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==Scientific search==
{{main|Astrobiology}}
The scientific search for extraterrestrial life is being carried out both directly and indirectly. {{As of|2017|09}}, 3,667 [[exoplanet]]s in 2,747 [[Planetary system|systems]] have been [[Exoplanet|identified]], and other planets and moons in the [[Solar System]] hold the potential for hosting primitive life such as [[microorganism]]s. As of 8 February 2021, an updated status of studies considering the possible detection of [[lifeform]]s on Venus (via [[phosphine]]) and Mars (via [[methane]]) was reported.<ref name="NYT-20210208">{{cite news |last1=Chang |first1=Kenneth |last2=Stirone |first2=Shannon |title=Life on Venus? The Picture Gets Cloudier – Despite doubts from many scientists, a team of researchers who said they had detected an unusual gas in the planet's atmosphere were still confident of their findings. |url=https://www.nytimes.com/2021/02/08/science/venus-life-phosphine.html |date=8 February 2021 |work=[[The New York Times]] |access-date=8 February 2021 }}</ref>

===Direct search===
[[File:NASA-WhatBiosignaturesDoesLifeProduce-20180625.jpg|thumb|upright|Lifeforms produce a variety of biosignatures that may be detectable by telescopes.<ref name="NASA-20180625">{{cite web |url=https://www.jpl.nasa.gov/news/news.php?feature=7171 |title=NASA Asks: Will We Know Life When We See It? |publisher=[[NASA]] |last1=Cofield |first1=Calla |last2=Chou |first2=Felicia |date=25 June 2018 |access-date=26 June 2018}}</ref><ref name="EA-20180625">{{cite news |url=https://ucrtoday.ucr.edu/54211 |title=UCR Team Among Scientists Developing Guidebook for Finding Life Beyond Earth |work=UCR Today |publisher=[[University of California, Riverside]] |first=Sarah |last=Nightingale |date=25 June 2018 |access-date=26 June 2018}}</ref>]]

