Rare Earth hypothesis

In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life (metazoa) on Earth required an improbable combination of astrophysical and geological events and circumstances. The term "Rare Earth" comes from Rare Earth: Why Complex Life Is Uncommon in the Universe (2000), a book by Peter Ward, a geologist and paleontologist, and Donald E. Brownlee, an astronomer and astrobiologist. Their book is the source for much of this article.

The rare earth hypothesis is the contrary of the widely accepted principle of mediocrity (also called the Copernican principle), advocated by Carl Sagan and Frank Drake, among others.[1] The principle of mediocrity concludes that the Earth is a typical rocky planet in a typical planetary system, located in an unexceptional 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 systems, 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.

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?"[2]

Contents

Why complex life may be very rare

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, the size of the planet, the advantage of a 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 bolide impacts, and whatever led to the still mysterious Cambrian explosion of animal phyla. The emergence of intelligent life may have required yet other rare events.

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 years. Such distances may preclude communication among any intelligent species evolving on such planets, which would solve the Fermi paradox.

The galactic habitable zone

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:

  1. Star metallicity declines. Metals (which in astronomy means all elements other than hydrogen and helium) are necessary to the formation of terrestrial planets.
  2. The X-ray and gamma ray radiation from the black hole at the galactic center, and from nearby neutron stars, 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 supernovae are common, will be unfit for the development of complex life.[4]
  3. 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 bolide. A sufficiently large impact may extinguish all complex life on a planet.

(1) rules out the outer reaches of a galaxy; (2) and (3) rule out galactic inner regions, globular clusters, and the spiral arms of spiral galaxies. These arms are not physical objects, but 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.

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 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.[5] calculate that the galactic habitable zone is a ring 7 to 9 kiloparsecs in diameter, that includes no more than 10% of the stars in the Milky Way.[6] 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.[7] would halve these numbers; he estimates that at most 5% of stars in the Milky Way fall in the galactic habitable zone.

The orbit of the Sun around the center of the Milky Way is indeed almost perfectly circular, with 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.[8] Some researchers have suggested that several mass extinctions do correspond with previous crossings of the spiral arms.[9]

A central star of the right character

The terrestrial example suggests complex life requires water in the liquid state and its planet must therefore be at an appropriate distance. This is the core of the notion of the habitable zone or Goldilocks Principle.[10] 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 (though sub-surface water, as suggested for Europa, Enceladus, and Ceres, may be possible at varying locations[11]). Kasting et al. (1993) estimate that the habitable zone for the Sun ranges from 0.95 to 1.15 astronomical units.[12]

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 (H2O), 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 only 387 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,[13] 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.[14]

It is then presumed a star needs to have rocky planets within its habitable zone. While the habitable zone of hot stars such as Sirius or Vega is wide, there are two problems:

  1. Given that rocky planets were (at the time Rare Earth was written) thought to form closer to their central stars, the planet probably forms too close to the star to lie within the habitable zone. This does not rule out life on a moon of a gas giant. Hot stars also emit much more ultraviolet radiation, which will ionize any planetary atmosphere.
  2. Hot stars, as mentioned above, have short lives, becoming red giants in as little as 1 Ga. This may not allow enough time for advanced life to evolve.

These considerations rule out the massive and powerful stars of type F6 to O (see stellar classification) as homes to evolved metazoan life.

Small red dwarf stars, on the other hand, have habitable zones with a small radius. This proximity causes one face of the planet to constantly face the star, and the other to always remain dark, a situation known as tidal lock. Tidal locking of a planetary hemisphere to its primary will cause one side of a planet to be extremely hot, while the other will be extremely cold. Planets within a habitable zone with a small radius are also at increased risk of solar flares (see Aurelia), which would tend to ionize the atmosphere and are otherwise inimical to complex life. Rare Earth proponents argue that this rules out the possibility of life in such systems, though some exobiologists have suggested that habitability may exist under the right circumstances. This is a central point of contention for the theory, since these late-K and M category stars make up about 82% of all hydrogen-burning stars.[15]

Rare Earth proponents argue that the stellar type of central stars that are "just right" ranges from F7 to K1. Such stars are not common: G type stars such as the Sun (between the hotter F and cooler K) comprise only 9%[15] of the hydrogen-burning stars in the Milky Way.

