View source for Extraterrestrial life

{{Short description|Life that does not originate on Earth}}
{{About|any kind of extraterrestrial life|aliens with human-like intelligence|Extraterrestrial intelligence}}
{{Use British English|date=June 2023}}
{{Use dmy dates|date=September 2019}}

{{Unsolved|astronomy|Could [[life]] have arisen elsewhere?<br />What are the [[Abiogenesis|requirements for life]]?<br />Are there [[exoplanet]]s like Earth?<br />How likely is the [[Extraterrestrial intelligence|evolution of intelligent life]]?}}

'''Extraterrestrial life''', or '''alien life''' (colloquially, '''alien'''), is [[life]] that originates from another world rather than on [[Earth]]. No extraterrestrial life has yet been scientifically conclusively detected. Such life might range from simple forms such as [[prokaryote]]s to [[Extraterrestrial intelligence|intelligent beings]], possibly bringing forth [[civilization]]s that might be [[Kardashev scale|far more, or far less, advanced]] than humans.<ref name="WP-20201231">{{cite news |last=Frank |first=Adam |date=31 December 2020 |title=A new frontier is opening in the search for extraterrestrial life – The reason we haven't found life elsewhere in the universe is simple: We haven't really looked until now. |newspaper=[[The Washington Post]] |url=https://www.washingtonpost.com/outlook/2020/12/31/breakthrough-listen-seti-technosignatures/ |url-access=subscription |access-date=1 January 2021}}</ref><ref name="NYT-20131118">{{cite news |last=Davies |author-link=Paul Davies |first=Paul |title=Are We Alone in the Universe? |url=https://www.nytimes.com/2013/11/19/opinion/are-we-alone-in-the-universe.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2013/11/19/opinion/are-we-alone-in-the-universe.html |archive-date=2022-01-01 |url-access=limited |date=18 November 2013 |work=[[The New York Times]] |access-date=20 November 2013}}{{cbignore}}</ref><ref>{{cite news |first=John |last=Pickrell |title=Top 10: Controversial pieces of evidence for extraterrestrial life |date=4 September 2006 |url=https://www.newscientist.com/article/dn9943-top-10-controversial-pieces-of-evidence-for-extraterrestrial-life.html |work=[[New Scientist]] |access-date=18 February 2011}}</ref> The [[Drake equation]] speculates about the existence of sapient life elsewhere in the universe. The science of extraterrestrial life is known as [[astrobiology]].

Speculation about the possibility of inhabited worlds beyond Earth dates back to antiquity. Early [[Christianity|Christian]] writers discussed the idea of a "plurality of worlds" as proposed by earlier thinkers such as [[Democritus]]; [[Augustine of Hippo|Augustine]] references [[Epicurus]]'s idea of innumerable worlds "throughout the boundless immensity of space" in ''[[The City of God]]''.<ref name=Crowe-2008>{{Cite book |last=Crowe |first=Michael J. |title=The extraterrestrial life debate, antiquity to 1915: a sourcebook |date=2008 |publisher=University of Notre Dame |isbn=978-0-268-02368-3 |location=Notre Dame, Ind}}</ref>

[[Pre-modern]] writers typically assumed extraterrestrial "worlds" were inhabited by living beings. [[William Vorilong]], in the 15th century, acknowledged the possibility [[Jesus]] could have visited extraterrestrial worlds to redeem their inhabitants.<ref name=Crowe-2008/>{{rp|26}} [[Nicholas of Cusa]] wrote in 1440 that Earth is "a brilliant star" like other celestial objects visible in space; which would appear similar to the [[Sun]], from an exterior perspective, due to a layer of "fiery brightness" in the outer layer of the atmosphere. He theorized all extraterrestrial bodies could be inhabited by men, plants, and animals, including the Sun.<ref>{{cite book|author=Nicholas of Cusa.|translator=Germain Heron |title=Of Learned Ignorance|pages=111–118|publisher=Routledge|date=1954}}</ref> [[René Descartes|Descartes]] wrote that there were no means to prove the stars were not inhabited by "intelligent creatures", but their existence was a matter of speculation.<ref name=Crowe-2008/>{{rp|67}}

When considering the [[atmospheric composition]] and ecosystems hosted by extraterrestrial bodies, extraterrestrial life can seem more speculation than reality, due to the harsh conditions and disparate chemical composition of the atmospheres,<ref>{{Citation |last=Catling |first=D.C. |title=Planetary Atmospheres |date=2015 |work=Treatise on Geophysics |pages=429–472 |url=https://linkinghub.elsevier.com/retrieve/pii/B9780444538024001858 |access-date=2024-04-17 |publisher=Elsevier |language=en |doi=10.1016/b978-0-444-53802-4.00185-8 |bibcode=2015trge.book..429C |isbn=978-0-444-53803-1}}</ref> when compared to the life-abundant Earth. However, there are many extreme and chemically harsh ecosystems on Earth that do support forms of life and are often hypothesized to be the origin of life on Earth. [[Hydrothermal vent]]s,<ref name=":0">{{Cite journal |last1=Shibuya |first1=Takazo |last2=Takai |first2=Ken |date=2022-11-16 |title=Liquid and supercritical CO2 as an organic solvent in Hadean seafloor hydrothermal systems: implications for prebiotic chemical evolution |journal=Progress in Earth and Planetary Science |language=en |volume=9 |issue=1 |doi=10.1186/s40645-022-00510-6 |doi-access=free |issn=2197-4284}}</ref> acidic hot springs,<ref>{{Cite journal |last1=Damer |first1=Bruce |last2=Deamer |first2=David |date=2020-04-01 |title=The Hot Spring Hypothesis for an Origin of Life |journal=Astrobiology |language=en |volume=20 |issue=4 |pages=429–452 |doi=10.1089/ast.2019.2045 |issn=1531-1074 |pmc=7133448 |pmid=31841362|bibcode=2020AsBio..20..429D }}</ref> and [[volcanic lakes]]<ref>{{Citation |last1=Mapelli |first1=Francesca |title=Microbial Life in Volcanic Lakes |date=2015 |work=Volcanic Lakes |pages=507–522 |editor-last=Rouwet |editor-first=Dmitri |url=http://link.springer.com/10.1007/978-3-642-36833-2_23 |access-date=2024-04-17 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |doi=10.1007/978-3-642-36833-2_23 |isbn=978-3-642-36832-5 |last2=Marasco |first2=Ramona |last3=Rolli |first3=Eleonora |last4=Daffonchio |first4=Daniele |last5=Donachie |first5=Stuart |last6=Borin |first6=Sara |hdl=2434/266460 |editor2-last=Christenson |editor2-first=Bruce |editor3-last=Tassi |editor3-first=Franco |editor4-last=Vandemeulebrouck |editor4-first=Jean|hdl-access=free }}</ref> are examples of life forming under difficult circumstances, provide parallels to the extreme environments on other planets and support the possibility of extraterrestrial life.

Since the mid-20th century, active research has taken place to look for signs of extraterrestrial life, encompassing searches for current and historic extraterrestrial life, and a narrower [[search for extraterrestrial intelligence|search for extraterrestrial intelligent life]]. Depending on the category of search, methods range from analysis of telescope and specimen data<ref name="NYT-20150106-DB">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |date=6 January 2015 |title=So Many Earth-Like Planets, So Few Telescopes |work=[[The New York Times]] |url=https://www.nytimes.com/2015/01/07/science/space/as-ranks-of-goldilocks-planets-grow-astronomers-consider-whats-next.html |url-access=limited |access-date=6 January 2015 |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2015/01/07/science/space/as-ranks-of-goldilocks-planets-grow-astronomers-consider-whats-next.html |archive-date=2022-01-01}}{{cbignore}}</ref> to radios used to detect and transmit communications.<ref>{{Cite web |last=Mann |first=Adam |date=2020-12-01 |title=Want to Talk to Aliens? Try Changing the Technological Channel beyond Radio |url=https://www.scientificamerican.com/article/want-to-talk-to-aliens-try-changing-the-technological-channel-beyond-radio/ |access-date=2024-05-10 |website=Scientific American |language=en}}</ref>

The concept of extraterrestrial life, particularly extraterrestrial intelligence, has had a major cultural impact, especially [[extraterrestrials in fiction]]. [[Science fiction]] has communicated scientific ideas, imagined a range of possibilities, and influenced public interest in and perspectives on extraterrestrial life. One shared space is the debate over the wisdom of attempting communication with extraterrestrial intelligence. Some encourage aggressive methods to try to contact intelligent extraterrestrial life. Others – citing the tendency of technologically advanced human societies to [[Slavery|enslave]] or destroy less advanced societies – argue it may be dangerous to actively draw attention to Earth.<ref>{{cite news |url=http://www.bbc.com/news/science-environment-31442952 |title=Scientists in US are urged to seek contact with aliens |work=BBC News |first=Pallab |last=Ghosh |date=12 February 2015}}</ref><ref>{{cite journal |title=Would Contact with Extraterrestrials Benefit or Harm Humanity? A Scenario Analysis |journal=Acta Astronautica |first1=Seth |last1=Baum |first2=Jacob |last2=Haqq-Misra |first3=Shawn |last3=Domagal-Goldman |volume=68 |issue=11 |pages=2114–2129 |date=June 2011 |doi=10.1016/j.actaastro.2010.10.012 |bibcode=2011AcAau..68.2114B |arxiv=1104.4462|s2cid=16889489 |issn = 0094-5765}}</ref>

==Context==
{{Life in the Universe}}
Initially, after the [[Big Bang]], the universe was too hot to allow life. It is estimated that the temperature of the universe was around 10 billion K at the one second mark.<ref>{{Cite web |date=2020-09-01 |title=Overview - NASA Science |url=https://science.nasa.gov/universe/overview/#:~:text=NASA-,Big%20Bang%20and%20Nucleosynthesis,of%20fog%20that%20scattered%20light. |access-date=2025-02-12 |language=en-US}}</ref> [[Chronology of the universe|15 million years later]], it cooled to temperate levels, but the elements that make up living things did not exist yet. The only freely available elements at that point were [[hydrogen]] and [[helium]]. [[Carbon]] and [[oxygen]] (and later, [[water]]) would not appear until 50 million years later, created through stellar fusion. At that point, the difficulty for life to appear was not the temperature, but the scarcity of free heavy elements.<ref>{{cite web |url= https://www.scientificamerican.com/article/when-did-life-first-emerge-in-the-universe/|title= When Did Life First Emerge in the Universe?|author= Avi Loeb|date= April 4, 2021|publisher= Scientific American|accessdate=April 17, 2023}}</ref> [[Planetary system]]s emerged, and the first [[organic compound]]s may have formed in the [[protoplanetary disk]] of [[cosmic dust|dust grains]] that would eventually create rocky planets like Earth. Although Earth was in a molten state after its birth and may have burned any organics that fell in it, it would have been more receptive once it cooled down.<ref>{{cite web |last=Moskowitz |first=Clara|author-link= Clara Moskowitz |title=Life's Building Blocks May Have Formed in Dust Around Young Sun |url=http://www.space.com/15089-life-building-blocks-young-sun-dust.html |date=29 March 2012 |publisher=[[Space.com]] |access-date=30 March 2012}}</ref> Once the right conditions on Earth were met, life started by a chemical process known as [[abiogenesis]]. Alternatively, life may have formed less frequently, then spread – by [[meteoroid]]s, for example – between [[habitable planet]]s in a process called [[panspermia]].<ref name="USRA-2010">{{cite conference |url=http://www.lpi.usra.edu/meetings/abscicon2010/pdf/5224.pdf |title=Panspermia: A Promising Field of Research |conference=Astrobiology Science Conference 2010: Evolution and Life: Surviving Catastrophes and Extremes on Earth and Beyond. 20–26 April 2010. League City, Texas. |first=P. H. |last=Rampelotto |date=April 2010 |bibcode=2010LPICo1538.5224R}}</ref><ref>{{cite book |first1=Guillermo |last1=Gonzalez |first2=Jay Wesley |last2=Richards |title=The privileged planet: how our place in the cosmos is designed for discovery |pages=343–345 |publisher=Regnery Publishing |year=2004 |isbn=978-0-89526-065-9 |url=https://books.google.com/books?id=KFdu4CyQ1k0C&pg=PA343}}</ref>