Scientists search for [[biosignature]]s within the [[Solar System]] by studying planetary surfaces and examining [[Meteoroid|meteorites]].<ref name="IJA-2014October">{{cite journal |last=Loeb |first=Abraham |author-link=Abraham Loeb |title=The Habitable Epoch of the Early Universe |date=October 2014 |journal=[[International Journal of Astrobiology]] |volume=13 |issue=4 |pages=337–339 |doi=10.1017/S1473550414000196 |arxiv=1312.0613 |bibcode=2014IJAsB..13..337L|citeseerx=10.1.1.748.4820 |s2cid=2777386 }}</ref><ref name="NYT-20141202">{{cite news |last=Dreifus |first=Claudia |author-link=Claudia Dreifus |title=Much-Discussed Views That Go Way Back – Avi Loeb Ponders the Early Universe, Nature and Life |url=https://www.nytimes.com/2014/12/02/science/avi-loeb-ponders-the-early-universe-nature-and-life.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2014/12/02/science/avi-loeb-ponders-the-early-universe-nature-and-life.html |archive-date=2022-01-01 |url-access=limited |date=2 December 2014 |work=[[The New York Times]] |access-date=3 December 2014}}{{cbignore}}</ref> Some claim to have identified evidence that microbial life has existed on Mars.<ref name=disbelief>{{cite web |title=Experts: Little Evidence of Life on Mars |url=http://dsc.discovery.com/news/2006/08/08/marslife_spa.html?category=space&guid=20060808100030 |last=Crenson |first=Matt |publisher=[[Associated Press]] |date=6 August 2006 |access-date=8 March 2011 |archive-url=https://web.archive.org/web/20110416094930/http://dsc.discovery.com/news/2006/08/08/marslife_spa.html?category=space&guid=20060808100030 |archive-date=16 April 2011 |url-status=dead }}</ref><ref name="life">{{cite journal |title=Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001 |journal=Science |first1=David S. |last1=McKay | first2=Everett K. Jr. |last2=Gibson |first3=Kathie L. |last3=Thomas-Keprta |first4=Hojatollah |last4=Vali |first5=Christopher S. |last5=Romanek |first6=Simon J. |last6=Clemett |first7=Xavier D. F. |last7=Chillier |first8=Claude R. |last8=Maechling |first9=Richard N. |last9=Zare |s2cid=40690489 |display-authors=5 |volume=273 |issue=5277 |pages=924–930 |date=August 1996 |doi=10.1126/science.273.5277.924 |bibcode=1996Sci...273..924M |pmid=8688069}}</ref><ref name="NASA-20140227">{{cite web |last=Webster |first=Guy |title=NASA Scientists Find Evidence of Water in Meteorite, Reviving Debate Over Life on Mars |url=http://www.jpl.nasa.gov/news/news.php?release=2014-065&1 |date=27 February 2014 |work=[[NASA]] |access-date=27 February 2014}}</ref><ref name="SP-20140228">{{cite web |last=Gannon |first=Megan |title=Mars Meteorite with Odd 'Tunnels' & 'Spheres' Revives Debate Over Ancient Martian Life |url=http://www.space.com/24834-strange-mars-meteorite-life-evidence-debate.html |date=28 February 2014 |work=[[Space.com]] |access-date=28 February 2014}}</ref> An experiment on the two [[Viking program|Viking]] Mars landers reported gas emissions from heated Martian soil samples that some scientists argue are consistent with the presence of living microorganisms.<ref name="Chambers">{{cite book |first=Paul |last=Chambers |title=Life on Mars; The Complete Story |place=London |publisher=Blandford |date=1999 |isbn=978-0-7137-2747-0 |url=https://archive.org/details/lifeonmarscomple00cham }}</ref> Lack of corroborating evidence from other experiments on the same samples suggests that a non-biological reaction is a more likely hypothesis.<ref name="Chambers"/><ref>{{cite journal |title=The Viking Biological Investigation: Preliminary Results |journal=Science |date=1 October 1976 |first1=Harold P. |last1=Klein |last2=Levin |first2=Gilbert V. |last3=Levin |first3=Gilbert V. |last4=Oyama |first4=Vance I. |last5=Lederberg |first5=Joshua |last6=Rich |first6=Alexander |last7=Hubbard |first7=Jerry S. |last8=Hobby |first8=George L. |last9=Straat |first9=Patricia A. |last10=Berdahl |first10=Bonnie J. |last11=Carle |first11=Glenn C. |last12=Brown |first12=Frederick S. |last13=Johnson |first13=Richard D. |volume=194 |issue=4260 |pages=99–105 |doi=10.1126/science.194.4260.99 |pmid=17793090 |bibcode=1976Sci...194...99K |s2cid=24957458 }}</ref><ref name="Beegle">{{cite journal |title=A Concept for NASA's Mars 2016 Astrobiology Field Laboratory |journal=Astrobiology |date=August 2007 |last1=Beegle |first1=Luther W. |last2=Wilson |first2=Michael G. |volume=7 |issue=4 |pmid=17723090 |pages=545–577 |doi=10.1089/ast.2007.0153 |bibcode=2007AsBio...7..545B |last3=Abilleira |first3=Fernando |last4=Jordan |first4=James F. |last5=Wilson |first5=Gregory R.}}</ref><ref>{{cite web |url=http://www.esa.int/SPECIALS/ExoMars/SEMK39JJX7F_0.html |title=ExoMars rover |publisher=ESA |access-date=14 April 2014 |archive-date=19 October 2012 |archive-url=https://web.archive.org/web/20121019105332/http://www.esa.int/SPECIALS/ExoMars/SEMK39JJX7F_0.html |url-status=dead }}</ref> In 1996, a controversial report stated that structures resembling [[Nanobacterium|nanobacteria]] were discovered in a meteorite, [[ALH84001]], formed of [[martian meteorite|rock ejected from Mars]].<ref name=disbelief/><ref name="life"/>

[[File:ALH84001 structures.jpg|thumb|left|Electron micrograph of Martian meteorite [[ALH84001]] showing structures that some scientists think could be fossilized bacteria-like life forms]]

