Rare Earth hypothesis

From The Book of THoTH (Leaves of Wisdom)

The rare Earth hypothesis asserts that the emergence of complex multicellular life (hereinafter "complex life") on Earth required such a fortuitous combination of astrophysical and geological circumstances that such life is probably very rare in the universe. The rare Earth hypothesis is explained in detail in the book Rare Earth: Why Complex Life Is Uncommon in the Universe, by Peter Ward, a geologist and paleontologist, and Donald Brownlee, an astronomer and astrobiologist.

The rare Earth hypothesis is the contrary of the principle of mediocrity, also called the Copernican principle, whose best known advocates are Carl Sagan and Frank Drake. The principle of mediocrity maintains that the Earth is a common sort of planet orbiting a common sort of star, both part of a common sort of planetary system, located in an unexceptional region of a large but conventional spiral galaxy. Ward and Brownlee argue to the contrary, namely that planets and planetary systems that are as friendly to complex life as are the Earth and its solar system, are probably extremely rare in the universe. The Earth could well be the only planet in our galaxy, the Milky Way, and even in the entire universe, with complex life on its surface.

If one also assumes that an Earth-like planet is required in order for complex life to evolve, then the rare Earth hypothesis solves the Fermi paradox (Webb 2002) ("if extraterrestrial aliens exist, why aren't they obvious?"): there is no evidence of aliens because none exist in the Milky Way. Hence the current lack of evidence of extraterrestrial civilizations.

Contents 1 Complex life is probably very rare 1.1 The galactic habitable zone 1.2 A central star of the right character 1.3 Planetary system 1.4 Planetary system habitable zone 1.5 Size of planet 1.6 Large moon 1.7 Magnetic field 1.8 Plate tectonics 1.9 Chemistry of the atmosphere 1.10 The mysterious Cambrian explosion 1.11 Impact frequency and evolution 1.12 Evolution must succeed 2 Microbes may be common 3 Concurring voices 4 Criticism 5 See also 6 References 7 External links

Complex life is probably very rare

Contra the principle of mediocrity, 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 solar 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, the mysterious Cambrian explosion of animal phyla, and rare bolide impacts as possible "evolutionary pumps." The evolution of intelligent life may require yet more rare circumstances.

In order for a small rocky planet to support complex life, the values of hundreds of variables must fall within narrow ranges. The universe is vast beyond ready human comprehension, so vast that it could contain multiple 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 preclude communication among any intelligent species that may evolve on such planets. This hypothetical rarity of Earth-like planets in the Milky Way would explain the Fermi paradox, namely the absence of evidence of extraterrestrial colonization.

The well-known Drake equation estimating the number of planets in the Milky Way harboring intelligent life, omits many of the factors described below and now believed important. Now the more factors are included in a Drake-like equation, the greater the likelihood that any single factor is near zero. And if any factor is near zero, so is the result. At present, most of the factors 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 spiral galaxy, and the home of the only intelligent species we know, namely ourselves. The rare earth hypothesis is perhaps little more than the argument that a properly specified Drake equation predicts that the Milky Way is unlikely to harbor more than one intelligent species having a fair grasp of technology. We may be alone in the universe.

The galactic habitable zone

A great deal of the known universe cannot support life because of elementary astrophysical and chemical considerations. Rare Earth refers to such regions as "dead zones." The exceptions make up the galactic habitable zones.

The density of stars near the center of the galaxy is so high that the amount of cosmic radiation there precludes the emergence of life. The complex and delicate chemistry of life also requires an environment lacking high energy (x-ray and gamma) radiation and fast moving particles. Quasars, neutron stars, magnetars, supernovae, and black holes emit lethal high energy radiation and fast moving particles. Hence the early universe, and regions where the stellar density is high, such as galactic centers and globular clusters, are all unfit for life.

