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| [[Image:The Earth seen from Apollo 17.jpg|200px|thumb|right| Are planets that support complex life, such as Earth, rare?]]
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| In [[planetary astronomy]] and [[astrobiology]], the '''Rare Earth hypothesis''' argues that the [[origin of life|emergence]] of complex [[multicellular life]] on [[Earth]] (and, subsequently, [[intelligence]]) required an improbable combination of [[astrophysics|astrophysical]] and [[geology|geological]] events and circumstances. The hypothesis argues that complex [[extraterrestrial life]] requires an [[Earth-like planet]] with similar circumstance and that few if any such planets exist. The term "Rare Earth" originates from ''Rare Earth: Why Complex Life Is Uncommon in the Universe'' (2000), a book by [[Peter Ward (paleontologist)|Peter Ward]], a geologist and paleontologist, and [[Donald E. Brownlee]], an astronomer and [[astrobiology|astrobiologist]].
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| The rare earth hypothesis is contrary to the [[principle of mediocrity]] (also called the [[Copernican principle]]), advocated by [[Carl Sagan]] and [[Frank Drake]], among others.<ref>{{harvnb|Ward|Brownlee|2000|pp=xxi–xxiii}}</ref> The principle of mediocrity states that the Earth is a typical rocky [[planet]] in a typical [[planetary system]], located in a non-exceptional region of a common [[barred-spiral galaxy]]. Hence, it is probable that the universe teems with complex life. Ward and Brownlee argue to the contrary: planets, [[planetary system]]s, and galactic regions that are as friendly to complex life as are the Earth, the Solar System, and our region of the [[Milky Way]] are very rare.
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| On 4 November 2013, astronomers reported, based on [[Kepler (spacecraft)|''Kepler'' space mission]] data, that there could be as many as 40 billion [[Terrestrial planet|Earth-sized]] [[extrasolar planets|planets]] orbiting in the [[habitable zone]]s of [[sun-like|sun-like stars]] and [[red dwarf stars]] within the [[Milky Way Galaxy]].<ref name="NYT-20131104">{{cite news |last=Overbye |first=Dennis|title=Far-Off Planets Like the Earth Dot the Galaxy|url=http://www.nytimes.com/2013/11/05/science/cosmic-census-finds-billions-of-planets-that-could-be-like-earth.html |date=4 November 2013 |work=[[New York Times]] |accessdate=5 November 2013 }}</ref><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|url=http://www.pnas.org/content/early/2013/10/31/1319909110 |date=31 October 2013 |journal=[[Proceedings of the National Academy of Sciences of the United States of America]]|doi=10.1073/pnas.1319909110 |accessdate=5 November 2013 }}</ref> 11 billion of these estimated planets may be orbiting sun-like stars.<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=November 4, 2013 |work=[[Los Angeles Times]] |accessdate=November 5, 2013 }}</ref> The nearest such planet may be 12 light-years away, according to the scientists.<ref name="NYT-20131104" /><ref name="PNAS-20131031" /> Nonetheless, by concluding that complex life is uncommon, the Rare Earth hypothesis is a possible solution to the [[Fermi paradox]]: "If extraterrestrial aliens are common, why aren't they obvious?"<ref name="Webb, Stephen 2002">{{harvnb|Webb|2002}}</ref>
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| == Rare Earth's requirements for complex life ==
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| The Rare Earth hypothesis argues that the emergence of complex life requires a host of fortuitous circumstances. A number of such circumstances are set out below under the following headings: galactic [[habitable zone]], a central star and planetary system having the requisite character, the circumstellar habitable zone, a right sized terrestrial planet, the advantage of a gas giant guardian and large satellite, conditions needed to assure the planet has a [[magnetosphere]] and [[plate tectonics]], the chemistry of the [[lithosphere]], [[atmosphere]], and oceans, the role of "evolutionary pumps" such as massive [[glaciation]] and rare [[Meteoroid#Bolide|bolide]] impacts, and whatever led to the still mysterious [[Cambrian explosion]] of animal [[phylum|phyla]]. The emergence of [[Evolution of human intelligence|intelligent life]] may have required yet other rare events. | |
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| 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 [[universe]] is so vast that it could contain many Earth-like planets. But if such planets exist, they are likely to be separated from each other by many thousands of [[light year]]s. Such distances may preclude communication among any intelligent species evolving on such planets, which would solve the [[Fermi paradox]].
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| === The right location in the right kind of galaxy ===
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| [[File:NGC 7331 zoomed.jpg|thumb|right|295px|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]]
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| ''Rare Earth'' suggests that much of the known universe, including large parts of our galaxy, cannot support complex life; Ward and Brownlee refer to such regions as "dead zones." Those parts of a galaxy where complex life is possible make up the [[galactic habitable zone]]. This zone is primarily a function of distance from the galactic center. As that distance increases:
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| # Star [[metallicity]] declines. Metals (which in astronomy means all elements other than hydrogen and helium) are necessary to the formation of [[terrestrial planets]].
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| # The [[X-ray]] and [[gamma ray]] radiation from the [[Supermassive black hole|black hole]] at the [[galactic center]], and from nearby [[neutron star]]s, becomes less intense. Radiation of this nature is considered dangerous to complex life, hence the Rare Earth hypothesis predicts that the early universe, and galactic regions where stellar density is high and [[supernova]]e are common, will be unfit for the development of complex life.<ref>{{harvnb|Ward|Brownlee|2000|pp=27–29}}</ref>
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| # Gravitational perturbation of planets and [[planetesimals]] 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 [[Meteoroid#Bolide|bolide]]. A sufficiently large impact may [[extinction|extinguish]] all complex life on a planet.
