Editing Extraterrestrial life (section)

==Scientific search==
{{main|Astrobiology}}
The science that searches and studies life in the universe, both on Earth and elsewhere, is called [[astrobiology]]. With the study of Earth's life, the only known form of life, astrobiology seeks to study how life starts and evolves and the requirements for its continuous existence. This helps to determine what to look for when searching for life in other celestial bodies. This is a complex area of study, and uses the combined perspectives of several scientific disciplines, such as [[astronomy]], [[biology]], [[chemistry]], [[geology]], [[Planetary oceanography|oceanography]], and [[atmospheric science]]s.<ref>{{cite web |url= https://depts.washington.edu/astrobio/wordpress/about-us/what-is-astrobiology/|title= What Is Astrobiology?|author= |date= |publisher= University of Washington|accessdate=April 28, 2023}}</ref>

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>

===Search for basic life===
[[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]]. 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> 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"/> Although all the unusual properties of the meteorite were eventually explained as the result of inorganic processes, the controversy over its discovery laid the groundwork for the development of astrobiology.<ref name=disbelief/>

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 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>

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>

A group of scientists at Cornell University started a catalog of microorganisms, with the way each one reacts to sunlight. The goal is to help with the search for similar organisms in exoplanets, as the starlight reflected by planets rich in such organisms would have a specific spectrum, unlike that of starlight reflected from lifeless planets. If Earth was studied from afar with this system, it would reveal a shade of green, as a result of the abundance of plants with photosynthesis.<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>

In August 2011, NASA studied [[meteorite]]s found on Antarctica, finding [[adenine]], [[guanine]], [[hypoxanthine]] and [[xanthine]]. Adenine and guanine are components of DNA, and the others are used in other biological processes. The studies ruled out pollution of the meteorites on Earth, as those components would not be freely available the way they were found in the samples. This discovery suggests that several [[organic molecules]] that serve as building blocks of life may be generated within asteroids and comets.<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 |archive-date=11 May 2020 |archive-url=https://web.archive.org/web/20200511192941/https://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html |url-status=dead }}</ref> In October 2011, scientists reported that [[cosmic dust]] contains complex [[organic compound]]s ("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> It is still unclear if those compounds played a role in the creation of life on Earth, but Sun Kwok, of the University of Hong Kong, thinks so. "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|IRAS 16293-2422]]'', which is located 400 light years from Earth.<ref>{{cite web |url= https://www.nationalgeographic.com/adventure/article/120829-sugar-space-planets-science-life|title= Sugar Found In Space: A Sign of Life?|author= Ker Than|date= August 30, 2012|publisher= National Geographic|accessdate=July 4, 2023}}</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>

