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{{Short description|Hypothesis that complex extraterrestrial life is improbable and extremely rare}}
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[[File:The Blue Marble (remastered).jpg|thumb|300px|The Rare Earth hypothesis argues that planets with complex life, like Earth, are exceptionally rare.]]
In planetary astronomy and [[Astronomy:Astrobiology|astrobiology]], the '''Rare Earth hypothesis''' argues that the origin of life and the [[Biology:Evolution of biological complexity|evolution of biological complexity]] such as [[Biology:Sexual reproduction|sexually reproducing]], [[Biology:Multicellular organism|multicellular organism]]s on [[Astronomy:Earth|Earth]] (and, subsequently, [[Philosophy:Human intelligence|human intelligence]]) required an improbable combination of [[Physics:Astrophysics|astrophysical]] and [[Earth:Geology|geological]] events and circumstances.

According to the hypothesis, complex [[Astronomy:Extraterrestrial life|extraterrestrial life]] is an improbable phenomenon and likely to be rare throughout the universe as a whole. The term "Rare Earth" originates from ''Rare Earth: Why Complex Life Is Uncommon in the Universe'' (2000), a book by [[Biography:Peter Ward (paleontologist)|Peter Ward]], a geologist and paleontologist, and [[Biography:Donald E. Brownlee|Donald E. Brownlee]], an astronomer and astrobiologist, both faculty members at the [[Organization:University of Washington|University of Washington]].

In the 1970s and 1980s, [[Biography:Carl Sagan|Carl Sagan]] and Frank Drake, among others, argued that Earth is a typical [[Astronomy:Terrestrial planet|rocky planet]] in a typical [[Astronomy:Planetary system|planetary system]], located in a non-exceptional region of a common [[Astronomy:Barred spiral galaxy|barred spiral galaxy]]. From the principle of mediocrity (extended from the [[Astronomy:Copernican principle|Copernican principle]]), they argued that the evolution of life on Earth, including human beings, was also typical, and therefore that the universe teems with complex life. However, Ward and Brownlee argue that planets, planetary systems, and galactic regions that are as accommodating for complex life as are the Earth, the [[Astronomy:Solar System|Solar System]], and our own [[Astronomy:Local Bubble|galactic region]] are not typical at all, but actually exceedingly rare.


==Fermi paradox==
{{Main|Astronomy:Fermi paradox}}
There is no reliable or reproducible evidence that [[Astronomy:Extraterrestrial life|extraterrestrial organisms]] of any kind have visited [[Astronomy:Earth|Earth]].<ref name="ConversationNoUFOs">{{cite news |last1=Tingay |first1=Steven |title=Is there evidence aliens have visited Earth? Here's what's come out of US congress hearings on 'unidentified aerial phenomena' |url=https://theconversation.com/is-there-evidence-aliens-have-visited-earth-heres-whats-come-out-of-us-congress-hearings-on-unidentified-aerial-phenomena-183443 |access-date=27 October 2022 |work=The Conversation |language=en}}</ref><ref name="NewYorkerAliens2021">{{cite magazine |last1=Kolbert |first1=Elizabeth |title=Have We Already Been Visited by Aliens? |url=https://www.newyorker.com/magazine/2021/01/25/have-we-already-been-visited-by-aliens |access-date=27 October 2022 |magazine=The New Yorker |date=14 January 2021}}</ref> No transmissions or evidence of intelligent extraterrestrial life have been detected or observed anywhere other than [[Astronomy:Earth|Earth]] in the [[Astronomy:Universe|Universe]]. This runs counter to the knowledge that the Universe is filled with a very large number of planets, some of which likely hold the conditions hospitable for life. Life typically expands until it fills all available niches.<ref name="AreWeAlone">{{Cite journal|url=https://adsabs.harvard.edu/full/1978QJRAS..19..277P|title=1978QJRAS..19..277P Page 277|journal=Quarterly Journal of the Royal Astronomical Society|bibcode=1978QJRAS..19..277P |last1=Papagiannis |first1=Michael D. |year=1978 |volume=19 |page=277 }}</ref> These contradictory facts form the basis for the [[Astronomy:Fermi paradox|Fermi paradox]], of which the Rare Earth hypothesis is one proposed solution.

==Requirements for complex life==
{{Life timeline}}
The Rare Earth hypothesis argues that the [[Biology:Evolution of biological complexity|evolution of biological complexity]] anywhere in the universe requires the coincidence of a large number of fortuitous circumstances, including, among others, a [[Astronomy:Galactic habitable zone|galactic habitable zone]]; a central star and planetary system having the requisite character (i.e. a [[Astronomy:Circumstellar habitable zone|circumstellar habitable zone]]); a terrestrial planet of the right mass; the advantage of one or more gas giant guardians like Jupiter and possibly a large [[Astronomy:Natural satellite|natural satellite]] to shield the planet from frequent impact events; conditions needed to ensure the planet has a [[Astronomy:Magnetosphere|magnetosphere]] and [[Earth:Plate tectonics|plate tectonics]]; a chemistry similar to that present in the Earth's [[Earth:Lithosphere|lithosphere]], [[Astronomy:Atmosphere|atmosphere]], and oceans; the influence of periodic "evolutionary pumps" such as massive glaciations and [[Astronomy:Bolide|bolide]] impacts; and whatever factors may have led to the emergence of eukaryotic [[Biology:Cell|cells]], [[Biology:Sexual reproduction|sexual reproduction]], and the [[Earth:Cambrian explosion|Cambrian explosion]] of [[Biology:Animal|animal]], [[Biology:Plant|plant]], and [[Biology:Fungi|fungi]] [[Biology:Phylum|phyla]]. The [[Biology:Human evolution|evolution of human beings]] and of [[Philosophy:Evolution of human intelligence|human intelligence]] may have required yet further specific events and circumstances, all of which are extremely unlikely to have happened were it not for the [[Earth:Cretaceous–Paleogene extinction event|Cretaceous–Paleogene extinction event]] 66 million years ago removing dinosaurs as the dominant terrestrial [[Biology:Vertebrate|vertebrate]]s.

In order for a small rocky planet to support complex life, Ward and Brownlee argue, the values of several variables must fall within narrow ranges. The [[Astronomy:Universe|universe]] is so vast that it might still contain many Earth-like planets, but if such planets exist, they are likely to be separated from each other by many thousands of [[Astronomy:Light-year|light-year]]s. Such distances may preclude communication among any intelligent species that may evolve on such planets, which would solve the [[Astronomy:Fermi paradox|Fermi paradox]]: "If extraterrestrial aliens are common, why aren't they obvious?"{{citation needed|date=December 2021}}

===The right location in the right kind of galaxy===
Rare Earth suggests that much of the known universe, including large parts of our galaxy, are "dead zones" unable to support complex life. Those parts of a galaxy where complex life is possible make up the [[Astronomy:Galactic habitable zone|galactic habitable zone]], which is primarily characterized by distance from the [[Galactic Center]].
# As that distance increases, star [[Astronomy:Metallicity|metallicity]] declines. Metals (which in astronomy refers to all elements other than hydrogen and helium) are necessary for the formation of [[Astronomy:Terrestrial planet|terrestrial planet]]s.
# The [[Physics:X-ray|X-ray]] and [[Physics:Gamma ray|gamma ray]] radiation from the [[Astronomy:Supermassive black hole|black hole]] at the galactic center, and from nearby [[Astronomy:Neutron star|neutron star]]s, becomes less intense as distance increases. Thus the early universe, and present-day galactic regions where stellar density is high and [[Astronomy:Supernova|supernova]]e are common, will be dead zones.<ref>{{harvnb|Ward|Brownlee|2000|pp=27–29}}</ref>
# Gravitational perturbation of planets and [[Astronomy:Planetesimal|planetesimal]]s by nearby stars becomes less likely as the density of stars decreases. Hence the further a planet lies from the Galactic Center or a spiral arm, the less likely it is to be struck by a large bolide which could [[Biology:Extinction event|extinguish]] all complex life on a planet.
{{multiple image|caption_align=center |align=left |direction=vertical |image1=NGC 7331 zoomed.jpg |width1=150px |caption1=Dense centers of galaxies such as [[Astronomy:NGC 7331|NGC 7331]] (often referred to as a "twin" of the [[Astronomy:Milky Way|Milky Way]]<ref>[http://www.spitzer.caltech.edu/Media/releases/ssc2004-12/ssc2004-12a.shtml 1 Morphology of Our Galaxy's 'Twin'] {{webarchive|url=https://web.archive.org/web/20060215171141/http://www.spitzer.caltech.edu/Media/releases/ssc2004-12/ssc2004-12a.shtml |date=15 February 2006 }} Spitzer Space Telescope, Jet Propulsion Laboratory, NASA.</ref>) have high radiation levels toxic to complex life. |image2=A Swarm of Ancient Stars - GPN-2000-000930.jpg |width2=150px |caption2=According to Rare Earth, globular clusters are unlikely to support life.}}
<!---
[[File:NGC 7331 zoomed.jpg|thumb|left|150px|The dense centre of galaxies such as NGC 7331 (often referred to as a "twin" of the Milky Way<ref>[http://www.spitzer.caltech.edu/Media/releases/ssc2004-12/ssc2004-12a.shtml 1 Morphology of Our Galaxy's 'Twin'] Spitzer Space Telescope, Jet Propulsion Laboratory, NASA.</ref>) have high levels of radiation which are dangerous to complex life]]
[[File:A Swarm of Ancient Stars - GPN-2000-000930.jpg|thumb|left|150px|According to Rare Earth, globular clusters are unlikely to support life.]]
--->
Item #1 rules out the outermost reaches of a galaxy; #2 and #3 rule out galactic inner regions. Hence a galaxy's habitable zone may be a relatively narrow ring of adequate conditions sandwiched between its uninhabitable center and outer reaches.

Also, a habitable planetary system must maintain its favorable location long enough for complex life to evolve. A star with an [[Astronomy:Orbital eccentricity|eccentric]] (elliptical or hyperbolic) galactic orbit will pass through some spiral arms, unfavorable regions of high star density; thus a life-bearing star must have a galactic orbit that is nearly circular, with a close synchronization between the orbital velocity of the star and of the spiral arms. This further restricts the galactic habitable zone within a fairly narrow range of distances from the Galactic Center. Lineweaver et al. calculate this zone to be a ring 7 to 9 [[Astronomy:Parsec|kiloparsecs]] in radius, including no more than 10% of the stars in the [[Astronomy:Milky Way|Milky Way]],{{sfn|Lineweaver|Fenner|Gibson|2004|pp=59-62}} about 20 to 40 billion stars. Gonzalez ''et al.''<ref name=Gonzalez>{{harvnb|Gonzalez|Brownlee|Ward|2001}}</ref> would halve these numbers; they estimate that at most 5% of stars in the Milky Way fall within the galactic habitable zone.

Approximately 77% of observed galaxies are spiral,<ref>{{cite journal |last=Loveday |first=J. |date=February 1996 |title=The APM Bright Galaxy Catalogue |journal=[[Organization:Monthly Notices of the Royal Astronomical Society|Monthly Notices of the Royal Astronomical Society]] |volume=278 |issue=4 |pages=1025–1048 |bibcode=1996MNRAS.278.1025L |arxiv=astro-ph/9603040 |doi=10.1093/mnras/278.4.1025|s2cid=15246554 }}</ref> two-thirds of all spiral galaxies are barred, and more than half, like the Milky Way, exhibit multiple arms.<ref name="mihalas1968">{{Cite book |author=D. Mihalas |date=1968 |title=Galactic Astronomy |publisher=W. H. Freeman |isbn=978-0-7167-0326-6}}</ref> According to Rare Earth, our own galaxy is unusually quiet and dim (see below), representing just 7% of its kind.<ref name="HammerPuech2007">{{cite journal |last1=Hammer |first1=F. |last2=Puech |first2=M. |last3=Chemin |first3=L. |last4=Flores |first4=H. |last5=Lehnert |first5=M. D. |title=The Milky Way, an Exceptionally Quiet Galaxy: Implications for the Formation of Spiral Galaxies |journal=The Astrophysical Journal |volume=662 |issue=1 |year=2007 |pages=322–334 |doi=10.1086/516727 |bibcode=2007ApJ...662..322H |arxiv=astro-ph/0702585|s2cid=18002823 }}</ref> Even so, this would still represent more than 200 billion galaxies in the known universe.

Our galaxy also appears unusually favorable in suffering fewer collisions with other galaxies over the last 10 billion years, which can cause more supernovae and other disturbances.<ref>{{cite web |title=Milky Way mysteries: Andromeda, our sibling rival |date=28 March 2012 |work=New Scientist |first=Stephen |last=Battersby |url=https://www.newscientist.com/article/mg21328586.500-milky-way-mysteries-andromeda-our-sibling-rival.html}}</ref> Also, the Milky Way's central [[Astronomy:Black hole|black hole]] seems to have neither too much nor too little activity.{{sfn|Scharf| 2012}}

The orbit of the Sun around the center of the Milky Way is indeed almost perfectly circular, with [[Astronomy:Galactic year|a period of 226 Ma]] (million years), closely matching the rotational period of the galaxy. However, the majority of stars in barred spiral galaxies populate the spiral arms rather than the halo and tend to move in gravitationally aligned orbits, so there is little that is unusual about the Sun's orbit. While the Rare Earth hypothesis predicts that the Sun should rarely, if ever, have passed through a spiral arm since its formation, astronomer Karen Masters has calculated that the orbit of the Sun takes it through a major spiral arm approximately every 100 million years.<ref>{{cite web |url=http://curious.astro.cornell.edu/physics/55-our-solar-system/the-sun/the-sun-in-the-milky-way/207-how-often-does-the-sun-pass-through-a-spiral-arm-in-the-milky-way-intermediate |title=How often does the Sun pass through a spiral arm in the Milky Way? |first=Karen |last=Masters |work=Curious About Astronomy}}</ref> Some researchers have suggested that several mass extinctions do indeed correspond with previous crossings of the spiral arms.<ref>{{harvnb|Dartnell|2007|p=75}}</ref>

===The right orbital distance from the right type of star===
[[File:Estimated extent of the Solar Systems habitable zone.png|thumb|According to the hypothesis, Earth has an improbable orbit in the very narrow habitable zone (dark green) around the Sun.]]
The terrestrial example suggests that complex life requires liquid water, the maintenance of which requires an orbital distance neither too close nor too far from the central star, another scale of [[Astronomy:Habitable zone|habitable zone]] or [[Unsolved:Goldilocks principle|Goldilocks principle]].<ref>{{cite journal |author=Hart, M.H. |title=Habitable Zones Around Main Sequence Stars |journal=Icarus |volume=37 |issue=1 |pages=351–7 |date=January 1979 |doi=10.1016/0019-1035(79)90141-6 |bibcode=1979Icar...37..351H}}</ref> 
The habitable zone varies with the star's type and age.

