Editing Extraterrestrial life (section)

==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>