During most of its [[stellar evolution]] stars combine hydrogen nuclei to make helium nuclei by stellar fusion, and the comparatively lighter weight of helium allows the star to release the extra energy. The process continues until the star uses all of its available fuel, with the speed of consumption being related to the size of the star. During its last stages, stars start combining helium nuclei to form carbon nuclei. The higher-sized stars can further combine carbon nuclei to create oxygen and silicon, oxygen into neon and sulfur, and so on until iron. In the end, the star blows much of its content back into the stellar medium, where it would join clouds that would eventually become new generations of stars and planets. Many of those materials are the raw components of life on Earth. As this process takes place in all the universe, said materials are ubiquitous in the cosmos and not a rarity from the Solar System.<ref>Bennet, pp. 60-63</ref>  

[[Earth]] is a planet in the [[Solar System]], a planetary system formed by a star at the center, the [[Sun]], and the objects that orbit it: other planets, moons, asteroids, and comets. The sun is part of the [[Milky Way]], a [[galaxy]]. The Milky Way is part of the [[Local Group]], a [[galaxy group]] that is in turn part of the [[Laniakea Supercluster]]. The [[universe]] is composed of all similar structures in existence.<ref>Bennett, p. 53</ref> The immense distances between celestial objects is a difficulty for the study of extraterrestrial life. So far, humans have only set foot on the [[Moon]] and sent robotic probes to other planets and moons in the Solar System. Although probes can withstand conditions that may be lethal to humans, the distances cause time delays: the ''[[New Horizons]]'' took nine years after launch to reach [[Pluto]].<ref name="bennett55">Bennet, p. 55</ref> No probe has ever reached extrasolar planetary systems. The ''[[Voyager 2]]'' has left the Solar System at a speed of 50,000 kilometers per hour, if it headed towards the [[Alpha Centauri]] system, the closest one to Earth at 4.4 light years, it would reach it in 100,000 years. Under current technology, such systems can only be studied by telescopes, which have limitations.<ref name="bennett55"/> It is estimated that [[dark matter]] has a larger amount of combined matter than stars and gas clouds, but as it plays no role on the stellar evolution of stars and planets, it is usually not taken into account by astrobiology.<ref>Bennet, pp. 57-58</ref> 

There is an area around a star, the [[circumstellar habitable zone]] or "Goldilocks zone", where water may be at the right temperature to exist in liquid form at a planetary surface. This area is neither too close to the star, where water would become steam, nor too far away, where water would be frozen as ice. However, although useful as an approximation, [[planetary habitability]] is complex and defined by several factors. Being in the habitable zone is not enough for a planet to be habitable, not even to actually have such liquid water. Venus is located in the habitable zone of the Solar System but does not have liquid water because of the conditions of its atmosphere. Jovian planets or [[gas giant]]s are not considered habitable even if they orbit close enough to their stars as [[hot Jupiter]]s, due to crushing atmospheric pressures.<ref name="Neighbors" /> The actual distances for the habitable zones vary according to the type of star, and even the [[solar activity]] of each specific star influences the local habitability. The type of star also defines the time the habitable zone will exist, as its presence and limits will change along with the star's stellar evolution.<ref>{{cite web |url= https://www.space.com/goldilocks-zone-habitable-area-life|title= Goldilocks zone: Everything you need to know about the habitable sweet spot|author= Vicky Stein|date= February 16, 2023|publisher= Space.com|accessdate=April 22, 2023}}</ref>

The Big Bang took place 13.8 billion years ago, the Solar System was formed 4.6 billion years ago, and the first hominids appeared 6 million years ago. Life on other planets may have started, evolved, given birth to extraterrestrial intelligences, and perhaps even faced a planetary extinction event millions or even billions of years ago. The brief times of existence of Earth's species, when considered from a cosmic perspective, may suggest that extraterrestrial life may be equally fleeting under such a scale.<ref>Bennet, p. 65</ref>  

During a period of about 7 million years, from about 10 to 17 million after the Big Bang, the background temperature was between {{cvt|373 and 273|K|C F}}, allowing the possibility of [[liquid water]] if any planets existed. 
[[Avi Loeb]] (2014) speculated that [[Abiogenesis|primitive life]] might in principle have appeared during this window, which he called "the Habitable Epoch of the Early Universe".<ref name="IJA-2014October">{{cite journal |last=Loeb |first=Abraham |author-link=Abraham Loeb |title=The Habitable Epoch of the Early Universe |url=https://www.cfa.harvard.edu/~loeb/habitable.pdf |date=October 2014 |journal=[[International Journal of Astrobiology]] |volume=13 |issue=4 |pages=337–339 |doi=10.1017/S1473550414000196 |access-date=15 December 2014 |bibcode=2014IJAsB..13..337L |arxiv = 1312.0613 |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 |date=2 December 2014 |work=[[The New York Times]] |access-date=3 December 2014 }}</ref>

Life on Earth is quite ubiquitous across the planet and has adapted over time to almost all the available environments in it, [[extremophile]]s and the [[deep biosphere]] thrive at even the most hostile ones. As a result, it is inferred that life in other celestial bodies may be equally adaptive. However, the origin of life is unrelated to its ease of adaptation and may have stricter requirements. A celestial body may not have any life on it, even if it were habitable.<ref>Aguilera Mochon, pp. 9–10</ref>

==Likelihood of existence==
{{Main|Drake equation|Extraterrestrial intelligence}}
It is unclear if life, and more importantly,  intelligent life in the cosmos is ubiquitous or rare. The hypothesis of ubiquitous extraterrestrial life relies on three main ideas. The first one, the [[size of the universe]] allows for plenty of planets to have a similar habitability to Earth, and the [[age of the universe]] gives enough time for a long process analog to the [[history of Earth]] to happen there. The second is that the chemical elements that make up life, such as carbon and water, are ubiquitous in the universe. The third is that the [[physical law]]s are universal, which means that the forces that would facilitate or prevent the existence of life would be the same ones as on Earth.<ref>Bennet, p. 51</ref> According to this argument, made by scientists such as [[Carl Sagan]] and [[Stephen Hawking]], it would be improbable for life ''not'' to exist somewhere else other than Earth.<ref>{{cite book |url=https://books.google.com/books?id=vgQj5D524PYC&pg=PA3 |title=Other Worlds, Other Universes |publisher=Health Research Books |editor1-first=Brad |editor1-last=Steiger |editor2-first=John |editor2-last=White |page=3 |date=1986 |isbn=978-0-7873-1291-6}}</ref><ref>{{cite book |first1=David |last1=Filkin |first2=Stephen W. |last2=Hawking |title=Stephen Hawking's universe: the cosmos explained |page=[https://archive.org/details/stephenhawkingsu00filk/page/194 194] |series=Art of Mentoring Series |publisher=Basic Books |year=1998 |isbn=978-0-465-08198-1 |url=https://archive.org/details/stephenhawkingsu00filk|url-access=registration }}</ref> This argument is embodied in the [[Copernican principle]], which states that Earth does not occupy a unique position in the Universe, and the [[mediocrity principle]], which states that there is nothing special about life on Earth.<ref>{{Cite book |url=https://books.google.com/books?id=aRkvNoDYtvEC&pg=PA300 |title=Chemical Evolution and the Origin of Life |publisher=Springer |first=Horst |last=Rauchfuss |others=trans. Terence N. Mitchell |date=2008 |isbn=978-3-540-78822-5}}</ref>

Other authors consider instead that life in the cosmos, or at least multicellular life, may be actually rare. The [[Rare Earth hypothesis]] maintains that life on Earth is possible because of a series of factors that range from the location in the galaxy and the configuration of the [[Solar System]] to local characteristics of the planet, and that it is unlikely that all such requirements are simultaneously met by another planet. The proponents of this hypothesis consider that very little evidence suggests the existence of extraterrestrial life and that at this point it is just a desired result and not a reasonable scientific explanation for any gathered data.<ref>Aguilera Mochón, p. 66</ref><ref>{{cite web |url= https://www.princeton.edu/news/2012/04/26/expectation-extraterrestrial-life-built-more-optimism-evidence-study-finds|title= Expectation of extraterrestrial life built more on optimism than evidence, study finds|author= Morgan Kelly|date= April 26, 2012|publisher= Princeton University|accessdate=April 22, 2023}}</ref>

In 1961, astronomer and astrophysicist [[Frank Drake]] devised the [[Drake equation]] as a way to stimulate scientific dialogue at a meeting on the [[search for extraterrestrial intelligence]] (SETI).<ref name='December 2002'>{{cite web |url=http://www.setileague.org/askdr/drake.htm |title=Chapter 3 – Philosophy: "Solving the Drake Equation |access-date=24 July 2015 |date=December 2002 |publisher=SETI League}}</ref>{{Better source needed|date=May 2023}} The Drake equation is a [[probability theory|probabilistic argument]] used to estimate the number of active, communicative extraterrestrial civilizations in the [[Milky Way]] [[galaxy]]. The Drake equation is:
:<math>N = R_{\ast} \cdot f_p \cdot n_e \cdot f_{\ell} \cdot f_i \cdot f_c \cdot L</math>
where:
:''N'' = the number of Milky Way galaxy civilizations already capable of communicating across [[interplanetary space]]

and
:''R''<sub>*</sub> = the average rate of [[star formation]] in [[Milky Way|our galaxy]]
:''f''<sub>''p''</sub> = the fraction of those stars that have [[planet]]s
:''n''<sub>''e''</sub> = the average number of planets that can potentially support life
:''f''<sub>''l''</sub> = the fraction of planets that actually support life
:''f''<sub>''i''</sub> = the fraction of planets with life that evolves to become [[intelligence|intelligent]] life (civilisations)
:''f''<sub>''c''</sub> = the fraction of civilizations that develop a technology to broadcast detectable signs of their existence into space
:''L'' = the length of time over which such civilizations broadcast detectable signals into space

Drake's proposed estimates are as follows, but numbers on the right side of the equation are agreed as speculative and open to substitution:

<math>10{,}000 = 5 \cdot 0.5 \cdot 2 \cdot 1 \cdot 0.2 \cdot 1 \cdot 10{,}000</math><ref name="NOVA">{{cite web |last=Aguirre |first=L. |date=1 July 2008 |title=The Drake Equation |url=https://www.pbs.org/wgbh/nova/origins/drake.html |work=[[Nova ScienceNow]] |publisher=[[PBS]] |access-date=7 March 2010}}</ref>{{Better source needed|date=May 2023}}

The Drake equation has proved controversial since, although it is written as a math equation, none of its values were known at the time. Although some values may eventually be measured, others are based on [[social science]]s and are not knowable by their very nature.<ref name='Burchell'>{{cite journal |title=W(h)ither the Drake equation? |last=Burchell |first=M. J. |journal=International Journal of Astrobiology |volume=5 |issue=3 |pages=243–250 |date=2006 |doi=10.1017/S1473550406003107 |bibcode=2006IJAsB...5..243B|s2cid=121060763 }}</ref> This does not allow one to make noteworthy conclusions from the equation.<ref>{{cite book |chapter=Chapter 6: What does a Martian look like? |title=Evolving the Alien: The Science of Extraterrestrial Life |publisher=John Wiley and Sons |location=Hoboken, NJ |first1=Jack |last1=Cohen |author1-link=Jack Cohen (biologist) |first2=Ian |last2=Stewart |author2-link=Ian Stewart (mathematician) |date=2002 |isbn=978-0-09-187927-3|title-link=Evolving the Alien: The Science of Extraterrestrial Life }}</ref>

Based on observations from the [[Hubble Space Telescope]], there are nearly 2 trillion galaxies in the observable universe.<ref>{{cite news |url=https://skyandtelescope.org/astronomy-news/universe-2-trillion-galaxies/ |title=About those 2 trillion new galaxies... |last=Macrobert |first=Alan |work=Sky & Telescope |date=13 October 2016 |access-date=24 May 2023}}</ref> It is estimated that at least ten percent of all Sun-like stars have a system of planets,<ref name="marcyprogth05">{{cite journal |first1=G. |last1=Marcy |first2=R. |last2=Butler |first3=D. |last3=Fischer |display-authors=etal |title=Observed Properties of Exoplanets: Masses, Orbits and Metallicities |journal=Progress of Theoretical Physics Supplement |year=2005 |volume=158 |pages=24–42 |url=http://ptp.ipap.jp/link?PTPS/158/24 |doi=10.1143/PTPS.158.24 |arxiv=astro-ph/0505003 |bibcode=2005PThPS.158...24M |s2cid=16349463 |url-status=dead |archive-url=https://web.archive.org/web/20081002085400/http://ptp.ipap.jp/link?PTPS%2F158%2F24 |archive-date=2 October 2008 }}</ref> i.e. there are {{val|6.25|e=18}} stars with planets orbiting them in the observable universe. Even if it is assumed that only one out of a billion of these stars has planets supporting life, there would be some 6.25&nbsp; billion life-supporting planetary systems in the observable universe. A 2013 study based on results from the [[Kepler (spacecraft)|''Kepler'']] spacecraft estimated that the Milky Way contains at least as many planets as it does stars, resulting in 100–400&nbsp; billion exoplanets.<ref name="Swift2013">{{cite journal |title=Characterizing the Cool KOIs. IV. Kepler-32 as a Prototype for the Formation of Compact Planetary Systems throughout the Galaxy |journal=The Astrophysical Journal |first1=Jonathan J. |last1=Swift |first2=John Asher |last2=Johnson |first3=Timothy D. |last3=Morton |first4=Justin R. |last4=Crepp |first5=Benjamin T. |last5=Montet |first6=Daniel C. |last6=Fabrycky |first7=Philip S. |last7=Muirhead |display-authors=5 |volume=764 |issue=1 |at=105 |date=January 2013 |doi=10.1088/0004-637X/764/1/105 |bibcode=2013ApJ...764..105S |arxiv=1301.0023|s2cid=43750666 }}</ref><ref name="space20130102">{{cite news |url=http://www.space.com/19103-milky-way-100-billion-planets.html |title=100 Billion Alien Planets Fill Our Milky Way Galaxy: Study |work=Space.com |date=2 January 2013 |access-date=10 March 2016 |url-status=dead |archive-url=https://web.archive.org/web/20130103060601/http://www.space.com/19103-milky-way-100-billion-planets.html |archive-date=3 January 2013 }}</ref> The [[Nebular hypothesis]] that explains the formation of the Solar System and other planetary systems would suggest that those can have several configurations, and not all of them may have rocky planets within the habitable zone.<ref>Bennet, p. 98</ref> 

The apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilisations and the lack of evidence for such civilisations is known as the [[Fermi paradox]].<ref name="NYT-20150803">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=The Flip Side of Optimism About Life on Other Planets |url=https://www.nytimes.com/2015/08/04/science/space/the-flip-side-of-optimism-about-life-on-other-planets.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2015/08/04/science/space/the-flip-side-of-optimism-about-life-on-other-planets.html |archive-date=2022-01-01 |url-access=limited |date=3 August 2015 |work=[[The New York Times]] |access-date=29 October 2015}}{{cbignore}}</ref> [[Dennis W. Sciama]] claimed that life's existence in the universe depends on various fundamental constants. [[Zhi-Wei Wang]] and [[Samuel L. Braunstein]] suggest that a random universe capable of supporting life is likely to be just barely able to do so, giving a potential explanation to the Fermi paradox.<ref>{{cite journal |last1=Wang |first1=Zhi-Wei |last2=Braunstein |first2=Samuel L. |year=2023 |title=Sciama's argument on life in a random universe and distinguishing apples from oranges |journal=Nature Astronomy |volume=7 |issue=2023 |pages=755–756 |doi=10.1038/s41550-023-02014-9 |arxiv=2109.10241 |bibcode=2023NatAs...7..755W }}</ref>

==Biochemical basis==
{{main|Hypothetical types of biochemistry}}
{{see also|Water#Effects on life}}
If extraterrestrial life exists, it could range from simple [[microorganism]]s and [[multicellular organism]]s similar to animals or plants, to complex [[alien intelligence]]s akin to [[human]]s. When scientists talk about extraterrestrial life, they consider all those types. Although it is possible that extraterrestrial life may have other configurations, scientists use the hierarchy of lifeforms from Earth for simplicity, as it is the only one known to exist.<ref>Bennett, p. 3</ref>

The first basic requirement for life is an environment with [[non-equilibrium thermodynamics]], which means that the [[thermodynamic equilibrium]] must be broken by a source of energy. The traditional sources of energy in the cosmos are the stars, such as for life on Earth, which depends on the energy of the sun. However, there are other alternative energy sources, such as [[volcano|volcanoe]]s, [[plate tectonics]], and [[hydrothermal vent]]s. There are ecosystems on Earth in deep areas of the ocean that do not receive sunlight, and take energy from [[black smoker]]s instead.<ref>Aguilera Mochón, p. 42</ref> [[Magnetic field]]s and [[radioactivity]] have also been proposed as sources of energy, although they would be less efficient ones.<ref>Aguilera Mochón, p. 58</ref>

Life on Earth requires water in a liquid state as a [[solvent]] in which biochemical reactions take place. It is highly unlikely that an [[abiogenesis]] process can start within a gaseous or solid medium: the atom speeds, either too fast or too slow, make it difficult for specific ones to meet and start chemical reactions. A liquid medium also allows the transport of nutrients and substances required for metabolism.<ref>Aguilera Mochón, p. 51</ref> Sufficient quantities of carbon and other elements, along with water, might enable the formation of living organisms on [[terrestrial planet]]s with a chemical make-up and temperature range similar to that of Earth.<ref>{{cite journal |last1=Bond |first1=Jade C. |last2=O'Brien |first2=David P. |last3=Lauretta |first3=Dante S. |title=The Compositional Diversity of Extrasolar Terrestrial Planets. I. In Situ Simulations |journal=The Astrophysical Journal |volume=715 |issue=2 |pages=1050–1070 |date=June 2010 |doi=10.1088/0004-637X/715/2/1050 |bibcode=2010ApJ...715.1050B |arxiv=1004.0971|s2cid=118481496 }}</ref><ref>{{cite journal |first=Norman R. |last=Pace |date=20 January 2001 |title=The universal nature of biochemistry |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=98 |issue=3 |pages=805–808 |doi=10.1073/pnas.98.3.805 |pmid=11158550 |bibcode=2001PNAS...98..805P |pmc=33372|doi-access=free }}</ref> Life based on [[ammonia]] rather than water has been suggested as an alternative, though this solvent appears less suitable than water. It is also conceivable that there are forms of life whose solvent is a liquid [[hydrocarbon]], such as [[methane]], [[ethane]] or [[propane]].<ref>{{cite book |chapter-url=http://www.nap.edu/read/11919/chapter/8#74 |chapter=6.2.2: Nonpolar Solvents |title=The Limits of Organic Life in Planetary Systems |publisher=The National Academies Press |author=National Research Council |page=74 |date=2007 |doi=10.17226/11919 |isbn=978-0-309-10484-5}}</ref>

Another unknown aspect of potential extraterrestrial life would be the [[chemical element]]s that would compose it. Life on Earth is largely composed of carbon, but there could be other [[hypothetical types of biochemistry]]. A replacement for carbon would need to be able to create complex molecules, store information required for evolution, and be freely available in the medium. To create [[DNA]], [[RNA]], or a close analog, such an element should be able to bind its atoms with many others, creating complex and stable molecules. It should be able to create at least three covalent bonds: two for making long strings and at least a third to add new links and allow for diverse information. Only nine elements meet this requirement: [[boron]], [[nitrogen]], [[phosphorus]], [[arsenic]], [[antimony]] (three bonds), [[carbon]], [[silicon]], [[germanium]] and [[tin]] (four bonds). As for abundance, carbon, nitrogen, and silicon are the most abundant ones in the universe, far more than the others. On [[Earth's crust]] the most abundant of those elements is silicon, in the [[Hydrosphere]] it is carbon and in the atmosphere, it is carbon and nitrogen. Silicon, however, has disadvantages over carbon. The molecules formed with silicon atoms are less stable, and more vulnerable to acids, oxygen, and light. An ecosystem of silicon-based lifeforms would require very low temperatures, high [[atmospheric pressure]], an atmosphere devoid of oxygen, and a solvent other than water. The low temperatures required would add an extra problem, the difficulty to kickstart a process of abiogenesis to create life in the first place.<ref>Aguilera Mochón, pp. 43–49</ref>  [[Norman Horowitz]], head of the Jet Propulsion Laboratory bioscience section for the Mariner and Viking missions from 1965 to 1976 considered that the great versatility of the [[carbon]] atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival of life on other planets.<ref>Horowitz, N.H. (1986). Utopia and Back and the search for life in the solar system. New York: W.H. Freeman and Company. ISBN 0-7167-1766-2</ref>  However, he also considered that the conditions found on [[Mars]] were incompatible with carbon based life. 