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| Item #1 rules out the outer reaches of a galaxy; #2 and #3 rule out galactic inner regions, [[globular cluster]]s,{{citation needed|date=December 2012}} and the [[spiral arm]]s of [[spiral galaxy|spiral galaxies]].{{citation needed|date=December 2012}}. These "arms" are regions of a galaxy characterized by a higher rate of star formation, moving very slowly through the galaxy in a wave-like manner. As one moves from the center of a galaxy to its furthest extremity, the ability to support life rises then falls. Hence the galactic habitable zone may be ring-shaped, sandwiched between its uninhabitable center and outer reaches.
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| While a planetary system may enjoy a location favorable to complex life, it must also maintain that location for a span of time sufficiently long for complex life to evolve. Hence a central star with a galactic orbit that steers clear of galactic regions where radiation levels are high, such as the galactic center and the spiral arms, would appear most favourable. If the central star's galactic orbit is [[orbital eccentricity|eccentric]] (elliptic or hyperbolic), it will pass through some spiral arms, but if the orbit is a near perfect circle and the orbital velocity equals the "rotational" velocity of the spiral arms, the star will drift into a spiral arm region only gradually—if at all. Therefore ''Rare Earth'' proponents conclude that a life-bearing star must have a galactic orbit that is nearly circular about the center of its galaxy. The required synchronization of the orbital velocity of a central star with the wave velocity of the spiral arms can occur only within a fairly narrow range of distances from the galactic center. This region is termed the "galactic habitable zone". Lineweaver et al.<ref>{{cite journal | last1 = Lineweaver | first1 = Charles H. | last2 = Fenner | first2 = Yeshe | last3 = Gibson | first3 = Brad K. | year = 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 | format = 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 }}</ref> calculate that the galactic habitable zone is a ring 7 to 9 [[parsec|kiloparsecs]] in diameter, that includes no more than 10% of the stars in the [[Milky Way]].<ref>{{harvnb|Ward|Brownlee|2000|p=32}}</ref> Based on conservative estimates of the total number of stars in the galaxy, this could represent something like 20 to 40 billion stars. Gonzalez, ''et al.''<ref name=Gonzalez>{{harvnb|Gonzalez|Brownlee|Ward|2001}}</ref> would halve these numbers; he estimates that at most 5% of stars in the Milky Way fall in the galactic habitable zone.
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| The orbit of the Sun around the center of the Milky Way is indeed almost perfectly circular, with [[Galactic year|a period of 226 Ma]] (1 Ma = 1 million years), one closely matching the rotational period of the galaxy. 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>[http://curious.astro.cornell.edu/question.php?number=402 How often does the Sun pass through a spiral arm in the Milky Way?], Karen Masters, ''Curious About Astronomy''</ref> Some researchers have suggested that several mass extinctions do correspond with previous crossings of the spiral arms.<ref>{{harvnb|Dartnell|2007|p=75}}</ref>
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| [[Andromeda Galaxy|Andromeda]] and the Milky Way have a similar mass, but whereas Andromeda is a typical spiral galaxy the Milky Way is unusually quiet and dim. It appears to have suffered fewer collisions with other galaxies over the last 10 billion years, and its peaceful history may have made it more hospitable to complex life than galaxies which have suffered more collisions, and consequently more supernovae and other disturbances.<ref>{{cite web |author= |title=Sibling Rivalry |date=31 March 2012 |work=New Scientist |url=http://www.newscientist.com/article/mg21328586.500-milky-way-mysteries-andromeda-our-sibling-rival.html}}</ref> The level of activity of the [[black hole]] at the centre of the Milky Way may also be important: too much or too little and the conditions for life may be even rarer. The Milky Way black hole appears to be just right.<ref>Scharf, 2012</ref>
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| === Orbiting at the right distance from the right type of star ===
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| The terrestrial example suggests that complex life requires water in the liquid state, and a central star's planet must therefore be at an appropriate distance. This is the core of the notion of the [[habitable zone]] or [[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 |url=http://www.sciencedirect.com/science/article/pii/0019103579901416|bibcode = 1979Icar...37..351H }}</ref> The habitable zone forms a ring around the central star. If a planet orbits its sun too closely or too far away, the surface temperature is incompatible with water being liquid.
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| The habitable zone varies with the type and age of the central star. The habitable zone for a main sequence star very gradually moves out over time until the star becomes a white dwarf, at which time the habitable zone vanishes. The habitable zone is closely connected to the [[greenhouse warming]] afforded by atmospheric water vapor ({{chem|H|2|O}}), carbon dioxide ({{CO2}}), and/or other greenhouse gases. Even though the Earth's atmosphere contains a water vapor concentration from 0% (in arid regions) to 4% (in rain forest and ocean regions) and -as of June 2013- only 400 parts per million of {{CO2}}, these small amounts suffice to raise the average surface temperature of the Earth by about 40 °C from what it would otherwise be,<ref>{{harvnb|Ward|Brownlee|2000|p=18}}</ref> with the dominant contribution being due to water vapor, which together with clouds makes up between 66% and 85% of Earth's greenhouse effect, with {{CO2}} contributing between 9% and 26% of the effect.<ref>{{cite web |first=Gavin |last=Schmidt |title=Water vapour: feedback or forcing? |date=6 April 2005 |publisher=RealClimate |url=http://www.realclimate.org/index.php/archives/2005/04/water-vapour-feedback-or-forcing/}}</ref>
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| Rocky planets must orbit within the habitable zone for life to form. Although the habitable zone of such hot stars as [[Sirius]] or [[Vega]] is wide:
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| # Rocky planets that form too close to the star to lie within the habitable zone cannot sustain life; however, life could arise on a [[natural satellite|moon]] of a gas giant. Hot stars also emit much more [[ultraviolet radiation]] that [[ionize]]s any planetary [[Celestial body atmosphere|atmosphere]].