In February 2005 NASA scientists reported they may have found some evidence of extraterrestrial life on Mars.<ref>{{cite news |url=http://www.space.com/scienceastronomy/mars_life_050216.html |title=Exclusive: NASA Researchers Claim Evidence of Present Life on Mars |last=Berger |first=Brian |date=2005-02-16 |work=Space.com}}</ref> The two scientists, Carol Stoker and Larry Lemke of NASA's [[Ames Research Center]], based their claim on methane signatures found in Mars's atmosphere resembling the methane production of some forms of primitive life on Earth, as well as on their own study of primitive life near the [[Rio Tinto river]] in Spain. NASA officials soon distanced NASA from the scientists' claims, and Stoker herself backed off from her initial assertions.<ref>{{cite news |url=http://www.spacetoday.net/Summary/2804 |title=NASA denies Mars life reports |publisher=spacetoday.net |date=2005-02-19}}</ref> Though such methane findings are still debated, support among some scientists for the existence of life on Mars exists.<ref name="Spotts">{{cite news |last=Spotts |first=Peter N. |title=Sea boosts hope of finding signs of life on Mars |newspaper=The Christian Science Monitor |date=28 February 2005 |url=http://www.csmonitor.com/2005/0228/p02s02-usgn.html |access-date=18 December 2006}}</ref>

In November 2011 NASA launched the [[Mars Science Laboratory]] that landed the ''Curiosity'' rover on Mars. It is designed to assess the past and present habitability on Mars using a variety of scientific instruments. The rover landed on Mars at [[Gale (crater)|Gale Crater]] in August 2012.<ref name="Gale Crater2">{{cite web |last1=Chow |first1=Dennis |title=NASA's Next Mars Rover to Land at Huge Gale Crater |url=http://www.space.com/12394-nasa-mars-rover-landing-site-unveiled.html |date=22 July 2011 |publisher=[[Space.com]] |access-date=22 July 2011}}</ref><ref name="Gale Crater3">{{cite news |last1=Amos |first1=Jonathan |title=Mars rover aims for deep crater |date=22 July 2011 |url=https://www.bbc.co.uk/news/science-environment-14249524 |work=[[BBC News]] |access-date=22 July 2011}}</ref>

The [[Gaia hypothesis]] stipulates that any planet with a robust population of life will have an atmosphere in chemical disequilibrium, which is relatively easy to determine from a distance by [[spectroscopy]]. However, significant advances in the ability to find and resolve light from smaller rocky worlds near their stars are necessary before such spectroscopic methods can be used to analyze extrasolar planets. To that effect, the [[Carl Sagan Institute]] was founded in 2014 and is dedicated to the atmospheric characterization of exoplanets in [[circumstellar habitable zone]]s.<ref>{{cite web |url=http://instituteforpalebluedots.com/2015/01/what-is-the-institute-for-pale-blue-dots/ |title=Introducing: The Carl Sagan Institute |last=Glaser |first=Linda |date=27 January 2015 |access-date=11 May 2015 |url-status=dead |archive-url=https://web.archive.org/web/20150227012302/http://instituteforpalebluedots.com/2015/01/what-is-the-institute-for-pale-blue-dots/ |archive-date=27 February 2015 }}</ref><ref name='CSI-Research'>{{cite web |url=http://instituteforpalebluedots.com/research/ |title=Carl Sagan Institute – Research |date=May 2015 |access-date=11 May 2015}}</ref> Planetary spectroscopic data will be obtained from telescopes like [[Wide Field Infrared Survey Telescope|WFIRST]] and [[Extremely Large Telescope|ELT]].<ref name="ColorCatalog">{{cite news |last=Cofield |first=Calla |url=http://www.space.com/28906-alien-life-earth-microbe-catalog.html |title=Catalog of Earth Microbes Could Help Find Alien Life |work=Space.com |date=30 March 2015 |access-date=11 May 2015}}</ref>

