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'<!-- The IAU uses the COMMONWEALTH ENGLISH spelling of "neighbourhood" please do not change --> Having "'''cleared the neighbourhood''' of its orbit" is a [[criterion]] for a celestial body to be considered a [[planet]] in the [[Solar System]]. This was one of the three criteria adopted by the [[International Astronomical Union]] (IAU) in its 2006 [[IAU definition of planet|definition of planet]].<ref name="IAU definition">{{cite news| url=http://www.iau.org/public_press/news/detail/iau0603/| title=IAU 2006 General Assembly: Result of the IAU Resolution votes| date=24 August 2006| accessdate = 2009-10-23| publisher=IAU}}</ref> In the end stages of [[Nebular hypothesis|planet formation]], a [[planet]] will have "cleared the neighbourhood" of its own [[Planet#Orbit|orbital zone]], meaning it has become gravitationally dominant, and there are no other bodies of comparable size other than its own [[natural satellite|satellites]] or those otherwise under its gravitational influence. A large body which meets the other criteria for a planet but has not cleared its neighbourhood is classified as a [[dwarf planet]]. This includes [[Pluto]], which shares its orbital neighbourhood with [[Kuiper belt]] objects such as the [[plutino]]s. The IAU's definition does not attach specific numbers or equations to this term, but the extent to which all the planets have cleared their neighbourhoods is much greater, by any measure, than that of any dwarf planet or any candidate for dwarf planet known so far. The phrase may be derived from a paper presented to the general assembly of the IAU in [[2000]] by [[Alan Stern]] and [[Harold F. Levison]]. The authors used several similar phrases as they developed a theoretical basis for determining if an object orbiting a [[star]] is likely to "clear its neighboring region" of [[planetesimal]]s, based on the object's [[mass]] and its [[orbital period]].<ref name="Stern 2002">{{cite journal | last=Stern | first=S. Alan | coauthors=and Levison, Harold F. | year=2002 | title=Regarding the criteria for planethood and proposed planetary classification schemes | url=http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf | format=[[PDF]] | journal=Highlights of Astronomy| volume=12 | pages=205–213, as presented at the XXIVth General Assembly of the IAU–2000 [Manchester, UK, 7 August-18 August 2000]}}</ref> Clearly distinguishing "planets" from "dwarf planets" and other [[minor planet]]s had become necessary because the IAU had adopted different rules for naming newly discovered major planets and newly discovered minor planets, without establishing a basis for telling them apart. The naming process for [[Eris (dwarf planet)|Eris]] stalled after the announcement of its discovery in [[2005]], pending clarification of this first step. ==Details== The phrase refers to an orbiting body (a planet or [[protoplanet]]) "sweeping out" its [[Planetary orbit|orbital]] region over time, by [[gravitation]]ally interacting with smaller [[celestial bodies|bodies]] nearby. Over many orbital cycles, a large body will tend to cause small bodies either to [[Accretion (astrophysics)|accrete]] with it, or to be disturbed to another orbit. As a consequence it does not then share its orbital region with other bodies of significant size, except for its own [[satellite]]s, or other bodies governed by its own gravitational influence. This latter restriction excludes objects whose orbits may cross but which will never collide with each other due to [[orbital resonance]], such as [[Jupiter]] and the [[Trojan asteroid]]s, [[Earth]] and [[3753 Cruithne]] or [[Neptune]] and the [[plutinos]].<ref name="Stern 2002"/> [[Steven Soter]] of the Department of Astrophysics, [[American Museum of Natural History]], has written that "''A heliocentric body with Λ > 1 [viz., a planet] has cleared a substantial fraction of small bodies out of its orbital neighborhood.''"<ref name="Soter 2006">{{cite web| url=http://arxiv.org/abs/astro-ph/0608359| title= What is a Planet?