Aged stars, such as red giants and white dwarfs, are also unlikely to support life. Red giants are common in globular clusters and elliptical galaxies. White dwarfs are mostly dying stars that have already gone through their red giant phase. The diameter of a red giant has substantially increased from its youth. If a planet was in the habitable zone during a star's youth and middle age, it will be fried when its parent star becomes a red giant (though theoretically planets at a much greater distance may become habitable).

The energy output of a star over its lifespan should only change very gradually; variable stars such as Cepheid variables, for instance, are highly unlikely to support life. If the central star's energy output suddenly decreases, even for a relatively short while, the planet's water may freeze. Conversely, if the central star's energy output significantly increases, the oceans may evaporate, resulting in a greenhouse effect; this may preclude the oceans from reforming.

There is no known way to achieve life without complex chemistry, and such chemistry requires metals, namely elements other than hydrogen or helium. This suggests a condition for life is a solar system rich in metals. The only known mechanism for creating and dispersing metals is a supernova explosion. The presence of metals in stars is revealed by their absorption spectrum, 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 formed when the universe was young, stars in most galaxies other than large spirals, and stars in the outer regions of all galaxies. Thus metal-rich central stars capable of supporting complex life are believed most common in the quiet suburbs of the larger spiral galaxies, regions hospitable to complex life for another reason, namely the absence of high radiation.[16]

Planetary system

A gas cloud capable of giving birth to a star can also give rise to gas-giant low-metallicity (Jovian) planets like Jupiter and Saturn. But Jovian planets have no hard surface of the kind believed necessary for complex life (their satellites may have hard surfaces, though). The Ward and Brownlee argument holds that a planetary system capable of sustaining complex life must be structured more or less like our solar system, with small and rocky inner planets, and Jovian outer ones. Recent research calls this line of argument into question, however.

Uncertainty over Jupiter

At the time of Ward and Brownlee's book, gas giants were thought to help support life by keeping asteroids away from the life-bearing planet. Hence a gas giant was thought to protect the inner rocky planets from asteroid bombardment. However, recent computer simulations on the matter suggest that the situation is more complex than this: A planet of Jupiter's mass still seems to provide increased protection against asteroids, but the total effect on all orbital bodies within the solar system is unclear.[18][19]

Disruption of orbit

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.

Newtonian dynamics can produce chaotic planetary orbits, especially in a system having large planets at high orbital eccentricity.[20]

The need for stable orbits rules out 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.[21]

Size of planet

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.[22]

Small rocky planets like Earth may be common according to astronomer Michael Meyer of the University of Arizona:

Our observations suggest that between 20% and 60% of Sun-like stars have evidence for the formation of rocky planets not unlike the processes we think led to planet Earth. That is very exciting.[23]

Meyer’s team found cosmic dust near recently-formed sun-like stars and sees this as a byproduct of the formation of rocky planets.

Large moon

The Moon is unusual because the other rocky planets in the Solar System either have no satellites (Mercury and Venus), or have tiny satellites that are probably captured asteroids (Mars).

The giant impact theory hypothesizes that the Moon resulted from the impact of a Mars-sized body, Theia, with the very young Earth. This giant impact also gave the Earth its axis tilt and velocity of rotation.[24] Rapid rotation reduces the daily variation in temperature and makes photosynthesis viable. The Rare Earth hypothesis further argues that the axis tilt cannot be too large or too small (relative to the 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. 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 variation in tilt would be chaotic, probably making complex life forms on land impossible.[25]

If the Earth had no Moon, the ocean tides 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 complex life, though this is far from certain.[26]

A large satellite also increases the likelihood of plate tectonics through the effect of tidal forces 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. However, there is strong evidence that plate tectonics existed on Mars in the past, without such a mechanism to initiate it.[27]

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. Recent work by Edward Belbruno and J. Richard Gott of Princeton University suggests that a suitable impacting body could form in a planet's trojan points (L4 or L5 Lagrangian point).[28]

Plate tectonics

A planet will not experience plate tectonics unless its chemical composition allows it. The only known long-lasting source of the required heat is radioactive decay occurring deep in the planet's interior. Continents must also be made up of less dense granitic rocks that "float" on underlying denser basaltic rock. Taylor[29] 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.