[[File:Green Bank 100m diameter Radio Telescope.jpg|thumb|The [[Green Bank Telescope]] is one of the [[radio telescope]]s used by the [[Breakthrough Listen]] project to search for alien communications.]]
In August 2011, findings by NASA, based on studies of [[meteorite]]s found on Earth, suggest DNA and [[RNA]] components ([[adenine]], [[guanine]] and related [[organic molecules]]), building blocks for life as we know it, may be formed extraterrestrially in [[outer space]].<ref name="Callahan">{{cite journal |last1=Callahan |first1=M.P. |last2=Smith |first2=K.E. |last3=Cleaves |first3=H.J. |last4=Ruzica |first4=J. |last5=Stern |first5=J.C. |last6=Glavin |first6=D.P. |last7=House |first7=C.H. |last8=Dworkin |first8=J.P. |date=11 August 2011 |title=Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases |doi=10.1073/pnas.1106493108 |volume=108 |issue=34 |journal=Proceedings of the National Academy of Sciences |pages=13995–13998 |bibcode=2011PNAS..10813995C |pmid=21836052 |pmc=3161613|doi-access=free }}</ref><ref name="Steigerwald">{{cite web |last=Steigerwald |first=John |title=NASA Researchers: DNA Building Blocks Can Be Made in Space |url=http://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html |publisher=[[NASA]] |date=8 August 2011 |access-date=10 August 2011}}</ref><ref name="DNA">{{cite web |title=DNA Building Blocks Can Be Made in Space, NASA Evidence Suggests |url=https://www.sciencedaily.com/releases/2011/08/110808220659.htm |date=9 August 2011 |website=[[ScienceDaily]] |access-date=9 August 2011}}</ref> In October 2011, scientists reported that [[cosmic dust]] contains complex [[organic compound|organic]] matter ("amorphous organic solids with a mixed [[aromatic]]-[[aliphatic]] structure") that could be created naturally, and rapidly, by [[stars]].<ref name="Space-20111026">{{cite web |last=Chow |first=Denise |title=Discovery: Cosmic Dust Contains Organic Matter from Stars |url=http://www.space.com/13401-cosmic-star-dust-complex-organic-compounds.html |date=26 October 2011 |publisher=[[Space.com]] |access-date=26 October 2011}}</ref><ref name="ScienceDaily-20111026">{{cite web |title=Astronomers Discover Complex Organic Matter Exists Throughout the Universe |url=https://www.sciencedaily.com/releases/2011/10/111026143721.htm |date=26 October 2011 |website=[[ScienceDaily]] |access-date=27 October 2011}}</ref><ref name="Nature-20111026">{{cite journal |last1=Kwok |first1=Sun |last2=Zhang |first2=Yong |title=Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features |date=26 October 2011 |journal=[[Nature (journal)|Nature]] |doi=10.1038/nature10542 |volume=479 |issue=7371 |pages=80–3 |bibcode=2011Natur.479...80K |pmid=22031328|s2cid=4419859 }}</ref> One of the scientists suggested that these compounds may have been related to the development of life on Earth and said that, "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."<ref name="Space-20111026"/>

In August 2012, and in a world first, astronomers at [[Copenhagen University]] reported the detection of a specific sugar molecule, [[glycolaldehyde]], in a distant star system. The molecule was found around the [[protostar|protostellar]] binary ''IRAS 16293-2422'', which is located 400 light years from Earth.<ref name="NG-20120829">{{cite journal |title=Sugar Found in Space |journal=National Geographic |last=Than |first=Ker |date=29 August 2012 |url=http://news.nationalgeographic.com/news/2012/08/120829-sugar-space-planets-science-life/ |access-date=31 August 2012}}</ref><ref name="AP-20120829">{{cite news |title=Sweet! Astronomers spot sugar molecule near star |url=http://apnews.excite.com/article/20120829/DA0V31D80.html |date=29 August 2012 |agency=Associated Press |access-date=31 August 2012}}</ref> Glycolaldehyde is needed to form [[ribonucleic acid]], or [[RNA]], which is similar in function to DNA. This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.<ref>{{cite journal |url=http://www.eso.org/public/archives/releases/sciencepapers/eso1234/eso1234a.pdf |title=Detection of the simplest sugar, glycolaldehyde, in a solar-type protostar with ALMA |journal=The Astrophysical Journal Letters |first1=Jes K. |last1=Jørgensen |first2=Cécile |last2=Favre |first3=Suzanne E. |last3=Bisschop |first4=Tyler L. |last4=Bourke |first5=Ewine F. |last5=van Dishoeck |first6=Markus |last6=Schmalzl |volume=757 |issue=1 |at=L4 |date=September 2012 |doi=10.1088/2041-8205/757/1/L4 |bibcode=2012ApJ...757L...4J |arxiv=1208.5498|s2cid=14205612 }}</ref>