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| # Hot stars, as mentioned above, may become [[red giant]]s before advanced life [[Evolution|evolve]]s on their planets.
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| These considerations rule out the massive and powerful stars of type F6 to O (see [[stellar classification]]) as homes to evolved metazoan life.
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| Small [[red dwarf]] stars conversely have small [[habitable zone]]s wherein planets are in [[tidal lock]]—one side always faces the star and becomes very hot and the other always faces away and becomes very cold—and are also at increased risk of solar flares (see [[Aurelia and Blue Moon#Aurelia|Aurelia]]) that would tend to ionize the atmosphere and be otherwise inimical to complex life. ''Rare Earth'' proponents argue that life therefore cannot arise in such systems and that only central stars that range from F7 to K1 stars 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. However, [[Planetary habitability#Red dwarf systems|some exobiologists have suggested]] that stars outside this range may give rise to life under the right circumstances; this possbility 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"/> [[Image:A Swarm of Ancient Stars - GPN-2000-000930.jpg|thumb|right|According to Rare Earth, globular clusters are unlikely to support life.]]
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| Such aged stars as [[red giant]]s and [[white dwarf]]s are also unlikely to support life. Red giants are common in globular clusters and [[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 a much greater distance [[Red_giant#Prospects_for_habitability|may become habitable]]).
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| An energy output that varies with the lifetime of the star will very likely prevent life (e.g., as [[Cepheid variable]]s). A sudden decrease, even if brief, may freeze the water of orbiting planets, and a significant increase may evaporate them and cause a [[greenhouse effect]] that may prevent the oceans from reforming.
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| Life without complex chemistry is unknown. Such chemistry requires [[metallicity|metals]], namely elements other than hydrogen or helium and thereby suggests that a planetary system rich in [[metallicity|metals]] is a necessity for life. The only known mechanism for creating and dispersing metals is a [[supernova]] explosion. The [[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. 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 quiet suburbs of the larger spiral galaxies—where radiation also happens to be weak.<ref>{{harvnb|Ward|Brownlee|2000|pp=15–33}}</ref>
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| === With the right arrangement of planets ===
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| [[Image:Jupiter.jpg|thumb|right|According to Rare Earth, without the presence of the massive gas giant Jupiter (fifth [[planet]] from the [[Sun]] and the [[Solar system by size|largest]]) complex life on Earth would not have arisen.]]
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| 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 and rocky inner planets and outer gas giants.<ref>{{Cite web| url=http://news.nationalgeographic.com/news/2007/08/070827-jupiter-comets_2.html | first=Anne | last=Minard | date= 27 August 2007 | title=Jupiter Both an Impact Source and Shield for Earth | accessdate = 2014-1-14 | quote=without the long, peaceful periods offered by Jupiter's shield, intelligent life on Earth would never have been able to take hold. }}</ref>
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| In addition, Rare Earth proponents have argued that the arrangement of the Solar System is not only rare but optimal as the large mass and gravitational attraction of the gas giants provide protection for the inner rocky planets from [[Small Solar System body]] impacts and [[asteroid]] bombardment.
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| === A continuously stable orbit ===
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| Rare Earth argues that a gas giant must not be too close to a body upon which life is developing, unless that body is one of its moons. Close placement of gas giant(s) could disrupt the [[orbit]] of a potential life-bearing planet, either directly or by drifting into the habitable zone.
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| Newtonian dynamics can produce [[n-body problem#Three-body problem|chaotic planetary orbits]], especially in a system having [[Jovian planets|large planets]] at high [[orbital eccentricity]].<ref>{{Cite web|url= http://www.astro.ku.dk/~tobiash/posters/Tobias_final_new.pdf |format=PDF|title= Chaos and Planet-Particle Dynamics within the Habitable Zone of Extrasolar Planetary Systems (A qualitative numerical stability study) |accessdate= 2007-10-31 |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>
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| The need for stable orbits rules out [[Extrasolar planet|stars with systems of planets]] that contain large planets with orbits close to the host star (called "[[hot Jupiters]]"). It is believed that hot Jupiters formed much further from their parent stars than they are now, and 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 Upsilon Andromedae), you begin to wonder if there might be something special about our solar system" (UCBerkeleyNews quoting Extra solar 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 |accessdate = 2007-10-31 }}</ref>
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| === A terrestrial planet of the right size ===
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| It is argued that life requires terrestrial planets like Earth and as gas giants lack such a surface, that complex life cannot arise there.<ref>pg 220 Ward & Brownlee</ref>
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| A planet that is too small cannot hold much of an atmosphere. Hence the surface temperature becomes more variable and the average temperature drops. Substantial and long-lasting oceans become impossible. A small planet will also tend to have a rough surface, with large mountains and deep canyons. The core will cool faster, and [[plate tectonics]] will either not last as long as they would on a larger planet or may not occur at all.<ref>{{harvnb|Lissauer|1999}}, as summarized by {{harvnb|Conway Morris|2003|p=92}}; also see {{harvnb|Comins|1993}}</ref>
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| === With plate tectonics ===
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| Rare Earth proponents argue that [[plate tectonics]] is essential for the emergence and sustenance of complex life.<ref>pg 191. Rare Earth: Why Complex Life is Uncommon in the Universe. By Peter D. Ward, Donald Brownlee</ref> Ward & Brownlee assert that biodiversity, global temperature regulation, carbon cycle and the magnetic field of the Earth that make it habitable for complex terrestrial life all depend on plate tectonics.<ref>pg 194. Rare Earth: Why Complex Life is Uncommon in the Universe. By Peter D. Ward, Donald Brownlee</ref>
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| Ward & Brownlee contend that the lack of mountain chains elsewhere in the Solar System is direct evidence that Earth is the only body with plate tectonics and as such the only body capable of supporting life.<ref>pg 200. Rare Earth: Why Complex Life is Uncommon in the Universe. By Peter D. Ward, Donald Brownlee</ref>
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| Plate tectonics is dependent on chemical composition and a long-lasting source of heat in the form of [[radioactive decay]] occurring deep in the planet's interior. Continents must also be made up of less dense [[felsic]] rocks that "float" on underlying denser [[mafic]] rock. Taylor<ref name=Taylor98/> emphasizes that [[subduction]] zones (an essential part of plate tectonics) require the lubricating action of ample water; on Earth, such zones exist only at the bottom of oceans.