| first=Steven| last=Soter| date=2006-08-16| accessdate = 2006-08-24}} submitted to The Astronomical Journal, 16 August 2006</ref> ''Λ'' ([[Lambda]]) is a parameter proposed by Stern and Levison<ref name="Stern 2002"/> that measures the extent to which a body scatters smaller masses out of its orbital zone over a long period of time. Mathematically ''Λ'' is defined as :<math>\Lambda = \frac{kM^2}{P}</math> where ''k'' is approximately constant and ''M'' and ''P'' are the scattering body's mass and orbital period, respectively. Two bodies are defined to share an '''orbital zone''' if their orbits cross a common radial distance from the primary, and their non-resonant periods differ by less than an order of magnitude. The order-of-magnitude similarity in period requirement excludes comets from the calculation, but the combined mass of the comets turn out to be negligible compared to the other small Solar System bodies anyway so their inclusion would have little impact on the results. Stern and Levison found a gap of five orders of magnitude in ''Λ'' between the smallest terrestrial planets and the largest asteroids and [[Kuiper Belt Objects]] (KBOs). Soter went on to propose a parameter he called the "'''planetary discriminant'''", designated with the symbol ''µ'' ([[mu (letter)|mu]]), that represents an experimental measure of the actual degree of cleanliness of the orbital zone. ''µ'' is calculated by dividing the mass of the candidate body by the total mass of the other objects that share its orbital zone. Here is a list of planets by planetary discriminant, as defined by Steven Soter, in decreasing order, where the planetary discriminant μ is the ratio between the mass of the body and the total mass of the other non-resonating and non-satellite bodies in the same orbital zone, as defined by Soter. Also listed is the Stern-Levison parameter Λ, the square of the mass over the orbital period, normalized to the Earth values (Λ/Λ_E). (Note that Λ_E ~ 1.5{{e|5}}, so that the unnormalized values of Λ for the eight planets defined by the IAU are orders of magnitude greater than 1, and the values of Λ for the dwarf planets are orders of magnitude less than 1.)<ref name="Soter 2006"/> {| class="sortable wikitable" |- !|Rank !|Name !|Stern-Levison<br/>parameter Λ</sub> !|Λ/Λ_E !|Planetary<br/>discriminant μ !|Mass ([[kilogram|kg]]) !|Type of object !|Λ = 1 at ([[Astronomical_unit|au]]) |- |1|| [[Earth]]||align=right|{{sort|0000153000|1.53{{Esp|5}}}}||align=right| {{sort|0001.00|1.00}} ||align=right| {{sort|1700000|1.7{{Esp|6}}}} ||align=right| {{sort|0005973600|5.9736{{Esp|24}}}} || 3rd planet ||align=right| {{nts|2870}} |- |2|| [[Venus]]||align=right|{{sort|0000166000|1.66{{Esp|5}}}}||align=right| {{sort|0001.08|1.08}} ||align=right| {{sort|1350000|1.35{{Esp|6}}}} ||align=right| {{sort|0004868500|4.8685{{Esp|24}}}} || 2nd planet ||align=right| {{nts|2180}} |- |3|| [[Jupiter]]||align=right|{{sort|1300000000|1.30{{Esp|9}}}}||align=right| {{sort|8510|8510}} ||align=right| {{sort|0625000|6.25{{Esp|5}}}} ||align=right| {{sort|1898600000|1.8986{{Esp|27}}}} || 5th planet ||align=right| {{nts|6220000}} |- |4|| [[Saturn]] ||align=right|{{sort|0046800000|4.68{{Esp|7}}}}||align=right| {{sort|0308|308}} ||align=right| {{sort|0190000|1.9{{Esp|5}}}} ||align=right| {{sort|0568460000|5.6846{{Esp|26}}}} || 6th planet ||align=right| {{nts|1250000}} |- |5|| [[Mars]] ||align=right|{{sort|0000000942|9.42{{Esp|2}}}}||align=right| {{sort|0000.0061|0.0061}} ||align=right| {{sort|0180000|1.8{{Esp|5}}}} ||align=right| {{sort|0000641850|6.4185{{Esp|23}}}} || 4th planet ||align=right| {{nts|146}} |- |6|| [[Mercury (planet)|Mercury]]||align=right|{{sort|0000001950|1.95{{Esp|3}}}}||align=right| {{sort|0000.0126|0.0126}} ||align=right| {{sort|0091000|9.1{{Esp|4}}}} ||align=right| {{sort|0000330220|3.3022{{Esp|23}}}} || 1st planet ||align=right| {{nts|60}} |- |7|| [[Uranus]]||align=right|{{sort|0000384000|3.84{{Esp|5}}}}||align=right| {{sort|0002.51|2.51}} ||align=right| {{sort|0029000|2.9{{Esp|4}}}} ||align=right| {{sort|0086832000|8.6832{{Esp|25}}}} || 7th planet ||align=right| {{nts|102000}} |- |8|| [[Neptune]] ||align=right|{{sort|0000273000|2.73{{Esp|5}}}}||align=right| {{sort|0001.79|1.79}} ||align=right| {{sort|0024000|2.4{{Esp|4}}}} ||align=right| {{sort|0102430000|1.0243{{Esp|26}}}} || 8th planet ||align=right| {{nts|127000}} |- |9|| [[Ceres (dwarf planet)|Ceres]]||align=right|{{sort|0000000000.000832|8.32{{Esp|&minus;4}}}}||align=right| {{sort|0000.0000000087|8.7{{Esp|&minus;9}}}} ||align=right| {{sort|0000000.33|0.33}} ||align=right| {{sort|0000000943|9.43{{Esp|20}}}} || 1st [[dwarf planet]] ||align=right| {{nts|0.0245}} |- |10|| [[Eris (dwarf