Even if extraterrestrial life is based on carbon and uses water as a solvent, like Earth life, it may still have a radically different [[biochemistry]].  Life is generally considered to be a product of [[natural selection]].  It has been proposed that to undergo natural selection a living entity must have the capacity to [[DNA replication|replicate]] itself, the capacity to avoid damage/decay, and the capacity to acquire and process resources in support of the first two capacities.<ref>Bernstein, Harris; Byerly, Henry C.; Hopf, Frederick A.; et al. (June 1983). "The Darwinian Dynamic". The Quarterly Review of Biology. 58 (2): 185–207. doi:10.1086/413216. JSTOR 2828805. S2CID 83956410</ref> Life on Earth started with an [[RNA world]] and later evolved to its current form, where some of the [[RNA]] tasks were transferred to [[DNA]] and [[proteins]]. Extraterrestrial life may still be stuck using RNA, or evolve into other configurations. It is unclear if our biochemistry is the most efficient one that could be generated, or which elements would follow a similar pattern.<ref>Aguilera Mochón, pp. 58–59</ref> However, it is likely that, even if cells had a different composition to those from Earth, they would still have a [[cell membrane]]. Life on Earth jumped from [[prokaryote]]s to [[eukaryote]]s and from [[unicellular organism]]s to multicellular organisms through [[evolution]]. So far no alternative process to achieve such a result has been conceived, even if hypothetical. Evolution requires life to be divided into individual organisms, and no alternative organisation has been satisfactorily proposed either. At the basic level, membranes define the limit of a cell, between it and its environment, while remaining partially open to exchange energy and resources with it.<ref>Aguilera Mochón, pp. 42–43</ref>

The evolution from simple cells to eukaryotes, and from them to multicellular lifeforms, is not guaranteed. The [[Cambrian explosion]] took place thousands of millions of years after the origin of life, and its causes are not fully known yet. On the other hand, the jump to multicellularity took place several times, which suggests that it could be a case of [[convergent evolution]], and so likely to take place on other planets as well. Palaeontologist [[Simon Conway Morris]] considers that convergent evolution would lead to kingdoms similar to our plants and animals, and that many features are likely to develop in alien animals as well, such as [[bilateral symmetry]], [[Limb (anatomy)|limbs]], [[Digestion|digestive systems]] and heads with [[sensory organ]]s.<ref name="AM6166"/> Scientists from the University of Oxford analysed it from the perspective of evolutionary theory and wrote in a study in the [[International Journal of Astrobiology]] that aliens may be similar to humans.<ref>{{cite web |url=http://www.ox.ac.uk/news/2017-10-31-aliens-may-be-more-us-we-think |title=Aliens may be more like us than we think |publisher=[[University of Oxford]] |date=31 October 2017}}</ref> The planetary context would also have an influence: a planet with higher [[gravity]] would have smaller animals, and other types of stars can lead to [[Hypothetical types of biochemistry#Non-green photosynthesizers|non-green photosynthesizers]]. The amount of energy available would also affect [[biodiversity]], as an ecosystem sustained by black smokers or hydrothermal vents would have less energy available than those sustained by a star's light and heat, and so its lifeforms would not grow beyond a certain complexity.<ref name="AM6166">Aguilera Mochón, pp. 61–66</ref> There is also research in assessing the capacity of life for developing intelligence. It has been suggested that this capacity arises with the number of potential [[Ecological niche|niches]] a planet contains, and that the complexity of life itself is reflected in the information density of planetary environments, which in turn can be computed from its niches.<ref>{{cite journal |title=Evolutionary exobiology: Towards the qualitative assessment of biological potential on exoplanets |journal=International Journal of Astrobiology |volume=18 |issue=3 |first1=David S. |last1=Stevenson |first2=Sean |last2=Large |date=25 October 2017 |doi=10.1017/S1473550417000349 |pages=204–208|s2cid=125275411 }}</ref>

=== Harsh environmental conditions on Earth harboring life ===
It is common knowledge that the conditions on other planets in the solar system, in addition to the many galaxies outside of the [[Milky Way galaxy]], are very harsh and seem to be too extreme to harbor any life.<ref>{{Cite web |title=Atmosphere - Planets, Composition, Pressure {{!}} Britannica |url=https://www.britannica.com/science/atmosphere/The-atmospheres-of-other-planets |access-date=2024-04-17 |website=www.britannica.com |language=en}}</ref> The environmental conditions on these planets can have intense [[UV radiation]] paired with extreme temperatures, lack of water,<ref>{{Cite journal |last1=Amils |first1=Ricardo |last2=González-Toril |first2=Elena |last3=Fernández-Remolar |first3=David |last4=Gómez |first4=Felipe |last5=Aguilera |first5=Ángeles |last6=Rodríguez |first6=Nuria |last7=Malki |first7=Mustafá |last8=García-Moyano |first8=Antonio |last9=Fairén |first9=Alberto G. |last10=de la Fuente |first10=Vicenta |last11=Luis Sanz |first11=José |date=February 2007 |title=Extreme environments as Mars terrestrial analogs: The Rio Tinto case |url=https://linkinghub.elsevier.com/retrieve/pii/S0032063306001826 |journal=Planetary and Space Science |language=en |volume=55 |issue=3 |pages=370–381 |doi=10.1016/j.pss.2006.02.006|bibcode=2007P&SS...55..370A }}</ref> and much more that can lead to conditions that don't seem to favor the creation or maintenance of extraterrestrial life. However, there has been much historical evidence that some of the earliest and most basic forms of life on Earth originated in some extreme environments<ref>{{Cite journal |last1=Daniel |first1=Isabelle |last2=Oger |first2=Philippe |last3=Winter |first3=Roland |date=2006 |title=Origins of life and biochemistry under high-pressure conditions |url=https://xlink.rsc.org/?DOI=b517766a |journal=Chemical Society Reviews |language=en |volume=35 |issue=10 |pages=858–875 |doi=10.1039/b517766a |pmid=17003893 |issn=0306-0012}}</ref> that seem unlikely to have harbored life at least at one point in Earth's history. Fossil evidence as well as many historical theories backed up by years of research and studies have marked environments like [[hydrothermal vent]]s or acidic hot springs as some of the first places that life could have originated on Earth.<ref>{{Cite journal |last1=Dong |first1=Hailiang |last2=Yu |first2=Bingsong |date=2007-09-01 |title=Geomicrobiological processes in extreme environments: A review |journal=Episodes |language=en |volume=30 |issue=3 |pages=202–216 |doi=10.18814/epiiugs/2007/v30i3/003 |issn=0705-3797|doi-access=free }}</ref> These environments can be considered extreme when compared to the typical ecosystems that the majority of life on Earth now inhabit, as hydrothermal vents are scorching hot due to the [[magma]] escaping from the [[Earth's mantle]] and meeting the much colder oceanic water. Even in today's world, there can be a diverse population of bacteria found inhabiting the area surrounding these hydrothermal vents<ref name=":1">{{Cite journal |last1=Georgieva |first1=Magdalena N. |last2=Little |first2=Crispin T.S. |last3=Maslennikov |first3=Valeriy V. |last4=Glover |first4=Adrian G. |last5=Ayupova |first5=Nuriya R. |last6=Herrington |first6=Richard J. |date=June 2021 |title=The history of life at hydrothermal vents |journal=Earth-Science Reviews |language=en |volume=217 |pages=103602 |doi=10.1016/j.earscirev.2021.103602|bibcode=2021ESRv..21703602G |doi-access=free }}</ref> which can suggest that some form of life can be supported even in the harshest of environments like the other planets in the solar system.

The aspects of these harsh environments that make them ideal for the origin of life on Earth, as well as the possibility of creation of life on other planets, is the [[chemical reaction]]s forming spontaneously. For example, the [[hydrothermal vent]]s found on the ocean floor are known to support many [[Chemosynthesis|chemosynthetic]] processes<ref name=":0" /> which allow organisms to utilize energy through reduced chemical compounds that fix carbon.<ref name=":1" /> In return, these reactions will allow for organisms to live in relatively low oxygenated environments while maintaining enough energy to support themselves. The early Earth environment was reducing<ref>{{Cite journal |last1=Zahnle |first1=Kevin J. |last2=Lupu |first2=Roxana |last3=Catling |first3=David C. |last4=Wogan |first4=Nick |date=2020-06-01 |title=Creation and Evolution of Impact-generated Reduced Atmospheres of Early Earth |journal=The Planetary Science Journal |volume=1 |issue=1 |pages=11 |doi=10.3847/PSJ/ab7e2c |doi-access=free |arxiv=2001.00095 |bibcode=2020PSJ.....1...11Z |issn=2632-3338}}</ref> and therefore, these carbon fixing compounds were necessary for the survival and possible [[origin of life on Earth]]. With the little amount of information that scientists have found regarding the atmosphere on other planets in the [[Milky Way galaxy]] and beyond, the atmospheres are most likely reducing or with very low oxygen levels,<ref>{{Cite journal |last1=Atreya |first1=S.K |last2=Mahaffy |first2=P.R |last3=Niemann |first3=H.B |last4=Wong |first4=M.H |last5=Owen |first5=T.C |date=February 2003 |title=Composition and origin of the atmosphere of Jupiter—an update, and implications for the extrasolar giant planets |url=https://linkinghub.elsevier.com/retrieve/pii/S0032063302001447 |journal=Planetary and Space Science |language=en |volume=51 |issue=2 |pages=105–112 |doi=10.1016/S0032-0633(02)00144-7|bibcode=2003P&SS...51..105A }}</ref> especially when compared with Earth's atmosphere. If there were the necessary elements and ions on these planets, the same carbon fixing, reduced chemical compounds occurring around hydrothermal vents could also occur on these planets' surfaces and possibly result in the origin of extraterrestrial life.