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| Ward & Brownlee and others such as Tilman Spohn of the German Space Research Centre Institute of Planetary Research<ref>http://www.space.com/4076-plate-tectonics-essential-alien-life.html</ref> argue that plate tectonics provides a means of [[Biogeochemical cycle|biochemical cycling]] which promotes complex life on Earth and that water is required to lubricate planetary plates.
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| === A large moon ===
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| The Moon is unusual because the other rocky planets in the Solar System either have no satellites ([[Mercury (planet)|Mercury]] and [[Venus]]), or have tiny satellites that are probably captured asteroids ([[Mars]]).
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| The [[giant impact theory]] hypothesizes that the Moon resulted from the impact of a [[Mars]]-sized body, [[Theia (planet)|Theia]], with the very young Earth. This giant impact also gave the Earth its [[Axis of rotation|axis tilt]] and velocity of rotation.<ref name=Taylor98>{{harvnb|Taylor|1998}}</ref> Rapid rotation reduces the daily variation in temperature and makes [[photosynthesis]] viable.{{citation needed|date=March 2013}} The ''Rare Earth'' hypothesis further argues that the axis tilt cannot be too large or too small (relative to the [[orbital plane (astronomy)|orbital plane]]). A planet with a large tilt will experience extreme seasonal variations in climate, unfriendly to complex life. 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 [[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>
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| If the Earth had no Moon, the ocean [[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 pool]]s, which may be essential for the formation of [[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 (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 |url=http://www.sciencedirect.com/science/article/pii/S001910350300383X}} It is taught less formally here: {{Cite web|url= http://abyss.uoregon.edu/~js/ast121/lectures/lec25.html |title= Origin of Life |accessdate=2007-10-31 |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>
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| A large satellite also increases the likelihood of [[plate tectonics]] through the effect of [[tidal force]]s on the planet's crust. The impact that formed the Moon may also have initiated plate tectonics, without which the [[continental crust]] would cover the entire planet, leaving no room for [[oceanic crust]]. It is possible that the large scale [[mantle convection]] needed to drive plate tectonics could not have emerged in the absence of crustal inhomogeneity.
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| If a giant impact is the only way for a rocky inner planet to acquire a large satellite, any planet in the circumstellar [[habitable zone]] will need to form as a [[double planet]] in order that there be an impacting object sufficiently massive to give rise in due course to a large satellite. An impacting object of this nature is not necessarily improbable.
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| === An evolutionary trigger for complex life ===
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| Regardless of whether planets with similar physical attributes to the Earth are rare or not, some argue that life usually remains as simple bacteria. Biochemist [[Nick Lane]] argues that simple cells ([[prokaryote]]s) emerged soon after earth's formation, but almost half the planet's life had passed before they evolved into complex ones ([[eukaryote]]s) and as all complex life has a common origin this event can only have happened once. In his view, [[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; two billion years ago, one simple cell incorporated itself into another, multiplied, and evolved into [[mitochondria]] that supplied the vast increase in available energy that enabled the evolution of complex life. If this incorporation occurred only once in four billion years or is otherwise unlikely, then life on most planets remains simple.<ref>Lane, 2012</ref>
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| ==Rare Earth equation==
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| The following discussion is adapted from Cramer.<ref>{{harvnb|Cramer|2000}}</ref> The Rare Earth equation is Ward and Brownlee's [[riposte]] to the [[Drake equation]]. It calculates <math>N</math>, the number of Earth-like planets in the Milky Way having complex life forms, as:
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| :<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 [[Milky Way]]. This number is not well-estimated, because the Milky Way's mass is not well estimated. Moreover, there is little information about the number of very small stars. ''N*'' is at least 100 billion, and may be as high as 500 billion, if there are many low visibility stars.
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| * <math>n_e</math> is the average number of planets in a star's habitable zone. This zone is fairly narrow, because 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.
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| 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 we have but one data point: the Earth, a rocky planet orbiting a [[Stellar classification#Class G|G2]] star in a quiet suburb of a large [[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/>).
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| * <math>f_p</math> is the fraction of stars in the [[Milky Way]] with planets.
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| * <math>f_{pm}</math> is the fraction of planets that are rocky ("metallic") rather than gaseous.
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| * <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.
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| * <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 very small.
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| * <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 [[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.
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| * <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.
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| * <math>f_j</math> is the fraction of planetary systems with large Jovian planets. This fraction could be large.
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| * <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 [[Cambrian explosion]] may be unusual, in which case this fraction would be small.
