/12/14/world/nasa-parker-solar-probe-sun-scn/index.html |archive-date=14 December 2021 |access-date=15 December 2021 |website=CNN}}</ref> and visited two dwarf planets ([[Pluto]] and [[Ceres (dwarf planet)|Ceres]]).<ref>{{Cite news |last1=Corum |first1=Jonathan |last2=Gröndahl |first2=Mika |last3=Parshina-Kottas |first3=Yuliya |date=13 July 2015 |title=New Horizons' Pluto Flyby |url=https://www.nytimes.com/interactive/2015/07/14/science/space/pluto-flyby.html,%20https://www.nytimes.com/interactive/2015/07/14/science/space/pluto-flyby.html |access-date=20 April 2022 |work=The New York Times |language=en-US |issn=0362-4331}}</ref><ref name="NASA-20180907">{{cite web |last1=McCartney |first1=Gretchen |last2=Brown |first2=Dwayne |last3=Wendel |first3=JoAnna |date=7 September 2018 |title=Legacy of NASA's Dawn, Near the End of its Mission |url=https://www.jpl.nasa.gov/news/news.php?feature=7231 |access-date=8 September 2018 |work=[[NASA]]}}</ref> To save on fuel, some space missions make use of [[Gravity assist|gravity assist maneuvers]], such as the two [[Voyager program|''Voyager'' probes]] accelerating when flyby planets in the outer Solar System<ref name=":7">{{Cite web |title=Basics of Spaceflight: A Gravity Assist Primer |url=https://science.nasa.gov/learn/basics-of-space-flight/primer/ |access-date=2 May 2024 |website=science.nasa.gov |language=en-US}}</ref> and the [[Parker Solar Probe]] decelerating closer towards the Sun after flyby with Venus.<ref>{{Cite web |date=4 October 2018 |title=Parker Solar Probe Changed the Game Before it Even Launched - NASA |url=https://www.nasa.gov/solar-system/parker-solar-probe-changed-the-game-before-it-even-launched/ |access-date=2 May 2024 |language=en-US}}</ref>

Humans have landed on the Moon during the [[Apollo program]] in the 1960s and 1970s<ref>{{Cite book |url=https://archive.org/details/guinnessworldrec00vari/page/13 |title=Guinness World Records 2010 |publisher=[[Bantam Books]] |year=2010 |isbn=978-0-553-59337-2 |editor-last=Glenday |editor-first=Craig |editor-link=Craig Glenday |location=New York}}</ref> and will return to the Moon in the 2020s with the [[Artemis program]].<ref name="sn-20230313">{{cite web |last=Foust |first=Jeff |date=13 March 2023 |title=NASA planning to spend up to $1 billion on space station deorbit module |url=https://spacenews.com/nasa-planning-to-spend-up-to-1-billion-on-space-station-deorbit-module/ |access-date=13 March 2023 |work=[[SpaceNews]]}}</ref> Discoveries in the 20th and 21st century has prompted the [[Definition of planet|redefinition of the term ''planet'']] in 2006, hence the demotion of Pluto to a dwarf planet,<ref name="NYT-20220118">{{cite news |last=Chang |first=Kenneth |date=18 January 2022 |title=Quiz - Is Pluto A Planet? - Who doesn't love Pluto? It shares a name with the Roman god of the underworld and a Disney dog. But is it a planet? - Interactive |url=https://www.nytimes.com/interactive/2022/science/is-pluto-a-planet.html |accessdate=18 January 2022 |work=[[The New York Times]]}}</ref> and further interest in [[trans-Neptunian object]]s.<ref name=":02">{{cite web |last=Spaceflight |first=Leonard David |date=9 January 2019 |title=A Wild 'Interstellar Probe' Mission Idea Is Gaining Momentum |url=https://www.space.com/42935-nasa-interstellar-probe-mission-idea.html |access-date=23 September 2019 |website=Space.com |language=en}}</ref>


== See also ==
== See also ==
{{portal|Solar System|Solar system.jpg}}
{{Portal|Solar system|Outer space|Astronomy
}}
<div style="-moz-column-count:2; column-count:2;">
* [[Astronomical symbols]]
* [[Interplanetary spaceflight]]
* [[Attributes of the largest solar system bodies]]
* [[Lists of geological features of the Solar System]]
* [[List of gravitationally rounded objects of the Solar System]]
* [[Celestia]] – 3D computer space-simulation program using OpenGL
* [[List of Solar System extremes]]
* [[Family Portrait (Voyager)]]
* [[List of Solar System objects by size]]
* [[Geological features of the solar system]]
* List of Solar System objects:
* [[Outline of the Solar System]]
* {{annotated link|Planetary mnemonic}}
**[[List of Solar System objects by mass|by mass]]
**[[List of named Solar System objects|by name]]
**[[List of Solar System objects|by orbit]]
**[[List of Solar System objects by radius|by radius]]
**[[List of Solar System objects by surface gravity|by surface gravity]]
* [[Numerical model of solar system]]
* [[Solar System in fiction]]
* [[Solar system model]]
* [[Space colonization]]
* [[Table of planets and dwarf planets in the Solar System]]
* [[The Parable of the Solar System Model]]
* [[Timeline of discovery of Solar System planets and their moons]]
</div>


== Notes ==
== Notes ==
{{Reflist|group=lower-alpha}}
<div class="references-small">
<ol type="a">
<li>{{Note_label|A|a|none}}[[Capitalization]] of the name varies. The [[International Astronomical Union|IAU]], the authoritative body regarding astronomical nomenclature, specifies [http://www.iau.org/SPELLING_OF_NAMES.240.0.html capitalizing the names of all individual astronomical objects] ('''Solar System'''). However, the name is commonly rendered in lower case ('''solar system''') including in the ''[[Oxford English Dictionary]]'', [http://www.m-w.com/dictionary/solar%20system ''Merriam-Webster's 11th Collegiate Dictionary''], and [http://www.britannica.com/eb/article-9110143 ''Encyclopædia Britannica''].</li>


== References ==
<li>{{Note_label|B|b|none}}The mass of the Solar System excluding the Sun, Jupiter and Saturn can be determined by adding together all the calculated masses for its largest objects and using rough calculations for the masses of the Oort cloud (estimated at roughly 3 Earth masses),<ref>{{cite web|title=ORIGIN AND DYNAMICAL EVOLUTION OF COMETS AND THEIR RESERVOIRS|author=Alessandro Morbidelli|work=CNRS, Observatoire de la Côte d’Azur|year=2006|url=http://arxiv.org/abs/astro-ph/0512256v1|accessdate=2007-08-03}}</ref> the Kuiper Belt (estimated at roughly 0.1 Earth mass)<ref name="Delsanti-Beyond_The_Planets"/> and the asteroid belt (estimated to be 0.0005 Earth mass)<ref name="Krasinsky2002" /> for a total, rounded upwards, of ~37 Earth masses, or 8.1 percent the mass in orbit around the Sun.</li>


=== Data sources ===<!-- Remind that info cited to these sources need to be updated monthly -->
<li>{{note_label|C|c|none}}Astronomers usually measure distances within the Solar System in [[astronomical unit]]s (AU). One AU is approximately the average distance between the Earth and the Sun, or {{convert|149598000|km|mi|abbr=on}}. Pluto is roughly 38&nbsp;AU from the Sun, while Jupiter lies at roughly 5.2&nbsp;AU. One [[light-year]] is 63,240&nbsp;AU.</li></ol></div>
{{Reflist|group=D}}