==Planetary habitability in the Solar System==
{{main|Planetary habitability in the Solar System}}
[[File:Habitable Worlds 2.jpg|thumb|Besides Earth, [[Mars]], [[Europa (moon)|Europa]] and [[Enceladus]] are the most likely places in the Solar System to find life.]] 
The Solar System has a wide variety of planets, dwarf planets, and moons, and each one is studied for its potential to host life. Each one has its own specific conditions that may benefit or harm life. So far, the only lifeforms found are those from Earth. No [[extraterrestrial intelligence]] other than [[human]]s exists or has ever existed within the Solar System.<ref>Bennett, pp. 3-4</ref> Astrobiologist Mary Voytek points out that it would be unlikely to find large ecosystems, as they would have already been detected by now.<ref name="Neighbors">{{cite web |url= https://exoplanets.nasa.gov/news/1665/life-in-our-solar-system-meet-the-neighbors/|title= Life in Our Solar System? Meet the Neighbors|author= Pat Brennan|date= November 10, 2020|publisher= NASA|accessdate=March 30, 2023}}</ref>

The inner Solar System is likely devoid of life. However, [[Venus]] is still of interest to astrobiologists, as it is a [[terrestrial planet]] that was likely similar to Earth in its early stages and developed in a different way. There is a [[greenhouse effect]], the surface is the hottest in the Solar System, sulfuric acid clouds, all surface liquid water is lost, and it has a thick carbon-dioxide atmosphere with huge pressure.<ref>{{Cite journal |last1=Marcq |first1=Emmanuel |last2=Mills |first2=Franklin P. |last3=Parkinson |first3=Christopher D. |last4=Vandaele |first4=Ann Carine |date=2017-11-30 |title=Composition and Chemistry of the Neutral Atmosphere of Venus |url=https://hal-insu.archives-ouvertes.fr/insu-01656562/file/EMFMCP.pdf |journal=Space Science Reviews |language=en |volume=214 |issue=1 |pages=10 |doi=10.1007/s11214-017-0438-5 |s2cid=255067610 |issn=1572-9672}}</ref> Comparing both helps to understand the precise differences that lead to beneficial or harmful conditions for life. And despite the conditions against [[life on Venus]], there are suspicions that microbial life-forms may still survive in high-altitude clouds.<ref name="Neighbors"/>

[[Mars]] is a cold and almost airless desert, inhospitable to life. However, recent studies revealed that [[water on Mars]] used to be quite abundant, forming rivers, lakes, and perhaps even oceans. Mars may have been habitable back then, and [[life on Mars]] may have been possible. But when the planetary core ceased to generate a magnetic field, solar winds removed the atmosphere and the planet became vulnerable to solar radiation. Ancient life-forms may still have left fossilised remains, and microbes may still survive deep underground.<ref name="Neighbors"/>

As mentioned, the gas giants and [[ice giants]] are unlikely to contain life. The most distant solar system bodies, found in the [[Kuiper Belt]] and outwards, are locked in permanent deep-freeze, but cannot be ruled out completely.<ref name="Neighbors"/>

Although the giant planets themselves are highly unlikely to have life, there is much hope to find it on moons orbiting these planets. [[Europa (moon)|Europa]], from the Jovian system, has a subsurface ocean below a thick layer of ice. [[Ganymede (moon)|Ganymede]] and [[Callisto (moon)|Callisto]] also have subsurface oceans, but life is less likely in them because water is sandwiched between layers of solid ice. Europa would have contact between the ocean and the rocky surface, which helps the chemical reactions. It may be difficult to dig so deep in order to study those oceans, though. [[Enceladus]], a tiny moon of [[Saturn]] with another subsurface ocean, may not need to be dug, as it releases water to space in [[eruption column]]s. The space probe ''[[Cassini–Huygens|Cassini]]'' flew inside one of these, but could not make a full study because NASA did not expect this phenomenon and did not equip the probe to study ocean water. Still, ''Cassini'' detected complex organic molecules, salts, evidence of hydrothermal activity, hydrogen, and methane.<ref name="Neighbors"/>

[[Titan (moon)|Titan]] is the only celestial body in the Solar System besides Earth that has liquid bodies on the surface. It has rivers, lakes, and rain of hydrocarbons, methane, and ethane, and even a cycle similar to Earth's [[water cycle]]. This special context encourages speculations about [[Life on Titan|lifeforms]] with different biochemistry, but the cold temperatures would make such chemistry take place at a very slow pace. Water is rock-solid on the surface, but Titan does have a subsurface water ocean like several other moons. However, it is of such a great depth that it would be very difficult to access it for study.<ref name="Neighbors"/>

==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>

==History and cultural impact==
{{main|History of the extraterrestrial life debate}}
{{see also|History of astronomy|Potential cultural impact of extraterrestrial contact}}

===Cosmic pluralism===

{{main|Cosmic pluralism}}

[[File:Epikouros BM 1843.jpg|thumb|upright|The Greek [[Epicurus]] proposed that other worlds may have their own animals and plants.]]

The modern concept of extraterrestrial life is based on assumptions that were not commonplace during the early days of [[history of astronomy|astronomy]]. The first explanations for the celestial objects seen in the [[night sky]] were based on mythology. Scholars from [[Ancient Greece]] were the first to consider that the universe is inherently understandable and rejected explanations based on supernatural incomprehensible forces, such as the myth of the Sun being pulled across the sky in the chariot of [[Apollo]]. They had not developed the [[scientific method]] yet and based their ideas on pure thought and speculation, but they developed precursor ideas to it, such as that explanations had to be discarded if they contradict observable facts. The discussions of those Greek scholars established many of the pillars that would eventually lead to the idea of extraterrestrial life, such as Earth being round and not flat. The cosmos was first structured in a [[geocentric model]] that considered that the sun and all other celestial bodies revolve around Earth. However, they did not consider them as worlds. In Greek understanding, the world was composed by both Earth and the celestial objects with noticeable movements. [[Anaximander]] thought that the cosmos was made from ''apeiron'', a substance that created the world, and that the world would eventually return to the cosmos.

Eventually two groups emerged, the ''[[Atomism|atomists]]'' that thought that matter at both Earth and the cosmos was equally made of small atoms of the [[classical elements]] (earth, water, fire and air), and the ''[[Aristotelian physics|Aristotelians]]'' who thought that those elements were exclusive of Earth and that the cosmos was made of a fifth one, the ''[[Aether (classical element)|aether]]''. Atomist [[Epicurus]] thought that the processes that created the world, its animals and plants should have created other worlds elsewhere, along with their own animals and plants. Aristotle thought instead that all the earth element naturally fell towards the center of the universe, and that would make it impossible for other planets to exist elsewhere. Under that reasoning, Earth was not only in the center, it was also the only planet in the universe.<ref>Bennett, pp. 16-23</ref>

Cosmic pluralism, the plurality of worlds, or simply pluralism, describes the philosophical belief in numerous "worlds" in addition to Earth, which might harbor extraterrestrial life. The earliest recorded assertion of extraterrestrial human life is found in ancient scriptures of [[Jainism]]. There are multiple "worlds" mentioned in Jain scriptures that support human life. These include, among others, ''Bharat Kshetra'', ''Mahavideh Kshetra'', ''Airavat Kshetra'', and ''Hari kshetra''.<ref name=crowe1999>{{cite book |url=https://books.google.com/books?id=J4TZPlihVUoC |title=The Extraterrestrial Life Debate, 1750–1900 |first=Michael J. |last=Crowe |publisher=Courier Dover Publications |date=1999 |isbn=978-0-486-40675-6}}</ref><ref>{{cite web |first=Benjamin D. |last=Wiker |date=4 November 2002 |title=Alien Ideas: Christianity and the Search for Extraterrestrial Life |url=http://www.crisismagazine.com/november2002/feature7.htm |work=Crisis Magazine |archive-url=https://web.archive.org/web/20030210140752/http://www.crisismagazine.com/november2002/feature7.htm |archive-date=10 February 2003}}</ref><ref name=Irwin>{{Cite book |title=The Arabian Nights: A Companion |first=Robert |last=Irwin |publisher=[[I.B. Tauris|Tauris Parke Paperbacks]] |year=2003 |isbn=978-1-86064-983-7 |page=204 & 209}}</ref> Medieval Muslim writers like [[Fakhr al-Din al-Razi]] and [[Muhammad al-Baqir]] supported cosmic pluralism on the basis of the [[Qur'an]].<ref name =Weintraub>David A. Weintraub (2014). "Islam," ''Religions and Extraterrestrial Life'' (pp 161–168). Springer International Publishing.</ref> [[Geoffrey Chaucer|Chaucer]]'s poem ''[[The House of Fame]]'' engaged in medieval thought experiments that postulated the plurality of worlds.<ref name="Gabrovsky 2016 p. 83">{{cite book | last=Gabrovsky | first=A.N. | title=Chaucer the Alchemist: Physics, Mutability, and the Medieval Imagination | publisher=Palgrave Macmillan US | series=The New Middle Ages | year=2016 | isbn=978-1-137-52391-4 | url=https://books.google.com/books?id=3JqkCgAAQBAJ&pg=PT83 | access-date=2023-05-14 | page=83}}</ref> However, those ideas about other worlds were different from the current knowledge about the structure of the universe, and did not postulate the existence of planetary systems other than the Solar System. When those authors talk about other worlds, they talk about places located at the center of their own systems, and with their own stellar vaults and cosmos surrounding them.<ref>Crowe, p. 4</ref>

The Greek ideas and the disputes between atomists and Aristotelians outlived the fall of the Greek empire. The [[Great Library of Alexandria]] compiled information about it, part of which was translated by Islamic scholars and thus survived the end of the Library. Baghdad combined the knowledge of the Greeks, the Indians, the Chinese and its own scholars, and the knowledge expanded through the [[Byzantine Empire]]. From there it eventually returned to Europe by the time of the [[Middle Ages]]. However, as the Greek atomist doctrine held that the world was created by random movements of atoms, with no need for a [[creator deity]], it became associated with [[atheism]], and the dispute intertwined with religious ones.<ref>Bennett, p. 24</ref> Still, the Church did not react to those topics in a homogeneous way, and there were stricter and more permissive views within the church itself.<ref name="Bennet31">Bennett, p. 31</ref>

The first known mention of the term 'panspermia' was in the writings of the 5th-century BC [[Ancient Greece|Greek]] philosopher [[Anaxagoras]]. He proposed the idea that life exists everywhere.<ref name="Anaxagoras">{{cite book |url= https://books.google.com/books?id=dhcQuqC8Y3kC&dq=panspermia+anaxagoras&pg=PA14|title= Life's Origin: The Beginnings of Biological Evolution|author= J. William Schopf|date= 2002|publisher= University of California Press|isbn= 9780520233911|accessdate=August 6, 2022}}</ref>

===Early modern period===
[[File:Galileo before the Holy Office - Joseph-Nicolas Robert-Fleury, 1847.png|upright=1.35|thumb|''Galileo before the Holy Office'', a 19th-century painting by [[Joseph-Nicolas Robert-Fleury]]]]
By the time of the [[late Middle Ages]] there were many known inaccuracies in the geocentric model, but it was kept in use because [[naked eye]] observations provided limited data. [[Nicolaus Copernicus]] started the [[Copernican Revolution]] by proposing that the planets revolve around the sun rather than Earth. His proposal had little acceptance at first because, as he kept the assumption that orbits were perfect circles, his model led to as many inaccuracies as the geocentric one. [[Tycho Brahe]] improved the available data with naked-eye observatories, which worked with highly complex [[sextant]]s and [[Quadrant (instrument)|quadrants]]. Tycho could not make sense of his observations, but [[Johannes Kepler]] did: orbits were not perfect circles, but ellipses. This knowledge benefited the Copernican model, which worked now almost perfectly. The invention of the [[telescope]] a short time later, perfected by [[Galileo Galilei]], clarified the final doubts, and the [[paradigm shift]] was completed.<ref>Bennet, pp. 24-27</ref> Under this new understanding, the notion of extraterrestrial life became feasible: if Earth is but just a planet orbiting around a star, there may be planets similar to Earth elsewhere. The astronomical study of distant bodies also proved that physical laws are the same elsewhere in the universe as on Earth, with nothing making the planet truly special.<ref>Bennet, p. 5</ref>