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| | |
| The Rare Earth equation, unlike the [[Drake equation]], does not factor the probability that complex life evolves into [[intelligent life]] that discovers technology (Ward and Brownlee are not [[evolutionary biology|evolutionary biologists]]). Barrow and Tipler<ref>{{BarrowTipler1986}} Section 3.2</ref> review the consensus among such biologists that the evolutionary path from primitive Cambrian [[chordate]]s, e.g. ''[[Pikaia]]'' to ''[[Homo sapiens]]'', was a highly improbable event. For example, the large [[brain]]s of humans have marked adaptive disadvantages, requiring as they do an expensive [[metabolism]], a long [[gestation period]], and a childhood lasting more than 25% of the average total life span. Other improbable features of humans include:
| |
| *Being the only extant [[bipedal]] land (non-avian) [[vertebrate]].{{dubious||Acc. to [[bipedal]], there are four bipedal species even among mammals alone|date=November 2011}} Combined with an unusual eye–hand coordination, this permits dextrous manipulations of the physical environment with the hands;
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| *A vocal apparatus far more expressive 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;
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| *The capability of formulating [[abstraction]]s to a degree permitting the invention of mathematics, and the discovery of science and technology. Only recently did humans acquire anything like their current scientific and technological sophistication.
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| | |
| ==Advocates==
| |
| Authors that advocate the Rare Earth hypothesis:
| |
| * Stuart Ross Taylor,<ref name=Taylor98/> a specialist on the solar system, firmly believes in the hypothesis, but its truth is not central to his purpose, which is to write a short introductory book on the solar system and its formation. Taylor concludes that the solar system is probably very unusual, because it resulted from so many chance factors and events.
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| * Stephen Webb,<ref name="Webb, Stephen 2002"/> a physicist, mainly presents and rejects candidate solutions for the [[Fermi paradox]]. The Rare Earth hypothesis emerges as one of the few solutions left standing by the end of the book.
| |
| * [[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> His main purpose, however, is to argue that if a planet does harbour life, intelligent beings something like humans are inevitable.<ref>Conway Morris, 2003, p. xv</ref>
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| * [[John D. Barrow]] and [[Frank J. Tipler]] (1986. 3.2, 8.7, 9), [[cosmologist]]s, vigorously defend the hypothesis that humans are likely to be the only intelligent life in the [[Milky Way]], and perhaps the entire universe. But this hypothesis is not central to their book ''The Anthropic Cosmological Principle'', a very thorough study of the [[anthropic principle]], and of how the laws of physics are peculiarly suited to enable the emergence of complexity in nature.
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| * [[Ray Kurzweil]], a computer pioneer and self-proclaimed [[Singularitarian]], argues in ''[[The Singularity Is Near]]'' that the coming [[Technological singularity|Singularity]] requires that Earth be the first planet on which sentient, 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 [[Technological singularity|Singularity]] and expanded to harness the full computational capacity of the physical universe.
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| * [[John Gribbin]], a prolific science writer, defends the hypothesis in a book devoted to it called ''Alone in the Universe: Why our planet is unique''.<ref>{{harvnb|Gribbin|2011}}</ref>
| |
| * [[Guillermo Gonzalez (astronomer)|Guillermo Gonzalez]], [[Astrophysics|astrophysicist]] who coined the term Galactic Habitable Zone uses the hypothesis in his book ''[[The Privileged Planet]]'' to promote the concept of [[intelligent design]]<ref>[http://www.reasons.org/resources/fff/2000issue04/index.shtml?main#measurability_of_the_universe The Measurability of the Universe––a Record of the Creator’s Design] By Guillermo Gonzalez, Facts for Faith Issue 4, 2000.</ref>
| |
| * [[Michael H. Hart]], [[astrophysicist]] who proposed a very narrow habitable zone based on climate studies edited the influential book "Extraterrestrials: Where are They" and authored "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>
| |
| | |
| ==Criticism==
| |
| Cases against the Rare Earth Hypothesis take various forms.
| |
| | |
| === Exoplanets with Earth-like properties are being discovered ===
| |
| {{see also|Estimated frequency of Earth-like planets}}
| |
| | |
| An increasing number of [[extrasolar planet]] discoveries are being made with 3,548 candidate planets now known as of August 2013. These discoveries and such tools as the [[Kepler (spacecraft)|Kepler space telescope]] aid estimating the frequency of Earth-like planets. Because life has not been found on other planets, and because the Copernican principle states that life should be common on these other Earth-like planets if the Copernican principle is true, the more Earth-like planets that are found without life increases the strength of the Rare Earth Hypothesis. In 2013 a study that was published in the journal Proceedings of the National Academy of Sciences calculated that about "one in five" of all [[sun]]-like [[star]]s are expected to have earthlike planets "within the [[habitable zone]]s of their stars"; 8.8 billion of them therefore exist in the Milky Way galaxy alone.<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 |publisher=[[nbcnews.com/]] |accessdate=2013-11-05}}</ref>
| |
| | |
| [[NASA]] and the [[SETI Institute]] now categorise Earth like planets with an [[Earth Similarity Index]] (ESI) of mass, radius and temperature.<ref>http://www.wired.co.uk/news/archive/2011-11/21/exoplanet-indices</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. November 22, 2011</ref>
| |
| | |
| Current technology limits the testing of important Rare Earth Criteria: surface water, tectonic plates, or a large moon, are currently undetectable, and few of the 146 documented exasolar systems have been found to resemble ours because Earth-sized planets are difficult to detect. However, a large moon and planetary arrangements that resemble that of the solar system are not necessarily important for the development of life in a system (see other reasons below).