===Indirect search===
Projects such as [[SETI]] are monitoring the galaxy for electromagnetic [[interstellar communication]]s from civilizations on other worlds.<ref name="Schenkel_2006">{{cite web |title=SETI Requires a Skeptical Reappraisal |last=Schenkel |first=Peter |publisher=Skeptical Inquirer |date=May–June 2006 |volume=30 |number=3 |access-date=28 June 2009 |url=http://www.csicop.org/si/show/seti_requires_a_skeptical_reappraisal/}}</ref><ref>{{cite web |title=Why SETI is science and UFOlogy is not |last=Moldwin |first=Mark |publisher=Skeptical Inquirer |date=November 2004 |url=http://www.csicop.org/si/2004-11/ |url-status=dead |archive-url=https://web.archive.org/web/20090313001822/http://www.csicop.org/si/2004-11/ |archive-date=13 March 2009 }}</ref> If there is an advanced extraterrestrial civilization, there is no guarantee that it is transmitting radio communications in the direction of Earth or that this information could be interpreted as such by humans. The length of time required for a signal to travel across the vastness of space means that any signal detected would come from the distant past.<ref>{{cite web |url=http://www.coseti.org/ |publisher=The Columbus [[Optical SETI]] Observatory |title=The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum}}</ref>

The presence of heavy elements in a star's light-spectrum is another potential [[biosignature]]; such elements would (in theory) be found if the star were being used as an incinerator/repository for nuclear waste products.<ref>{{cite journal |last1=Whitmire |first1=Daniel P. |last2=Wright |first2=David P. |title=Nuclear waste spectrum as evidence of technological extraterrestrial civilizations |journal=Icarus |date=April 1980 |volume=42 |issue=1 |pages=149–156 |doi=10.1016/0019-1035(80)90253-5 |bibcode=1980Icar...42..149W}}</ref>

===Extrasolar planets===
{{Main|Exoplanet}}
{{See also|List of multiplanetary systems}}
[[File:Glieseupdated.jpg|thumb|Artist's impression of [[Gliese 581 c]], the first [[terrestrial planet|terrestrial extrasolar planet]] discovered within its star's habitable zone]]
[[File:NASA-KeplerSpaceTelescope-ArtistConcept-20141027.jpg|thumb|Artist's impression of the [[Kepler (spacecraft)|Kepler telescope]]]]

Some astronomers search for [[exoplanet|extrasolar planets]] that may be conducive to life, narrowing the search to [[terrestrial planet]]s within the [[Circumstellar habitable zone|habitable zones]] of their stars.<ref name="Earth-likeplanet1">{{cite web |url=http://planet.iap.fr/OB05390.news.html |date=25 January 2006 |title=Discovery of OGLE 2005-BLG-390Lb, the first cool rocky/icy exoplanet |work=IAP.fr}}</ref><ref name="GlieseSpace">{{cite news |url=http://www.space.com/3728-major-discovery-planet-harbor-water-life.html |title=Major Discovery: New Planet Could Harbor Water and Life |work=Space.com |first=Ker |last=Than |date=24 April 2007}}</ref> Since 1992, over four thousand exoplanets have been discovered ({{Extrasolar planet counts|planet_count}} planets in {{Extrasolar planet counts|system_count}} [[planetary system]]s including {{Extrasolar planet counts|multiplanetsystem_count}} [[List of multiplanetary systems|multiple planetary systems]] as of {{Extrasolar planet counts|asof}}).<ref name="Encyclopaedia"/>
The extrasolar planets so far discovered range in size from that of [[terrestrial planet]]s similar to Earth's size to that of gas giants larger than Jupiter.<ref name="Encyclopaedia">{{cite encyclopedia |title=Interactive Extra-solar Planets Catalog |encyclopedia=[[The Extrasolar Planets Encyclopaedia]] |url=http://exoplanet.eu/catalog.php |last=Schneider |first=Jean |date=10 September 2011 |access-date=30 January 2012}}</ref> The number of observed exoplanets is expected to increase greatly in the coming years.<ref>{{cite news |url=http://www.space.com/15160-alien-planet-kepler-mission-2016.html |title=NASA Extends Planet-Hunting Kepler Mission Through 2016 |work=Space.com |first=Mike |last=Wall |date=4 April 2012}}</ref>