== References ==
=== Other sources ===
{{reflist|3}}
{{Reflist}}


== External links ==
== External links ==
{{Spoken Wikipedia|date=31 May 2021|En-Solar System-article.ogg}}
{{Commonscat|Solar system}}
{{Library resources box
{{sisterlinks|Solar system}}
|by=no
*[http://solarsystem.nasa.gov/planets/profile.cfm?Object=SolarSys&Display=Overview Solar System Profile] by [http://solarsystem.nasa.gov/index.cfm NASA's Solar System Exploration]
|onlinebooksabout=yes
*[http://space.jpl.nasa.gov NASA's Solar System Simulator]
|others=
*[http://www.jpl.nasa.gov/solar_system NASA/JPL Solar System main page]
|about=yes
*[http://www.nineplanets.org/ The Nine Planets – Comprehensive Solar System site by Bill Arnett]
|label=Solar System
*[http://www.space.com/solarsystem/ SPACE.com: All About the Solar System]
|viaf= |lccn= |lcheading=Solar system |wikititle=
*[http://www.classzone.com/books/earth_science/terc/content/visualizations/es2701/es2701page01.cfm?chapter_no=27 Illustration of the distance between planets]
}}
*[http://www.co-intelligence.org/newsletter/comparisons.html Illustration comparing the sizes of the planets with each other, the sun, and other stars]
* {{Cite EB1911|wstitle=Solar System|volume=25 |pages=157–158 |short=x}}
* [http://www.fourmilab.ch/cgi-bin/uncgi/Solar/ Solar System Live] (an interactive [[orrery]])
* [http://www.joshworth.com/a-tediously-accurate-map-of-the-solar-system/ If the Moon were only 1 Pixel: A Tediously Accurate Map of the Solar System (web based scroll map scaled to the Moon being 1 pixel)]
* [http://janus.astro.umd.edu/javadir/orbits/ssv.html Solar System Viewer] (animation)
* [https://eyes.nasa.gov/apps/solar-system NASA's Eyes on the Solar System]
* [http://www.psrd.hawaii.edu/ Planetary Science Research Discoveries] Educational journal with articles about Solar System bodies
* [https://solarsystem.nasa.gov/ NASA's Solar System Exploration]
* [http://space.jpl.nasa.gov NASA's Solar System Simulator]


{{Sister bar|Solar System|d=Q544|n=Category:Space|s=Category:Planetary astronomy}}

{{Solar system table}}
{{Solar system|state=expanded}}
{{Earth's location}}
{{Earth's location}}
{{Astronomy navbox}}
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{{featured article}}
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[[Category:Solar System| ]]
[[Category:Solar System| ]]
[[Category:Planetary science]]
[[Category:Planetary science]]
[[Category:Space science]]
[[Category:Space science]]
[[Category:Planetary systems with eight confirmed planets]]

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[[af:Sonnestelsel]]
[[als:Sonnensystem]]
[[ar:نظام شمسي]]
[[frp:Sistèmo solèro]]
[[ast:Sistema solar]]
[[az:Günəş sistemi]]
[[be:Сонечная сістэма]]
[[be-x-old:Сонечная сыстэма]]
[[bs:Sunčev sistem]]
[[br:Koskoriad an Heol]]
[[bg:Слънчева система]]
[[ca:Sistema solar]]
[[cv:Хĕвел системи]]
[[cs:Sluneční soustava]]
[[cy:Cysawd yr Haul]]
[[da:Solsystem]]
[[de:Sonnensystem]]
[[dv:އިރަވީ ނިޒާމު]]
[[et:Päikesesüsteem]]
[[el:Ηλιακό σύστημα]]
[[es:Sistema Solar]]
[[eo:Sunsistemo]]
[[eu:Eguzki-sistema]]
[[fa:سامانه خورشیدی]]
[[fi:Aurinkokunta]]
[[fo:Sólskipanin]]
[[fr:Système solaire]]
[[fy:Sinnestelsel]]
[[fur:Sisteme solâr]]
[[ga:Grianchóras]]
[[gan:太陽系]]
[[gv:Corys ny Greiney]]
[[gd:An Crios-Grèine]]
[[gl:Sistema Solar]]
[[gu:સૂર્યમંડળ]]
[[haw:Kahua o nā hōkū]]
[[hy:Արեգակնային համակարգ]]
[[hi:सौर मंडल]]
[[he:מערכת השמש]]
[[ht:Sistèm solè]]
[[hr:Sunčev sustav]]
[[io:Sunala sistemo]]
[[ilo:Sistema a Solar]]
[[id:Tata surya]]
[[ia:Systema solar]]
[[ie:Solari sistema]]
[[is:Sólkerfið]]
[[it:Sistema solare]]
[[ja:太陽系]]
[[jv:Tata surya]]
[[ka:მზის სისტემა]]
[[km:ប្រព័ន្ធព្រះអាទិត្យ]]
[[ko:태양계]]
[[kn:ಜ್ಯೋತಿರ್ಮಂಡಲ]]
[[ku:Pergala rojê]]
[[la:Systema solare]]
[[lv:Saules sistēma]]
[[lb:Sonnesystem]]
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[[hu:Naprendszer]]
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[[ml:സൗരയൂഥം]]
[[mt:Sistema Solari]]
[[mr:सूर्यमाला]]
[[ms:Sistem Suria]]
[[mn:Нарны аймаг]]
[[my:Solar System]]
[[nah:Tōnatiuh Īchān]]
[[nl:Zonnestelsel]]
[[nds-nl:Zunnesysteem]]
[[ne:सौर्यमण्डल]]
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[[nds:Sünnsystem]]
[[pl:Układ Słoneczny]]
[[pt:Sistema Solar]]
[[ksh:Sunnesystem]]
[[ro:Sistem solar]]
[[qu:Inti Llika]]
[[ru:Солнечная система]]
[[se:Beaivvášgoddi]]
[[sco:Solar seestem]]
[[sq:Sistemi diellor]]
[[scn:Sistema sulari]]
[[si:සෞරග්‍රහ මණ්ඩලය]]
[[simple:Solar system]]
[[sk:Slnečná sústava]]
[[sl:Osončje]]
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[[sh:Sunčev sistem]]
[[sw:Mfumo wa jua na sayari zake]]
[[sv:Solsystemet]]
[[tl:Sistemang Pang-araw]]
[[ta:சூரியக் குடும்பம்]]
[[te:సౌరమండలము]]
[[th:ระบบสุริยะ]]
[[vi:Hệ Mặt Trời]]
[[tg:Системаи офтобӣ]]
[[tr:Güneş Sistemi]]
[[uk:Сонячна система]]
[[ur:نظام شمسی]]
[[uz:Quyosh tizimi]]
[[vec:Sistema sołare]]
[[wa:Sistinme solrece]]
[[wo:Nosteeg jant]]
[[yi:זון סיסטעם]]
[[yo:Ètò òòrùn]]
[[zh-yue:太陽系]]
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[[zh-min-nan:Thài-iông-hē]]

Latest revision as of 14:23, 14 December 2024

Solar System
The Sun, planets, moons and dwarf planets[a]
(true color, size to scale, distances not to scale)
Age4.568 billion years[b]
Location
Nearest star
Population
StarsSun
Planets
Known dwarf planets
Known natural satellites758[D 3]
Known minor planets1,368,528[D 4]
Known comets4,591[D 4]
Planetary system
Star spectral typeG2V
Frost line~5 AU[5]
Semi-major axis of outermost planet30.07 AU[D 5] (Neptune)
Kuiper cliff50–70 AU[3][4]
Heliopausedetected at 120 AU[6]
Hill sphere1.1 pc (230,000 AU)[7] – 0.865 pc (178,419 AU)[8]
Orbit about Galactic Center
Invariable-to-galactic plane inclination~60°, to the ecliptic[c]
Distance to
Galactic Center
24,000–28,000 ly
[9]
Orbital speed
720,000 km/h (450,000 mi/h)[10]
Orbital period~230 million years[10]

The Solar System[d] is the gravitationally bound system of the Sun and the objects that orbit it.[11] It formed about 4.6 billion years ago when a dense region of a molecular cloud collapsed, forming the Sun and a protoplanetary disc. The Sun is a typical star that maintains a balanced equilibrium by the fusion of hydrogen into helium at its core, releasing this energy from its outer photosphere. Astronomers classify it as a G-type main-sequence star.