| |
| | |
| ===Oxygen is not a requirement for multicellular life===
| |
| {{see also|Alternative biochemistry|Great Oxygenation Event}}
| |
| Multicellular life, e.g., [[Anaerobic organism|anaerobic]] metazoa, can exist without oxygen (despite Ward & Brownlee's now disproven contrary assertion <ref>Ward & Brownlee 2000, p. 217</ref>). Three multicellular species, including [[Spinoloricus nov. sp.]] discovered in the [[hypersaline]] [[Anoxic waters|anoxic]] [[L'Atalante basin]] at the bottom of the [[Mediterranean Sea]] in 2010, appear to metabolise with hydrogen instead of oxygen, lacking [[mitochondria]] and instead using [[hydrogenosomes]].<ref>[http://news.nationalgeographic.com/news/2010/04/100416-oxygen-free-complex-animals-mediterranean/ Oxygen-Free Animals Discovered-A First, National Geographic news]</ref><ref name="pmid20370908">{{cite journal |author=Danovaro R, Dell'anno A, Pusceddu A, Gambi C, Heiner I, 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 |url=http://www.biomedcentral.com/1741-7007/8/30}}</ref>
| |
| | |
| === Anthropic reasoning ===
| |
| 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 (scientist)|Jack Cohen]], believe this assumption too restrictive and unimaginative; they see it as a form of [[begging the question|circular reasoning]].
| |
| | |
| According to [[David Darling (astronomer)|David Darling]], the Rare Earth hypothesis is neither [[hypothesis]] nor [[prediction]], but merely a description of how life arose on Earth.<ref name="Darling">{{Cite book| last=Darling| first= David| year= 2001| title= Life Everywhere: The Maverick Science of Astrobiology| publisher=Basic Books/Perseus| isbn=0-585-41822-5}}</ref> In his view Ward and Brownlee have done nothing more than select the factors that best suit their case.
| |
| | |
| <blockquote>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></blockquote>
| |
| | |
| Critics also point to a link between the Rare Earth Hypothesis and the creationist ideas of intelligent design.<ref>Frazier, Kendrick. 'Was the 'Rare Earth' Hypothesis Influenced by a Creationist?' The Skeptical Inquirer. November 1, 200</ref>
| |
| | |
| === Alternative habitats for complex Life ===
| |
| {{see also|Alternative biochemistry}}
| |
| [[Image:Blacksmoker in Atlantic Ocean.jpg|thumb|upright|Complex life may exist elsewhere in environments similar to those found around [[black smoker]]s on Earth.]]
| |
| | |
| Rare Earth proponents argue that simple life may be common, though complex life requires specific environmental conditions to arise. Some argue that complex life may exist in such diverse habitats as those beyond the Solar System's habitable zone and on non-planetary bodies where both water and an active energy source may exist. For example, sub-surface water habitats that are warmed by [[tidal heating]] may exist on [[Europa (moon)|Europa]] and [[Enceladus (moon)|Enceladus]].<ref>{{cite journal |author=Reynolds, R.T.; McKay, C.P.; 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 |year=1987 |doi=10.1016/0273-1177(87)90364-4 |url=http://www.sciencedirect.com/science/article/pii/0273117787903644|bibcode = 1987AdSpR...7..125R }}</ref><ref>For a detailed critique of the Rare Earth hypothesis along these lines, see {{harvnb|Cohen|Stewart|2002}}.</ref> Some theories on the origin of life on Earth indicate that complex life evolved in such environments before arising on the surface.
| |
| | |
| === Uncertainty over Jupiter's role ===
| |
| The assertion that Jupiter's mass guards of the terrestrial planets from impacts has been challenged. Since Rare Earth, the 2005 [[Nice model]] and 2007 [[Nice 2 model]] have provided computer modelling of planetary formation. A study by Horner & 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.
| |
| | coauthors=Jones, B.W.
| |
| | title=Jupiter – friend or foe? I: the asteroids
| |
| | journal=International Journal of Astrobiology
| |
| | year=2008 | volume=7 | issue=3&4 | pages=251–261
| |
| | url=http://xxx.lanl.gov/ftp/arxiv/papers/0806/0806.2795.pdf
| |
| | format=PDF
| |
| | doi=10.1017/S1473550408004187 |bibcode = 2008IJAsB...7..251H |arxiv = 0806.2795 }}</ref>
| |
| | |
| === Necessity of tectonics ===
| |
| | |
| Ward & Brownlee argue that [[tectonics]] is necessary to support [[biogeochemical cycles]] required for intelligent life and that such geological features are unique to Earth and that such processes do not occur elsewhere citing the lack of any observable [[orogenic]] evidence. Though growing evidence suggests that such activity is not unique to Earth. Recent evidence points to similar activity either having occurred or continuing to occur on other terrestrial objects including Mars,<ref>[http://newsroom.ucla.edu/portal/ucla/ucla-scientist-discovers-plate-237303.aspx UCLA scientist discovers plate tectonics on Mars]
| |
| By Stuart Wolpert August 09, 2012</ref> Venus,<ref>[http://www3.imperial.ac.uk/earthscienceandengineering/research/iarc/theplanets/platetectonicsonvenus Richard Ghail, ''Plate tectonics on Venus,'' Imperial College London, Department faculty page]</ref> Titan,<ref>[http://www.nasa.gov/mission_pages/cassini/media/cassini-20061212.html Massive Mountain Range Imaged on Saturn's Moon Titan] NASA 12.12.06</ref><ref name="ChenChen2010">{{cite journal|last1=Chen|first1=Chao|last2=Chen|first2=Bo|last3=Ping|first3=JinSong|last4=Liang|first4=Qing|last5=Huang|first5=Qian|last6=Zhao|first6=WenJin|last7=Zhang|first7=ChangDa|title=The interpretation of gravity anomaly on lunar Apennines|journal=Science in China Series G: Physics, Mechanics and Astronomy|volume=52|issue=12|year=2010|pages=1824–1832|issn=1672-1799|doi=10.1007/s11433-009-0281-0}}</ref> Europa<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|year=2000|pages=17551|issn=0148-0227|doi=10.1029/1999JE001147}}</ref> and the Moon.<ref name="ChenChen2010"/> Several more natural satellites exhibit similar processes though that may have different mechanisms.