For advanced life, the star must also be highly stable, which is typical of middle star life, about 4.6 billion years old. Proper [[Astronomy:Metallicity|metallicity]] and size are also important to stability. The Sun has a low (0.1%) [[Astronomy:Solar luminosity|luminosity]] variation. To date, no solar twin star, with an exact match of the Sun's luminosity variation, has been found, though some come close. The star must also have no stellar companions, as in [[Astronomy:Binary star|binary systems]], which would disrupt the orbits of any planets. Estimates suggest 50% or more of all star systems are binary.<ref>{{cite web |url=https://science.nasa.gov/science-news/science-at-nasa/2013/08jan_sunclimate/ |publisher=NASA |title=Science Solar Variability and Terrestrial Climate |date=8 January 2013 |first=Tony |last=Phillips}}</ref><ref name="astro.unl.edu">{{Cite web|url=http://astro.unl.edu/classaction/animations/stellarprops/stellarlum.html|title=Stellar Luminosity Calculator|website=astro.unl.edu}}</ref><ref name="nap.edu">{{Cite book|url=https://nap.nationalacademies.org/catalog/13519/the-effects-of-solar-variability-on-earths-climate-a-workshop|title=The Effects of Solar Variability on Earth's Climate: A Workshop Report|first=National Research|last=Council|date=18 September 2012|doi=10.17226/13519 |isbn=978-0-309-26564-5 |via=nap.nationalacademies.org}}</ref><ref>{{Cite web|url=https://scienceblogs.com/startswithabang/2013/06/05/most-of-earths-twins-arent-identical-or-even-close|title=Most of Earth's twins aren't identical, or even close! &#124; ScienceBlogs|website=scienceblogs.com}}</ref> The habitable zone for a [[Astronomy:Main sequence|main sequence]] star very gradually moves out over its lifespan until the star becomes a [[Astronomy:White dwarf|white dwarf]] and the habitable zone vanishes.

The liquid water and other gases available in the habitable zone bring the benefit of the [[Earth:Greenhouse effect|greenhouse effect]]. Even though the [[Earth:Atmosphere of Earth|Earth's atmosphere]] contains a water vapor concentration from 0% (in arid regions) to 4% (in rainforest and ocean regions) and – as of November 2022 – only 417.2 parts per million of {{CO2}},<ref>[https://research.noaa.gov/article/ArtMID/587/ArticleID/2914/ Global atmospheric carbon dioxide levels continue to rise, NOAA Research news]</ref> these small amounts suffice to raise the average surface temperature by about 40&nbsp;°C,<ref>{{harvnb|Ward|Brownlee|2000|p=18}}</ref> with the dominant contribution being due to water vapor.

Rocky planets must orbit within the habitable zone for life to form. Although the habitable zone of such hot stars as [[Astronomy:Sirius|Sirius]] or [[Astronomy:Vega|Vega]] is wide, hot stars also emit much more ultraviolet radiation that ionizes any planetary atmosphere. Such stars may also become [[Astronomy:Red giant|red giant]]s before advanced life [[Biology:Evolution|evolve]]s on their planets. These considerations rule out the massive and powerful stars of type F6 to O (see [[Astronomy:Stellar classification|stellar classification]]) as homes to evolved metazoan life.

Conversely, small [[Astronomy:Red dwarf|red dwarf]] stars have small [[Astronomy:Habitable zone|habitable zone]]s wherein planets are in [[Astronomy:Tidal locking|tidal lock]], with one very hot side always facing the star and another very cold side always facing away, and they are also at increased risk of solar flares (see Aurelia). As such, it is disputed whether they can support life. Rare Earth proponents claim that only stars from F7 to K1 types are hospitable. Such stars are rare: G type stars such as the Sun (between the hotter F and cooler K) comprise only 9%<ref name="RECONS1">[http://joy.chara.gsu.edu/RECONS/TOP100.posted.htm] The One Hundred Nearest Star Systems, Research Consortium on Nearby Stars.</ref> of the hydrogen-burning stars in the Milky Way.

Such aged stars as [[Astronomy:Red giant|red giant]]s and [[Astronomy:White dwarf|white dwarf]]s are also unlikely to support life. Red giants are common in [[Astronomy:Globular cluster|globular cluster]]s and [[Astronomy:Elliptical galaxy|elliptical galaxies]]. White dwarfs are mostly dying stars that have already completed their red giant phase. Stars that become red giants expand into or overheat the habitable zones of their youth and middle age (though theoretically planets at much greater distances [[Astronomy:Red giant#Prospects for habitability|may then become habitable]]).

An energy output that varies with the lifetime of the star will likely prevent life (e.g., as [[Astronomy:Cepheid variable|Cepheid variable]]s). A sudden decrease, even if brief, may freeze the water of orbiting planets, and a significant increase may evaporate it and cause a [[Earth:Greenhouse effect|greenhouse effect]] that prevents the oceans from reforming.

All known life requires the complex chemistry of [[Astronomy:Metallicity|metallic]] elements. The [[Physics:Absorption spectrum|absorption spectrum]] of a star reveals the presence of metals within, and studies of stellar spectra reveal that many, perhaps most, stars are poor in metals. Because heavy metals originate in [[Astronomy:Supernova|supernova]] explosions, metallicity increases in the universe over time. Low metallicity characterizes the early universe: globular clusters and other stars that formed when the universe was young, stars in most galaxies other than large [[Spiral galaxy|spirals]], and stars in the outer regions of all galaxies. Metal-rich central stars capable of supporting complex life are therefore believed to be most common in the less dense regions of the larger spiral galaxies—where radiation also happens to be weak.<ref name="Ward 2000 15–33">{{harvnb|Ward|Brownlee|2000|pp=15–33}}</ref>

===The right arrangement of planets around the star===
[[File:Planets2013.svg|right|thumb|Depiction of the Sun and planets of the Solar System and the sequence of planets. Rare Earth argues that without such an arrangement, in particular the presence of the massive gas giant Jupiter (the fifth planet from the Sun and the largest), complex life on Earth would not have arisen.]]
Rare Earth proponents argue that a planetary system capable of sustaining complex life must be structured more or less like the Solar System, with small, rocky inner planets and massive outer gas giants.<ref>{{Cite web |url=http://news.nationalgeographic.com/news/2007/08/070827-jupiter-comets_2.html |archive-url=https://web.archive.org/web/20140203015506/http://news.nationalgeographic.com/news/2007/08/070827-jupiter-comets_2.html |url-status=dead |archive-date=3 February 2014 |first=Anne |last=Minard |date=27 August 2007 |title=Jupiter Both an Impact Source and Shield for Earth |access-date=14 January 2014 |quote=without the long, peaceful periods offered by Jupiter's shield, intelligent life on Earth would never have been able to take hold.}}</ref> Without the protection of such "celestial vacuum cleaner" planets with strong gravitational pulls, other planets would be subject to more frequent catastrophic asteroid collisions.

Observations of exoplanets have shown that arrangements of planets similar to the [[Astronomy:Solar System|Solar System]] are rare. Most [[Astronomy:Planetary system|planetary system]]s have super-Earths, several times larger than Earth, close to their star, whereas the Solar System's inner region has only a few small rocky planets and none inside Mercury's orbit. Only 10% of stars have giant planets similar to Jupiter and Saturn, and those few rarely have stable, nearly circular orbits distant from their star. [[Biography:Konstantin Batygin|Konstantin Batygin]] and colleagues argue that these features can be explained if, early in the history of the Solar System, Jupiter and Saturn drifted towards the Sun, sending showers of planetesimals towards the super-Earths which sent them spiralling into the Sun, and ferrying icy building blocks into the terrestrial region of the Solar System which provided the building blocks for the rocky planets. The two giant planets then drifted out again to their present positions. In the view of Batygin and his colleagues: "The concatenation of chance events required for this delicate choreography suggest that small, Earth-like rocky planets – and perhaps life itself – could be rare throughout the cosmos."{{sfn|Batygin|Laughlin|Morbidelli2016|pp=23-24}}

===A continuously stable orbit===
Rare Earth proponents argue that a gas giant also must not be too close to a body where life is developing. Close placement of one or more gas giants could disrupt the orbit of a potential life-bearing planet, either directly or by drifting into the habitable zone.

Newtonian dynamics can produce [[Physics:N-body problem#Three-body problem|chaotic planetary orbits]], especially in a system having large planets at high [[Astronomy:Orbital eccentricity|orbital eccentricity]].<ref>{{Cite web |url=http://www.astro.ku.dk/~tobiash/posters/Tobias_final_new.pdf |title=Chaos and Planet-Particle Dynamics within the Habitable Zone of Extrasolar Planetary Systems (A qualitative numerical stability study) |access-date=31 October 2007 |last=Hinse |first=T.C. |publisher=Niels Bohr Institute |quote=Main simulation results observed: [1] The presence of high-order mean-motion resonances for large values of giant planet eccentricity [2] Chaos dominated dynamics within the habitable zone(s) at large values of giant planet mass.}}</ref>

The need for stable orbits rules out stars with planetary systems that contain large planets with orbits close to the host star (called "hot Jupiters"). It is believed that hot Jupiters have migrated inwards to their current orbits. In the process, they would have catastrophically disrupted the orbits of any planets in the habitable zone.<ref>"Once you realize that most of the known extrasolar planets have highly eccentric orbits (like the planets in [[Astronomy:Upsilon Andromedae|Upsilon Andromedae]]), you begin to wonder if there might be something special about our solar system" (UCBerkeleyNews quoting extrasolar planetary researcher Eric Ford.) {{Cite web |url=http://www.berkeley.edu/news/media/releases/2005/04/13_planet.shtml |first=Robert |last=Sanders |date=13 April 2005 |title=Wayward planet knocks extrasolar planets for a loop |access-date=31 October 2007}}</ref> To exacerbate matters, hot Jupiters are much more common orbiting F and G class stars.<ref>[http://www.solstation.com/habitable.htm Sol Company, Stars and Habitable Planets, 2012] {{webarchive |url=https://web.archive.org/web/20110628175616/http://www.solstation.com/habitable.htm |date=28 June 2011}}</ref>

===A terrestrial planet of the right size===
[[File:Size planets comparison.jpg|upright=1.4|thumb|left|Planets of the Solar System, shown to scale. Rare Earth argues that complex life cannot exist on large gaseous planets like Jupiter and Saturn (top row) or Uranus and Neptune (top middle) or smaller planets such as Mars and Mercury.]]
The Rare Earth hypothesis argues that life requires [[Astronomy:Terrestrial planet|terrestrial planet]]s like Earth, and since gas giants lack such a surface, that complex life cannot arise there.<ref>{{harvnb|Ward|Brownlee|2000|p=220}}</ref>

A planet that is too small cannot maintain much atmosphere, rendering its surface temperature low and variable and oceans impossible. A small planet will also tend to have a rough surface, with large mountains and deep canyons. The core will cool faster, and [[Earth:Plate tectonics|plate tectonics]] may be brief or entirely absent. A planet that is too large will retain too dense an atmosphere, like [[Astronomy:Venus|Venus]]. Although Venus is similar in size and mass to Earth, its surface atmospheric pressure is 92 times that of Earth, and its surface temperature is 735 K (462&nbsp;°C; 863&nbsp;°F). The early Earth once had a similar atmosphere, but may have lost it in the [[Astronomy:Giant-impact hypothesis|giant impact event]] which formed the [[Astronomy:Moon|Moon]].<ref>{{harvnb|Lissauer|1999}}, as summarized by {{harvnb|Conway Morris|2003|p=92}}; also see {{harvnb|Comins|1993}}</ref>

===Plate tectonics===
[[File:Great American Biotic Interchange examples.svg|thumb|The Great American Interchange on Earth, approximately 3.5 to 3 Ma, an example of species competition, resulting from continental plate interaction]]
[[File:Magnetosphere Levels.svg|thumb|right|An artist's rendering of the structure of Earth's magnetic field-magnetosphere that protects Earth's life from [[Physics:Solar radiation|solar radiation]]. 1)&nbsp;Bow shock. 2)&nbsp;Magnetosheath. 3)&nbsp;Magnetopause. 4)&nbsp;Magnetosphere. 5)&nbsp;Northern tail lobe. 6)&nbsp;Southern tail lobe. 7)&nbsp;Plasmasphere.]]
Rare Earth proponents argue that [[Earth:Plate tectonics|plate tectonics]] and a strong [[Earth:Earth's magnetic field|magnetic field]] are essential for [[Biology:Biodiversity|biodiversity]], global temperature regulation, and the [[Earth:Carbon cycle|carbon cycle]].<ref>{{harvnb|Ward|Brownlee|2000|p=194}}</ref> The lack of [[Earth:Mountain chain|mountain chain]]s elsewhere in the Solar System is evidence that Earth is the only body which now has plate tectonics, and thus the only one capable of supporting life.<ref>{{harvnb|Ward|Brownlee|2000|p=200}}</ref>

Plate tectonics depend on the right chemical composition and a long-lasting source of heat from [[Physics:Radioactive decay|radioactive decay]]. Continents must be made of less dense [[Earth:Felsic|felsic]] rocks that "float" on underlying denser [[Earth:Mafic|mafic]] rock. Taylor<ref name=Taylor98/> emphasizes that tectonic [[Earth:Subduction|subduction]] zones require the lubrication of oceans of water. Plate tectonics also provide a means of [[Biology:Biogeochemical cycle|biochemical cycling]].<ref>[http://www.space.com/4076-plate-tectonics-essential-alien-life.html Plate Tectonics Could Be Essential for Alien Life], By Lee Pullen, Astrobiology Magazine, February 19, 2009, SPACE.com</ref>

Plate tectonics and, as a result, [[Earth:Continental drift|continental drift]] and the creation of separate landmasses would create diversified [[Earth:Ecosystem|ecosystem]]s and [[Biology:Biodiversity|biodiversity]], one of the strongest defenses against extinction.<ref>{{aut|Ward, R. D. & Brownlee, D.}} 2000. ''[http://hyperphysics.phy-astr.gsu.edu/hbase/geophys/platelif.html Plate tectonics essential for complex evolution]'' - Rare Earth - Copernicus Books</ref> An example of species diversification and later competition on Earth's continents is the [[Earth:Great American Interchange|Great American Interchange]]. North and Middle America drifted into [[Earth:South America|South America]] at around 3.5 to 3 Ma. The [[Earth:Fauna|fauna]] of South America had already evolved separately for about 30 million years, since [[Earth:Antarctica|Antarctica]] separated, but, after the merger, many species were wiped out, mainly in South America, by competing [[Earth:North America|North America]]n animals.