The largest objects that orbit the Sun are the eight planets. In order from the Sun, they are four terrestrial planets (Mercury, Venus, Earth and Mars); two gas giants (Jupiter and Saturn); and two ice giants (Uranus and Neptune). All terrestrial planets have solid surfaces. Inversely, all giant planets do not have a definite surface, as they are mainly composed of gases and liquids. Over 99.86% of the Solar System's mass is in the Sun and nearly 90% of the remaining mass is in Jupiter and Saturn.

There is a strong consensus among astronomers[e] that the Solar System has at least nine dwarf planets: Ceres, Orcus, Pluto, Haumea, Quaoar, Makemake, Gonggong, Eris, and Sedna. There are a vast number of small Solar System bodies, such as asteroids, comets, centaurs, meteoroids, and interplanetary dust clouds. Some of these bodies are in the asteroid belt (between Mars's and Jupiter's orbit) and the Kuiper belt (just outside Neptune's orbit).[f] Six planets, seven dwarf planets, and other bodies have orbiting natural satellites, which are commonly called 'moons'.

The Solar System is constantly flooded by the Sun's charged particles, the solar wind, forming the heliosphere. Around 75–90 astronomical units from the Sun,[g] the solar wind is halted, resulting in the heliopause. This is the boundary of the Solar System to interstellar space. The outermost region of the Solar System is the theorized Oort cloud, the source for long-period comets, extending to a radius of 2,000–200,000 AU. The closest star to the Solar System, Proxima Centauri, is 4.25 light-years (269,000 AU) away. Both stars belong to the Milky Way galaxy.

Formation and evolution

Past

Diagram of the early Solar System's protoplanetary disk, out of which Earth and other Solar System bodies formed

The Solar System formed at least 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud.[b] This initial cloud was likely several light-years across and probably birthed several stars.[14] As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elements fused by previous generations of stars.[15]

As the pre-solar nebula[15] collapsed, conservation of angular momentum caused it to rotate faster. The center, where most of the mass collected, became increasingly hotter than the surroundings.[14] As the contracting nebula spun faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU[14][16] and a hot, dense protostar at the center.[17][18] The planets formed by accretion from this disc,[19] in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies. Hundreds of protoplanets may have existed in the early Solar System, but they either merged or were destroyed or ejected, leaving the planets, dwarf planets, and leftover minor bodies.[20][21]

Due to their higher boiling points, only metals and silicates could exist in solid form in the warm inner Solar System close to the Sun (within the frost line). They eventually formed the rocky planets of Mercury, Venus, Earth, and Mars. Because these refractory materials only comprised a small fraction of the solar nebula, the terrestrial planets could not grow very large.[20]

The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid. The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium, the lightest and most abundant elements.[20] Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud.[20]

Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion.[22] As helium accumulates at its core, the Sun is growing brighter;[23] early in its main-sequence life its brightness was 70% that of what it is today.[24] The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved: the thermal pressure counterbalancing the force of gravity. At this point, the Sun became a main-sequence star.[25] Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space.[23]

Following the dissipation of the protoplanetary disk, the Nice model proposes that gravitational encounters between planetisimals and the gas giants caused each to migrate into different orbits. This led to dynamical instability of the entire system, which scattered the planetisimals and ultimately placed the gas giants in their current positions. During this period, the grand tack hypothesis suggests that a final inward migration of Jupiter dispersed much of the asteroid belt, leading to the Late Heavy Bombardment of the inner planets.[26][27]

Present and future

The Solar System remains in a relatively stable, slowly evolving state by following isolated, gravitationally bound orbits around the Sun.[28] Although the Solar System has been fairly stable for billions of years, it is technically chaotic, and may eventually be disrupted. There is a small chance that another star will pass through the Solar System in the next few billion years. Although this could destabilize the system and eventually lead millions of years later to expulsion of planets, collisions of planets, or planets hitting the Sun, it would most likely leave the Solar System much as it is today.[29]

The current Sun compared to its peak size in the red-giant phase

The Sun's main-sequence phase, from beginning to end, will last about 10 billion years for the Sun compared to around two billion years for all other subsequent phases of the Sun's pre-remnant life combined.[30] The Solar System will remain roughly as it is known today until the hydrogen in the core of the Sun has been entirely converted to helium, which will occur roughly 5 billion years from now. This will mark the end of the Sun's main-sequence life. At that time, the core of the Sun will contract with hydrogen fusion occurring along a shell surrounding the inert helium, and the energy output will be greater than at present. The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a red giant. Because of its increased surface area, the surface of the Sun will be cooler (2,600 K (4,220 °F) at its coolest) than it is on the main sequence.[30]

The expanding Sun is expected to vaporize Mercury as well as Venus, and render Earth and Mars uninhabitable (possibly destroying Earth as well).[31][32] Eventually, the core will be hot enough for helium fusion; the Sun will burn helium for a fraction of the time it burned hydrogen in the core. The Sun is not massive enough to commence the fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will be ejected into space, leaving behind a dense white dwarf, half the original mass of the Sun but only the size of Earth.[30] The ejected outer layers may form a planetary nebula, returning some of the material that formed the Sun—but now enriched with heavier elements like carbon—to the interstellar medium.[33][34]

General characteristics

Astronomers sometimes divide the Solar System structure into separate regions. The inner Solar System includes Mercury, Venus, Earth, Mars, and the bodies in the asteroid belt. The outer Solar System includes Jupiter, Saturn, Uranus, Neptune, and the bodies in the Kuiper belt.[35] Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.[36]

Composition

The principal component of the Solar System is the Sun, a G-type main-sequence star that contains 99.86% of the system's known mass and dominates it gravitationally.[37] The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, asteroids, and comets) together comprise less than 0.002% of the Solar System's total mass.[h]

The Sun is composed of roughly 98% hydrogen and helium,[41] as are Jupiter and Saturn.[42][43] A composition gradient exists in the Solar System, created by heat and light pressure from the early Sun; those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points. Objects farther from the Sun are composed largely of materials with lower melting points.[44] The boundary in the Solar System beyond which those volatile substances could coalesce is known as the frost line, and it lies at roughly five times the Earth's distance from the Sun.[5]

Orbits

Animations of the Solar System's inner planets orbiting. Each frame represents 2 days of motion.
Animations of the Solar System's outer planets orbiting. This animation is 100 times faster than the inner planet animation.

The planets and other large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic. Smaller icy objects such as comets frequently orbit at significantly greater angles to this plane.[45][46] Most of the planets in the Solar System have secondary systems of their own, being orbited by natural satellites called moons. All of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent. The four giant planets have planetary rings, thin discs of tiny particles that orbit them in unison.[47]

As a result of the formation of the Solar System, planets and most other objects orbit the Sun in the same direction that the Sun is rotating. That is, counter-clockwise, as viewed from above Earth's north pole.[48] There are exceptions, such as Halley's Comet.[49] Most of the larger moons orbit their planets in prograde direction, matching the direction of planetary rotation; Neptune's moon Triton is the largest to orbit in the opposite, retrograde manner.[50] Most larger objects rotate around their own axes in the prograde direction relative to their orbit, though the rotation of Venus is retrograde.[51]

To a good first approximation, Kepler's laws of planetary motion describe the orbits of objects around the Sun.[52]: 433–437  These laws stipulate that each object travels along an ellipse with the Sun at one focus, which causes the body's distance from the Sun to vary over the course of its year. A body's closest approach to the Sun is called its perihelion, whereas its most distant point from the Sun is called its aphelion.[53]: 9-6  With the exception of Mercury, the orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits. Kepler's laws only account for the influence of the Sun's gravity upon an orbiting body, not the gravitational pulls of different bodies upon each other. On a human time scale, these perturbations can be accounted for using numerical models,[53]: 9-6  but the planetary system can change chaotically over billions of years.[54]

The angular momentum of the Solar System is a measure of the total amount of orbital and rotational momentum possessed by all its moving components.[55] Although the Sun dominates the system by mass, it accounts for only about 2% of the angular momentum.[56][57] The planets, dominated by Jupiter, account for most of the rest of the angular momentum due to the combination of their mass, orbit, and distance from the Sun, with a possibly significant contribution from comets.[56]

Distances and scales

To-scale diagram of distance between planets, with the white bar showing orbital variations. The size of the planets is not to scale.