planet)|Eris]]||align=right|{{sort|0000000000.00215|2.15{{Esp|&minus;3}}}}||align=right| {{sort|0000.000000035|1.33{{Esp|&minus;8}}}} ||align=right| {{sort|0000000.10|0.10}} ||align=right| {{sort|0000016700|1.67{{Esp|22}}}} || 3rd [[dwarf planet]] ||align=right| {{nts|1.13}} |- |11|| [[Pluto]] ||align=right|{{sort|0000000000.00295|2.95{{Esp|&minus;3}}}}||align=right| {{sort|0000.0000000195|1.95{{Esp|&minus;8}}}} ||align=right| {{sort|0000000.077|0.077}} ||align=right| {{sort|0000012900|1.29{{Esp|22}}&nbsp;±&nbsp;10%}} || 2nd [[dwarf planet]] ||align=right| {{nts|0.812}} |- |12|| [[Makemake (dwarf planet)|Makemake]]||align=right|{{sort|0000000000.000222|2.22{{Esp|&minus;4}}}}||align=right|{{sort|0000.00000000145|1.45{{Esp|&minus;9}}}}||align=right| {{sort|0000000.02|0.02<ref name=est>Calculated using the estimate for the mass of the Kuiper belt found in [http://adsabs.harvard.edu/abs/2007MNRAS.tmp...24I Iorio, 2007] of 0.033 Earth masses</ref>}} ||align=right| {{sort|0000004000|~4{{Esp|21}}}} || 4th [[dwarf planet]] ||align=right| {{nts|0.168}} |- |13|| [[Haumea (dwarf planet)|Haumea]] ||align=right|{{sort|0000000000.000268|2.68{{Esp|&minus;4}}}}||align=right| {{sort|0000.00000000172|1.72{{Esp|&minus;9}}}} ||align=right| {{sort|0000000.02|0.02<ref name=est/>}} ||align=right| {{sort|0000004200|4.2&nbsp;±&nbsp;0.1{{Esp|21}}}}|| 5th [[dwarf planet]] ||align=right| {{nts|0.179}} |} ==Controversy== [[Image:TheKuiperBelt 75AU All.svg|right|thumb|400px|Orbits of celestial bodies in the Kuiper Belt with approximate distances and inclination. Objects marked with red are in orbital resonances with Neptune, with Pluto (the largest red circle) located in the "spike" of plutinos at the 2:3 resonance]] Stern, currently leading the [[NASA]] ''[[New Horizons]]'' mission to Pluto, disagrees with the reclassification of Pluto on the basis that—like Pluto—[[Earth]], [[Mars]], Jupiter and Neptune have not cleared their orbital neighbourhoods either. Earth co-orbits with 10,000 [[near-Earth asteroid]]s, and Jupiter has 100,000 [[Trojan_asteroid|Trojan asteroids]] in its orbital path. "If Neptune had cleared its zone, Pluto wouldn't be there," he now says.<ref>{{cite web| url=http://news.bbc.co.uk/2/hi/science/nature/5283956.stm| title=Pluto vote 'hijacked' in revolt| publisher=BBC News| date=25 August 2006| accessdate = 2006-09-03| first=Paul| last= Rincon}}</ref> However, in 2000 Stern himself wrote, "we define an ''überplanet'' as a planetary body in orbit about a star that is dynamically important enough to have cleared its neighboring planetesimals ..." and a few paragraphs later, "From a dynamical standpoint, our solar system clearly contains 8 überplanets"—including Earth, Mars, Jupiter, and Neptune.<ref name="Stern 2002"/> Most planetary scientists understand "clearing the neighborhood" to refer to an object being the dominant mass in its vicinity, for instance Earth being many times more massive than all of the NEAs combined, and Neptune "dwarfing" Pluto and the rest of the [[Kuiper Belt Objects|KBO]]s.<ref name="Soter 2006"/> Stern and Levison's paper shows that it is possible to estimate whether an object is likely to dominate its neighborhood given only the object's mass and orbital period, known values even for extrasolar planets. In any case, the recent IAU definition specifically limits itself only to objects orbiting the Sun.<ref name="IAU definition"/> ==See also== *[[Planet]] *[[List of Solar System objects in hydrostatic equilibrium]] *[[List of Solar System objects by size]] *[[List of Solar System objects]] *[[List of noteworthy asteroids]] *[[Definition of planet]] *[[2006 definition of planet]] *[[Mesoplanet]] ==References== <!-- ---------------------------------------------------------- See http://en.wikipedia.org/wiki/Wikipedia:Footnotes for a discussion of different citation methods and how to generate footnotes using the <ref> </ref> and <reference /> tags ----------------------------------------------------------- --> <div class="references-small"> <references /> </div> ==External links== * [[Ottawa Citizen]]: [http://www.canada.com/ottawacitizen/news/story.html?id=a9591aed-f19f-4ac3-a324-1f8bb46d9379&p=2 The case against Pluto] (P. Surdas Mohit) Thursday, August 24, 2006 {{Solar System}} [[Category:Planetary science]] [[Category:Planets]] [[de:Planetarische Diskriminante]] [[fr:Élimination du voisinage]] [[it:Dominanza orbitale]] [[hu:Bolygópálya tisztára söprése]] [[zh:清除鄰近的小天體]]'