===A large moon===
[[File:Porto Covo February 2009-2.jpg|thumb|left|Tide pools resulting from the tidal interactions of the Moon are said to have promoted the evolution of complex life.]]
The Moon is unusual because the other rocky planets in the Solar System either have no satellites ([[Astronomy:Mercury (planet)|Mercury]] and [[Astronomy:Venus|Venus]]), or only relatively tiny satellites which are probably captured asteroids ([[Astronomy:Mars|Mars]]). After [[Astronomy:Charon (moon)|Charon]], the Moon is also the largest natural satellite in the Solar System relative to the size of its parent body, being 27% the size of Earth.<ref>{{Cite web|last=October 2017|first=Tim Sharp 28|date=2017-10-28|title=How Big is the Moon?|url=https://www.space.com/18135-how-big-is-the-moon.html|access-date=2021-08-03|website=Space.com|language=en}}</ref>

The [[Astronomy:Giant-impact hypothesis|giant-impact theory]] hypothesizes that the Moon resulted from the impact of a [[Astronomy:Mars|Mars]]-sized body, dubbed [[Astronomy:Theia (planet)|Theia]], with the young Earth. This giant impact also gave the Earth its [[Astronomy:Axial tilt|axial tilt]] (inclination) and velocity of rotation.<ref name=Taylor98>{{harvnb|Taylor|1998}}</ref> Rapid rotation reduces the daily variation in temperature and makes [[Earth:Photosynthesis|photosynthesis]] viable.<ref>{{cite web |url=http://www.scientificamerican.com/article/earth-rotation-summer-solstice/ |title=Fact or Fiction: The Days (and Nights) Are Getting Longer |first=Adam |last=Hadhazy |date=14 June 2010 |work=Scientific American}}</ref> The ''Rare Earth'' hypothesis further argues that the axial tilt cannot be too large or too small (relative to the [[Astronomy:Orbital plane|orbital plane]]). A planet with a large tilt will experience extreme seasonal variations in climate. A planet with little or no tilt will lack the stimulus to evolution that climate variation provides.{{citation needed|date=July 2013}} In this view, the Earth's tilt is "just right". The gravity of a large satellite also stabilizes the planet's tilt; without this effect, the [[Physics:Precession|variation in tilt]] would be [[Chaos theory|chaotic]], probably making complex life forms on land impossible.<ref>{{harvnb|Dartnell|2007|pp=69–70}}</ref>

If the Earth had no Moon, the ocean [[Astronomy:Tide|tide]]s resulting solely from the Sun's gravity would be only half that of the lunar tides. A large satellite gives rise to tidal pools, which may be essential for the formation of [[Biology:Macromolecule|complex life]], though this is far from certain.<ref>A formal description of the hypothesis is given in: {{Cite journal |last=Lathe |first=Richard |date=March 2004 |title=Fast tidal cycling and the origin of life |journal=Icarus |volume=168 |issue=1 |pages=18–22 |quote=tidal cycling, resembling the polymerase chain reaction (PCR) mechanism, could only replicate and amplify DNA-like polymers. This mechanism suggests constraints on the evolution of extra-terrestrial life. |doi=10.1016/j.icarus.2003.10.018 |bibcode=2004Icar..168...18L }} It is taught less formally here: {{Cite web |url=http://abyss.uoregon.edu/~js/ast121/lectures/lec25.html |title=Origin of Life |access-date=31 October 2007 |first=James |last=Schombert |publisher=University of Oregon |quote=with the vastness of the Earth's oceans it is statistically very improbable that these early proteins would ever link up. The solution is that the huge tides from the Moon produced inland tidal pools, which would fill and evaporate on a regular basis to produce high concentrations of amino acids.}}</ref>

A large satellite also increases the likelihood of [[Earth:Plate tectonics|plate tectonics]] through the effect of [[Physics:Tidal force|tidal force]]s on the planet's crust.{{Citation needed|date=March 2017}} The impact that formed the Moon may also have initiated plate tectonics, without which the [[Earth:Continental crust|continental crust]] would cover the entire planet, leaving no room for [[Earth:Oceanic crust|oceanic crust]].{{Citation needed|date=March 2017}} It is possible that the large-scale [[Physics:Mantle convection|mantle convection]] needed to drive plate tectonics could not have emerged if the crust had a uniform composition. A further theory indicates that such a large moon may also contribute to maintaining a planet's magnetic shield by continually acting upon a metallic planetary core as dynamo, thus protecting the surface of the planet from charged particles and cosmic rays, and helping to ensure the atmosphere is not stripped over time by solar winds.{{citation needed|date=October 2018}}

===An atmosphere===
[[File:Earth's atmosphere.svg|thumb|right|Earth's atmosphere]]
A terrestrial planet must be the right size, like Earth and Venus, in order to retain an atmosphere. On Earth, once the giant impact of [[Astronomy:Theia (planet)|Theia]] thinned [[Earth:Atmosphere of Earth|Earth's atmosphere]], other events were needed to make the atmosphere capable of sustaining life. The [[Astronomy:Late Heavy Bombardment|Late Heavy Bombardment]] reseeded Earth with water lost after the impact of Theia.<ref>{{cite news |url=http://www.space.com/27969-earth-water-from-asteroids-not-comets.html |work=Space.com |title=Most of Earth's Water Came from Asteroids, Not Comets |first=Charles Q. |last=Choi |date=10 December 2014}}</ref> The development of an [[Physics:Ozone layer|ozone layer]] generated a protective shield against [[Physics:Ultraviolet|ultraviolet]] (UV) sunlight.<ref>{{cite web |url=http://disc.sci.gsfc.nasa.gov/ozone/additional/science-focus/about-ozone/ozone_formation.shtml |publisher=NASA |title=Formation of the Ozone Layer}}</ref><ref>{{Cite web|url=https://disc.gsfc.nasa.gov/information/glossary?title=Ozone|title=GES DISC|website=disc.gsfc.nasa.gov}}</ref> [[Chemistry:Nitrogen|Nitrogen]] and [[Chemistry:Carbon dioxide|carbon dioxide]] are needed in a correct ratio for life to form.<ref name="emsley360">Emsley, p. 360</ref> [[Astronomy:Lightning|Lightning]] is needed for [[Biology:Nitrogen fixation|nitrogen fixation]].<ref name="Lightning">{{Cite book |title=Lightning: Physics and Effects |first1=Vladimir A. |last1=Rakov |last2=Uman |first2=Martin A. |publisher=Cambridge University Press |date=2007 |isbn=978-0-521-03541-5 |page=508 |url=https://books.google.com/books?id=TuMa5lAa3RAC&pg=PA508}}</ref> The gaseous carbon dioxide needed for life comes from sources such as [[Earth:Volcano|volcano]]es and [[Earth:Geyser|geyser]]s. Carbon dioxide is preferably needed at relatively low levels (currently at approximately 400 [[Parts per million|ppm]] on Earth) because at high levels it is poisonous.<ref>{{cite web |url=http://earthobservatory.nasa.gov/Features/CarbonCycle/page5.php |publisher=NASA |title=Effects of Changing the Carbon Cycle|date=16 June 2011 }}</ref><ref>{{Cite web|url=https://www.ivhhn.org/index.php?option=com_content&view=article&id=84|title=Home &#124; IVHHN|website=www.ivhhn.org}}</ref> Precipitation is needed to have a stable water cycle.<ref>{{cite web |url=https://pmm.nasa.gov/education/water-cycle |publisher=NASA |title=The Water Cycle |first=Gail |last=Skofronick-Jackson|author-link=Gail Skofronick Jackson}}</ref> A proper atmosphere must reduce [[Earth:Diurnal temperature variation|diurnal temperature variation]].<ref>{{cite web |url=http://www.nasa.gov/mission_pages/noaa-n/climate/climate_weather.html |publisher=NASA |title=What's the Difference Between Weather and Climate? |date=1 February 2005}}</ref><ref>{{cite web |url=http://www.nasa.gov/mission_pages/sunearth/science/atmosphere-layers2.html |publisher=NASA |title=Earth's Atmospheric Layers |date=21 January 2013}}</ref>

===One or more evolutionary triggers for complex life===
[[File:Evolsex-dia1a.svg|thumb|This diagram illustrates the ''twofold cost of sex''. If each individual were to contribute to the same number of offspring (two), ''(a)'' the sexual population remains the same size each generation, whereas ''(b)'' the asexual population doubles in size each generation.]]
Regardless of whether planets with similar physical attributes to the Earth are rare or not, some argue that life tends not to evolve into anything more complex than simple bacteria without being provoked by rare and specific circumstances. Biochemist Nick Lane argues that simple cells ([[Biology:Prokaryote|prokaryote]]s) emerged soon after Earth's formation, but since almost half the planet's life had passed before they evolved into complex ones ([[Biology:Eukaryote|eukaryote]]s), all of whom share a common ancestor, this event can only have happened once. According to some views, [[Biology:Prokaryote|prokaryote]]s lack the cellular architecture to evolve into eukaryotes because a bacterium expanded up to eukaryotic proportions would have tens of thousands of times less energy available to power its metabolism. Two billion years ago, one simple cell incorporated itself into another, multiplied, and evolved into [[Biology:Mitochondria|mitochondria]] that supplied the vast increase in available energy that enabled the evolution of complex eukaryotic life. If this incorporation occurred only once in four billion years or is otherwise unlikely, then life on most planets remains simple.{{sfn|Lane| 2012}} An alternative view is that the evolution of mitochondria was environmentally triggered, and that mitochondria-containing organisms appeared soon after the first traces of atmospheric oxygen.<ref>Martin, W. & Mentel, M. (2010)[http://www.nature.com/scitable/topicpage/the-origin-of-mitochondria-14232356# The Origin of Mitochondria. Nature Education] 3(9):58</ref>

The evolution and persistence of [[Biology:Evolution of sexual reproduction|sexual reproduction]] is another mystery in biology. The purpose of [[Biology:Sexual reproduction|sexual reproduction]] is unclear, as in many organisms it has a 50% cost (fitness disadvantage) in relation to asexual reproduction.<ref>Ridley M (2004) Evolution, 3rd edition. Blackwell Publishing, p. 314.</ref> [[Biology:Mating type|Mating type]]s (types of [[Biology:Gamete|gamete]]s, according to their compatibility) may have arisen as a result of [[Physics:Anisogamy|anisogamy]] (gamete dimorphism), or the male and female sexes may have evolved before anisogamy.<ref>T. Togashi, P. Cox (Eds.) ''The Evolution of Anisogamy''. Cambridge University Press, Cambridge; 2011, p. 22-29.</ref><ref>Beukeboom, L. & Perrin, N. (2014). ''The Evolution of Sex Determination''. Oxford University Press, p. 25 [https://books.google.com/books?id=d4cLBAAAQBAJ&pg=PA25]. Online resources, [http://www.oup.co.uk/companion/beukeboom].</ref> It is also unknown why most sexual organisms use a binary [[Earth:Mating system|mating system]],<ref>{{cite journal |last1=Czárán |first1=T.L. |last2=Hoekstra |first2=R.F. |year=2006 |title=Evolution of sexual asymmetry |journal=BMC Evolutionary Biology |volume=4 |pages=34–46 |doi=10.1186/1471-2148-4-34|pmid=15383154 |pmc=524165 |doi-access=free }}</ref> and why some organisms have gamete dimorphism. [[Biography:Charles Darwin|Charles Darwin]] was the first to suggest that [[Biology:Sexual selection|sexual selection]] drives [[Biology:Speciation|speciation]]; without it, complex life would probably not have evolved.