Oort Cloud

Almost all the comets we see derive from the Oort Cloud and few, if any, have hyperbolic orbits which suggest origins from interstellar space, and thus from other solar systems. This is puzzling as the number of comets in our Solar System runs into billions, and many escape into interstellar space, so we would expect to see comets from other solar systems. In the view of J. C. Brandt and R. D. Chapman, although other explanations are possible, "Perhaps solar systems like ours are the exception rather than the rule."[30]

Rare Earth equation

The following discussion is adapted from Cramer.[31] The Rare Earth equation is Ward and Brownlee's riposte to the Drake equation. It calculates N, the number of Earth-like planets in the Milky Way having complex life forms, as:

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}[32]

where:

We assume N^* \cdot n_e = 5\cdot10^{11}. 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 10−10 and could plausibly be as small as 10−12. In the latter case, N could be as small as 0 or 1. Ward and Brownlee do not actually calculate the value of N, 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 G2 star in a quiet suburb of a large barred spiral galaxy, and the home of the only intelligent species we know, namely ourselves.

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 biologists). Barrow and Tipler[33] review the consensus among such biologists that the evolutionary path from primitive Cambrian chordates, e.g. Pikaia to Homo sapiens, was a highly improbable event. For example, the large brains 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:

Advocates

Books that advocate the Rare Earth hypothesis:

Criticism

Criticisms of the Rare Earth Hypothesis take various forms.

Exoplanets are common

Over 700 exoplanets are known as of late 2011 and more are continually discovered.[37] Alan Boss of the Carnegie Institution of Science estimates there may be 100 billion terrestrial planets in our Milky Way Galaxy alone.[38] Boss believes many could have simple lifeforms and there could be thousands of civilizations in our galaxy. He uses an estimate that each sun-like star has on average one Earth-like planet.

The claimed discovery of Gliese 581 g, a Goldilocks planet only 20 light-years from Earth, has been used as an argument against the Rare Earth hypothesis. With such proximity to Earth, exoplanetologists estimated that the likelihood of finding an Earth-like planet in any given system in our galaxy is 10-20%.[39] However, subsequent research has put the existence of Gliese 581 g into question.[40]

The discovery of Kepler-22b, a super-Earth orbiting in the habitable zone of a Sun-like star, has now been confirmed.[41]

Evolutionary biology

Central to the Rare Earth hypothesis is the following claim about evolutionary biology: while microbes of some sort could well be common in the universe, complex life is unlikely to be. Yet to date, the only evolutionary biologist to speak to the hypothesis at any length is Simon Conway Morris (2003). 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 circular reasoning (see Alternative biochemistry, a speculative biochemistry of alien life forms). Earth-like planets may indeed be very rare, but non carbon-based complex life could possibly emerge in other environments.[42] According to David Darling, the Rare Earth hypothesis is neither hypothesis nor prediction, but merely a description of how life arose on Earth.[43] In his view Ward and Brownlee have done nothing more than select the factors that best suit their case.

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.[44]

Impacts

There are also studies showing that Jupiter has caused more impacts on Earth than it has prevented, invalidating the Rare Earth argument of Jupiter-like planets as necessary protectors.[45] The role of Jupiter, however, has since been revised by the Nice model.

Oxygen

In the hypersaline anoxic L'Atalante basin at the bottom of the Mediterranean Sea, multicellular animals have been discovered that use hydrogen instead of oxygen. They have no mitochondria as we know it, but use hydrogenosomes instead. They live their entire lives and reproduce without oxygen, invalidating the Rare Earth claim that all multicellular animal life requires oxygen.[46][47]