The radius of the Sun is 0.0047 AU (700,000 km; 400,000 mi).[58] Thus, the Sun occupies 0.00001% (1 part in 107) of the volume of a sphere with a radius the size of Earth's orbit, whereas Earth's volume is roughly 1 millionth (10−6) that of the Sun. Jupiter, the largest planet, is 5.2 AU from the Sun and has a radius of 71,000 km (0.00047 AU; 44,000 mi), whereas the most distant planet, Neptune, is 30 AU from the Sun.[43][59]

With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and the orbit of the next nearest object to the Sun. For example, Venus is approximately 0.33 AU farther out from the Sun than Mercury, whereas Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances, like the Titius–Bode law[60] and Johannes Kepler's model based on the Platonic solids,[61] but ongoing discoveries have invalidated these hypotheses.[62]

Some Solar System models attempt to convey the relative scales involved in the Solar System in human terms. Some are small in scale (and may be mechanical—called orreries)—whereas others extend across cities or regional areas.[63] The largest such scale model, the Sweden Solar System, uses the 110-meter (361-foot) Avicii Arena in Stockholm as its substitute Sun, and, following the scale, Jupiter is a 7.5-meter (25-foot) sphere at Stockholm Arlanda Airport, 40 km (25 mi) away, whereas the farthest current object, Sedna, is a 10 cm (4 in) sphere in Luleå, 912 km (567 mi) away.[64][65] At that scale, the distance to Proxima Centauri would be roughly 8 times further than the Moon is from Earth.

If the Sun–Neptune distance is scaled to 100 metres (330 ft), then the Sun would be about 3 cm (1.2 in) in diameter (roughly two-thirds the diameter of a golf ball), the giant planets would be all smaller than about 3 mm (0.12 in), and Earth's diameter along with that of the other terrestrial planets would be smaller than a flea (0.3 mm or 0.012 in) at this scale.[66]

Habitability

Comparison of the habitable zones of the Solar System and TRAPPIST-1, an ultracool red dwarf star known to have seven terrestrial planets in stable orbits around the star.
Comparison of the habitable zones for different stellar temperatures, with a sample of known exoplanets plus the Earth, Mars, and Venus. From top to bottom are an F-type main-sequence star, a yellow dwarf (G-type main-sequence star), an orange dwarf (K-type main-sequence star), a typical red dwarf, and an ultra-cool dwarf.

Besides solar energy, the primary characteristic of the Solar System enabling the presence of life is the heliosphere and planetary magnetic fields (for those planets that have them). These magnetic fields partially shield the Solar System from high-energy interstellar particles called cosmic rays. The density of cosmic rays in the interstellar medium and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic-ray penetration in the Solar System varies, though by how much is unknown.[67]

The zone of habitability of the Solar System is conventionally located in the inner Solar System, where planetary surface or atmospheric temperatures admit the possibility of liquid water.[68] Habitability might be possible in subsurface oceans of various outer Solar System moons.[69]

Comparison with extrasolar systems

Compared to many extrasolar systems, the Solar System stands out in lacking planets interior to the orbit of Mercury.[70][71] The known Solar System lacks super-Earths, planets between one and ten times as massive as the Earth,[70] although the hypothetical Planet Nine, if it does exist, could be a super-Earth orbiting in the edge of the Solar System.[72]

Uncommonly, it has only small terrestrial and large gas giants; elsewhere planets of intermediate size are typical—both rocky and gas—so there is no "gap" as seen between the size of Earth and of Neptune (with a radius 3.8 times as large). As many of these super-Earths are closer to their respective stars than Mercury is to the Sun, a hypothesis has arisen that all planetary systems start with many close-in planets, and that typically a sequence of their collisions causes consolidation of mass into few larger planets, but in case of the Solar System the collisions caused their destruction and ejection.[70][73]

The orbits of Solar System planets are nearly circular. Compared to many other systems, they have smaller orbital eccentricity.[70] Although there are attempts to explain it partly with a bias in the radial-velocity detection method and partly with long interactions of a quite high number of planets, the exact causes remain undetermined.[70][74]

Sun

White ball of plasma
The Sun in true white color

The Sun is the Solar System's star and by far its most massive component. Its large mass (332,900 Earth masses),[75] which comprises 99.86% of all the mass in the Solar System,[76] produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium.[77] This releases an enormous amount of energy, mostly radiated into space as electromagnetic radiation peaking in visible light.[78][79]

Because the Sun fuses hydrogen at its core, it is a main-sequence star. More specifically, it is a G2-type main-sequence star, where the type designation refers to its effective temperature. Hotter main-sequence stars are more luminous but shorter lived. The Sun's temperature is intermediate between that of the hottest stars and that of the coolest stars. Stars brighter and hotter than the Sun are rare, whereas substantially dimmer and cooler stars, known as red dwarfs, make up about 75% of the fusor stars in the Milky Way.[80]

The Sun is a population I star, having formed in the spiral arms of the Milky Way galaxy. It has a higher abundance of elements heavier than hydrogen and helium ("metals" in astronomical parlance) than the older population II stars in the galactic bulge and halo.[81] Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, whereas stars born later have more. This higher metallicity is thought to have been crucial to the Sun's development of a planetary system because the planets formed from the accretion of "metals".[82]

The region of space dominated by the Solar magnetosphere is the heliosphere, which spans much of the Solar System. Along with light, the Sun radiates a continuous stream of charged particles (a plasma) called the solar wind. This stream spreads outwards at speeds from 900,000 kilometres per hour (560,000 mph) to 2,880,000 kilometres per hour (1,790,000 mph),[83] filling the vacuum between the bodies of the Solar System. The result is a thin, dusty atmosphere, called the interplanetary medium, which extends to at least 100 AU.[84]

Activity on the Sun's surface, such as solar flares and coronal mass ejections, disturbs the heliosphere, creating space weather and causing geomagnetic storms.[85] Coronal mass ejections and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. The interaction of this magnetic field and material with Earth's magnetic field funnels charged particles into Earth's upper atmosphere, where its interactions create aurorae seen near the magnetic poles.[86] The largest stable structure within the heliosphere is the heliospheric current sheet, a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium.[87][88]

Inner Solar System

The inner Solar System is the region comprising the terrestrial planets and the asteroids.[89] Composed mainly of silicates and metals,[90] the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn. This region is within the frost line, which is a little less than 5 AU from the Sun.[45]

Inner planets

Venus and Earth about the same size, Mars is about 0.55 times as big and Mercury is about 0.4 times as big
The four terrestrial planets Mercury, Venus, Earth and Mars

The four terrestrial or inner planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of refractory minerals such as silicates—which form their crusts and mantles—and metals such as iron and nickel which form their cores. Three of the four inner planets (Venus, Earth, and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features, such as rift valleys and volcanoes.[91]