The [[Kepler (spacecraft)|Kepler space telescope]] has also detected a few thousand<ref name=keplersite>{{cite web |title=NASA – Kepler |url=http://www.nasa.gov/mission_pages/kepler/main/index.html |access-date=4 November 2013 |url-status=dead |archive-url=https://web.archive.org/web/20131105082102/http://www.nasa.gov/mission_pages/kepler/main/index.html |archive-date=5 November 2013 }}</ref><ref name="usher">{{cite web |last1=Harrington |first1=J. D. |last2=Johnson |first2=M. |date=4 November 2013 |title=NASA Kepler Results Usher in a New Era of Astronomy |url=http://www.nasa.gov/press/2013/november/nasa-kepler-results-usher-in-a-new-era-of-astronomy/}}</ref> candidate planets,<ref>{{Cite journal |doi=10.1088/0067-0049/206/1/5 |arxiv=1212.2915 |title=Detection of Potential Transit Signals in the First 12 Quarters of ''Kepler'' Mission Data |journal=The Astrophysical Journal Supplement Series |volume=206 |issue=1 |pages=5 |year=2013 |last1=Tenenbaum |first1=P. |last2=Jenkins |first2=J. M. |last3=Seader |first3=S. |last4=Burke |first4=C. J. |last5=Christiansen |first5=J. L. |last6=Rowe |first6=J. F. |last7=Caldwell |first7=D. A. |last8=Clarke |first8=B. D. |last9=Li |first9=J. | last10 = Quintana | first10 = E. V. |last11=Smith |first11=J. C. |last12=Thompson |first12=S. E. |last13=Twicken |first13=J. D. |last14=Borucki |first14=W. J. |last15=Batalha |first15=N. M. |last16=Cote |first16=M. T. |last17=Haas |first17=M. R. |last18=Hunter |first18=R. C. |last19=Sanderfer |first19=D. T. | last20 = Girouard | first20 = F. R. |last21=Hall |first21=J. R. |last22=Ibrahim |first22=K. |last23=Klaus |first23=T. C. |last24=McCauliff |first24=S. D. |last25=Middour |first25=C. K. |last26=Sabale |first26=A. |last27=Uddin |first27=A. K. |last28=Wohler |first28=B. |last29=Barclay |first29=T. | last30 = Still | first30 = M. |bibcode=2013ApJS..206....5T|s2cid=250885680 }}</ref><ref name=mygoditsfullofplanets>{{cite press release | url=http://phl.upr.edu/press-releases/mygoditsfullofplanetstheyshouldhavesentapoet | title=My God, it's full of planets! They should have sent a poet. | publisher=Planetary Habitability Laboratory, University of Puerto Rico at Arecibo | date=3 January 2012 | access-date=25 July 2015 | archive-date=25 July 2015 | archive-url=https://web.archive.org/web/20150725135354/http://phl.upr.edu/press-releases/mygoditsfullofplanetstheyshouldhavesentapoet | url-status=dead }}</ref> of which about 11% may be [[false positive]]s.<ref>{{cite journal |arxiv=1310.2133 |last1=Santerne |first1=A. |last2=Díaz |first2=R. F. |last3=Almenara |first3=J.-M. |last4=Lethuillier |first4=A. |last5=Deleuil |first5=M. |last6=Moutou |first6=C. |title=Astrophysical false positives in exoplanet transit surveys: Why do we need bright stars? |journal=Sf2A-2013: Proceedings of the Annual Meeting of the French Society of Astronomy and Astrophysics |date=2013|pages=555 |bibcode=2013sf2a.conf..555S }}</ref>