Life can only evolve on planets whose orbits are stable over long periods of time and hence free of gravitational disruption. This rules out regions of galaxies where stars are close enough to one another to permit them to disrupt the planetary systems of nearby stars. Such regions include globular clusters, the inner regions of galaxies, and even the spiral arms of spiral galaxies. Given that a planetary system happens to enjoy a location propitious for complex life, it must maintain that location for the billions of years required for complex life to emerge. This requires that a planetary system have a nearly circular orbit about the center of its galaxy, and have an orbital velocity equal to the rotational velocity of the spiral arms. Otherwise a planetary system not in a spiral arm will gradually drift into an inhospitable spiral arm. The required synchronization of the orbital velocity of a planetary system with the rotational velocity of the galaxy containing it can occur only within a fairly narrow range of distances from the galactic center. According to Guillermo Gonzalez (2003), more than 95% of stars in the Milky Way do not satisfy this criterion.

The central star must be in the suburbs of its galaxy; and not in its city or the countryside. The central star must have a galactic orbit that steers clear of the galaxy's spiral arms, where supernovae and their attendant radiation hazards are more common. If the orbit is eccentric (egg-shaped), it will pass through the arms. The orbit of the central star must also steer clear of the energetic galactic center and its high radiation levels. The orbit of the Sun around the center of the Milky Way is almost perfectly circular, with a period of 226Ma, one closely matching the rotational period of the galaxy. Such close matches are possible only for stars located in a narrow ring about the galactic center, making up the galactic habitable zone. The Sun's orbit is so perfect that it has remained clear of the spiral arms of the Milky Way over its entire 4.6Ba lifetime.

A central star of the right character

Aged stars, such as red giants and white dwarfs, cannot support life. Red giants are common in globular clusters and elliptical galaxies. White dwarfs are mostly dying stars that were either too small to support life, or have already gone through their red giant phase. The diameter of a red giant has substantially increased from what it was over the previous 5-10Ba. If a planet was in the habitable zone during a star's youth and middle age, it will be fried to crisp when its parent star becomes a red giant.

The central star must have the right size. Large stars emit too much ultraviolet radiation, which precludes life. Large stars have lifespans too short to permit complex life to evolve. Large stars live for millions, not billions, of years, too short of a time for evolution to do its work. They explode as supernovas, after which they become neutron stars or black holes. Small stars, on the other hand, have habitable zones with a small radius. A small orbital radius causes one face of the planet to constantly face the star, and the other to always remain dark, a situation, known as tidal lock; the Moon is a case in point. Tidal lock rules out axial rotation; hence one side of a planet in an otherwise habitable zone will be too hot, the other too cold, ruling out liquid water and an atmosphere.

The central star cannot be a multiple star system, for which the stability of planetary orbits is problematic. Most stars are either too weak to support life, or are part of multiple star systems, or both.

The energy output of a star over its lifespan must change only very gradually; hence it cannot be a variable star such as a Cepheid variable. If the central star's energy output suddenly decreases, even for a relatively short while, the planet's water freezes. This situation is irreversible; the ice will not melt when the star resumes its normal energy output, because the planet's icy surface has too high of an albedo; most of the star’s energy reaching the planet will be reflected back into space. Conversely, if the central star's energy output temporarily increases, the oceans will evaporate, resulting in an irreversible greenhouse effect that precludes the oceans from ever reforming.

A necessary condition for life is a solar system rich in metals, elements other than hydrogen, helium, and lithium. There is no known way to achieve life without complex chemistry, and such chemistry requires metals. If a star is poor in metals, we infer that any associated planetary system is likewise poor in metals. 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. The only known mechanism for creating and dispersing metals is a supernova explosion. Hence metals are not all that common. This condition 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.

In order to have rocky planets like the Earth, a central star must have condensed out of a nebula that was fairly metal-rich. Only gas giant planets will condense out of a metal-poor nebula; such a nebula simply lacks the material required to form terrestrial planets. A lack of metals excludes the outer part of any galaxy. On the other hand, if a star is too rich in metals, its rocky planets may be large enough, and hence have sufficient gravitation, to accrete gas envelopes that make them similar to gas giants.