  • Mercury (0.31–0.59 AU from the Sun)[D 6] is the smallest planet in the Solar System. Its surface is grayish, with an expansive rupes (cliff) system generated from thrust faults and bright ray systems formed by impact event remnants.[92] The surface has widely varying temperature, with the equatorial regions ranging from −170 °C (−270 °F) at night to 420 °C (790 °F) during sunlight. In the past, Mercury was volcanically active, producing smooth basaltic plains similar to the Moon.[93] It is likely that Mercury has a silicate crust and a large iron core.[94][95] Mercury has a very tenuous atmosphere, consisting of solar-wind particles and ejected atoms.[96] Mercury has no natural satellites.[97]
  • Venus (0.72–0.73 AU)[D 6] has a reflective, whitish atmosphere that is mainly composed of carbon dioxide. At the surface, the atmospheric pressure is ninety times as dense as on Earth's sea level.[98] Venus has a surface temperatures over 400 °C (752 °F), mainly due to the amount of greenhouse gases in the atmosphere.[99] The planet lacks a protective magnetic field to protect against stripping by the solar wind, which suggests that its atmosphere is sustained by volcanic activity.[100] Its surface displays extensive evidence of volcanic activity with stagnant lid tectonics.[101] Venus has no natural satellites.[97]
  • Earth (0.98–1.02 AU)[D 6] is the only place in the universe where life and surface liquid water are known to exist.[102] Earth's atmosphere contains 78% nitrogen and 21% oxygen, which is the result of the presence of life.[103][104] The planet has a complex climate and weather system, with conditions differing drastically between climate regions.[105] The solid surface of Earth is dominated by green vegetation, deserts and white ice sheets.[106][107][108] Earth's surface is shaped by plate tectonics that formed the continental masses.[93] Earth's planetary magnetosphere shields the surface from radiation, limiting atmospheric stripping and maintaining life habitability.[109]
  • Mars (1.38–1.67 AU)[D 6] has a radius about half of that of Earth.[116] Most of the planet is red due to iron oxide in Martian soil,[117] and the polar regions are covered in white ice caps made of water and carbon dioxide.[118] Mars has an atmosphere composed mostly of carbon dioxide, with surface pressure 0.6% of that of Earth, which is sufficient to support some weather phenomena.[119] During the Mars year (687 Earth days), there are large surface temperature swings on the surface between −78.5 °C (−109.3 °F) to 5.7 °C (42.3 °F). The surface is peppered with volcanoes and rift valleys, and has a rich collection of minerals.[120][121] Mars has a highly differentiated internal structure, and lost its magnetosphere 4 billion years ago.[122][123] Mars has two tiny moons:[124]
    • Phobos is Mars's inner moon. It is a small, irregularly shaped object with a mean radius of 11 km (7 mi). Its surface is very unreflective and dominated by impact craters.[D 7][125] In particular, Phobos's surface has a very large Stickney impact crater that is roughly 4.5 km (2.8 mi) in radius.[126]
    • Deimos is Mars's outer moon. Like Phobos, it is irregularly shaped, with a mean radius of 6 km (4 mi) and its surface reflects little light.[D 8][D 9] However, the surface of Deimos is noticeably smoother than Phobos because the regolith partially covers the impact craters.[127]

Asteroids

Asteroid populations depicted: near-Earth asteroids, Earth trojans, Mars trojans, main asteroid belt, Jupiter trojans, Jupiter Greeks, Jupiter Hilda's triangle
Overview of the inner Solar System up to Jupiter's orbit

Asteroids except for the largest, Ceres, are classified as small Solar System bodies and are composed mainly of carbonaceous, refractory rocky and metallic minerals, with some ice.[128][129] They range from a few meters to hundreds of kilometers in size. Many asteroids are divided into asteroid groups and families based on their orbital characteristics. Some asteroids have natural satellites that orbit them, that is, asteroids that orbit larger asteroids.[130]

Asteroid belt

The asteroid belt occupies a torus-shaped region between 2.3 and 3.3 AU from the Sun, which lies between the orbits of Mars and Jupiter. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.[140] The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometer in diameter.[141] Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth.[40] The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident.[142]

The four largest asteroids: Ceres, Vesta, Pallas, Hygiea. Only Ceres and Vesta have been visited by a spacecraft and thus have a detailed picture.

Below are the descriptions of the three largest bodies in the asteroid belt. They are all considered to be relatively intact protoplanets, a precursor stage before becoming a fully-formed planet (see List of exceptional asteroids):[143][144][145]

Hilda asteroids are in a 3:2 resonance with Jupiter; that is, they go around the Sun three times for every two Jovian orbits.[159] They lie in three linked clusters between Jupiter and the main asteroid belt.

Trojans are bodies located within another body's gravitationally stable Lagrange points: L4, 60° ahead in its orbit, or L5, 60° behind in its orbit.[160] Every planet except Mercury and Saturn is known to possess at least 1 trojan.[161][162][163] The Jupiter trojan population is roughly equal to that of the asteroid belt.[164] After Jupiter, Neptune possesses the most confirmed trojans, at 28.[165]

Outer Solar System

The outer region of the Solar System is home to the giant planets and their large moons. The centaurs and many short-period comets orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles such as water, ammonia, and methane, than planets of the inner Solar System because their lower temperatures allow these compounds to remain solid, without significant sublimation.[20]

Outer planets

Jupiter and Saturn is about 2 times bigger than Uranus and Neptune, 10 times bigger than Venus and Earth, 20 times bigger than Mars and 25 times bigger than Mercury
The outer planets Jupiter, Saturn, Uranus and Neptune, compared to the inner planets Earth, Venus, Mars, and Mercury at the bottom right

The four outer planets, called giant planets or Jovian planets, collectively make up 99% of the mass orbiting the Sun.[h] All four giant planets have multiple moons and a ring system, although only Saturn's rings are easily observed from Earth.[91] Jupiter and Saturn are composed mainly of gases with extremely low melting points, such as hydrogen, helium, and neon,[166] hence their designation as gas giants.[167] Uranus and Neptune are ice giants,[168] meaning they are largely composed of 'ice' in the astronomical sense (chemical compounds with melting points of up to a few hundred kelvins[166] such as water, methane, ammonia, hydrogen sulfide, and carbon dioxide.[169]) Icy substances comprise the majority of the satellites of the giant planets and small objects that lie beyond Neptune's orbit.[169][170]

  • Jupiter (4.95–5.46 AU)[D 6] is the biggest and most massive planet in the Solar System. On its surface, there are orange-brown and white cloud bands moving via the principles of atmospheric circulation, with giant storms swirling on the surface such as the Great Red Spot and white 'ovals'. Jupiter possesses a strong enough magnetosphere to redirect ionizing radiation and cause auroras on its poles.[171] As of 2024, Jupiter has 95 confirmed satellites, which can roughly be sorted into three groups:
    • The Amalthea group, consisting of Metis, Adrastea, Amalthea, and Thebe. They orbit substantially closer to Jupiter than other satellites.[172] Materials from these natural satellites are the source of Jupiter's faint ring.[173]
    • The Galilean moons, consisting of Ganymede, Callisto, Io, and Europa. They are the largest moons of Jupiter and exhibit planetary properties.[174]
    • Irregular satellites, consisting of substantially smaller natural satellites. They have more distant orbits than the other objects.[175]
  • Saturn (9.08–10.12 AU)[D 6] has a distinctive visible ring system orbiting around its equator composed of small ice and rock particles. Like Jupiter, it is mostly made of hydrogen and helium.[176] At its north and south poles, Saturn has peculiar hexagon-shaped storms larger than the diameter of Earth. Saturn has a magnetosphere capable of producing weak auroras. As of 2024, Saturn has 146 confirmed satellites, grouped into:
    • Ring moonlets and shepherds, which orbit inside or close to Saturn's rings. A moonlet can only partially clear out dust in its orbit,[177] while the ring shepherds are able to completely clear out dust, forming visible gaps in the rings.[178]
    • Inner large satellites Mimas, Enceladus, Tethys, and Dione. These satellites orbit within Saturn's E ring. They are composed mostly of water ice and are believed to have differentiated internal structures.[179]
    • Trojan moons Calypso and Telesto (trojans of Tethys), and Helene and Polydeuces (trojans of Dione). These small moons share their orbits with Tethys and Dione, leading or trailing either.[180][181]
    • Outer large satellites Rhea, Titan, Hyperion, and Iapetus.[179] Titan is the only satellite in the Solar System to have a substantial atmosphere.[182]
    • Irregular satellites, consisting of substantially smaller natural satellites. They have more distant orbits than the other objects. Phoebe is the largest irregular satellite of Saturn.[183]
  • Uranus (18.3–20.1 AU),[D 6] uniquely among the planets, orbits the Sun on its side with an axial tilt >90°. This gives the planet extreme seasonal variation as each pole points alternately toward and then away from the Sun.[184] Uranus' outer layer has a muted cyan color, but underneath these clouds are many mysteries about its climate, such as unusually low internal heat and erratic cloud formation. As of 2024, Uranus has 28 confirmed satellites, divided into three groups:
    • Inner satellites, which orbit inside Uranus' ring system.[185] They are very close to each other, which suggests that their orbits are chaotic.[186]
    • Large satellites, consisting of Titania, Oberon, Umbriel, Ariel, and Miranda.[187] Most of them have roughly equal amounts of rock and ice, except Miranda, which is made primarily of ice.[188]
    • Irregular satellites, having more distant and eccentric orbits than the other objects.[189]
  • Neptune (29.9–30.5 AU)[D 6] is the furthest planet known in the Solar System. Its outer atmosphere has a slightly muted cyan color, with occasional storms on the surface that look like dark spots. Like Uranus, many atmospheric phenomena of Neptune are unexplained, such as the thermosphere's abnormally high temperature or the strong tilt (47°) of its magnetosphere. As of 2024, Neptune has 16 confirmed satellites, divided into two groups:
    • Regular satellites, which have circular orbits that lie near Neptune's equator.[183]
    • Irregular satellites, which as the name implies, have less regular orbits. One of them, Triton, is Neptune's largest moon. It is geologically active, with erupting geysers of nitrogen gas, and possesses a thin, cloudy nitrogen atmosphere.[190][182]