See also

Notes

  1. ^ Brownlee and Ward (2000), pp. xxi–xxiii.
  2. ^ a b Webb, Stephen, 2002. If the universe is teeming with aliens, where is everybody? Fifty solutions to the Fermi paradox and the problem of extraterrestrial life. Copernicus Books (Springer Verlag)
  3. ^ 1 Morphology of Our Galaxy's 'Twin' Spitzer Space Telescope, Jet Propulsion Laboratory, NASA.
  4. ^ Brownlee, Donald. Ward, Peter D. "Rare Earth", pages 27 – 29. Copernicus. 2000.
  5. ^ Lineweaver, Charles H.; Fenner, Yeshe; Gibson, Brad K. (2004). "The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way" (PDF). Science 303 (5654): 59–62. arXiv:astro-ph/0401024. Bibcode 2004Sci...303...59L. doi:10.1126/science.1092322. PMID 14704421. http://astronomy.swin.edu.au/GHZ/GHZ_astroph.pdf. 
  6. ^ Brownlee, Donald. Ward, Peter D. "Rare Earth", pages 32. Copernicus. 2000.
  7. ^ a b Gonzalez, Guillermo; Brownlee, Donald; Ward, Peter (2001). "The Galactic Habitable Zone: Galactic Chemical Evolution". Icarus 152: 185–200. arXiv:astro-ph/0103165. Bibcode 2001Icar..152..185G. doi:10.1006/icar.2001.6617. 
  8. ^ How often does the Sun pass through a spiral arm in the Milky Way?, Karen Masters, Curious About Astronomy
  9. ^ Dartnell, Lewis, Life in the Universe, One World, Oxford, 2007, p. 75.
  10. ^ Hart, M. "Habitable Zones Around Main Sequence Stars," Icarus, 37, 351 (1979).
  11. ^ Reynolds, R. T., McKay, C. P., and Kasting, J. F. "Europa, Tidally Heated Oceans, and Habitable Zones Around Giant Planets," Advances in Space Research, 7 (5), 125 (1987).
  12. ^ Kasting, James; Whitmire, D. P.; Reynolds, R. T. (1993). "Habitable zones around main sequence stars". Icarus 101 (1): 108–28. Bibcode 1993Icar..101..108K. doi:10.1006/icar.1993.1010. PMID 11536936. 
  13. ^ Brownlee, Donald. Ward, Peter D. "Rare Earth", page 18. Copernicus. 2000.
  14. ^ http://www.realclimate.org/index.php/archives/2005/04/water-vapour-feedback-or-forcing/
  15. ^ a b [1] The One Hundred Nearest Star Systems, Research Consortium on Nearby Stars.
  16. ^ Brownlee, Donald. Ward, Peter D. "Rare Earth", pages 15–33. Copernicus. 2000.
  17. ^ Jupiter, entry in the Oxford English Dictionary, prepared by J. A. Simpson and E. S. C. Weiner, vol. 8, second edition, Oxford: Clarendon Press, 1989. ISBN 0-19-861220-6 (vol. 8), ISBN 0-19-861186-2 (set.)
  18. ^ Jupiter – friend or foe? II: the Centaurs Jupiter http://arxiv.org/pdf/0903.3305
  19. ^ Horner, J.; Jones, B.W. (2008). "Jupiter – friend or foe? I: the asteroids". International Journal of Astrobiology 7 (3&4): 251–261. Bibcode 2008IJAsB...7..251H. doi:10.1017/S1473550408004187. http://xxx.lanl.gov/ftp/arxiv/papers/0806/0806.2795.pdf. Retrieved 2009-05-15. 
  20. ^ Hinse, T.C.. "Chaos and Planet-Particle Dynamics within the Habitable Zone of Extrasolar Planetary Systems (A qualitative numerical stability study)" (PDF). Niels Bohr Institute. http://www.astro.ku.dk/~tobiash/posters/Tobias_final_new.pdf. Retrieved 2007-10-31. "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." 
  21. ^ "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.) Sanders, Robert (13 April 2005). "Wayward planet knocks extrasolar planets for a loop". http://www.berkeley.edu/news/media/releases/2005/04/13_planet.shtml. Retrieved 2007-10-31. 
  22. ^ Lissauer 1999, as summarized by Conway Morris 2003: 92; also see Comins 1993
  23. ^ Planet-hunters set for big bounty, BBC
  24. ^ Taylor 1998
  25. ^ Dartnell, pp. 69–70,
  26. ^ A formal description of the hypothesis is given in: Lathe, Richard (March 2004). "Fast tidal cycling and the origin of life". Icarus 168 (1): 18–22. Bibcode 2004Icar..168...18L. doi:10.1016/j.icarus.2003.10.018. "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."  It is taught less formally here: Schombert, James. "Origin of Life". University of Oregon. http://abyss.uoregon.edu/~js/ast121/lectures/lec25.html. Retrieved 2007-10-31. "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" .