===The right time in evolutionary history===
[[File:Timeline evolution of life.svg|left|thumb|Timeline of evolution; human writing exists for only 0.000218% of Earth's history.]] 
While life on Earth is regarded to have spawned relatively early in the planet's history, the evolution from multicellular to intelligent organisms took around 800 million years.<ref>{{in lang|en}} [http://www.ita.uni-heidelberg.de/~ulm/papers/ECh7.pdf 800 million years for complex organ evolution] - [[Organization:Heidelberg University|Heidelberg University]]</ref> Civilizations on Earth have existed for about 12,000 years, and radio communication reaching space has existed for little more than 100 years. Relative to the age of the Solar System (~4.57 Ga) this is a short time, in which extreme climatic variations, super volcanoes, and large meteorite impacts were absent. These events would severely harm intelligent life, as well as life in general. For example, the [[Earth:Permian–Triassic extinction event|Permian-Triassic mass extinction]], caused by widespread and continuous volcanic eruptions in an area the size of Western Europe, led to the extinction of 95% of known species around 251.2 Ma ago. About 65 million years ago, the [[Earth:Chicxulub crater|Chicxulub]] impact at the [[Earth:Cretaceous–Paleogene boundary|Cretaceous–Paleogene boundary]] (~65.5 Ma) on the Yucatán peninsula in Mexico led to a mass extinction of the most advanced species at that time.

==Rare Earth equation==
The following discussion is adapted from Cramer.<ref>{{harvnb|Cramer|2000}}</ref> The Rare Earth equation is Ward and Brownlee's riposte to the [[Astronomy:Drake equation|Drake equation]]. It calculates <math>N</math>, the number of Earth-like planets in the Milky Way having complex life forms, as:
[[File:Pikaia gracilens B.jpg|thumb|upright|According to Rare Earth, the Cambrian explosion that saw extreme diversification of [[Biology:Chordata|chordata]] from simple forms like Pikaia (pictured) was an improbable event.]]
:<math>N=N^* \cdot n_e \cdot f_g \cdot f_p \cdot f_{pm} \cdot f_i \cdot f_c \cdot f_l \cdot f_m \cdot f_j \cdot f_{me}</math><ref>{{harvnb|Ward|Brownlee|2000|pp=271–5}}</ref>

where:
* ''N*'' is the number of stars in the [[Astronomy:Milky Way|Milky Way]]. This number is not well-estimated, because the Milky Way's mass is not well estimated, with little information about the number of small stars. ''N*'' is at least 100 billion, and may be as high as 500 billion, if there are many low visibility stars.
* <math>n_e</math> is the average number of planets in a star's habitable zone. This zone is fairly narrow, being constrained by the requirement that the average planetary temperature be consistent with water remaining liquid throughout the time required for complex life to evolve. Thus, <math>n_e</math>=1 is a likely upper bound.
We assume <math>N^* \cdot n_e=5\cdot10^{11}</math>. The Rare Earth hypothesis can then be viewed as asserting that the product of the other nine Rare Earth equation factors listed below, which are all fractions, is no greater than 10<sup>−10</sup> and could plausibly be as small as 10<sup>−12</sup>. In the latter case, <math>N</math> could be as small as 0 or 1. Ward and Brownlee do not actually calculate the value of <math>N</math>, because the numerical values of quite a few of the factors below can only be conjectured. They cannot be estimated simply because [[Astronomy:Anthropic principle|we have but one data point]]: the Earth, a rocky planet orbiting a [[Astronomy:Stellar classification#Class G|G2]] star in a quiet suburb of a large [[Astronomy:Barred spiral galaxy|barred spiral galaxy]], and the home of the only intelligent species we know; namely, ourselves.
* <math>f_g</math> is the fraction of stars in the galactic habitable zone (Ward, Brownlee, and Gonzalez estimate this factor as 0.1<ref name=Gonzalez/>).
* <math>f_p</math> is the fraction of stars in the [[Astronomy:Milky Way|Milky Way]] with planets.
* <math>f_{pm}</math> is the fraction of planets that are rocky ("metallic") rather than gaseous.
* <math>f_i</math> is the fraction of habitable planets where microbial life arises. Ward and Brownlee believe this fraction is unlikely to be small.
* <math>f_c</math> is the fraction of planets where complex life evolves. For 80% of the time since microbial life first appeared on the Earth, there was only bacterial life. Hence Ward and Brownlee argue that this fraction may be small.
* <math>f_l</math> is the fraction of the total lifespan of a planet during which complex life is present. Complex life cannot endure indefinitely, because the energy put out by the sort of star that allows complex life to emerge gradually rises, and the central star eventually becomes a [[Astronomy:Red giant|red giant]], engulfing all planets in the planetary habitable zone. Also, given enough time, a catastrophic extinction of all complex life becomes ever more likely.
* <math>f_m</math> is the fraction of habitable planets with a large moon. If the giant impact theory of the Moon's origin is correct, this fraction is small.
* <math>f_j</math> is the fraction of planetary systems with large Jovian planets. This fraction could be large.
* <math>f_{me}</math> is the fraction of planets with a sufficiently low number of extinction events. Ward and Brownlee argue that the low number of such events the Earth has experienced since the [[Earth:Cambrian explosion|Cambrian explosion]] may be unusual, in which case this fraction would be small.{{citation needed|date=December 2021}}

The Rare Earth equation, unlike the [[Astronomy:Drake equation|Drake equation]], does not factor the probability that complex life evolves into [[Philosophy:Intelligence|intelligent life]] that discovers technology. Barrow and Tipler review the consensus among such biologists that the evolutionary path from primitive Cambrian chordates, e.g., ''[[Biology:Pikaia|Pikaia]]'' to ''Homo sapiens'', was a highly improbable event. For example, the large [[Biology:Brain|brain]]s of humans have marked adaptive disadvantages, requiring as they do an expensive [[Biology:Metabolism|metabolism]], a long gestation period, and a childhood lasting more than 25% of the average total life span.{{sfn|Barrow|Tipler|1986|loc=Section 3.2}} Other improbable features of humans include:
*Being one of a handful of extant bipedal land (non-avian) [[Biology:Vertebrate|vertebrate]]. Combined with an unusual [[Biology:Eye–hand coordination|eye–hand coordination]], this permits dextrous manipulations of the physical environment with the hands;
*A vocal apparatus far more expressive{{citation needed|reason=What about dolphins? Their ability to communicate complex ideas is indisputable. Also, dolphins clearly have culture.|date=August 2016}} than that of any other mammal, enabling speech. Speech makes it possible for humans to interact cooperatively, to share knowledge, and to acquire a culture;
*The capability of formulating [[Philosophy:Abstraction|abstraction]]s to a degree permitting the invention of [[Mathematics|mathematics]], and the discovery of [[Science|science]] and [[Engineering:Technology|technology]]. Only recently did humans acquire anything like their current scientific and technological sophistication.

==Advocates==
Writers who support the Rare Earth hypothesis:
* Stuart Ross Taylor,<ref name=Taylor98/> a specialist on the Solar System, firmly believed in the hypothesis. Taylor concludes that the Solar System is probably unusual, because it resulted from so many chance factors and events.
* Stephen Webb,<ref name="Webb, Stephen 2002">{{harvnb|Webb|2002}}</ref> a physicist, mainly presents and rejects candidate solutions for the [[Astronomy:Fermi paradox|Fermi paradox]]. The Rare Earth hypothesis emerges as one of the few solutions left standing by the end of the book{{clarify|date=April 2020}}
* [[Biography:Simon Conway Morris|Simon Conway Morris]], a paleontologist, endorses the Rare Earth hypothesis in chapter 5 of his ''Life's Solution: Inevitable Humans in a Lonely Universe'',<ref>{{harvnb|Conway Morris|2003|loc=Ch. 5}}</ref> and cites Ward and Brownlee's book with approval.<ref>Conway Morris, 2003, p. 344, n. 1</ref>
* [[Biography:John D. Barrow|John D. Barrow]] and [[Biography:Frank J. Tipler|Frank J. Tipler]], cosmologists, vigorously defend the hypothesis that humans are likely to be the only intelligent life in the [[Astronomy:Milky Way|Milky Way]], and perhaps the entire universe. But this hypothesis is not central to their book ''The Anthropic Cosmological Principle'', a thorough study of the [[Astronomy:Anthropic principle|anthropic principle]] and of how the laws of physics are peculiarly suited to enable the emergence of complexity in nature.{{sfn|Barrow|Tipler|1986|loc=Sections 3.2, 8.7, 9}}
* [[Biography:Ray Kurzweil|Ray Kurzweil]], a computer pioneer and self-proclaimed Singularitarian, argues in his 2005 book ''[[Philosophy:The Singularity Is Near|The Singularity Is Near]]'' that the coming [[Philosophy:Technological singularity|Singularity]] requires that Earth be the first planet on which sapient, technology-using life evolved. Although other Earth-like planets could exist, Earth must be the most evolutionarily advanced, because otherwise we would have seen evidence that another culture had experienced the Singularity and expanded to harness the full computational capacity of the physical universe.
* [[Biography:John Gribbin|John Gribbin]], a prolific science writer, defends the hypothesis in ''Alone in the Universe: Why our planet is unique'' (2011).<ref>{{harvnb|Gribbin|2011}}</ref>
* Michael H. Hart, [[Biography:Astrophysicist|astrophysicist]] who proposed a narrow habitable zone based on climate studies, edited the influential 1982 book ''Extraterrestrials: Where are They'' and authored one of its chapters "Atmospheric Evolution, the Drake Equation and DNA: Sparse Life in an Infinite Universe".<ref>Extraterrestrials: Where are They? 2nd ed., Eds. Ben Zuckerman and Michael H. Hart (Cambridge: Press Syndicate of the University of Cambridge, 1995), 153.</ref>
*Marc J. Defant, professor of geochemistry and volcanology, elaborated on several aspects of the rare Earth hypothesis in his TEDx talk entitled: Why We are Alone in the Galaxy.<ref>{{Cite web|url=https://www.youtube.com/watch?v=_nCOhrYV7eg&t=890s/|title=YouTube|website=[[Company:YouTube|YouTube]]|language=en|access-date=2018-06-15}}</ref> He also wrote in his book in 1998: "I do not believe that we were the destined outcome of evolution. In fact, we are probably the result of an incredible number of chance circumstances (one example is the meteorite impact at the end of the Cretaceous which probably destroyed the dinosaurs and led to mammal domination). The coincidental nature of our evolution should be clear from this book. I might even contend that so many "coincidences" had to take place during the history of the universe, that intelligent life on this planet may be the only life in our universe. I do not mean to suggest that we must have been "created." I mean to say that maybe there is not as much chance of finding life in our galaxy or universe as some would have us believe. We may be it."<ref>Defant, M., 1998, Voyage of Discovery: From the Big Bang to the Ice Age: Mancorp Publishing, Tampa, FL, 314 p.</ref>
* [[Biography:Brian Cox (physicist)|Brian Cox]], physicist and popular science celebrity confesses his support for the hypothesis in his 2014 BBC production of the ''Human Universe''.
* [[Biography:Richard Dawkins|Richard Dawkins]], [[Biology:Evolutionary biology|evolutionary biologist]], notes the Fermi paradox in his book, ''The Greatest Show on Earth'', while discussing how life first evolved on Earth. Although we do not yet know the precise process for how life first began on Earth, Dawkins's view is that it is an implausible theory (i.e., improbable) given we have not encountered any evidence for life existing elsewhere in the universe. He concludes that life is probably very rare throughout the universe.<ref>{{cite book |last= Dawkins|first= Richard|date= 2009|title= The Greatest Show on Earth: The Evidence for Evolution|url= |location= London|publisher= Transworld Publishers|pages= 421–422|isbn= 9780552775243}}</ref>

=={{anchor|Criticism}}Criticism==
Cases against the Rare Earth hypothesis take various forms.