Centaurs

The centaurs are icy, comet-like bodies whose semi-major axes are longer than Jupiter's and shorter than Neptune's (between 5.5 and 30 AU). These are former Kuiper belt and scattered disc objects (SDOs) that were gravitationally perturbed closer to the Sun by the outer planets, and are expected to become comets or be ejected out of the Solar System.[39] While most centaurs are inactive and asteroid-like, some exhibit cometary activity, such as the first centaur discovered, 2060 Chiron, which has been classified as a comet (95P) because it develops a coma just as comets do when they approach the Sun.[191] The largest known centaur, 10199 Chariklo, has a diameter of about 250 km (160 mi) and is one of the few minor planets possessing a ring system.[192][193]

Trans-Neptunian region

Beyond the orbit of Neptune lies the area of the "trans-Neptunian region", with the doughnut-shaped Kuiper belt, home of Pluto and several other dwarf planets, and an overlapping disc of scattered objects, which is tilted toward the plane of the Solar System and reaches much further out than the Kuiper belt. The entire region is still largely unexplored. It appears to consist overwhelmingly of many thousands of small worlds—the largest having a diameter only a fifth that of Earth and a mass far smaller than that of the Moon—composed mainly of rock and ice. This region is sometimes described as the "third zone of the Solar System", enclosing the inner and the outer Solar System.[194]

Kuiper belt

Plot of objects around the Kuiper belt and other asteroid populations. J, S, U and N denotes Jupiter, Saturn, Uranus and Neptune.
Orbit classification of Kuiper belt objects. Some clusters that is subjected to orbital resonance are marked.

The Kuiper belt is a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice.[195] It extends between 30 and 50 AU from the Sun. It is composed mainly of small Solar System bodies, although the largest few are probably large enough to be dwarf planets.[196] There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km (30 mi), but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of Earth.[39] Many Kuiper belt objects have satellites,[197] and most have orbits that are substantially inclined (~10°) to the plane of the ecliptic.[198]

The Kuiper belt can be roughly divided into the "classical" belt and the resonant trans-Neptunian objects.[195] The latter have orbits whose periods are in a simple ratio to that of Neptune: for example, going around the Sun twice for every three times that Neptune does, or once for every two. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 to 47.7 AU.[199] Members of the classical Kuiper belt are sometimes called "cubewanos", after the first of their kind to be discovered, originally designated 1992 QB1, (and has since been named Albion); they are still in near primordial, low-eccentricity orbits.[200]

There is strong consensus among astronomers that five members of the Kuiper belt are dwarf planets.[196][201] Many dwarf planet candidates are being considered, pending further data for verification.[202]

  • Pluto (29.7–49.3 AU) is the largest known object in the Kuiper belt. Pluto has a relatively eccentric orbit, inclined 17 degrees to the ecliptic plane. Pluto has a 2:3 resonance with Neptune, meaning that Pluto orbits twice around the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called plutinos.[203] Pluto has five moons: Charon, Styx, Nix, Kerberos, and Hydra.[204]
    • Charon, the largest of Pluto's moons, is sometimes described as part of a binary system with Pluto, as the two bodies orbit a barycenter of gravity above their surfaces (i.e. they appear to "orbit each other").
  • Orcus (30.3–48.1 AU), is in the same 2:3 orbital resonance with Neptune as Pluto, and is the largest such object after Pluto itself.[205] Its eccentricity and inclination are similar to Pluto's, but its perihelion lies about 120° from that of Pluto. Thus, the phase of Orcus's orbit is opposite to Pluto's: Orcus is at aphelion (most recently in 2019) around when Pluto is at perihelion (most recently in 1989) and vice versa.[206] For this reason, it has been called the anti-Pluto.[207][208] It has one known moon, Vanth.[209]
  • Haumea (34.6–51.6 AU) was discovered in 2005.[210] It is in a temporary 7:12 orbital resonance with Neptune.[205] Haumea possesses a ring system, two known moons named Hiʻiaka and Namaka, and rotates so quickly (once every 3.9 hours) that it is stretched into an ellipsoid. It is part of a collisional family of Kuiper belt objects that share similar orbits, which suggests a giant impact on Haumea ejected fragments into space billions of years ago.[211]
  • Makemake (38.1–52.8 AU), although smaller than Pluto, is the largest known object in the classical Kuiper belt (that is, a Kuiper belt object not in a confirmed resonance with Neptune). Makemake is the brightest object in the Kuiper belt after Pluto. Discovered in 2005, it was officially named in 2009.[212] Its orbit is far more inclined than Pluto's, at 29°.[213] It has one known moon, S/2015 (136472) 1.[214]
  • Quaoar (41.9–45.5 AU) is the second-largest known object in the classical Kuiper belt, after Makemake. Its orbit is significantly less eccentric and inclined than those of Makemake or Haumea.[205] It possesses a ring system and one known moon, Weywot.[215]

Scattered disc

The orbital eccentricities and inclinations of the scattered disc population compared to the classical and resonant Kuiper belt objects

The scattered disc, which overlaps the Kuiper belt but extends out to near 500 AU, is thought to be the source of short-period comets. Scattered-disc objects are believed to have been perturbed into erratic orbits by the gravitational influence of Neptune's early outward migration. Most scattered disc objects have perihelia within the Kuiper belt but aphelia far beyond it (some more than 150 AU from the Sun). SDOs' orbits can be inclined up to 46.8° from the ecliptic plane.[216] Some astronomers consider the scattered disc to be merely another region of the Kuiper belt and describe scattered-disc objects as "scattered Kuiper belt objects".[217] Some astronomers classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.[218]

Currently, there is strong consensus among astronomers that two of the bodies in the scattered disc are dwarf planets:

  • Eris (38.3–97.5 AU) is the largest known scattered disc object and the most massive known dwarf planet. Eris's discovery contributed to a debate about the definition of a planet because it is 25% more massive than Pluto[219] and about the same diameter. It has one known moon, Dysnomia. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane at an angle of 44°.[220]
  • Gonggong (33.8–101.2 AU) is a dwarf planet in a comparable orbit to Eris, except that it is in a 3:10 resonance with Neptune.[D 10] It has one known moon, Xiangliu.[221]

Extreme trans-Neptunian objects

The current orbits of Sedna, 2012 VP113, Leleākūhonua (pink), and other very distant objects (red, brown and cyan) along with the predicted orbit of the hypothetical Planet Nine (dark blue)

Some objects in the Solar System have a very large orbit, and therefore are much less affected by the known giant planets than other minor planet populations. These bodies are called extreme trans-Neptunian objects, or ETNOs for short.[222] Generally, ETNOs' semi-major axes are at least 150–250 AU wide.[222][223] For example, 541132 Leleākūhonua orbits the Sun once every ~32,000 years, with a distance of 65–2000 AU from the Sun.[D 11]