The new ideas were met with resistance from the Catholic church. Galileo was [[Galileo affair|tried]] for the heliocentric model, which was considered heretical, and forced to recant it.<ref>Bennett, p. 29</ref> The best-known early-modern proponent of ideas of extraterrestrial life was the Italian philosopher [[Giordano Bruno]], who argued in the 16th century for an infinite universe in which every star is surrounded by its own [[planetary system]]. Bruno wrote that other worlds "have no less virtue nor a nature different to that of our earth" and, like Earth, "contain animals and inhabitants".<ref>{{cite web |url=http://www.positiveatheism.org/hist/brunoiuw0.htm#IUW0III |title=Giordano Bruno: On the Infinite Universe and Worlds (De l'Infinito Universo et Mondi) Introductory Epistle: Argument of the Third Dialogue |access-date=4 October 2014 |url-status=dead |archive-url=https://web.archive.org/web/20141013120648/http://www.positiveatheism.org/hist/brunoiuw0.htm#IUW0III |archive-date=13 October 2014 }}</ref> Bruno's belief in the plurality of worlds was one of the charges leveled against him by the [[Venetian Holy Inquisition]], which trialed and executed him.<ref name="AM8">Aguilera Mochon, p. 8</ref>

The heliocentric model was further strengthened by the postulation of the [[theory of gravity]] by Sir [[Isaac Newton]]. This theory provided the mathematics that explains the motions of all things in the universe, including planetary orbits. By this point, the geocentric model was definitely discarded. By this time, the use of the scientific method had become a standard, and new discoveries were expected to provide evidence and rigorous mathematical explanations. Science also took a deeper interest in the mechanics of natural phenomena, trying to explain not just the way nature works but also the reasons for working that way.<ref>Bennet, p. 30</ref>

There was very little actual discussion about extraterrestrial life before this point, as the Aristotlean ideas remained influential while geocentrism was still accepted. When it was finally proved wrong, it not only meant that Earth was not the center of the universe, but also that the lights seen in the sky were not just lights, but physical objects. The notion that life may exist in them as well soon became an ongoing topic of discussion, although one with no practical ways to investigate.<ref>Bennet, pp. 30-32</ref>

The possibility of extraterrestrials remained a widespread speculation as scientific discovery accelerated. [[William Herschel]], the discoverer of [[Uranus]], was one of many 18th–19th-century astronomers who believed that the [[Solar System]] is populated by alien life. Other scholars of the period who championed "cosmic pluralism" included [[Immanuel Kant]] and [[Benjamin Franklin]]. At the height of the [[Age of Enlightenment|Enlightenment]], even the [[Sun]] and Moon were considered candidates for extraterrestrial inhabitants.<ref>{{Cite web |title=Peoples & Creatures of the Moon {{!}} Life on Other Worlds {{!}} Articles and Essays {{!}} Finding Our Place in the Cosmos: From Galileo to Sagan and Beyond {{!}} Digital Collections {{!}} Library of Congress |url=https://www.loc.gov/collections/finding-our-place-in-the-cosmos-with-carl-sagan/articles-and-essays/life-on-other-worlds/peoples-and-creatures-of-the-moon |access-date=2024-05-10 |website=Library of Congress, Washington, D.C. 20540 USA}}</ref><ref>{{Cite web |last=Parkyn |first=Joel L. |date=April 2019 |title=The Devine Pedagogy: Theological Explorations of Intelligent Extraterrestrial Life |url=https://ore.exeter.ac.uk/repository/bitstream/handle/10871/120770/ParkynJ.pdf?sequence=2 |access-date=May 10, 2024 |website=ore.exeter.ac.uk}}</ref>

===19th century===
[[File:Lowell Mars channels.jpg|thumb|Artificial Martian channels, depicted by Percival Lowell]]
Speculation about life on Mars increased in the late 19th century, following telescopic observation of apparent [[Martian canals]] – which soon, however, turned out to be optical illusions.<ref>{{cite journal |title=Experiments as to the actuality of the "Canals" observed on Mars |journal=Monthly Notices of the Royal Astronomical Society |first1=J. E. |last1=Evans |first2=E. W. |last2=Maunder |author2-link=Edward Walter Maunder |volume=63 |issue=8 |pages=488–499 |date=June 1903 |doi=10.1093/mnras/63.8.488 |bibcode=1903MNRAS..63..488E|url=https://zenodo.org/record/1431867 |doi-access=free }}</ref> Despite this, in 1895, American astronomer [[Percival Lowell]] published his book ''Mars,'' followed by ''Mars and its Canals'' in 1906, proposing that the canals were the work of a long-gone civilisation.<ref name="Wallace1907">{{cite book |url=http://people.wku.edu/charles.smith/wallace/S730.htm |title=Is Mars Habitable? A Critical Examination of Professor Lowell's Book "Mars and Its Canals," With an Alternative Explanation |publisher=Macmillan |location=London |first=Alfred Russel |last=Wallace |date=1907 |oclc=8257449}}</ref> 

[[Spectroscopy|Spectroscopic]] analysis of Mars's atmosphere began in earnest in 1894, when U.S. astronomer [[William Wallace Campbell]] showed that neither water nor oxygen was present in the [[Martian atmosphere]].<ref name="chambers">{{Cite book |first=Paul |last=Chambers |title=Life on Mars; The Complete Story |place=London |publisher=Blandford |year=1999 |isbn=978-0-7137-2747-0 |url=https://archive.org/details/lifeonmarscomple00cham }}</ref> By 1909 better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal hypothesis.<ref>{{Cite web |title=Seeing and Interpreting Martian Oceans and Canals {{!}} Life on Other Worlds {{!}} Articles and Essays {{!}} Finding Our Place in the Cosmos: From Galileo to Sagan and Beyond {{!}} Digital Collections {{!}} Library of Congress |url=https://www.loc.gov/collections/finding-our-place-in-the-cosmos-with-carl-sagan/articles-and-essays/life-on-other-worlds/seeing-and-interpreting-martian-oceans-and-canals |access-date=2024-05-10 |website=Library of Congress, Washington, D.C. 20540 USA}}</ref>

As a consequence of the belief in the [[spontaneous generation]] there was little thought about the conditions of each celestial body: it was simply assumed that life would thrive anywhere. This theory was disproved by [[Louis Pasteur]] in the 19th century. Popular belief in thriving alien civilisations elsewhere in the solar system still remained strong until [[Mariner 4]] and [[Mariner 9]] provided close images of Mars, which debunked forever the idea of the existence of Martians and decreased the previous expectations of finding alien life in general.<ref>Aguilera Mochon, pp. 8–9</ref> The end of the spontaneous generation belief forced to investigate the origin of life. Although [[abiogenesis]] is the more accepted theory, a number of authors reclaimed the term "panspermia" and proposed that life was brought to Earth from elsewhere.<ref name="Anaxagoras"/> Some of those authors are [[Jöns Jacob Berzelius]] (1834),<ref>{{Cite journal |title=Analysis of the Alais meteorite and implications about life in other worlds |journal=[[Liebigs Annalen|Annalen der Chemie und Pharmacie]] |first=Jöns Jacob |last=Berzelius |author-link=Jöns Jacob Berzelius |volume=10 |pages=134–135 |date=1834}}</ref> [[William Thomson, 1st Baron Kelvin|Kelvin]] (1871),<ref>{{Cite journal |title=The British Association Meeting at Edinburgh |journal=Nature |first=William |last=Thomson |author-link=William Thomson, 1st Baron Kelvin |volume=4 |issue=92 |pages=261–278 |date=August 1871 |doi=10.1038/004261a0 |bibcode=1871Natur...4..261. |quote=We must regard it as probably to the highest degree that there are countless seed-bearing meteoritic stones moving through space. |pmc=2070380}}</ref> [[Hermann von Helmholtz]] (1879)<ref>{{cite journal |title=Darwin's Contribution to the Development of the Panspermia Theory |journal=Astrobiology |first=René |last=Demets |volume=12 |issue=10 |pages=946–950 |date=October 2012 |doi=10.1089/ast.2011.0790 |pmid=23078643 |bibcode=2012AsBio..12..946D}}</ref> and, somewhat later, by [[Svante Arrhenius]] (1903).<ref>{{cite book |url=https://archive.org/details/worldsinmakingev00arrhrich |title=Worlds in the Making: The Evolution of the Universe |publisher=Harper & Brothers |first=Svante |last=Arrhenius |others=trans. H. Borns |date=March 1908 |oclc=1935295}}</ref>

The science fiction genre, although not so named during the time, developed during the late 19th century. The expansion of the genre of [[extraterrestrials in fiction]] influenced the popular perception over the real-life topic, making people eager to jump to conclusions about the discovery of aliens. Science marched at a slower pace, some discoveries fueled expectations and others dashed excessive hopes. For example, with the advent of telescopes, most structures seen on the Moon or Mars were immediately attributed to Selenites or Martians, and later ones (such as more powerful telescopes) revealed that all such discoveries were natural features.<ref name="AM8"/> A famous case is the [[Cydonia (Mars)|Cydonia]] region of Mars, first imaged by the ''[[Viking 1]]'' orbiter. The low-resolution photos showed a rock formation that resembled a human face, but later spacecraft took photos in higher detail that showed that there was nothing special about the site.<ref>{{cite web |url= https://www.space.com/17191-face-on-mars.html|title= The Face on Mars: Fact & Fiction|author= Nola Taylor Tillman |date= August 20, 2012|publisher= Space.com|accessdate=September 18, 2022}}</ref>

===Recent history===
{{see also|Space exploration}}
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| footer            = Some major international efforts to search for extraterrestrial life, clockwise from top left:
*The search for [[extrasolar planet]]s (image: [[Kepler space telescope|''Kepler'' telescope]])
*[[SETI|Listening for extraterrestrial signals indicating intelligence]] (image: [[Allen Telescope Array|Allen array]])
*[[Robotic spacecraft|Robotic exploration]] of the [[Solar System]] (image: [[Curiosity (rover)|''Curiosity'' rover]] on [[Mars]])
}}
The search and study of extraterrestrial life became a science of its own, [[astrobiology]]. Also known as ''exobiology'', this discipline is studied by the [[NASA]], the [[ESA]], the [[INAF]], and others. Astrobiology studies life from Earth as well, but with a cosmic perspective. For example, [[abiogenesis]] is of interest to astrobiology, not because of the origin of life on Earth, but for the chances of a similar process taking place in other celestial bodies. Many aspects of life, from its definition to its chemistry, are analyzed as either likely to be similar in all forms of life across the cosmos or only native to Earth.<ref>Aguilera Mochon, pp. 10–11</ref> Astrobiology, however, remains constrained by the current lack of extraterrestrial life-forms to study, as all life on Earth comes from the same ancestor, and it is hard to infer general characteristics from a group with a single example to analyse.<ref>{{cite web|url= https://www.nasa.gov/vision/universe/starsgalaxies/life%27s_working_definition.html|title= Life's Working Definition: Does It Work?|author= |date= 2002|publisher= NASA|accessdate= January 17, 2022|archive-date= 26 May 2018|archive-url= https://web.archive.org/web/20180526161726/https://www.nasa.gov/vision/universe/starsgalaxies/life%27s_working_definition.html|url-status= dead}}</ref>