| |
| | |
| Many Rare Earth proponents argue that the Earth's plate tectonics would probably not exist if not for the tidal forces of the moon. However the hypothesis that the moon's tidal influence initiated Earth's plate tectonics remains unproven. Additionally, strong evidence suggests that plate tectonics existed on [[Mars]], which does not currently have a large companion.<ref>{{Cite web| url=http://www.nasa.gov/centers/goddard/news/topstory/2005/mgs_plates.html| date= 10 December 2005| title=New Map Provides More Evidence Mars Once Like Earth}}</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>{{Citation
| |
| | title = Does a Planet Need Life to Create Continents?
| |
| | url = http://www.astrobio.net/exclusive/5909/does-a-planet-need-life-to-create-continents
| |
| | year = 2014
| |
| | author = Choi, Charles Q.
| |
| | journal = Astrobiology Magazine
| |
| | accessdate = 2014-01-06
| |
| }}</ref>
| |
| | |
| === Giant impacts may not be rare nor necessary for rotational speed===
| |
| Recent work by [[Edward Belbruno]] and [[J. Richard Gott]] of Princeton University suggests that giant impacts such as those that formed the Moon can indeed form in planetary [[trojan points]] ({{L4}} or {{L5}} [[Lagrangian point]]) which means that similar circumstances may occur in other planetary systems.<ref>{{Cite journal|first=E.|last= Belbruno|coauthors=J. Richard Gott III| journal= The Astronomical Journal| volume= 129| issue=3| pages= 1724–45| year=2005| title=Where Did The Moon Come From?| arxiv=astro-ph/0405372| doi=10.1086/427539| bibcode=2005AJ....129.1724B}}</ref>
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| | |
| 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 "[[Tidal deceleration#Tidal deceleration|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, in 1 billion 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. {{Citation Needed|date=December 2013}}
| |
| | |
| ==See also==
| |
| <div style="-moz-column-count:3; column-count:3;">
| |
| * [[Astrochemistry]]
| |
| * [[Astrogeology]]
| |
| * [[Extrasolar planet]]
| |
| * [[Extraterrestrial life]]
| |
| * [[Evolving the Alien: The Science of Extraterrestrial Life]]
| |
| * [[Goldilocks planet]]
| |
| * [[Great Oxygenation Event]]
| |
| * [[History of Earth]]
| |
| * The [[mediocrity principle]] and [[cosmic pluralism]] are the [[Dialectic|antithesis]] of the Rare Earth hypothesis.
| |
| * [[Metaphysical naturalism]]
| |
| * [[Neocatastrophism]]
| |
| * [[Origin of life]]
| |
| * [[Panspermia]]
| |
| * [[Planetary habitability]]
| |
| * [[Precambrian]]
| |
| * [[Snowball earth]]
| |
| * [[Timetable of the Precambrian]]
| |
| </div>
| |
| | |
| ==Notes==
| |
| {{Reflist|2}}
| |
| | |
| ==References==
| |
| {{refbegin}}
| |
| *[http://news.bbc.co.uk/1/hi/sci/tech/7249884.stm 'Hundreds of worlds' in Milky Way]
| |
| *{{BarrowTipler1986}}
| |
| * Cirkovic, Milan M., and Bradbury, Robert J., 2006, "[http://www.anthropic-principle.com/preprints/milan-seti.pdf Galactic Gradients, Postbiological Evolution, and the Apparent Failure of SETI,]" New Astronomy, vol. 11, pp. 628–639.
| |
| *{{cite book |last=Comins |first=Neil F. |title=What if the moon didn't exist? Voyages to Earths that might have been |publisher=HarperCollins |year=1993 |ref=harv}}
| |
| *{{cite book |first=Simon |last=Conway Morris |authorlink=Simon Conway Morris |title=Life's Solution: Inevitable Humans in a Lonely Universe |publisher=Cambridge University Press |year=2003 |isbn=0 521 82704 3|ref=harv}}
| |
| *{{cite book |first1=Jack |last1=Cohen |authorlink1=Jack Cohen (scientist) |first2=Ian |last2=Stewart |author2-link=Ian Stewart (mathematician) |title=What Does a Martian Look Like: The Science of Extraterrestrial Life |publisher=Ebury Press |year=2002 |isbn=0-09-187927-2 |ref=harv}}
| |
| *{{cite web |last=Cramer |first=John G. |title=The 'Rare Earth' Hypothesis |date=September 2000 |work=Analog Science Fiction & Fact Magazine |url=http://www.npl.washington.edu/av/altvw102.html |ref=harv}}
| |
| *{{cite book |last=Dartnell |first=Lewis |title=Life in the Universe, a Beginner's Guide |publisher=One World |location=Oxford |year=2007 |ref=harv}}
| |
| *{{cite journal | last1 = Gonzalez | first1 = Guillermo | authorlink = Guillermo Gonzalez (astronomer) | last2 = Brownlee | first2 = Donald | last3 = Ward | first3 = Peter | year = 2001 | title = The Galactic Habitable Zone: Galactic Chemical Evolution | journal = Icarus | volume = 152 | pages = 185–200 | bibcode = 2001Icar..152..185G | doi = 10.1006/icar.2001.6617 |arxiv = astro-ph/0103165 |issue=1 |month=July |url=http://www.sciencedirect.com/science/article/pii/S0019103501966175 |ref=harv}}
| |
| *{{cite book |last=Gribbin |first=John |title=Alone in the Universe: Why our planet is unique |publisher=Wiley |year=2011 |ref=harv}}
| |
| *{{cite journal | last1 = Kasting | first1 = James | authorlink = James Kasting | last2 = Whitmire | first2 = D. P. | last3 = Reynolds | first3 = R. T. | year = 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 |ref=harv}}
| |
| *{{cite journal | last1 = Kirschvink | first1 = Joseph L. | last2 = Ripperdan | first2 = Robert L. | last3 = Evans | first3 = David A. | year = 1997 | title = Evidence for a Large-Scale Reorganization of Early Cambrian Continental Masses by Inertial Interchange True Polar Wander | url = http://www.sciencemag.org/cgi/content/abstract/277/5325/541 | journal = Science | volume = 277 | issue = 5325| pages = 541–45 | doi = 10.1126/science.277.5325.541 }}