There is at least one planet on average per star.<ref name="Nature-20120111">{{Cite journal |display-authors=1 |last1=Cassan |first1=A. |last2=Kubas |first2=D. |last3=Beaulieu |first3=J. -P. |last4=Dominik |first4=M. |last5=Horne |first5=K. |last6=Greenhill |first6=J. |last7=Wambsganss |first7=J. |last8=Menzies |first8=J. |last9=Williams |first9=A. | last10 = Jørgensen |doi=10.1038/nature10684 | first10 = U. G. |last11=Udalski |first11=A. |last12=Bennett |first12=D. P. |last13=Albrow |first13=M. D. |last14=Batista |first14=V. |last15=Brillant |first15=S. |last16=Caldwell |first16=J. A. R. |last17=Cole |first17=A. |last18=Coutures |first18=C. |last19=Cook |first19=K. H. | last20 = Dieters | first20 = S. |last21=Prester |first21=D. D. |last22=Donatowicz |first22=J. |last23=Fouqué |first23=P. |last24=Hill |first24=K. |last25=Kains |first25=N. |last26=Kane |first26=S. |last27=Marquette |first27=J. -B. |last28=Martin |first28=R. |last29=Pollard |first29=K. R. | last30 = Sahu | first30 = K. C. |title=One or more bound planets per Milky Way star from microlensing observations |journal=Nature |volume=481 |issue=7380 |pages=167–169 |date=11 January 2012 |pmid=22237108 |bibcode=2012Natur.481..167C |arxiv=1202.0903|s2cid=2614136 }}</ref> About 1 in 5 [[Solar analog|Sun-like stars]]<ref group=lower-alpha name=footnoteA>For the purpose of this 1 in 5 statistic, "Sun-like" means [[G-type star]]. Data for Sun-like stars wasn't available so this statistic is an extrapolation from data about [[K-type star]]s</ref> have an "Earth-sized"<ref group=lower-alpha name=footnoteB>For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii</ref> planet in the [[habitable zone]],<ref group=lower-alpha name=footnoteC>For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun).</ref> with the nearest expected to be within 12 light-years distance from Earth.<ref name="ucb1in5">{{cite web |last=Sanders |first=R. |date=4 November 2013 |title=Astronomers answer key question: How common are habitable planets? |url=http://newscenter.berkeley.edu/2013/11/04/astronomers-answer-key-question-how-common-are-habitable-planets/ |work=newscenter.berkeley.edu}}</ref><ref name="earthsunhzprev">{{cite journal |last1=Petigura |first1=E. A. |last2=Howard |first2=A. W. |last3=Marcy |first3=G. W. |date=2013 |title=Prevalence of Earth-size planets orbiting Sun-like stars |journal=[[Proceedings of the National Academy of Sciences]] |volume=110 |issue=48 |pages=19273–19278 |arxiv=1311.6806 |bibcode=2013PNAS..11019273P |doi=10.1073/pnas.1319909110 |pmid=24191033 |pmc=3845182|doi-access=free }}</ref> Assuming 200&nbsp;billion stars in the Milky Way,<ref group=lower-alpha name=footnoteD>About 1/4 of stars are GK Sun-like stars. The number of stars in the galaxy is not accurately known, but assuming 200&nbsp;billion stars in total, the Milky Way would have about 50&nbsp;billion Sun-like (GK) stars, of which about 1 in 5 (22%) or 11&nbsp;billion would be Earth-sized in the habitable zone. Including red dwarfs would increase this to 40&nbsp;billion.</ref> that would be 11&nbsp;billion potentially habitable Earth-sized planets in the Milky Way, rising to 40&nbsp;billion if [[red dwarf]]s are included.<ref>{{cite news |last=Khan |first=Amina |title=Milky Way may host billions of Earth-size planets |url=https://www.latimes.com/science/la-sci-earth-like-planets-20131105,0,2673237.story |date=4 November 2013 |work=[[Los Angeles Times]] |access-date=5 November 2013}}</ref> The [[rogue planet]]s in the Milky Way possibly number in the trillions.<ref>{{cite journal |last1=Strigari |first1=L. E. |last2=Barnabè |first2=M. |last3=Marshall |first3=P. J. |last4=Blandford |first4=R. D. |title=Nomads of the Galaxy |date=2012 |volume=423 |issue=2 |pages=1856–1865 |journal=[[Monthly Notices of the Royal Astronomical Society]] |arxiv=1201.2687 |bibcode=2012MNRAS.423.1856S |doi=10.1111/j.1365-2966.2012.21009.x|s2cid=119185094 }} estimates 700 objects >10<sup>−6</sup> solar masses (roughly the mass of Mars) per main-sequence star between 0.08 and 1 Solar mass, of which there are billions in the Milky Way.</ref>

The nearest known exoplanet is [[Proxima Centauri b]], located {{convert|4.2|ly|pc|lk=on}} from Earth in the southern [[constellation]] of [[Centaurus]].<ref name="nyt20160824">{{cite news |url=https://www.nytimes.com/2016/08/25/science/earth-planet-proxima-centauri.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2016/08/25/science/earth-planet-proxima-centauri.html |archive-date=2022-01-01 |url-access=limited |title=One Star Over, a Planet That Might Be Another Earth |work=The New York Times |first=Kenneth |last=Chang |date=24 August 2016 |access-date=4 September 2016}}{{cbignore}}</ref>