  27. ^ "New Map Provides More Evidence Mars Once Like Earth". 10 December 2005. http://www.nasa.gov/centers/goddard/news/topstory/2005/mgs_plates.html. 
  28. ^ Belbruno, E.; J. Richard Gott III (2005). "Where Did The Moon Come From?". The Astronomical Journal 129 (3): 1724–1745. arXiv:astro-ph/0405372. Bibcode 2005AJ....129.1724B. doi:10.1086/427539. 
  29. ^ Taylor, Stuart Ross, 1998. Destiny or Chance: Our Solar System and Its Place in the Cosmos. Cambridge Univ. Press
  30. ^ J. C. Brandt & R. D. Chapman, Rendezvous in Space: the Science of Comets, Freeman, New York, 1992, p. 170, quoted in Simon Conway Morris, The Crucible of Creation: The Burgess Shale and the Rise of Animals, Oxford University Press, 1998, p. 224
  31. ^ Cramer (2000)
  32. ^ Brownlee, Donald. Ward, Peter D. "Rare Earth", pages 271–275. Copernicus. 2000.
  33. ^
    [Edit this reference]
    Barrow, John D.; Tipler, Frank J. (19 May 1988). The Anthropic Cosmological Principle. foreword by John A. Wheeler. Oxford: Oxford University Press. ISBN 9780192821478. LC 87-28148. http://books.google.com/books?id=uSykSbXklWEC&printsec=frontcover. Retrieved 31 December 2009.  Section 3.2
  34. ^ Taylor, Stuart Ross, 1998. Destiny or Chance: Our Solar System and Its Place in the Cosmos. Cambridge Univ. Press.
  35. ^ Simon Conway Morris, 2003. Life's Solution. Cambridge Univ. Press. See chpt. 5; many references.
  36. ^ John Gribbin, Alone in the Universe: Why our planet is unique, Wiley, 2011
  37. ^ Schneider, Jean. "Interactive Extra-solar Planets Catalog". The Extrasolar Planets Encyclopedia. http://exoplanet.eu/catalog.php. 
  38. ^ Galaxy may be full of 'Earths,' alien life
  39. ^ Vogt, Steven S.; Butler, R. Paul; Rivera, Eugenio J.; Haghighipour, Nader; Henry, Gregory W.; Williamson, Michael H. (2010-09-29). "The Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581". Astrophysical Journal 723: 954–965. arXiv:1009.5733. Bibcode 2010ApJ...723..954V. doi:10.1088/0004-637X/723/1/954. 
  40. ^ Forveille, T.; Bonfils, X.; Delfosse, X.; Alonso, R.; Udry, S.; Bouchy, F.; Gillon, M.; Lovis, C.; Neves, V.; Mayor, M.; Pepe, F.; Queloz, D.; Santos, N. C.; Segransan, D.; Almenara, J. M.; eeg, H.; Rabus. M. (2011-09-12). "The HARPS search for southern extra-solar planets XXXII. Only 4 planets in the Gl~581 system". arXiv:1109.2505v1 [astro-ph.EP].  "...Our dataset therefore has strong diagnostic power for planets with the parameters of Gl 581f and Gl 581g, and we conclude that the Gl 581 system is unlikely to contain planets with those characteristics..."
  41. ^ "NASA - NASA's Kepler Confirms Its First Planet in Habitable Zone of Sun-like Star". NASA Press Release. http://www.nasa.gov/centers/ames/news/releases/2011/11-99AR.html. Retrieved 6 December 2011. 
  42. ^ For a detailed critique of the Rare Earth hypothesis along these lines, see Cohen and Ian Stewart (2002).
  43. ^ Darling, David (2001). Life Everywhere: The Maverick Science of Astrobiology. Basic Books/Perseus. ISBN 0585418225. 
  44. ^ Darling, p. 103
  45. ^ Horner, J.; Jones, B. W. (2008). "Jupiter - friend or foe? I: the asteroids". International Journal of Astrobiology 7 (3–4): 251–261.
  46. ^ Oxygen-Free Animals Discovered-A First, National Geographic news
  47. ^ Danovaro R, Dell'anno A, Pusceddu A, Gambi C, Heiner I, Kristensen RM (April 2010). "The first metazoa living in permanently anoxic conditions". BMC Biology 8 (1): 30. doi:10.1186/1741-7007-8-30. PMC 2907586. PMID 20370908. http://www.biomedcentral.com/1741-7007/8/30. 

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