The 20th century came with great technological advances, speculations about future [[Hypothetical technology|hypothetical technologies]], and an increased basic knowledge of science by the general population thanks to [[science divulgation]] through the mass media. The public interest in extraterrestrial life and the lack of discoveries by mainstream science led to the emergence of [[pseudoscience]]s that provided affirmative, if questionable, answers to the existence of aliens. [[Ufology]] claims that many [[unidentified flying object]]s (UFOs) would be spaceships from alien species, and [[ancient astronauts]] hypothesis claim that aliens would have visited Earth in antiquity and prehistoric times but people would have failed to understand it by then.<ref>Aguilera Mochon, p. 10</ref> Most UFOs or [[List of UFO sightings|UFO sightings]]<ref>{{cite journal |first=Anne |last=Cross |title=The Flexibility of Scientific Rhetoric: A Case Study of UFO Researchers |journal=Qualitative Sociology |volume=27 |issue=1 |pages=3–34 |doi=10.1023/B:QUAS.0000015542.28438.41 |year=2004|s2cid=144197172 }}</ref> can be readily explained as sightings of Earth-based aircraft (including [[Black project|top-secret aircraft]]), known [[astronomical object]]s or weather phenomenons, or as [[hoax]]es.<ref>{{cite journal |first=Philippe |last=Ailleris |date=January–February 2011 |title=The lure of local SETI: Fifty years of field experiments |journal=Acta Astronautica |volume=68 |issue=1–2 |pages=2–15 |doi=10.1016/j.actaastro.2009.12.011 |bibcode=2011AcAau..68....2A}}</ref>

Looking beyond the pseudosciences, [[Lewis White Beck]] strove to elevate the level of public discourse on the topic of extraterrestrial life by tracing the evolution of philosophical thought over the centuries from ancient times into the modern era. His review of the contributions made by [[Lucretius]], [[Plutarch]], [[Aristotle]], [[Copernicus]], [[Immanuel Kant]], [[John Wilkins]],<!-- The text of Beck's article cited here does say "Thomas Wilkins", but the source cited in that article is a work by John Wilkins. --> [[Charles Darwin]] and [[Karl Marx]] demonstrated that even in modern times, humanity could be profoundly influenced in its search for extraterrestrial life by subtle and comforting archetypal ideas which are largely derived from firmly held religious, philosophical and existential belief systems. On a positive note, however,  Beck further argued that even if the search for extraterrestrial life proves to be unsuccessful, the endeavor itself could have beneficial consequences by assisting humanity in its attempt to actualize superior ways of living here on Earth.<ref>{{Cite journal |last=Beck |first=Lewis White |date=1971 |title=Extraterrestrial Intelligent Life |url=https://www.jstor.org/stable/3129745 |journal=Proceedings and Addresses of the American Philosophical Association |volume=45 |pages=5–21 |doi=10.2307/3129745|jstor=3129745 }}</ref>
   
By the 21st century, it was accepted that multicellular life in the Solar System can only exist on Earth, but the interest in extraterrestrial life increased regardless. This is a result of the advances in several sciences. The knowledge of planetary habitability allows to consider on scientific terms the likelihood of finding life at each specific celestial body, as it is known which features are beneficial and harmful for life. Astronomy and telescopes also improved to the point exoplanets can be confirmed and even studied, increasing the number of search places. Life may still exist elsewhere in the Solar System in unicellular form, but the advances in spacecraft allow to send robots to study samples in situ, with tools of growing complexity and reliability. Although no extraterrestrial life has been found and life may still be just a rarity from Earth, there are scientific reasons to suspect that it can exist elsewhere, and technological advances that may detect it if it does.<ref>Bennett, p. 4</ref>

Many scientists are optimistic about the chances of finding alien life. In the words of SETI's Frank Drake, "All we know for sure is that the sky is not littered with powerful microwave transmitters".<ref>{{cite web |url=http://antarcticaedu.com/bio2024.htm |title=LECTURE 4: MODERN THOUGHTS ON EXTRATERRESTRIAL LIFE |work=The University of Antarctica |access-date=25 July 2015}}</ref> Drake noted that it is entirely possible that advanced technology results in communication being carried out in some way other than conventional radio transmission. At the same time, the data returned by space probes, and giant strides in detection methods, have allowed science to begin delineating [[Planetary habitability|habitability criteria]] on other worlds, and to confirm that at least other planets are plentiful, though aliens remain a question mark. The [[Wow! signal]], detected in 1977 by a SETI project, remains a subject of speculative debate.<ref>{{Cite web |date=2020-12-02 |title=Did the Wow! signal come from this star? {{!}} Space {{!}} EarthSky |url=https://earthsky.org/space/source-of-wow-signal-in-1977-sunlike-star-2mass-19281982-2640123/ |access-date=2024-05-10 |website=earthsky.org |language=en-US}}</ref>

On the other hand, other scientists are pessimistic. [[Jacques Monod#Philosophical contributions|Jacques Monod]] wrote that "Man knows at last that he is alone in the indifferent immensity of the universe, whence which he has emerged by chance".<ref>{{cite web |url= https://www.scientificamerican.com/article/the-cosmos-might-be-mostly-devoid-of-life/|title= The Cosmos Might Be Mostly Devoid of Life|author= Paul Davies|date= September 1, 2016|publisher= Scientific American|accessdate=July 8, 2022}}</ref> In 2000, geologist and [[paleontologist]] [[Peter Ward (paleontologist)|Peter Ward]] and [[Astrobiology|astrobiologist]] [[Donald Brownlee]] published a book entitled ''[[Rare Earth (book)|Rare Earth: Why Complex Life is Uncommon in the Universe]]''.<ref>{{cite book |title=Rare Earth: Why Complex Life is Uncommon in the Universe |publisher=Copernicus |first1=Peter |last1=Ward |first2=Donald |last2=Brownlee |date=2000 |bibcode=2000rewc.book.....W |isbn=978-0-387-98701-9}}</ref>{{Better source needed|reason=The significance of the book on the history of thoughts about alien life should not be the book itself|date=July 2023}} In it, they discussed the [[Rare Earth hypothesis]], in which they claim that Earth-like life is rare in the [[universe]], whereas [[bacteria|microbial]] life is common. Ward and Brownlee are open to the idea of evolution on other planets that is not based on essential Earth-like characteristics such as DNA and carbon.

As for the possible risks, theoretical physicist [[Stephen Hawking]] warned in 2010 that humans should not try to contact alien life forms. He warned that aliens might pillage Earth for resources. "If aliens visit us, the outcome would be much as when [[Christopher Columbus|Columbus]] landed in [[Americas|America]], which didn't turn out well for the [[Indigenous peoples of the Americas|Native Americans]]", he said.<ref>{{cite news |url=http://news.bbc.co.uk/2/hi/uk_news/8642558.stm |work=BBC News |title=Hawking warns over alien beings |date=25 April 2010 |access-date=2 May 2010}}</ref> [[Jared Diamond]] had earlier expressed similar concerns.<ref>{{cite book |chapter=Chapter 12 |title=The Third Chimpanzee: The Evolution and Future of the Human Animal |publisher=Harper Perennial |first=Jared M. |last=Diamond |date=2006 |isbn=978-0-06-084550-6}}</ref> On 20 July 2015, Hawking and Russian billionaire [[Yuri Milner]], along with the [[SETI Institute]], announced a well-funded effort, called the [[Breakthrough Initiatives]], to expand efforts to search for extraterrestrial life. The group contracted the services of the 100-meter [[Robert C. Byrd]] [[Green Bank Telescope]] in West Virginia in the United States and the 64-meter [[Parkes Telescope]] in New South Wales, Australia.<ref name="AP-20150720">{{cite news |url=http://apnews.excite.com/article/20150720/eu--britain-extraterrestrials-e52c157915.html |title=Searching for ET: Hawking to look for extraterrestrial life |work=Excite! |agency=Associated Press |last=Katz |first=Gregory |date=20 July 2015 |access-date=20 July 2015}}</ref> On 13 February 2015, scientists (including [[Geoffrey Marcy]], [[Seth Shostak]], [[Frank Drake]] and [[David Brin]]) at a convention of the [[American Association for the Advancement of Science]], discussed [[Active SETI]] and whether transmitting a message to possible intelligent extraterrestrials in the [[Cosmos]] was a good idea;<ref name="NYT-20150213">{{cite news |url=https://www.nytimes.com/aponline/2015/02/13/science/ap-us-sci-calling-the-cosmos.html |title=Should We Call the Cosmos Seeking ET? Or Is That Risky? |work=[[The New York Times]] |agency=Associated Press |first=Seth |last=Borenstein |date=13 February 2015 |archive-url=https://web.archive.org/web/20150214152940/http://www.nytimes.com/aponline/2015/02/13/science/ap-us-sci-calling-the-cosmos.html |archive-date=14 February 2015 |url-status=dead }}</ref><ref name="BBC-20150212">{{cite news |last=Ghosh |first=Pallab |title=Scientist: 'Try to contact aliens' |url=https://www.bbc.com/news/science-environment-31442952 |date=12 February 2015 |work=[[BBC News]] |access-date=12 February 2015}}</ref> one result was a statement, signed by many, that a "worldwide scientific, political and humanitarian discussion must occur before any message is sent".<ref name="UCB-20150213">{{cite web |url=http://setiathome.berkeley.edu/meti_statement_0.html |title=Regarding Messaging To Extraterrestrial Intelligence (METI) / Active Searches For Extraterrestrial Intelligence (Active SETI) |publisher=[[University of California, Berkeley]] |date=13 February 2015 |access-date=14 February 2015}}</ref>

==Government responses==
{{see also|Planetary protection}}

The 1967 [[Outer Space Treaty]] and the 1979 [[Moon Agreement]] define rules of [[planetary protection]] against potentially hazardous extraterrestrial life. [[COSPAR]] also provides guidelines for planetary protection.<ref>{{Cite news |last=Matignon |first=Louis |date=29 May 2019 |title=The French anti-UFO Municipal Law of 1954 |url=https://www.spacelegalissues.com/the-french-anti-ufo-municipal-law-of-1954/ |url-status=dead |archive-url=https://web.archive.org/web/20210427042210/https://www.spacelegalissues.com/the-french-anti-ufo-municipal-law-of-1954/ |archive-date=27 April 2021 |access-date=26 March 2021 |work=Space Legal Issues}}</ref> A committee of the [[United Nations Office for Outer Space Affairs]] had in 1977 discussed for a year strategies for interacting with extraterrestrial life or intelligence. The discussion ended without any conclusions. As of 2010, the UN lacks response mechanisms for the case of an extraterrestrial contact.<ref>{{Cite web |date=14 October 2010 |title=Press Conference by Director of Office for Outer Space Affairs |url=https://www.un.org/press/en/2010/101014_Othman.doc.htm |website=UN Press}}</ref>

One of the NASA divisions is the Office of Safety and Mission Assurance (OSMA), also known as the Planetary Protection Office. A part of its mission is to "rigorously preclude backward contamination of Earth by extraterrestrial life."<ref>{{Cite magazine |last1=Kluger |first1=Jeffrey |date=March 2, 2020 |title=Coronavirus Could Preview What Will Happen When Alien Life Reaches Earth |url=https://time.com/5793520/coronavirus-alien-life/ |magazine=Time}}</ref>