This population is divided into three subgroups by astronomers. The scattered ETNOs have perihelia around 38–45 AU and an exceptionally high eccentricity of more than 0.85. As with the regular scattered disc objects, they were likely formed as result of gravitational scattering by Neptune and still interact with the giant planets. The detached ETNOs, with perihelia approximately between 40–45 and 50–60 AU, are less affected by Neptune than the scattered ETNOs, but are still relatively close to Neptune. The sednoids or inner Oort cloud objects, with perihelia beyond 50–60 AU, are too far from Neptune to be strongly influenced by it.[222]

Currently, there is one ETNO that is classified as a dwarf planet:

  • Sedna (76.2–937 AU) was the first extreme trans-Neptunian object to be discovered. It is a large, reddish object, and it takes ~11,400 years for Sedna to complete one orbit. Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper belt because its perihelion is too distant to have been affected by Neptune's migration.[224] The sednoid population is named after Sedna.[222]

Edge of the heliosphere

Diagram of the Sun's magnetosphere and helioshealth

The Sun's stellar-wind bubble, the heliosphere, a region of space dominated by the Sun, has its boundary at the termination shock. Based on the Sun's peculiar motion relative to the local standard of rest, this boundary is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind.[225] Here the solar wind collides with the interstellar medium[226] and dramatically slows, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath.[225]

The heliosheath has been theorized to look and behave very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind to possibly several thousands of AU.[227][228] Evidence from the Cassini and Interstellar Boundary Explorer spacecraft has suggested that it is forced into a bubble shape by the constraining action of the interstellar magnetic field,[229][230] but the actual shape remains unknown.[231]

The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics of interactions with the interstellar medium as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU farther than the southern hemisphere.[225] The heliopause is considered the beginning of the interstellar medium.[84] Beyond the heliopause, at around 230 AU, lies the bow shock: a plasma "wake" left by the Sun as it travels through the Milky Way.[232] Large objects outside the heliopause remain gravitationally bound to the Sun, but the flow of matter in the interstellar medium homogenizes the distribution of micro-scale objects.[84]

Miscellaneous populations

Comets

Comet Hale–Bopp seen in 1997

Comets are small Solar System bodies, typically only a few kilometers across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye.[233]

Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are thought to originate in the Kuiper belt, whereas long-period comets, such as Hale–Bopp, are thought to originate in the Oort cloud. Many comet groups, such as the Kreutz sungrazers, formed from the breakup of a single parent.[234] Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult.[235] Old comets whose volatiles have mostly been driven out by solar warming are often categorized as asteroids.[236]

Meteoroids, meteors and dust

The planets, zodiacal light and meteor shower (top left of image)

Solid objects smaller than one meter are usually called meteoroids and micrometeoroids (grain-sized), with the exact division between the two categories being debated over the years.[237] By 2017, the IAU designated any solid object having a diameter between ~30 micrometers and 1 meter as meteoroids, and depreciated the micrometeoroid categorization, instead terms smaller particles simply as 'dust particles'.[238]

Some meteoroids formed via disintegration of comets and asteroids, while a few formed via impact debris ejected from planetary bodies. Most meteoroids are made of silicates and heavier metals like nickel and iron.[239] When passing through the Solar System, comets produce a trail of meteoroids; it is hypothesized that this is caused either by vaporization of the comet's material or by simple breakup of dormant comets. When crossing an atmosphere, these meteoroids will produce bright streaks in the sky due to atmospheric entry, called meteors. If a stream of meteoroids enter the atmosphere on parallel trajectories, the meteors will seemingly 'radiate' from a point in the sky, hence the phenomenon's name: meteor shower.[240]

The inner Solar System is home to the zodiacal dust cloud, which is visible as the hazy zodiacal light in dark, unpolluted skies. It may be generated by collisions within the asteroid belt brought on by gravitational interactions with the planets; a more recent proposed origin is materials from planet Mars.[241] The outer Solar System hosts a cosmic dust cloud. It extends from about 10 AU to about 40 AU, and was probably created by collisions within the Kuiper belt.[242][243]

Boundary region and uncertainties

An artist's impression of the Oort cloud, a region still well within the sphere of influence of the Solar System, including a depiction of the much further inside Kuiper belt (inset); the sizes of objects are over-scaled for visibility.

Much of the Solar System is still unknown. Regions beyond thousands of AU away are still virtually unmapped and learning about this region of space is difficult. Study in this region depends upon inferences from those few objects whose orbits happen to be perturbed such that they fall closer to the Sun, and even then, detecting these objects has often been possible only when they happened to become bright enough to register as comets.[244] Many objects may yet be discovered in the Solar System's uncharted regions.[245]

The Oort cloud is a theorized spherical shell of up to a trillion icy objects that is thought to be the source for all long-period comets.[246][247] No direct observation of the Oort cloud is possible with present imaging technology.[248] It is theorized to surround the Solar System at roughly 50,000 AU (~0.9 ly) from the Sun and possibly to as far as 100,000 AU (~1.8 ly). The Oort cloud is thought to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events, such as collisions, the gravitational effects of a passing star, or the galactic tide, the tidal force exerted by the Milky Way.[246][247]

As of the 2020s, a few astronomers have hypothesized that Planet Nine (a planet beyond Neptune) might exist, based on statistical variance in the orbit of extreme trans-Neptunian objects.[249] Their closest approaches to the Sun are mostly clustered around one sector and their orbits are similarly tilted, suggesting that a large planet might be influencing their orbit over millions of years.[250][251][252] However, some astronomers said that this observation might be credited to observational biases or just sheer coincidence.[253] An alternative hypothesis has a close flyby of another star disrupting the outer Solar System.[254]

The Sun's gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light-years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU.[255] Most of the mass is orbiting in the region between 3,000 and 100,000 AU.[256] The furthest known objects, such as Comet West, have aphelia around 70,000 AU from the Sun.[257] The Sun's Hill sphere with respect to the galactic nucleus, the effective range of its gravitational influence, is thought to extend up to a thousand times farther and encompasses the hypothetical Oort cloud.[258] It was calculated by G. A. Chebotarev to be 230,000 AU.[7]

The Solar System (left) within the interstellar medium, with the different regions and their distances on a logarithmic scale

Celestial neighborhood

Diagram of the Local Interstellar Cloud, the G-Cloud and surrounding stars. As of 2022, the precise location of the Solar System in the clouds is an open question in astronomy.[259]

Within 10 light-years of the Sun there are relatively few stars, the closest being the triple star system Alpha Centauri, which is about 4.4 light-years away and may be in the Local Bubble's G-Cloud.[260] Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the closest star to the Sun, the small red dwarf Proxima Centauri, orbits the pair at a distance of 0.2 light-years. In 2016, a potentially habitable exoplanet was found to be orbiting Proxima Centauri, called Proxima Centauri b, the closest confirmed exoplanet to the Sun.[261]

The Solar System is surrounded by the Local Interstellar Cloud, although it is not clear if it is embedded in the Local Interstellar Cloud or if it lies just outside the cloud's edge.[262] Multiple other interstellar clouds exist in the region within 300 light-years of the Sun, known as the Local Bubble.[262] The latter feature is an hourglass-shaped cavity or superbubble in the interstellar medium roughly 300 light-years across. The bubble is suffused with high-temperature plasma, suggesting that it may be the product of several recent supernovae.[263]

The Local Bubble is a small superbubble compared to the neighboring wider Radcliffe Wave and Split linear structures (formerly Gould Belt), each of which are some thousands of light-years in length.[264] All these structures are part of the Orion Arm, which contains most of the stars in the Milky Way that are visible to the unaided eye.[265]

Groups of stars form together in star clusters, before dissolving into co-moving associations. A prominent grouping that is visible to the naked eye is the Ursa Major moving group, which is around 80 light-years away within the Local Bubble. The nearest star cluster is Hyades, which lies at the edge of the Local Bubble. The closest star-forming regions are the Corona Australis Molecular Cloud, the Rho Ophiuchi cloud complex and the Taurus molecular cloud; the latter lies just beyond the Local Bubble and is part of the Radcliffe wave.[266]

Stellar flybys that pass within 0.8 light-years of the Sun occur roughly once every 100,000 years. The closest well-measured approach was Scholz's Star, which approached to ~50,000 AU of the Sun some ~70 thousands years ago, likely passing through the outer Oort cloud.[267] There is a 1% chance every billion years that a star will pass within 100 AU of the Sun, potentially disrupting the Solar System.[268]

Galactic position

Diagram of the Milky Way, with galactic features and the relative position of the Solar System labeled.