=== The hypothesis appears anthropocentric ===
The hypothesis concludes, more or less, that complex life is rare because it can evolve only on the surface of an Earth-like planet or on a suitable satellite of a planet. Some biologists, such as Jack Cohen, believe this assumption too restrictive and unimaginative; they see it as a form of [[Begging the question|circular reasoning]].{{sfn|Cohen|Stewart|2002}}{{pn|date=December 2023}}

According to David Darling, the Rare Earth hypothesis is neither [[Hypothesis|hypothesis]] nor [[Prediction|prediction]], but merely a description of how life arose on Earth.<ref>{{harvnb|Darling|2001|p=}}</ref> In his view, Ward and Brownlee have done nothing more than select the factors that best suit their case.
{{cquote|What matters is not whether there's anything unusual about the Earth; there's going to be something idiosyncratic about every planet in space. What matters is whether any of Earth's circumstances are not only unusual but also essential for complex life. So far we've seen nothing to suggest there is.<ref>{{harvnb|Darling|2001|p=103}}</ref>}}

Critics also argue that there is a link between the Rare Earth hypothesis and the unscientific idea of [[Unsolved:Intelligent design|intelligent design]].<ref>Frazier, Kendrick. 'Was the 'Rare Earth' Hypothesis Influenced by a Creationist?' The Skeptical Inquirer. 1 November 2001</ref>

===Exoplanets around main sequence stars are being discovered in large numbers===
An increasing number of extrasolar planet discoveries are being made, with {{Extrasolar planet counts|planet_count}} planets in {{Extrasolar planet counts|system_count}} planetary systems known as of {{Extrasolar planet counts|asof}}.<ref name="Encyclopaedia">{{cite encyclopedia |title=Interactive Extra-solar Planets Catalog |url=https://exoplanet.eu/catalog/ |last=Schneider |encyclopedia=[[Astronomy:Extrasolar Planets Encyclopaedia|Extrasolar Planets Encyclopaedia]] |first=Jean}}</ref> Rare Earth proponents argue life cannot arise outside Sun-like systems, due to [[Astronomy:Tidal locking|tidal locking]] and [[Physics:Ionizing radiation|ionizing radiation]] outside the F7–K1 range. However, [[Astronomy:Planetary habitability#Red dwarf systems|some exobiologists have suggested]] that stars outside this range may give [[Biology:Abiogenesis|rise to life]] under the right circumstances; this possibility is a central point of contention to the theory because these late-K and M category stars make up about 82% of all hydrogen-burning stars.<ref name="RECONS1"/>

Current technology limits the testing of important Rare Earth criteria: surface water, tectonic plates, a large moon and [[Astronomy:Biosignature|biosignature]]s are currently undetectable. Though planets the size of Earth are difficult to detect and classify, scientists now think that rocky planets are common around Sun-like stars.<ref name="HowardSanchis-Ojeda2013">{{cite journal |last1=Howard |first1=Andrew W. |display-authors=0 |title=A rocky composition for an Earth-sized exoplanet |journal=Nature |volume=503 |issue=7476 |year=2013 |pages=381–384 |doi=10.1038/nature12767 |pmid=24172898 |arxiv=1310.7988 |bibcode=2013Natur.503..381H|s2cid=4450760 }}</ref> The [[Astronomy:Earth Similarity Index|Earth Similarity Index]] (ESI) of mass, radius and temperature provides a means of measurement, but falls short of the full Rare Earth criteria.<ref>{{cite web |url=https://www.wired.co.uk/news/archive/2011-11/21/exoplanet-indices |title=Exoplanet hunters propose system to find life-supporting worlds |access-date=2015-09-18 |url-status=dead |archive-url=https://web.archive.org/web/20150925005119/http://www.wired.co.uk/news/archive/2011-11/21/exoplanet-indices |archive-date=25 September 2015 |df=dmy-all }}</ref><ref>Stuart Gary [http://www.abc.net.au/news/2011-11-22/new-recipe-in-search-for-alien-life/3686408/ New approach in search for alien life] ABC Online. 22 November 2011</ref>

===Rocky planets orbiting within habitable zones may not be rare===
[[File:PIA19827-Kepler-SmallPlanets-HabitableZone-20150723.jpg|thumb|Planets similar to Earth in size are being found in relatively large number in the habitable zones of similar stars. The 2015 infographic depicts [[Astronomy:Kepler-62e|Kepler-62e]], Kepler-62f, Kepler-186f, Kepler-296e, Kepler-296f, Kepler-438b, Kepler-440b, Kepler-442b, Kepler-452b.<ref name="NASA-20150106">{{cite web |last1=Clavin |first1=Whitney |last2=Chou |first2=Felicia |last3=Johnson |first3=Michele |title=NASA's Kepler Marks 1,000th Exoplanet Discovery, Uncovers More Small Worlds in Habitable Zones |url=http://www.jpl.nasa.gov/news/news.php?release=2015-003 |date=6 January 2015 |work=NASA |access-date=6 January 2015}}</ref>]]

Some argue that Rare Earth's estimates of rocky planets in habitable zones (<math>n_e</math> in the Rare Earth equation) are too restrictive. [[Biography:James Kasting|James Kasting]] cites the [[Astronomy:Titius–Bode law|Titius–Bode law]] to contend that it is a misnomer to describe habitable zones as narrow when there is a 50% chance of at least one planet orbiting within one.<ref>{{harvnb|Kasting|2001|pp=123}}</ref> In 2013, astronomers using the [[Astronomy:Kepler space telescope|Kepler space telescope]]'s data estimated that about one-fifth of G-type and K-type [[Astronomy:Star|star]]s (sun-like stars and orange dwarfs) are expected to have an [[Astronomy:Terrestrial planet|Earth-sized]] or [[Astronomy:Super-Earth|super-Earth]]-sized planet ({{val|1|-|2}} [[Astronomy:Earth radius|Earths wide]]) close to an Earth-like orbit ({{val|0.25|-|4|u=[[Astronomy:Solar constant|''F''<sub>🜨</sub>]]}}),<ref name="PNAS-20131031">{{cite journal |last1=Petigura |first1=Eric A. |last2=Howard |first2=Andrew W. |last3=Marcy |first3=Geoffrey W. |title=Prevalence of Earth-size planets orbiting Sun-like stars |date=31 October 2013 |journal=Proceedings of the National Academy of Sciences of the United States of America |doi=10.1073/pnas.1319909110 |arxiv=1311.6806 |bibcode=2013PNAS..11019273P |volume=110 |issue=48 |pages=19273–19278 |pmid=24191033 |pmc=3845182|doi-access=free }}</ref> yielding about 8.8 billion of them for the entire Milky Way Galaxy.<ref name="ChoiCQ">{{cite web |last1=Borenstein |first1=Seth |url=http://www.nbcnews.com/science/8-8-billion-habitable-earth-size-planets-exist-milky-way-8C11529186 |title=8.8 billion habitable Earth-size planets exist in Milky Way alone |date=4 November 2013 |work=NBC News |access-date=5 November 2013}}</ref><ref name="NYT-20131104">{{cite news |last=Overbye |first=Dennis |title=Far-Off Planets Like the Earth Dot the Galaxy |url=https://www.nytimes.com/2013/11/05/science/cosmic-census-finds-billions-of-planets-that-could-be-like-earth.html |date=4 November 2013 |work={{wipe|New York Times}} |access-date=5 November 2013}}</ref><ref name="LATimes-20131104">{{cite news |last=Khan |first=Amina |title=Milky Way may host billions of Earth-size planets |url=http://www.latimes.com/science/la-sci-earth-like-planets-20131105,0,2673237.story |date=4 November 2013 |work={{wipe|Los Angeles Times}} |access-date=5 November 2013}}</ref>

===Uncertainty over Jupiter's role===
The requirement for a system to have a Jovian planet as protector (Rare Earth equation factor <math>f_j</math>) has been challenged, affecting the number of proposed extinction events (Rare Earth equation factor <math>f_{me}</math>). Kasting's 2001 review of Rare Earth questions whether a Jupiter protector has any bearing on the incidence of complex life.<ref name="Kasting 2001">{{harvnb|Kasting|2001|pp=118–120}}</ref> Computer modelling including the 2005 [[Astronomy:Nice model|Nice model]] and 2007 [[Astronomy:Nice 2 model|Nice 2 model]] yield inconclusive results in relation to Jupiter's gravitational influence and impacts on the inner planets.<ref name="Brumfiel2007">{{cite journal |last1=Brumfiel |first1=Geoff |title=Jupiter's protective pull questioned |journal=News@nature |year=2007 |doi=10.1038/news070820-11|s2cid=121623523 |doi-access=free }}</ref> A study by Horner and Jones (2008) using computer simulation found that while the total effect on all orbital bodies within the Solar System is unclear, Jupiter has caused more impacts on Earth than it has prevented.<ref name=horner2008>{{Cite journal |last=Horner |first=J. |author2=Jones, B.W. |title=Jupiter – friend or foe? I: the asteroids |journal=International Journal of Astrobiology |date=2008 |volume=7 |issue=3&4 |pages=251–261 |doi=10.1017/S1473550408004187 |bibcode=2008IJAsB...7..251H |arxiv=0806.2795|s2cid=8870726 }}</ref> [[Astronomy:Lexell's Comet|Lexell's Comet]], a 1770 near miss that passed closer to Earth than any other comet in recorded history, was known to be caused by the gravitational influence of Jupiter.<ref name="Astrobiology 2012">{{cite web |url=http://www.astrobio.net/news-exclusive/villain-in-disguise-jupiters-role-in-impacts-on-earth/ |title=Villain in disguise: Jupiter's role in impacts on Earth |last=Cooper |first=Keith |date=12 March 2012 |access-date=2 September 2015 |url-status=usurped |archive-url=https://web.archive.org/web/20200919094006/https://www.astrobio.net/news-exclusive/villain-in-disguise-jupiters-role-in-impacts-on-earth/ |archive-date=2020-09-19}}</ref> 

===Plate tectonics may not be unique to Earth or a requirement for complex life===
[[File:NH-Pluto-SputnikPlanum-HillaryMontes-NorgayMontes-20150714.jpg|thumb|left|Geological discoveries like the active features of Pluto's [[Astronomy:Tombaugh Regio|Tombaugh Regio]] appear to contradict the argument that geologically active worlds like Earth are rare.<ref name="NASA-20150724-lg">{{cite web |last=Gipson |first=Lillian |title=New Horizons Discovers Flowing Ices on Pluto |url=http://www.nasa.gov/feature/new-horizons-discovers-flowing-ices-on-pluto |date=24 July 2015 |work=NASA |access-date=24 July 2015}}</ref>]]
Ward and Brownlee argue that for complex life to evolve (Rare Earth equation factor <math>f_c</math>), [[Earth:Tectonics|tectonics]] must be present to generate biogeochemical cycles, and predicted that such geological features would not be found outside of Earth, pointing to a lack of observable mountain ranges and subduction.<ref>{{harvnb|Ward|Brownlee|2000|pp=191–193}}</ref> There is, however, no scientific consensus on the evolution of plate tectonics on Earth. Though it is believed that tectonic motion first began around three billion years ago,<ref name="Kranendonk2011">{{Cite journal |last1=Kranendonk |first1=V. |last2=Martin |first2=J. |title=Onset of Plate Tectonics |doi=10.1126/science.1208766 |journal=Science |volume=333 |issue=6041 |pages=413–414 |year=2011 |pmid=21778389 |bibcode=2011Sci...333..413V|s2cid=206535429 }}</ref> by this time photosynthesis and oxygenation had already begun. Furthermore, recent studies point to plate tectonics as an episodic planetary phenomenon, and that life may evolve during periods of "stagnant-lid" rather than plate tectonic states.<ref name="O’NeillLenardic2016">{{cite journal |last1=O’Neill |first1=Craig |last2=Lenardic |first2=Adrian |last3=Weller |first3=Matthew |last4=Moresi |first4=Louis |last5=Quenette |first5=Steve |last6=Zhang |first6=Siqi |title=A window for plate tectonics in terrestrial planet evolution? |journal=Physics of the Earth and Planetary Interiors |volume=255 |year=2016 |pages=80–92 |doi=10.1016/j.pepi.2016.04.002|bibcode=2016PEPI..255...80O |doi-access=free |hdl=1911/90517 |hdl-access=free }}</ref>