In 2016, the Chinese Government released a white paper detailing its [[Chinese Space Program|space program]]. According to the document, one of the research objectives of the program is the search for extraterrestrial life.<ref>{{Cite web |last=Wheeler |first=Michelle |date=14 July 2017 |title=Is China The Next Space Superpower? |url=https://particle.scitech.org.au/space/china-next-space-superpower/ |work=Particle}}</ref> It is also one of the objectives of the Chinese [[Five-hundred-meter Aperture Spherical Telescope]] (FAST) program.<ref>{{Cite web |title=China Focus: Earth's largest radio telescope to search for "new worlds" outside solar system |url=http://www.xinhuanet.com/english/2019-07/11/c_138218290.htm |url-status=dead |archive-url=https://web.archive.org/web/20190711131334/http://www.xinhuanet.com/english/2019-07/11/c_138218290.htm |archive-date=11 July 2019}}</ref>

In 2020, [[Dmitry Rogozin]], the head of the [[Roskosmos|Russian space agency]], said the search for extraterrestrial life is one of the main goals of deep space research. 
He also acknowledged the possibility of existence of primitive life on other planets of the Solar System.<ref>{{Cite web |title=Рогозин допустил существование жизни на Марсе и других планетах Солнечной системы |url=https://tass.ru/kosmos/9160789 |website=ТАСС}}</ref>

The [[CNES|French space agency]] has an office for the study of "non-identified aero spatial phenomena".<ref name="newscientist.com">{{Cite web |date=22 March 2007 |title=France opens up its UFO files |url=https://www.newscientist.com/article/dn11443-france-opens-up-its-ufo-files/ |website=New Scientist}}</ref><ref>{{Cite news |last=Bockman |first=Chris |date=4 November 2014 |title=Why the French state has a team of UFO hunters |url=https://www.bbc.com/news/magazine-29755919 |work=BBC News}}</ref> The agency is maintaining a publicly accessible database of such phenomena, with over 1600 detailed entries. According to the head of the office, the vast majority of entries have a mundane explanation; but for 25% of entries, their extraterrestrial origin can neither be confirmed nor denied.<ref name="newscientist.com" />

In 2020, chairman of the [[Israel Space Agency]] [[Isaac Ben-Israel]] stated that the probability of detecting life in outer space is "quite large". But he disagrees with his former colleague [[Haim Eshed]] who stated that there are contacts between an advanced alien civilisation and some of Earth's governments.<ref>{{Cite web |last=Jeffay |first=Nathan |date=10 December 2020 |title=Israeli space chief says aliens may well exist, but they haven't met humans |url=https://www.timesofisrael.com/israeli-space-chief-says-aliens-may-well-exist-but-they-havent-met-humans/ |website=[[The Times of Israel]]}}</ref>

==In fiction==
{{main|Extraterrestrials in fiction}}
[[File:Grey Aliens Drawing.jpg|thumb|[[Grey alien]]s are a common way to depict extraterrestrials in fiction.]]
Although the idea of extraterrestrial peoples became feasible once astronomy developed enough to understand the nature of planets, they were not thought of as being any different from humans. Having no scientific explanation for the [[origin of mankind]] and its relation to other species, there was no reason to expect them to be any other way. This was changed by the 1859 book ''[[On the Origin of Species]]'' by [[Charles Darwin]], which proposed the [[theory of evolution]]. Now with the notion that evolution on other planets may take other directions, [[science fiction]] authors created bizarre aliens, clearly distinct from humans. A usual way to do that was to add body features from other animals, such as insects or octopuses. Costuming and special effects feasibility alongside budget considerations forced films and TV series to tone down the fantasy, but these limitations lessened since the 1990s with the advent of [[computer-generated imagery]] (CGI), and later on as CGI became more effective and less expensive.<ref name="early">{{cite web |url= https://www.bbc.com/future/article/20231019-the-weird-aliens-of-early-science-fiction|title= The weird aliens of early science fiction|author= Zaria Gorvett|date= October 22, 2023|publisher= BBC|accessdate=January 25, 2024}}</ref>

Real-life events sometimes captivate people's imagination and this influences the works of fiction. For example, during the [[Barney and Betty Hill incident]], the first recorded claim of an [[alien abduction]], the couple reported that they were abducted and experimented on by aliens with oversized heads, big eyes, pale grey skin, and small noses, a description that eventually became the [[grey alien]] archetype once used in works of fiction.<ref name="early"/>

==See also==
{{columns-list|
*[[Assembly theory]]
*[[Carbon chauvinism]]
*[[First contact (anthropology)]]
*[[Hemolithin]]
*[[Hypothetical types of biochemistry]]
*[[Life origination beyond planets]]
*[[Outline of extraterrestrial life]]
*[[Quiet and loud aliens]]
*[[Sentiocentrism]]
*[[Speciesism]]
*[[Uncontacted peoples]]
 }}

==Notes==
{{reflist|group=lower-alpha}}

==References==
{{reflist|30em}}

==Further reading==
{{refbegin}}
*{{cite book |last=Aguilera Mochón |first=Juan Antonio |date=2016 |title=La vida no terrestre |trans-title=The non-terrestrial life |url= |language=Spanish |location= |publisher=RBA |isbn=978-84-473-8665-9}}
*{{cite book |last=Baird |first=John C. |year=1987 |title=The Inner Limits of Outer Space: A Psychologist Critiques Our Efforts to Communicate With Extraterrestrial Beings |location=Hanover |publisher=University Press of New England |isbn=978-0-87451-406-3 |url=https://archive.org/details/innerlimitsofout00bair}}
*{{cite book |last=Bennett |first=Jeffrey |author-link= |date=2017 |title=Life in the universe |url= |location=United States |publisher=Pearson |pages=3–4 |isbn=978-0-13-408908-9}}
*{{cite book |last1=Cohen |first1=Jack |author-link1=Jack Cohen (scientist) |last2=Stewart |first2=Ian |author-link2=Ian Stewart (mathematician) |year=2002 |title=Evolving the Alien: The Science of Extraterrestrial Life |publisher=Ebury Press |isbn=978-0-09-187927-3 |title-link=Evolving the Alien}}
*{{cite book |last=Crowe |first=Michael J. |title=The Extraterrestrial Life Debate, 1750–1900 |publisher=Cambridge |year=1986 |isbn=978-0-521-26305-4}}
*{{cite book |last=Dick |first=Steven J. |title=Plurality of Worlds: The Extraterrestrial Life Debate from Democratus to Kant |publisher=Cambridge |year=1984}}
*{{cite book |last=Dick |first=Steven J. |title=The Biological Universe: The Twentieth Century Extraterrestrial Life Debate and the Limits of Science |publisher=Cambridge |year=1996 |isbn=978-0-521-34326-8}}
*{{cite book |last=Dick |first=Steven J. |title=Life on Other Worlds: The 20th Century Extraterrestrial Life Debate |publisher=Cambridge |year=2001 |isbn=978-0-521-79912-6}}
*{{cite book |last1=Dick |first1=Steven J. |first2=James E. |last2=Strick |title=The Living Universe: NASA And the Development of Astrobiology |url=https://archive.org/details/livinguniversena0000dick |url-access=registration |publisher=Rutgers |year=2004 |isbn=978-0-8135-3447-3}}
*{{cite book |last=Fasan |first=Ernst |title=Relations with alien intelligences&nbsp;– the scientific basis of metalaw |publisher=Berlin Verlag |location=Berlin |year=1970}}
*{{cite book |last=Goldsmith |first=Donald |year=1997 |title=The Hunt for Life on Mars |location=New York |publisher=A Dutton Book |isbn=978-0-525-94336-5 |url=https://archive.org/details/huntforlifeonmar00gold}}
*[[John Gribbin|Gribbin, John]], "Alone in the Milky Way: Why we are probably the only intelligent life in the galaxy", ''[[Scientific American]]'', vol. 319, no. 3 (September 2018), pp.&nbsp;94–99.
*{{cite book |last=Grinspoon |first=David |year=2003 |title=Lonely Planets: The Natural Philosophy of Alien Life |publisher=HarperCollins |isbn=978-0-06-018540-4}}
*{{cite book |last=Lemnick |first=Michael T. |year=1998 |title=Other Worlds: The Search for Life in the Universe |location=New York |publisher=A Touchstone Book |bibcode=1998owsl.book.....L}}
*{{cite book |last=Michaud |first=Michael |title=Contact with Alien Civilizations&nbsp;– Our Hopes and Fears about Encountering Extraterrestrials. |publisher=Springer |location=Berlin |year=2006 |isbn=978-0-387-28598-6 |url-access=registration |url=https://archive.org/details/contactwithalien0000mich}}
*{{cite book |last=Pickover |first=Cliff |author-link=Cliff Pickover |year=2003 |title=The Science of Aliens |location=New York |publisher=Basic Books |isbn=978-0-465-07315-3}}
*{{cite book |last=Roth |first=Christopher F. |year=2005 |title=Ufology as Anthropology: Race, Extraterrestrials, and the Occult |work=E.T. Culture: Anthropology in Outerspaces |editor=Debbora Battaglia |location=Durham, NC |publisher=Duke University Press}}
*{{cite book |last1=Sagan |first1=Carl |author-link1=Carl Sagan |first2=I. S. |last2=Shklovskii |author-link2=I.S. Shklovskii |title=Intelligent Life in the Universe |publisher=Random House |year=1966}}
*{{cite book |last=Sagan |first=Carl |title=Communication with Extraterrestrial Intelligence |publisher=MIT Press |year=1973 |isbn=978-0-262-19106-7 |title-link=Communication with Extraterrestrial Intelligence}}
*{{cite book |first=Peter D. |last=Ward |title=Life as we do not know it-the NASA search for (and synthesis of) alien life |publisher=Viking |location=New York |year=2005 |isbn=978-0-670-03458-1 |url=https://archive.org/details/lifeaswedonotkno00ward}}
*{{cite book |last=Tumminia |first=Diana G. |title=Alien Worlds&nbsp;– Social and Religious Dimensions of Extraterrestrial Contact |publisher=Syracuse University Press |location=Syracuse |year=2007 |isbn=978-0-8156-0858-5 |url-access=registration |url=https://archive.org/details/alienworldssocia0000unse}}
{{refend}}

==External links==
{{Commons category|Extraterrestrial life}}
{{Wikiquote}}
{{Wikisource portal|Extraterrestrial life}}
* [https://astrobiology.nasa.gov/ Astrobiology at NASA]
* [https://europeanastrobiology.eu/ European Astrobiology Institute]

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{{Astrobiology}}
{{Molecules detected in outer space}}
{{Interstellar messages}}
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{{DEFAULTSORT:Extraterrestrial Life}}
[[Category:Astrobiology]]
[[Category:Interstellar messages]]
[[Category:Search for extraterrestrial intelligence]]
[[Category:Unsolved problems in biology]]
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[[Category:Biological hypotheses]]
[[Category:Biology controversies]]
[[Category:Scientific speculation]]
[[Category:Outer space]]