The Solar System is located in the Milky Way, a barred spiral galaxy with a diameter of about 100,000 light-years containing more than 100 billion stars.[269] The Sun is part of one of the Milky Way's outer spiral arms, known as the Orion–Cygnus Arm or Local Spur.[270][271] It is a member of the thin disk population of stars orbiting close to the galactic plane.[272]

Its speed around the center of the Milky Way is about 220 km/s, so that it completes one revolution every 240 million years.[269] This revolution is known as the Solar System's galactic year.[273] The solar apex, the direction of the Sun's path through interstellar space, is near the constellation Hercules in the direction of the current location of the bright star Vega.[274] The plane of the ecliptic lies at an angle of about 60° to the galactic plane.[c]

The Sun follows a nearly circular orbit around the Galactic Center (where the supermassive black hole Sagittarius A* resides) at a distance of 26,660 light-years,[276] orbiting at roughly the same speed as that of the spiral arms.[277] If it orbited close to the center, gravitational tugs from nearby stars could perturb bodies in the Oort cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. In this scenario, the intense radiation of the Galactic Center could interfere with the development of complex life.[277]

The Solar System's location in the Milky Way is a factor in the evolutionary history of life on Earth. Spiral arms are home to a far larger concentration of supernovae, gravitational instabilities, and radiation that could disrupt the Solar System, but since Earth stays in the Local Spur and therefore does not pass frequently through spiral arms, this has given Earth long periods of stability for life to evolve.[277] However, according to the controversial Shiva hypothesis, the changing position of the Solar System relative to other parts of the Milky Way could explain periodic extinction events on Earth.[278][279]

Discovery and exploration

The motion of 'lights' moving across the sky is the basis of the classical definition of planets: wandering stars.

Humanity's knowledge of the Solar System has grown incrementally over the centuries. Up to the Late Middle AgesRenaissance, astronomers from Europe to India believed Earth to be stationary at the center of the universe[280] and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first person known to have developed a mathematically predictive heliocentric system.[281][282]

Heliocentrism did not triumph immediately over geocentrism, but the work of Copernicus had its champions, notably Johannes Kepler. Using a heliocentric model that improved upon Copernicus by allowing orbits to be elliptical, and the precise observational data of Tycho Brahe, Kepler produced the Rudolphine Tables, which enabled accurate computations of the positions of the then-known planets. Pierre Gassendi used them to predict a transit of Mercury in 1631, and Jeremiah Horrocks did the same for a transit of Venus in 1639. This provided a strong vindication of heliocentrism and Kepler's elliptical orbits.[283][284]

In the 17th century, Galileo publicized the use of the telescope in astronomy; he and Simon Marius independently discovered that Jupiter had four satellites in orbit around it.[285] Christiaan Huygens followed on from these observations by discovering Saturn's moon Titan and the shape of the rings of Saturn.[286] In 1677, Edmond Halley observed a transit of Mercury across the Sun, leading him to realize that observations of the solar parallax of a planet (more ideally using the transit of Venus) could be used to trigonometrically determine the distances between Earth, Venus, and the Sun.[287] Halley's friend Isaac Newton, in his magisterial Principia Mathematica of 1687, demonstrated that celestial bodies are not quintessentially different from Earthly ones: the same laws of motion and of gravity apply on Earth and in the skies.[52]: 142 

Solar System diagram made by Emanuel Bowen in 1747. At that time, Uranus, Neptune, nor the asteroid belts have been discovered yet. Orbits of planets are drawn to scale, but the orbits of moons and the size of bodies are not.

The term "Solar System" entered the English language by 1704, when John Locke used it to refer to the Sun, planets, and comets.[288] In 1705, Halley realized that repeated sightings of a comet were of the same object, returning regularly once every 75–76 years. This was the first evidence that anything other than the planets repeatedly orbited the Sun,[289] though Seneca had theorized this about comets in the 1st century.[290] Careful observations of the 1769 transit of Venus allowed astronomers to calculate the average Earth–Sun distance as 93,726,900 miles (150,838,800 km), only 0.8% greater than the modern value.[291]

Uranus, having occasionally been observed since 1690 and possibly from antiquity, was recognized to be a planet orbiting beyond Saturn by 1783.[292] In 1838, Friedrich Bessel successfully measured a stellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism.[293] Neptune was identified as a planet some years later, in 1846, thanks to its gravitational pull causing a slight but detectable variation in the orbit of Uranus.[294] Mercury's orbital anomaly observations led to searches for Vulcan, a planet interior of Mercury, but these attempts were quashed with Albert Einstein's theory of general relativity in 1915.[295]

In the 20th century, humans began their space exploration around the Solar System, starting with placing telescopes in space since the 1960s.[296] By 1989, all eight planets have been visited by space probes.[297] Probes have returned samples from comets[298] and asteroids,[299] as well as flown through the Sun's corona[300] and visited two dwarf planets (Pluto and Ceres).[301][302] To save on fuel, some space missions make use of gravity assist maneuvers, such as the two Voyager probes accelerating when flyby planets in the outer Solar System[303] and the Parker Solar Probe decelerating closer towards the Sun after flyby with Venus.[304]

Humans have landed on the Moon during the Apollo program in the 1960s and 1970s[305] and will return to the Moon in the 2020s with the Artemis program.[306] Discoveries in the 20th and 21st century has prompted the redefinition of the term planet in 2006, hence the demotion of Pluto to a dwarf planet,[307] and further interest in trans-Neptunian objects.[308]

See also

Notes

  1. ^ The Asteroid Belt, Kuiper Belt, and Scattered Disc are not added because the individual asteroids are too small to be shown on the diagram.
  2. ^ a b The date is based on the oldest inclusions found to date in meteorites, 4568.2+0.2
    −0.4
    million years, and is thought to be the date of the formation of the first solid material in the collapsing nebula.[13]
  3. ^ a b If is the angle between the north pole of the ecliptic and the north galactic pole then:

    where = 27° 07′ 42.01″ and = 12h 51m 26.282s are the declination and right ascension of the north galactic pole,[275] whereas = 66° 33′ 38.6″ and = 18h 0m 00s are those for the north pole of the ecliptic. (Both pairs of coordinates are for J2000 epoch.) The result of the calculation is 60.19°.
  4. ^ Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar System" and "solar system" structures in their naming guidelines document Archived 25 July 2021 at the Wayback Machine. The name is commonly rendered in lower case ('solar system'), as, for example, in the Oxford English Dictionary and Merriam-Webster's 11th Collegiate Dictionary Archived 27 January 2008 at the Wayback Machine.
  5. ^ The International Astronomical Union's Minor Planet Center has yet to officially list Orcus, Quaoar, Gonggong, and Sedna as dwarf planets as of 2024.
  6. ^ For more classifications of Solar System objects, see List of minor-planet groups and Comet § Classification.
  7. ^ The scale of the Solar System is sufficiently large that astronomers use a custom unit to express distances. The astronomical unit, abbreviated AU, is equal to 150,000,000 km; 93,000,000 mi. This is what the distance from the Earth to the Sun would be if the planet's orbit were perfectly circular.[12]
  8. ^ a b The mass of the Solar System excluding the Sun, Jupiter and Saturn can be determined by adding together all the calculated masses for its largest objects and using rough calculations for the masses of the Oort cloud (estimated at roughly 3 Earth masses),[38] the Kuiper belt (estimated at 0.1 Earth mass)[39] and the asteroid belt (estimated to be 0.0005 Earth mass)[40] for a total, rounded upwards, of ~37 Earth masses, or 8.1% of the mass in orbit around the Sun. With the combined masses of Uranus and Neptune (~31 Earth masses) subtracted, the remaining ~6 Earth masses of material comprise 1.3% of the total orbiting mass.

References

Data sources

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