Recent evidence also points to similar activity either having occurred or continuing to occur elsewhere. The [[Astronomy:Geology of Pluto|geology of Pluto]], for example, described by Ward and Brownlee as "without mountains or volcanoes ... devoid of volcanic activity",<ref name="Ward 2000 15–33"/> has since been found to be quite the contrary, with a geologically active surface possessing organic molecules<ref name="SternCunningham2012">{{cite journal |last1=Stern |first1=S. A. |title=First Ultraviolet Reflectance Spectra of Pluto and Charon by Thehubble Space Telescopecosmic Origins Spectrograph: Detection of Absorption Features and Evidence for Temporal Change |last2=Cunningham |first2=N. J. |last3=Hain |first3=M. J. |last4=Spencer |first4=J. R. |last5=Shinn |first5=A. |journal=The Astronomical Journal |volume=143 |issue=1 |year=2012 |pages=22 |doi=10.1088/0004-6256/143/1/22 |bibcode=2012AJ....143...22S|doi-access=free }}</ref> and mountain ranges<ref name="Hand2015">{{cite journal |last1=Hand |first1=Eric |title=UPDATED: Pluto's icy face revealed, spacecraft 'phones home' |journal=Science |year=2015 |doi=10.1126/science.aac8847}}</ref> like [[Astronomy:Tenzing Montes|Tenzing Montes]] and [[Astronomy:Hillary Montes|Hillary Montes]] comparable in relative size to those of Earth, and observations suggest the involvement of endogenic processes.<ref name="BarrCollins2015">{{cite journal |last1=Barr |first1=Amy C. |last2=Collins |first2=Geoffrey C. |title=Tectonic activity on Pluto after the Charon-forming impact |journal=Icarus |volume=246 |year=2015 |pages=146–155 |doi=10.1016/j.icarus.2014.03.042 |bibcode=2015Icar..246..146B |arxiv=1403.6377|s2cid=118634502 }}</ref> Plate tectonics has been suggested as a hypothesis for the [[Astronomy:Martian dichotomy|Martian dichotomy]], and in 2012 geologist An Yin put forward evidence for active plate [[Astronomy:Tectonics of Mars|tectonics on Mars]].<ref name="Yin2012">{{cite journal |last1=Yin |first1=A. |title=Structural analysis of the Valles Marineris fault zone: Possible evidence for large-scale strike-slip faulting on Mars |journal=Lithosphere |volume=4 |issue=4 |year=2012 |pages=286–330 |doi=10.1130/L192.1|bibcode=2012Lsphe...4..286Y |doi-access=free }}</ref> Europa has long been suspected to have plate tectonics<ref name="GreenbergGeissler2000">{{cite journal |last1=Greenberg |first1=Richard |last2=Geissler |first2=Paul |last3=Tufts |first3=B. Randall |last4=Hoppa |first4=Gregory V. |title=Habitability of Europa's crust: The role of tidal-tectonic processes |journal=Journal of Geophysical Research |volume=105 |issue=E7 |date=2000 |pages=17551 |doi=10.1029/1999JE001147 |bibcode=2000JGR...10517551G|doi-access=free }}</ref> and in 2014 NASA announced evidence of active subduction.<ref>{{cite web |url=http://www.jpl.nasa.gov/news/news.php?feature=4285 |title=Scientists Find Evidence of 'Diving' Tectonic Plates on Europa |author=<!--Staff writer(s); no by-line.--> |date=8 September 2014 |website=www.jpl.nasa.gov |publisher=NASA |access-date=30 August 2015 }}</ref> Like Europa, analysis of the surface of Jupiter's largest moon [[Astronomy:Ganymede (moon)|Ganymede]] strike-strip faulting and surface materials of possible endogenic origin suggests that plate tectonics has also taken place there.<ref name="CameronSmith-Konter2018">{{cite journal|last1=Cameron|first1=Marissa E.|last2=Smith-Konter|first2=Bridget R.|last3=Burkhard|first3=Liliane|last4=Collins|first4=Geoffrey C.|last5=Seifert|first5=Fiona|last6=Pappalardo|first6=Robert T.|title=Morphological mapping of Ganymede: Investigating the role of strike-slip tectonics in the evolution of terrain types|journal=Icarus|volume=315|year=2018|pages=92–114|issn=0019-1035|doi=10.1016/j.icarus.2018.06.024|bibcode=2018Icar..315...92C|s2cid=125288991}}</ref>
<ref name="LigierParanicas2019">{{cite journal|last1=Ligier|first1=N.|last2=Paranicas|first2=C.|last3=Carter|first3=J.|last4=Poulet|first4=F.|last5=Calvin|first5=W.M.|last6=Nordheim|first6=T.A.|last7=Snodgrass|first7=C.|last8=Ferellec|first8=L.|title=Surface composition and properties of Ganymede: Updates from ground-based observations with the near-infrared imaging spectrometer SINFONI/VLT/ESO|journal=Icarus|volume=333|year=2019|pages=496–515|issn=0019-1035|doi=10.1016/j.icarus.2019.06.013|arxiv=1910.07445|bibcode=2019Icar..333..496L|s2cid=204734477|url=http://oro.open.ac.uk/62031/1/1-s2.0-S0019103519300053-main.pdf}}</ref>
In 2017, scientists studying the [[Astronomy:Geology of Charon|geology of Charon]] confirmed that icy plate tectonics also operated on Pluto's largest moon.<ref name="Space.com 2017-01-25">{{cite news |last=Emspak |first=Jesse |url=http://www.space.com/35449-charon-icy-plate-tectonics.htm |title=Pluto's Moon Charon Had Its Own, Icy Plate Tectonics |work=Space.com |date=25 January 2017 |access-date=26 January 2017}}</ref> Since 2017 several studies of the [[Astronomy:Geodynamics of Venus|geodynamics of Venus]] have also found that contrary to the view that the lithosphere of Venus is static, that it is actually being deformed via active processes similar to plate tectonics, though with less subduction, implying that geodynamics are not a rare [[Astronomy:Geodynamics of terrestrial exoplanets|occurrence in Earth sized bodies]].<ref name="ByrneGhail2018">{{cite conference|last1=Byrne|first1=Paul K. |last2=Ghail|first2=Richard C. |last3=Şengör|first3=A.M. Celâl |last4=James|first4=Peter B. |last5=Klimczak|first5=Christian |last6=Solomon|first6=Sean C. |title=The Globally Fragmented, Mobile Lithosphere of Venus May Resemble the Permobile Tectonic Regime of Archean Earth |series=Geological Society of America Abstracts with Programs  |conference=GSA Annual Meeting 2018 |year=2018|issn=0016-7592|doi=10.1130/abs/2018AM-323063}}</ref><ref name="Hansen2018">{{cite journal|last1=Hansen|first1=Vicki L.|title=Global tectonic evolution of Venus, from exogenic to endogenic over time, and implications for early Earth processes|journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences|volume=376|issue=2132|year=2018|pages=20170412|issn=1364-503X|doi=10.1098/rsta.2017.0412|pmid=30275161|bibcode=2018RSPTA.37670412H|s2cid=52900029|doi-access=free}}</ref>

Kasting suggests that there is nothing unusual about the occurrence of plate tectonics in large rocky planets and liquid water on the surface as most should generate internal heat even without the assistance of radioactive elements.<ref name="Kasting 2001"/> Studies by Valencia<ref>{{Cite journal |first1=Diana |last1=Valencia |first2=Richard J. |last2=O'Connell |first3=Dimitar D |last3=Sasselov |date=November 2007 |title=Inevitability of Plate Tectonics on Super-Earths |journal=Astrophysical Journal Letters |volume=670 |issue=1 |pages=L45–L48 |doi=10.1086/524012 |arxiv=0710.0699 |bibcode=2007ApJ...670L..45V|s2cid=9432267 }}</ref> and Cowan<ref name="CowanAbbot2014">{{cite journal |last1=Cowan |first1=Nicolas B. |title=Water Cycling Between Ocean and Mantle: Super-Earths Need Not be Waterworlds |last2=Abbot |first2=Dorian S. |journal=The Astrophysical Journal |volume=781 |issue=1 |year=2014 |pages=27 |doi=10.1088/0004-637X/781/1/27 |bibcode=2014ApJ...781...27C |arxiv=1401.0720|s2cid=56272100 }}</ref> suggest that plate tectonics may be inevitable for terrestrial planets Earth-sized or larger, that is, [[Astronomy:Super-Earth|Super-Earth]]s, which are now known to be more common in planetary systems.<ref name="MayorUdry2011">{{cite journal |last1=Mayor |first1=M. |last2=Udry |first2=S. |last3=Pepe |first3=F. |last4=Lovis |first4=C. |title=Exoplanets: the quest for Earth twins |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=369 |issue=1936 |year=2011 |pages=572–81 |doi=10.1098/rsta.2010.0245 |pmid=21220281 |bibcode=2011RSPTA.369..572M|doi-access=free }}</ref>

===Free oxygen may be neither rare nor a prerequisite for multicellular life===
[[File:Spinoloricus.png|thumb|upright|100px|Animals in the genus ''[[Biology:Spinoloricus|Spinoloricus]]'' are thought to defy the paradigm that all animal life on earth needs oxygen.]]
The hypothesis that molecular oxygen, necessary for [[Biology:Animal|animal]] life, is rare and that a [[Earth:Great Oxygenation Event|Great Oxygenation Event]] (Rare Earth equation factor <math>f_c</math>) could only have been triggered and sustained by tectonics, appears to have been invalidated by more recent discoveries.

Ward and Brownlee ask "whether oxygenation, and hence the rise of animals, would ever have occurred on a world where there were no continents to erode".<ref name="Ward & Brownlee 2000, p. 217">Ward & Brownlee 2000, p. 217</ref> Extraterrestrial free oxygen has recently been detected around other solid objects, including Mercury,<ref>{{cite journal |last=Killen |first=Rosemary |author2=Cremonese, Gabrielle |author3=Lammer, Helmut |title=Processes that Promote and Deplete the Exosphere of Mercury |year=2007 |journal=Space Science Reviews |volume=132 |issue=2–4 |pages=433–509 |doi=10.1007/s11214-007-9232-0 |ref=Killen2007 |bibcode=2007SSRv..132..433K |s2cid=121944553 |url=https://boris.unibe.ch/25351/ |display-authors=0}}</ref> Venus,<ref name="GröllerShematovich2010">{{cite journal |last1=Gröller |first1=H. |last2=Shematovich |first2=V. I. |last3=Lichtenegger |first3=H. I. M. |last4=Lammer |first4=H. |last5=Pfleger |first5=M. |last6=Kulikov |first6=Yu. N. |last7=Macher |first7=W. |last8=Amerstorfer |first8=U. V. |last9=Biernat |first9=H. K. |title=Venus' atomic hot oxygen environment |journal=Journal of Geophysical Research |volume=115 |issue=E12 |pages=E12017 |year=2010 |doi=10.1029/2010JE003697 |bibcode=2010JGRE..11512017G|doi-access=free }}</ref> Mars,<ref name="MahaffyWebster2013">{{cite journal |last1=Mahaffy |first1=P. R. |display-authors=0. |title=Abundance and Isotopic Composition of Gases in the Martian Atmosphere from the Curiosity Rover |journal=Science |volume=341 |issue=6143 |year=2013 |pages=263–266 |doi=10.1126/science.1237966 |pmid=23869014 |bibcode=2013Sci...341..263M|s2cid=206548973 }}</ref> Jupiter's four [[Astronomy:Galilean moons|Galilean moons]],<ref name="SpencerCalvin1995">{{cite journal |last1=Spencer |first1=John R. |last2=Calvin |first2=Wendy M. |last3=Person |first3=Michael J. |title=Charge-coupled device spectra of the Galilean satellites: Molecular oxygen on Ganymede |journal=Journal of Geophysical Research |volume=100 |issue=E9 |year=1995 |pages=19049 |doi=10.1029/95JE01503 |bibcode=1995JGR...10019049S|url=https://zenodo.org/record/1231368 }}</ref> Saturn's moons Enceladus,<ref name="EspositoBarth2004">{{cite journal |last1=Esposito |first1=Larry W. |display-authors=0 |title=The Cassini Ultraviolet Imaging Spectrograph Investigation |journal=Space Science Reviews |volume=115 |issue=1–4 |year=2004 |pages=299–361 |doi=10.1007/s11214-004-1455-8 |bibcode=2004SSRv..115..299E|s2cid=9806513 }}</ref> Dione<ref name="TokarJohnson2012">{{cite journal |last1=Tokar |first1=R. L. |last2=Johnson |first2=R. E. |last3=Thomsen |first3=M. F. |last4=Sittler |first4=E. C. |last5=Coates |first5=A. J. |last6=Wilson |first6=R. J. |last7=Crary |first7=F. J. |last8=Young |first8=D. T. |last9=Jones |first9=G. H. |title=Detection of exospheric O2+at Saturn's moon Dione |journal=Geophysical Research Letters |volume=39 |issue=3 |year=2012 |pages=n/a |doi=10.1029/2011GL050452 |bibcode=2012GeoRL..39.3105T|url=http://discovery.ucl.ac.uk/1343201/1/2011GL050452.pdf |doi-access=free }}</ref><ref name="GleinBaross2015">{{cite journal |last1=Glein |first1=Christopher R. |last2=Baross |first2=John A. |last3=Waite |first3=J. Hunter |title=The pH of Enceladus' ocean |journal=Geochimica et Cosmochimica Acta |volume=162 |year=2015 |pages=202–219 |doi=10.1016/j.gca.2015.04.017 |bibcode=2015GeCoA.162..202G |arxiv=1502.01946|s2cid=119262254 }}</ref> and Rhea<ref name="TeolisJones2010">{{cite journal |last1=Teolis |display-authors=0 |title=Cassini Finds an Oxygen-Carbon Dioxide Atmosphere at Saturn's Icy Moon Rhea |journal=Science |volume=330 |issue=6012 |year=2010 |pages=1813–1815 |doi=10.1126/science.1198366 |pmid=21109635 |bibcode=2010Sci...330.1813T|s2cid=206530211 |doi-access=free }}</ref> and even the atmosphere of a comet.<ref>[https://gizmodo.com/theres-primordial-oxygen-leaking-from-rosettas-comet-1739333271 There's Primordial Oxygen Leaking From Rosetta's Comet], By Maddie Stone, Oct 31, 2015, Gizmodo Australia</ref> This has led scientists to speculate whether processes other than photosynthesis could be capable of generating an environment rich in free oxygen. Wordsworth (2014) concludes that oxygen generated other than through [[Astronomy:Photodissociation|photodissociation]] may be likely on Earth-like exoplanets, and could actually lead to false positive detections of life.<ref name="HallStrobel1995">{{cite journal |last1=Hall |first1=D. T. |last2=Strobel |first2=D. F. |last3=Feldman |first3=P. D. |last4=McGrath |first4=M. A. |last5=Weaver |first5=H. A. |title=Detection of an oxygen atmosphere on Jupiter's moon Europa |journal=Nature |volume=373 |issue=6516 |year=1995 |pages=677–679 |doi=10.1038/373677a0 |pmid=7854447 |bibcode=1995Natur.373..677H|s2cid=4258306 }}</ref> Narita (2015) suggests [[Physics:Photocatalysis|photocatalysis]] by [[Chemistry:Titanium dioxide|titanium dioxide]] as a geochemical mechanism for producing oxygen atmospheres.<ref name="NaritaEnomoto2015">{{cite journal |last1=Narita |first1=Norio |last2=Enomoto |first2=Takafumi |last3=Masaoka |first3=Shigeyuki |last4=Kusakabe |first4=Nobuhiko |title=Titania may produce abiotic oxygen atmospheres on habitable exoplanets |journal=Scientific Reports |volume=5 |year=2015 |pages=13977 |doi=10.1038/srep13977 |arxiv=1509.03123 |bibcode=2015NatSR...513977N |pmid=26354078 |pmc=4564821}}</ref>