In December 2023, astronomers reported the first time discovery, in the [[Plume (fluid dynamics)|plume]]s of [[Enceladus]], moon of the planet [[Saturn]], of [[hydrogen cyanide]], a possible chemical essential for [[life]]<ref name="ATL-20231205">{{cite news |last=Green |first=Jaime |title=What Is Life? - The answer matters in space exploration. But we still don't really know. |url=https://www.theatlantic.com/science/archive/2023/12/defining-life-existentialism-scientific-theory/676238/ |date=5 December 2023 |work=[[The Atlantic]] |url-status=live |archiveurl=https://archive.today/20231205121742/https://www.theatlantic.com/science/archive/2023/12/defining-life-existentialism-scientific-theory/676238/ |archivedate=5 December 2023 |accessdate=15 December 2023 }}</ref> as we know it, as well as other [[organic molecule]]s, some of which are yet to be better identified and understood. According to the researchers, "these [newly discovered] compounds could potentially support extant [[Microorganism|microbial communities]] or drive complex [[organic synthesis]] leading to the [[origin of life]]."<ref name="NYT-20231214kc">{{cite news |last=Chang |first=Kenneth |title=Poison Gas Hints at Potential for Life on an Ocean Moon of Saturn - A researcher who has studied the icy world said "the prospects for the development of life are getting better and better on Enceladus." |url=https://www.nytimes.com/2023/12/14/science/enceladus-moon-cyanide-life-saturn.html |date=14 December 2023 |work=[[The New York Times]] |url-status=live |archiveurl=https://archive.today/20231214210144/https://www.nytimes.com/2023/12/14/science/enceladus-moon-cyanide-life-saturn.html |archivedate=14 December 2023 |accessdate=15 December 2023 }}</ref><ref name="NA-20231214">{{cite journal |author=Peter, Jonah S. |display-authors=et al. |title=Detection of HCN and diverse redox chemistry in the plume of Enceladus |url=https://www.nature.com/articles/s41550-023-02160-0 |date=14 December 2023 |journal=[[Nature Astronomy]] |volume=8 |issue=2 |pages=164–173 |doi=10.1038/s41550-023-02160-0 |url-status=live |archiveurl=https://archive.today/20231215144349/https://www.nature.com/articles/s41550-023-02160-0 |archivedate=15 December 2023 |accessdate=15 December 2023 |arxiv=2301.05259 |bibcode=2024NatAs...8..164P |s2cid=255825649 }}</ref>

===Search for extraterrestrial intelligences===
{{main|Search for extraterrestrial intelligence}}
[[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.]]
Although most searches are focused on the biology of extraterrestrial life, an extraterrestrial intelligence capable enough to develop a [[civilization]] may be detectable by other means as well. Technology may generate [[technosignature]]s, effects on the native planet that may not be caused by natural causes. There are three main types of techno-signatures considered: [[interstellar communication]]s, effects on the atmosphere, and planetary-sized structures such as [[Dyson sphere]]s.<ref name="techno">{{cite web |url= https://exoplanets.nasa.gov/news/1765/searching-for-signs-of-intelligent-life-technosignatures/|title= Searching for Signs of Intelligent Life: Technosignatures|author= Pat Brennan|date= |publisher= NASA|accessdate=July 4, 2023}}</ref>

Organizations such as the [[SETI Institute]] search the cosmos for potential forms of communication. They started with [[radio waves]], and now search for [[laser pulse]]s as well. The challenge for this search is that there are natural sources of such signals as well, such as gamma-ray bursts and supernovae, and the difference between a natural signal and an artificial one would be in its specific patterns. Astronomers intend to use [[artificial intelligence]] for this, as it can manage large amounts of data and is devoid of biases and preconceptions.<ref name="techno"/> Besides, even if there is an advanced extraterrestrial civilization, there is no guarantee that it is transmitting radio communications in the direction of Earth. The length of time required for a signal to travel across space means that a potential answer may arrive decades or centuries after the initial message.<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 atmosphere of Earth is rich in [[nitrogen dioxide]] as a result of [[air pollution]], which can be detectable. The natural abundance of carbon, which is also relatively reactive, makes it likely to be a basic component of the development of a potential extraterrestrial technological civilization, as it is on Earth. [[Fossil fuel]]s may likely be generated and used on such worlds as well. The abundance of [[chlorofluorocarbon]]s in the atmosphere can also be a clear technosignature, considering their role in [[ozone depletion]]. [[Light pollution]] may be another technosignature, as multiple lights on the night side of a rocky planet can be a sign of advanced technological development. However, modern telescopes are not strong enough to study exoplanets with the required level of detail to perceive it.<ref name="techno"/>

The [[Kardashev scale]] proposes that a civilization may eventually start consuming energy directly from its local star. This would require giant structures built next to it, called Dyson spheres. Those speculative structures would cause an excess infrared radiation, that telescopes may notice. The infrared radiation is typical of young stars, surrounded by dusty [[protoplanetary disk]]s that will eventually form planets. An older star such as the Sun would have no natural reason to have excess infrared radiation.<ref name="techno"/> 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 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>