{{asof|March 2014}}, the [[List of exoplanet extremes#Planetary characteristics|least massive exoplanet]] known is [[PSR B1257+12 A]], which is about twice the mass of the [[Moon]]. The [[List of exoplanet extremes#Planetary characteristics|most massive planet]] listed on the [[NASA Exoplanet Archive]] is [[DENIS-P J082303.1-491201 b]],<ref name="CT-Exo-2014">{{cite web |title=DENIS-P J082303.1-491201 b |url=http://exoplanetarchive.ipac.caltech.edu/cgi-bin/DisplayOverview/nph-DisplayOverview?objname=DENIS-P+J082303.1-491201+b&type=CONFIRMED_PLANET |work=[[Caltech]] |access-date=8 March 2014}}</ref><ref name="HU-201308">{{Cite journal |last1=Sahlmann |first1=J. |last2=Lazorenko |first2=P. F. |last3=Ségransan |first3=D. |last4=Martín |first4=Eduardo L. |last5=Queloz |first5=D. |last6=Mayor |first6=M. |last7=Udry |first7=S. |title=Astrometric orbit of a low-mass companion to an ultracool dwarf |volume=556 |pages=133 |bibcode=2013A&A...556A.133S |date=August 2013 |arxiv=1306.3225 |journal=Astronomy & Astrophysics |doi=10.1051/0004-6361/201321871|s2cid=119193690 }}</ref> about 29 times the mass of [[Jupiter]], although according to most definitions of a [[planet]], it is too massive to be a planet and may be a [[brown dwarf]] instead. Almost all of the planets detected so far are within the Milky Way, but there have also been a few possible detections of [[extragalactic planet]]s. The study of [[planetary habitability]] also considers a wide range of other factors in determining the suitability of a planet for hosting life.<ref name="NYT-20150106-DB"/>

One sign that a planet probably already contains life is the presence of an atmosphere with significant amounts of [[oxygen]], since that gas is highly reactive and generally would not last long without constant replenishment. This replenishment occurs on Earth through photosynthetic organisms. One way to analyze the atmosphere of an exoplanet is through [[spectrography]] when it [[Transit (astronomy)|transit]]s its star, though this might only be feasible with dim stars like [[white dwarf]]s.<ref name="hscfa20130225">{{cite web |url=https://www.cfa.harvard.edu/news/2013-06 |title=Future Evidence for Extraterrestrial Life Might Come from Dying Stars |publisher=Harvard-Smithsonian Center for Astrophysics |first1=David A. |last1=Aguilar |first2=Christine |last2=Pulliam |date=25 February 2013 |access-date=9 June 2017 |id=Release 2013-06}}</ref>

===Terrestrial analysis===
The science of [[astrobiology]] considers life on Earth as well, and in the broader astronomical context. In 2015, "remains of [[Biotic material|biotic life]]" were found in 4.1&nbsp;billion-year-old rocks in [[Western Australia]], when the [[Early Earth|young Earth]] was about 400 million years old.<ref name="AP-20151019">{{cite news |last=Borenstein |first=Seth |title=Hints of life on what was thought to be desolate early Earth |url=http://apnews.excite.com/article/20151019/us-sci--earliest_life-a400435d0d.html |date=19 October 2015 |work=[[Excite (web portal)|Excite]] |location=Yonkers, NY |publisher=[[Mindspark Interactive Network]] |agency=[[Associated Press]] |url-status=dead |archive-url=https://web.archive.org/web/20151023200248/http://apnews.excite.com/article/20151019/us-sci--earliest_life-a400435d0d.html |archive-date=23 October 2015 |access-date=8 October 2018}}</ref><ref name="PNAS-20151014-pdf">{{cite journal |last1=Bell |first1=Elizabeth A. |last2=Boehnike |first2=Patrick |last3=Harrison |first3=T. Mark |last4=Mao |first4=Wendy L. |display-authors=3 |date=19 October 2015 |title=Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon |journal=Proc. Natl. Acad. Sci. U.S.A. |doi=10.1073/pnas.1517557112 |issn=1091-6490 |pmid=26483481 |pmc=4664351 |volume=112 |issue=47 |pages=14518–21 |bibcode=2015PNAS..11214518B |doi-access=free }} Early edition, published online before print.</ref> According to one of the researchers, "If life arose relatively quickly on Earth, then it could be common in the [[universe]]."<ref name="AP-20151019" />