| |
| *{{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 |year=2003 }}
| |
| *{{cite journal|last=Lane|first=Nick|authorlink=Nick Lane|journal=New Scientist|title=Life: is it inevitable or just a fluke?|url=http://www.newscientist.com/article/mg21428700.100-life-is-it-inevitable-or-just-a-fluke.html?full=true|date=28 June 2012|issue=2870|accessdate=1 JUuly 2012}}
| |
| *{{cite journal | last1 = Lineweaver | first1 = Charles H. | last2 = Fenner | first2 = Yeshe | last3 = Gibson | first3 = Brad K. | year = 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 | format = 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 }}
| |
| *{{cite journal |last=Lissauer |first=J.J. |title=How common are habitable planets? |journal=Nature |volume=402 |issue=6761 Suppl |pages=C11–4 |date=December 1999 |pmid=10591221 |doi=10.1038/35011503 |ref=harv}}
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| *{{cite arXiv |last=Prantzos |first=Nikos |eprint=astro-ph |title=On the Galactic Habitable Zone |class=astro-ph |year=2006 |doi=10.1007/s11214-007-9236-9 }} in Bada, J. et al., eds., ''Strategies for Life Detection''. To appear in ''Space Science Reviews''.
| |
| *{{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 |year=1993 |edition=2nd}}
| |
| *{{cite journal|first=Caleb|last=Scharf|url=http://www.scientificamerican.com/article.cfm?id=how-black-holes-shape-galaxies-stars-planets-around-them|journal=Scientific American|date=17 July 2012|title=How Black Holes Shape the Galaxies, Stars and Planets around Them}}
| |
| *{{cite book |last=Taylor |first=Stuart Ross |title=Destiny or Chance: Our Solar System and Its Place in the [[Cosmos]] |publisher=Cambridge University Press |year=1998 |ref=harv}}
| |
| *{{cite journal | last1 = Tipler | first1 = Frank J. | authorlink = Frank J. Tipler | author-separator =, | year = 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 }}
| |
| *{{cite book |last1=Ward |first1=Peter D. |last2=Brownlee |first2=Donald |title=Rare Earth: Why Complex Life is Uncommon in the Universe |publisher=Copernicus Books (Springer Verlag) |year=2000 |isbn=0-387-98701-0 |ref=harv}}
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| *{{cite book |last=Webb |first=Stephen |title=Where is Everybody? (If the universe is teeming with aliens, Where is Everybody?: Fifty solutions to the Fermi paradox and the problem of extraterrestrial life) |publisher=Copernicus Books (Springer Verlag) |year=2002 |ref=harv }}
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| * {{cite journal | last1 = Stenger | first1 = Victor | authorlink = Victor Stenger | author-separator =, | year = 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 }}
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| {{refend}}
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| | |
| ==External links==
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| * [http://replay.web.archive.org/20081019163719/http://www.astro.washington.edu/rareearth/ Home page] of ''Rare Earth'' (archival)
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| * Reviews of ''Rare Earth'':
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| ** [http://www.setileague.org/reviews/rarearth.htm Athena Andreadis], PhD in molecular biology.
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| ** [http://www.findarticles.com/p/articles/mi_m2843/is_6_25/ai_79794362 Kendrick Frazier], editor, ''Skeptical Inquirer''.
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| *"[http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=139 Galactic Habitable Zone,]" ''Astrobiology Magazine'', May 18, 2001.
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| * [[Gregg Easterbrook]], "[http://www.theatlantic.com/issues/88aug/easterbr.htm Are We Alone?]" ''[[The Atlantic Monthly]]'', August 1988. Article that anticipates REH in some respects.
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| *Solstation.com: "[http://www.solstation.com/habitable.htm Stars and Habitable Planets.]"
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| *{{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=June 1, 1999 |archiveurl=http://web.archive.org/web/19991011230719/http://www.chron.com/content/interactive/space/astronomy/news/1999/ds/990602.html |archivedate=1999-10-11}}
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| *Breitbart.com, "[http://www.breitbart.com/article.php?id=paUniverse_sun14_parallel_universes&show_article=1&cat=0 Parallel universes exist,]" Sept. 23 2007.
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| * {{Cite news |url=http://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}}
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| {{Extraterrestrial life}}
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| {{DEFAULTSORT:Rare Earth Hypothesis}}
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| [[Category:Earth]]
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| [[Category:Astrobiology]]
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| [[Category:Origin of life]]
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| [[Category:Fermi paradox]]
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| [[Category:Hypotheses]]
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| [[Category:Astronomical hypotheses]]
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| {{Link GA|zh}}
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| {{Link FA|sh}}
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