Since Ward & Brownlee's assertion that "there is irrefutable evidence that oxygen is a necessary ingredient for animal life",<ref name="Ward & Brownlee 2000, p. 217"/> [[Biology:Anaerobic organism|anaerobic]] metazoa have been found that indeed do metabolise without oxygen. ''[[Biology:Spinoloricus cinziae|Spinoloricus cinziae]]'', for example, a species discovered in the hypersaline [[Earth:Anoxic waters|anoxic]] L'Atalante basin at the bottom of the [[Place:Mediterranean Sea|Mediterranean Sea]] in 2010, appears to metabolise with hydrogen, lacking [[Biology:Mitochondria|mitochondria]] and instead using hydrogenosomes.<ref>{{Cite web|url=http://news.nationalgeographic.com/news/2010/04/100416-oxygen-free-complex-animals-mediterranean/|archive-url=https://web.archive.org/web/20100418230614/http://news.nationalgeographic.com/news/2010/04/100416-oxygen-free-complex-animals-mediterranean/|url-status=dead|archive-date=18 April 2010|title=Oxygen-Free Animals Discovered-A First, National Geographic news}}</ref><ref name="pmid20370908">{{cite journal |display-authors=4 |author=Danovaro R |author2=Dell'anno A |author3=Pusceddu A |author4=Gambi C |author5=Heiner I |author6=Kristensen RM |title=The first metazoa living in permanently anoxic conditions |journal=BMC Biology |volume=8 |issue=1 |pages=30 |date=April 2010 |pmid=20370908 |pmc=2907586 |doi=10.1186/1741-7007-8-30 |doi-access=free }}</ref> Studies since 2015 of the eukaryotic genus ''[[Biology:Monocercomonoides|Monocercomonoides]]'' that lack mitochondrial organelles are also significant as there are no detectable signs that mitochondria are part of the organism.<ref name=":0">{{Cite journal |last1=Karnkowska |first1=Anna |last2=Vacek |first2=Vojtěch |last3=Zubáčová |first3=Zuzana |last4=Treitli |first4=Sebastian C. |last5=Petrželková |first5=Romana |last6=Eme |first6=Laura |last7=Novák |first7=Lukáš |last8=Žárský |first8=Vojtěch |last9=Barlow |first9=Lael D. |last10=Herman |first10=Emily K. |last11=Soukal |first11=Petr |year=2016 |title=A Eukaryote without a Mitochondrial Organelle |journal=Current Biology |language=en |volume=26 |issue=10 |pages=1274–1284 |doi=10.1016/j.cub.2016.03.053|pmid=27185558 |s2cid=3933236 |doi-access=free }}</ref> Since then further eukaryotes, particularly [[Biology:Parasite|parasite]]s, have been identified to be completely absent of mitochondrial genome, such as the 2020 discovery in ''[[Biology:Henneguya zschokkei|Henneguya zschokkei]]''.<ref name="10.1073/pnas.1909907117">{{Cite journal |last1=Yahalomi |first1=Dayana |last2=Atkinson |first2=Stephen D.|last3=Neuhof|first3=Moran |last4=Chang |first4=E. Sally |last5=Philippe |first5=Hervé |last6=Cartwright |first6=Paulyn |last7=Bartholomew |first7=Jerri L. |last8=Huchon |first8=Dorothée |date=2020-02-19 |title=A cnidarian parasite of salmon (Myxozoa: ''Henneguya'') lacks a mitochondrial genome |journal=Proceedings of the National Academy of Sciences |volume=117 |issue=10 |pages=5358–5363 |doi=10.1073/pnas.1909907117 |issn=0027-8424 |pmid=32094163|pmc=7071853 |bibcode=2020PNAS..117.5358Y |doi-access=free }}</ref> Further investigation into alternative metabolic pathways used by these organisms appear to present further problems for the premise.

Stevenson (2015) has proposed other membrane alternatives for complex life in worlds without oxygen.<ref name="StevensonLunine2015">{{cite journal |last1=Stevenson |first1=J. |last2=Lunine |first2=Jonathan I. |last3=Clancy |first3=P. |title=Membrane alternatives in worlds without oxygen: Creation of an azotosome |journal=Science Advances |volume=1 |issue=1 |year=2015 |pages=e1400067 |doi=10.1126/sciadv.1400067 |bibcode=2015SciA....1E0067S |pmid=26601130 |pmc=4644080}}</ref> In 2017, scientists from the [[Organization:NASA Astrobiology Institute|NASA Astrobiology Institute]] discovered the necessary chemical preconditions for the formation of azotosomes on Saturn's moon Titan, a world that lacks atmospheric oxygen.<ref>[https://astrobiology.nasa.gov/news/nasa-finds-moon-of-saturn-has-chemical-that-could-form-membranes/ NASA Finds Moon of Saturn Has Chemical That Could Form ‘Membranes’], Written by NASA; Rob Garner, Astrobiology</ref> Independent studies by Schirrmeister and by Mills concluded that Earth's multicellular life existed prior to the Great Oxygenation Event, not as a consequence of it.<ref name="Schirrmeisterde Vos2013">{{cite journal |last1=Schirrmeister |first1=B. E. |last2=de Vos |first2=J. M. |last3=Antonelli |first3=A. |last4=Bagheri |first4=H. C. |title=Evolution of multicellularity coincided with increased diversification of cyanobacteria and the Great Oxidation Event |journal=Proceedings of the National Academy of Sciences |volume=110 |issue=5 |year=2013 |pages=1791–1796 |doi=10.1073/pnas.1209927110 |bibcode=2013PNAS..110.1791S |pmid=23319632 |pmc=3562814|doi-access=free }}</ref><ref name="MillsWard2014">{{cite journal |last1=Mills |first1=D. B. |last2=Ward |first2=L. M. |last3=Jones |first3=C. |last4=Sweeten |first4=B. |last5=Forth |first5=M. |last6=Treusch |first6=A. H. |last7=Canfield |first7=D. E. |title=Oxygen requirements of the earliest animals |journal=Proceedings of the National Academy of Sciences |volume=111 |issue=11 |year=2014 |pages=4168–4172 |doi=10.1073/pnas.1400547111 |bibcode=2014PNAS..111.4168M |pmid=24550467 |pmc=3964089|doi-access=free }}</ref>

NASA scientists Hartman and McKay argue that plate tectonics may in fact slow the rise of oxygenation (and thus stymie complex life rather than promote it).<ref>Hartman H, McKay CP "Oxygenic photosynthesis and the oxidation state of Mars." Planet Space Sci. 1995 Jan-Feb;43(1-2):123-8.</ref> Computer modelling by Tilman Spohn in 2014 found that plate tectonics on Earth may have arisen from the effects of complex life's emergence, rather than the other way around as the Rare Earth might suggest. The action of lichens on rock may have contributed to the formation of subduction zones in the presence of water.<ref name=Choi2014>{{Cite journal|title=Does a Planet Need Life to Create Continents?
 |url=http://www.astrobio.net/exclusive/5909/does-a-planet-need-life-to-create-continents |date=2014 |author=Choi, Charles Q. |journal=Astrobiology Magazine |access-date=6 January 2014 |url-status=usurped |archive-url=https://web.archive.org/web/20140106174352/http://www.astrobio.net/exclusive/5909/does-a-planet-need-life-to-create-continents |archive-date=2014-01-06}}</ref> Kasting argues that if oxygenation caused the Cambrian explosion then any planet with oxygen producing photosynthesis should have complex life.<ref>{{harvnb|Kasting|2001|p=130}}</ref>

===A magnetosphere may not be rare or a requirement===
The importance of Earth's magnetic field to the development of complex life has been disputed. The origin of Earth's magnetic field remains a mystery<ref name="BorlinaWeiss2020">{{cite journal|last1=Borlina|first1=Cauê S.|last2=Weiss|first2=Benjamin P.|last3=Lima|first3=Eduardo A.|last4=Tang|first4=Fengzai|last5=Taylor|first5=Richard J. M.|last6=Einsle|first6=Joshua F.|last7=Harrison|first7=Richard J.|last8=Fu|first8=Roger R.|last9=Bell|first9=Elizabeth A.|last10=Alexander|first10=Ellen W.|last11=Kirkpatrick|first11=Heather M.|last12=Wielicki|first12=Matthew M.|last13=Harrison|first13=T. Mark|last14=Ramezani|first14=Jahandar|last15=Maloof|first15=Adam C.|title=Reevaluating the evidence for a Hadean-Eoarchean dynamo|journal=Science Advances|volume=6|issue=15|year=2020|pages=eaav9634|issn=2375-2548|doi=10.1126/sciadv.aav9634|pmid=32284988|pmc=7141829|bibcode=2020SciA....6.9634B}}</ref> though the presence of a magnetosphere appears to be relatively common for larger planetary mass objects as all Solar System planets larger than Earth possess one.<ref name="Planetary Shields: Magnetospheres">{{cite web |title=Planetary Shields: Magnetospheres |url=https://mobile.arc.nasa.gov/public/iexplore/missions/pages/yss/november2011.html |publisher=NASA |access-date=5 January 2020}}</ref> There is increasing evidence of present or past magnetic activity in terrestrial bodies such as the Moon, Ganymede, Mercury and Mars.<ref name="BreuerLabrosse2010">{{cite journal|last1=Breuer|first1=Doris|last2=Labrosse|first2=Stephane|last3=Spohn|first3=Tilman|title=Thermal Evolution and Magnetic Field Generation in Terrestrial Planets and Satellites|journal=Space Science Reviews|volume=152|issue=1–4|year=2010|pages=449–500|issn=0038-6308|doi=10.1007/s11214-009-9587-5|bibcode=2010SSRv..152..449B|s2cid=53541047}}</ref> Without sufficient measurement present studies rely heavily on modelling methods developed in 2006 by Olson & Christensen to predict field strength.<ref name="McIntyreLineweaver2019">{{cite journal|last1=McIntyre|first1=Sarah R N|last2=Lineweaver|first2=Charles H|last3=Ireland|first3=Michael J|title=Planetary Magnetism as a Parameter in Exoplanet Habitability|journal=Monthly Notices of the Royal Astronomical Society|year=2019|issn=0035-8711|doi=10.1093/mnras/stz667|arxiv=1903.03123}}</ref> Using a sample of 496 planets such models predict [[Astronomy:Kepler-186f|Kepler-186f]] to be one of few of Earth size that would support a magnetosphere (though such a field around this planet has not currently been confirmed).<ref name="McIntyreLineweaver2019" /> However current recent empirical evidence points to the occurrence of much larger and more powerful fields than those found in our Solar System, some of which cannot be explained by these models.<ref name="CauleyShkolnik2019">{{cite journal|last1=Cauley|first1=P. Wilson|last2=Shkolnik|first2=Evgenya L.|last3=Llama|first3=Joe|last4=Lanza|first4=Antonino F.|title=Magnetic field strengths of hot Jupiters from signals of star–planet interactions|journal=Nature Astronomy|volume=3|issue=12|year=2019|pages=1128–1134|issn=2397-3366|doi=10.1038/s41550-019-0840-x|arxiv=1907.09068|bibcode=2019NatAs...3.1128C|s2cid=198147426}}</ref><ref name="KaoHallinan2018">{{cite journal|last1=Kao|first1=Melodie M.|last2=Hallinan|first2=Gregg|last3=Pineda|first3=J. Sebastian|last4=Stevenson|first4=David|last5=Burgasser|first5=Adam|title=The Strongest Magnetic Fields on the Coolest Brown Dwarfs|journal=The Astrophysical Journal Supplement Series|volume=237|issue=2|year=2018|pages=25|issn=1538-4365|doi=10.3847/1538-4365/aac2d5|arxiv=1808.02485|bibcode=2018ApJS..237...25K|s2cid=118898602|url=https://resolver.caltech.edu/CaltechAUTHORS:20180808-151343056 |doi-access=free }}</ref>

Kasting argues that the atmosphere provides sufficient protection against cosmic rays even during times of magnetic pole reversal and atmosphere loss by sputtering.<ref name="Kasting 2001"/> Kasting also dismisses the role of the magnetic field in the evolution of eukaryotes, citing the age of the oldest known magnetofossils.<ref>{{harvnb|Kasting|2001|pp=128–129}}</ref>

===A large moon may be neither rare nor necessary===
The requirement of a large moon (Rare Earth equation factor <math>f_m</math>) has also been challenged. Even if it were required, such an occurrence may not be as unique as predicted by the Rare Earth Hypothesis. Work by Edward Belbruno and [[Biography:J. Richard Gott|J. Richard Gott]] of Princeton University suggests that giant impactors such as those that may have formed the [[Astronomy:Moon|Moon]] can indeed form in planetary trojan points ({{L4}} or {{L5}} [[Astronomy:Lagrangian point|Lagrangian point]]) which means that similar circumstances may occur in other planetary systems.<ref>{{Cite journal |first=E. |last=Belbruno |author2=J. Richard Gott III |journal=The Astronomical Journal |volume=129 |issue=3 |pages=1724–1745 |date=2005 |title=Where Did The Moon Come From? |arxiv=astro-ph/0405372 |doi=10.1086/427539 |bibcode=2005AJ....129.1724B|s2cid=12983980 }}</ref>

[[File:Artist's concept of collision at HD 172555.jpg|thumb|left|Collision between two planetary bodies (artist concept)]]

The assertion that the Moon's stabilization of Earth's obliquity and spin is a requirement for complex life has been questioned. Kasting argues that a moonless Earth would still possess habitats with climates suitable for complex life and questions whether the spin rate of a moonless Earth can be predicted.<ref name="Kasting 2001"/> Although the giant impact theory posits that the impact forming the Moon increased Earth's rotational speed to make a day about 5 hours long, the Moon has slowly "stolen" much of this speed to reduce Earth's solar day since then to about 24 hours and continues to do so: in 100 million years Earth's solar day will be roughly 24 hours 38 minutes (the same as Mars's solar day); in 1 billion years, 30 hours 23 minutes. Larger secondary bodies would exert proportionally larger tidal forces that would in turn decelerate their primaries faster and potentially increase the solar day of a planet in all other respects like Earth to over 120 hours within a few billion years. This long solar day would make effective heat dissipation for organisms in the tropics and subtropics extremely difficult in a similar manner to tidal locking to a red dwarf star. Short days (high rotation speed) cause high wind speeds at ground level. Long days (slow rotation speed) cause the day and night temperatures to be too extreme.<ref>[http://news.discovery.com/earth/what-would-happen-if-our-planet-became-tidally-locked-130202.htm discovery.com What If Earth Became Tidally Locked? 2 February 2013]</ref>

