Editing Extraterrestrial life (section)

===Extrasolar planets===
{{Main|Exoplanet}}
{{See also|List of potentially habitable exoplanets}}
[[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]]
Some astronomers search for [[exoplanet|extrasolar planets]] that may be conducive to life, narrowing the search to [[terrestrial planet]]s within the 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 systems 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 planets similar to Earth's size to that of gas giants larger than Jupiter.<ref name="Encyclopaedia">{{cite encyclopedia |title=Interactive Extra-solar Planets Catalog |url=https://exoplanet.eu/catalog/ |last=Schneider |first=Jean |date=10 September 2011 |encyclopedia=[[Extrasolar Planets Encyclopaedia]] |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>{{Better source needed|reason=The reference said in 2016 that the Kepler telescope will continue searching for planets, but it has been retired in 2018. We need a reference that is not outdated.|date=July 2023}} The [[Kepler space telescope]] has also detected a few thousand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 main-sequence star|G-type star]]. Data for Sun-like stars wasn't available so this statistic is an extrapolation from data about [[K-type main-sequence star|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|doi-access=free |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>

{{as of|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 analyse 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>