Many Rare Earth proponents argue that the Earth's plate tectonics would probably not exist if not for the tidal forces of the Moon or the impact of [[Astronomy:Theia (planet)|Theia]] (prolonging mantle effects).<ref name=":1">Ward & Brownlee 2000, p. 233</ref><ref name=":2">{{cite news |last=Nick |first=Hoffman |url=http://www.spacedaily.com/news/life-01x1.html |title=The Moon And Plate Tectonics: Why We Are Alone |work=Space Daily |date=11 June 2001 |access-date=8 August 2015}}</ref> The hypothesis that the Moon's tidal influence initiated or sustained Earth's plate tectonics remains unproven, though at least one study implies a temporal correlation to the formation of the Moon.<ref name="TurnerRushmer2014">{{cite journal |last1=Turner |first1=S. |last2=Rushmer |first2=T. |last3=Reagan |first3=M. |last4=Moyen |first4=J.-F. |title=Heading down early on? Start of subduction on Earth |journal=Geology |volume=42 |issue=2 |year=2014 |pages=139–142 |doi=10.1130/G34886.1|bibcode=2014Geo....42..139T }}</ref> Evidence for the past existence of plate tectonics on planets like Mars<ref>Stuart Wolpert (9 August 2012). [http://newsroom.ucla.edu/portal/ucla/ucla-scientist-discovers-plate-237303.aspx "UCLA scientist discovers plate tectonics on Mars"]</ref> which may never have had a large moon would counter this argument, although plate tectonics may fade anyway before a moon is relevant to life.<ref name=":1" /><ref name=":2" /> Kasting argues that a large moon is not required to initiate plate tectonics.<ref name="Kasting 2001"/>

===Complex life may arise in alternative habitats===
[[File:Blacksmoker in Atlantic Ocean.jpg|thumb|right|Complex life may exist in environments similar to black smokers on Earth.]]
Rare Earth proponents argue that simple life may be common, though complex life requires specific environmental conditions to arise. Critics consider life could arise on a [[Astronomy:Natural satellite|moon]] of a gas giant, though this is less likely if life requires volcanicity. The moon must have stresses to induce tidal heating, but not so dramatic as seen on Jupiter's Io. However, the moon is within the gas giant's intense radiation belts, sterilizing any biodiversity before it can get established. [[Biography:Dirk Schulze-Makuch|Dirk Schulze-Makuch]] disputes this, hypothesizing alternative biochemistries for alien life.<ref name="Schulze-MakuchIrwin2008">{{cite book |author1=Dirk Schulze-Makuch |author2=Louis Neal Irwin |title=Life in the Universe: Expectations and Constraints |url=https://books.google.com/books?id=qdjoTGAvrkkC&pg=PA1 |date=2 October 2008 |publisher=Springer Science & Business Media |isbn=978-3-540-76816-6 |pages=162}}</ref> While Rare Earth proponents argue that only microbial extremophiles could exist in subsurface habitats beyond Earth, some argue that complex life can also arise in these environments. Examples of extremophile animals such as the ''[[Biology:Hesiocaeca methanicola|Hesiocaeca methanicola]]'', an animal that inhabits ocean floor [[Chemistry:Methane clathrate|methane clathrate]]s substances more commonly found in the outer Solar System, the [[Biology:Tardigrade|tardigrade]]s which can survive in the vacuum of space<ref name="NYT-20150907">{{cite news |last=Dean |first=Cornelia |title=The Tardigrade: Practically Invisible, Indestructible 'Water Bears' |url=https://www.nytimes.com/2015/09/08/science/the-tardigrade-water-bear.html |date=7 September 2015 |work={{wipe|New York Times}} |access-date=7 September 2015}}</ref> or ''[[Biology:Halicephalobus mephisto|Halicephalobus mephisto]]'' which exists in crushing pressure, scorching temperatures and extremely low oxygen levels 3.6 kilometres ( 2.2 miles) deep in the Earth's crust,<ref>{{cite web |url=http://news.nationalgeographic.com/news/2011/06/110601-deepest-worm-earth-devil-science-animals-life/ |archive-url=https://web.archive.org/web/20110604125906/http://news.nationalgeographic.com/news/2011/06/110601-deepest-worm-earth-devil-science-animals-life |url-status=dead |archive-date=4 June 2011 |title=New "Devil Worm" Is Deepest-Living Animal Species evolved to withstand heat and crushing pressure |last1=Mosher |first1=Dave |date=2 June 2011 |website=National Geographic News }}</ref> are sometimes cited by critics as complex life capable of thriving in "alien" environments. [[Biography:Jill Tarter|Jill Tarter]] counters the classic counterargument that these species adapted to these environments rather than arose in them, by suggesting that we cannot assume conditions for life to emerge which are not actually known.<ref name="Tarter 2011">{{cite web |url=http://www.sunypress.edu/pdf/62267.pdf |title=Exoplanets, Extremophiles, and the Search for Extraterrestrial Intelligence |last=Tarter |first=Jill |publisher=State University of New York Press |access-date=11 September 2015}}</ref> There are suggestions that complex life could arise in sub-surface conditions which may be similar to those where life may have arisen on Earth, such as the [[Astronomy:Tidal heating|tidally heated]] subsurfaces of Europa or Enceladus.<ref>{{cite journal |author=Reynolds, R.T. |author2=McKay, C.P. |author3=Kasting, J.F. |title=Europa, Tidally Heated Oceans, and Habitable Zones Around Giant Planets |journal=Advances in Space Research |volume=7 |issue=5 |pages=125–132 |date=1987 |doi=10.1016/0273-1177(87)90364-4 |pmid=11538217 |bibcode=1987AdSpR...7e.125R}}</ref><ref>For a detailed critique of the Rare Earth hypothesis along these lines, see {{harvnb |Cohen |Stewart |2002}}.</ref> Ancient circumvental ecosystems such as these support complex life on Earth such as ''[[Biology:Riftia pachyptila|Riftia pachyptila]]'' that exist completely independent of the surface biosphere.<ref name="Smil2003">{{cite book |author=Vaclav Smil |title=The Earth's Biosphere: Evolution, Dynamics, and Change |url=https://books.google.com/books?id=8ntHWPMUgpMC |year=2003 |publisher=MIT Press |isbn=978-0-262-69298-4 |page=166}}</ref>

==Notes==
{{Reflist|30em}}

==References==
{{Refbegin}}
*{{BarrowTipler1986}}
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*{{cite book |last=Dartnell |first=Lewis |title=Life in the Universe, a Beginner's Guide |publisher=One World |location=Oxford |date=2007}}
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*{{cite journal |last1=Lineweaver |first1=Charles H. |last2=Fenner |first2=Yeshe |last3=Gibson |first3=Brad K. |date=2004 |title=The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way |url=http://astronomy.swin.edu.au/GHZ/GHZ_astroph.pdf |journal=Science |volume=303 |issue=5654 |pages=59–62 |doi=10.1126/science.1092322 |pmid=14704421 |arxiv=astro-ph/0401024 |bibcode=2004Sci...303...59L |s2cid=18140737 |url-status=dead |archive-url=https://web.archive.org/web/20060712034717/http://astronomy.swin.edu.au/GHZ/GHZ_astroph.pdf |archive-date=12 July 2006 |df=dmy-all }}
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{{Refend}}

==Further reading==
* {{cite journal |last1=Cirkovic |first1=Milan M. |last2=Bradbury |first2=Robert J. |year=2006  |title=Galactic Gradients, Postbiological Evolution, and the Apparent Failure of SETI |url=http://www.anthropic-principle.com/preprints/milan-seti.pdf |journal=New Astronomy |volume=11  |issue= 8|pages=628–639 |arxiv=astro-ph/0506110 |bibcode= 2006NewA...11..628C|doi=10.1016/j.newast.2006.04.003|s2cid=1540494 }}
*{{cite journal |last1=Kasting |first1=James |last2=Whitmire |first2=D. P. |last3=Reynolds |first3=R. T. |date=1993 |title=Habitable zones around main sequence stars |journal=Icarus |volume=101 |issue=1 |pages=108–28 |doi=10.1006/icar.1993.1010 |pmid=11536936 |bibcode=1993Icar..101..108K}}
*{{cite journal |last1=Kirschvink |first1=Joseph L. |last2=Ripperdan |first2=Robert L. |last3=Evans |first3=David A. |date=1997 |title=Evidence for a Large-Scale Reorganization of Early Cambrian Continental Masses by Inertial Interchange True Polar Wander |journal=Science |volume=277 |issue=5325 |pages=541–45 |doi=10.1126/science.277.5325.541|s2cid=177135895 }}
*{{cite book |last=Knoll |first=Andrew H |title=Life on a Young Planet: The First Three Billion Years of Evolution on Earth |publisher=Princeton University Press |date=2003}}
*{{cite journal |last=Prantzos |first=Nikos |title=On the Galactic Habitable Zone |doi=10.1007/s11214-007-9236-9 |editor=Bada, J. |display-editors=etal |journal=Space Science Reviews |date=March 2008 |volume=135 |issue=1–4 |pages=313–322 |arxiv=astro-ph/0612316 |bibcode=2008SSRv..135..313P|s2cid=119441813 }}
*{{cite journal |last1=Raymond |first1=Sean N. |last2=Scalo |first2=John |last3=Meadows |first3=Victoria S. |title=A Decreased Probability of Habitable Planet Formation around Low-Mass Stars |journal=The Astrophysical Journal |date=November 2007 |volume=669 |issue=1 |pages=606–614 |doi=10.1086/521587 |arxiv=0707.1711|bibcode=2007ApJ...669..606R |s2cid=1247176 }}
*{{cite book |last=Ross |first=Hugh |chapter=Some of the parameters of the galaxy-sun-earth-moon system necessary for advanced life |title=The Creator and the Cosmos |publisher=NavPress |location=Colorado Springs CO |date=1993 |edition=2nd}}
* {{cite journal |last1=Stenger |first1=Victor |name-list-style=vanc |date=1999 |title=The Anthropic Coincidences: A Natural Explanation |url=http://www.colorado.edu/philosophy/vstenger/Cosmo/anthro_skintel.html |journal=The Skeptical Intelligencer |volume=3 |page=3 |url-status=dead |archive-url=https://web.archive.org/web/20071112225213/http://www.colorado.edu/philosophy/vstenger/Cosmo/anthro_skintel.html |archive-date=12 November 2007 |df=dmy-all }}
*{{cite journal |last1=Tipler |first1=Frank J. |name-list-style=vanc |date=2003 |title=Intelligent Life in Cosmology |journal=International Journal of Astrobiology |volume=2 |issue=2 |pages=141–8 |doi=10.1017/S1473550403001526 |bibcode=2003IJAsB...2..141T |arxiv=0704.0058|s2cid=119283361 }}
*{{cite book |last=Waltham |first=David |title=Lucky Planet |publisher=Basic Books |date=2013|url=http://davidwaltham.com/lucky-planet/}} A defense of the Rare Earth Hypothesis by a UK geologist.
*Henderson, Lawrence Joseph (1913). ''The Fitness of the Environment''. The Macmillan Company
*Gonzales, Guillermo; Richards, Jay W (2004). ''The Privileged Planet''. Regnery Publishing, Inc.

==External links==
{{Library resources box}}

* [https://web.archive.org/web/20081019163719/http://www.astro.washington.edu/rareearth/ Home page] of ''Rare Earth'' (archival)
* Reviews of ''Rare Earth'':
** [http://www.setileague.org/reviews/rarearth.htm Athena Andreadis], PhD in molecular biology.
** [https://web.archive.org/web/20050921120946/http://www.findarticles.com/p/articles/mi_m2843/is_6_25/ai_79794362 Kendrick Frazier], editor, ''Skeptical Inquirer''.
* {{cite web |url=http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=139 |archive-url=https://web.archive.org/web/20030515003838/http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=139 |url-status=usurped |archive-date=15 May 2003 |title=Galactic Habitable Zone |work=Astrobiology Magazine |date=18 May 2001}}
* Gregg Easterbrook, "[https://www.theatlantic.com/issues/88aug/easterbr.htm Are We Alone?]" ''The Atlantic Monthly'', August 1988. Article that anticipates REH in some respects.
*Solstation.com: "[https://web.archive.org/web/20110628175616/http://www.solstation.com/habitable.htm Stars and Habitable Planets.]"
*{{Cite news |last=Recer |first=Paul |url=http://www.chron.com/content/interactive/space/astronomy/news/1999/ds/990602.html |title=Radio astronomers measure sun's orbit around Milky Way |agency=Associated Press |work=Houston Chronicle |date=1 June 1999 |archive-url=https://web.archive.org/web/19991011230719/http://www.chron.com/content/interactive/space/astronomy/news/1999/ds/990602.html |archive-date=11 October 1999}}
* {{Cite news |url=https://www.bbc.co.uk/news/science-environment-16068809 |title=Life on Earth: Is our planet special? |first=Howard |last=Falcon-Lang |work=BBC News |date=9 December 2011}}
*{{cite web |url=http://www.gresham.ac.uk/lectures-and-events/are-we-alone-the-search-for-life-beyond-the-earth |first=Ian |last=Morison |title=Are We Alone? The search for life beyond the Earth |publisher=Gresham College |date=24 September 2014}}
*{{cite web |url=https://www.scientificamerican.com/article/earths-tectonic-activity-may-be-crucial-for-life-and-rare-in-our-galaxy/?WT.mc_id=SA_DD_20170720 |title=Earth's Tectonic Activity May Be Crucial for Life--and Rare in Our Galaxy |work=Scientific American |first=Shannon |last=Hall |date=20 July 2017}}

{{Astrobiology}}
{{Interstellar messages}}
{{Extraterrestrial life|state=expanded}}

{{DEFAULTSORT:Rare Earth Hypothesis}}
[[Category:Earth]]
[[Category:Astrobiology]]
[[Category:Origin of life]]
[[Category:Fermi paradox]]
[[Category:Hypotheses]]
[[Category:Astronomical hypotheses]]

{{Sourceattribution|Rare Earth hypothesis}}