Planetary system

A gas cloud capable of giving birth to a star can also give rise to gas giant (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). Hence 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 Jovian outer ones.

Thanks to its gravitational force, a gas giant ejects the debris from planet formation into the equivalent of the Kuiper belt and Oort cloud. Hence a gas giant helps protect the inner rocky planets from asteroid bombardment. However, a gas giant must not be too close to a body upon which life is developing, unless that body is one of its moons. A gas giant must also not be too close to another gas giant. Either placement of the gas giant(s) could disrupt the orbit of a potential life-bearing planet, either directly or by drifting into the habitable zone.

Newtonian dynamics predict that all planetary orbits will tend to be chaotic, in which case it is unlikely that such orbits will remain predictable and nearly circular for a span of time needed for complex life to evolve. This tendency to chaos is much stronger when orbits are eccentric, especially the orbits of large planets. The need for stable orbits rules out planetary systems resembling those that have been discovered in recent years, namely systems with a large planet with a small orbit. Such planets are known as hot Jupiters. It is believed that hot Jupiters formed much further from their parent stars than they are now, and have gradually migrated inwards to their current orbits. In the process, they would have gravely disrupted the orbits of any inner planets in the habitable zone.

Planetary systems, especially their outer regions, are believed riddled with comets and asteroids which inevitably collide with planets. Such collisions, known as bolide impacts, can be highly disruptive for complex life. The asteroid collision that is believed to have created the KT boundary is a case in point. One way or another, bolide impacts must be rare (but nonexistent is not necessarily for the best either; see below) during the billions of years required for complex life to emerge. The frequency of bolide impacts on inner planets is reduced if there are lifeless planets at the right distance from the central star and with sufficient gravity either to attract comets and asteroids to themselves or to eject them from the planetary system.

Hence a planetary system capable of supporting complex life must include at least one large outer planet. Jupiter's large mass has attracted many (nearly all?) of the asteroids that would have otherwise hit Earth since the end of the period of asteroid bombardment. But planetary systems with too many Jovian planets, or with a single one that is too large, are likely to be unstable, in which case the likely fate of a rocky inner planet able to support life is to plunge into the central star or to be ejected into interstellar space.

Planetary system habitable zone

Complex life requires water in the liquid state. A planet with complex life must be at distance from its central star compatible with water being liquid. This is the core of the notion of habitable zone. 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. If the Earth's distance from the Sun differed from its actual value by more than 5% to15%, all water on earth would soon freeze or boil.

Kasting et al (1993) estimate that the habitable zone for the Sun ranges from 0.95 to 1.15 astronomical units. 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 carbon dioxide (CO₂). Even though the Earth's atmosphere contains only 350 parts per million of CO₂, that trace amount suffices to raise the average surface temperature of the Earth by about 40 C from what it would otherwise be (Ward and Brownlee 2000: 18).

A star with the correct metallicity 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 tend to form closer to their central stars, the minimum radius of the habitable zone may be greater than the orbital radius of any rocky planet. 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 have short lives, becoming red giants in as little as 1Ba. This may not allow enough time for advanced life to evolve.

These considerations rule out the massive and powerful start of type F6 to O (see stellar classification).

The habitable zone of a cool central star is narrow and lies close to the star, which reduces the likelihood that the zone includes a rocky planet. Any planet lying within the habitable zone would be vulnerable solar flares and X-rays (see Aurelia). Both conditions would tend to ionize the atmosphere and are otherwise inimical to complex life. These considerations rule out the 90% of stars that are red dwarves.

It turns out 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 5% of the stars in the Milky Way.

Size of planet

A planet that is too small cannot hold an atmosphere. One too massive would attract a gas giant-like atmosphere, and is more vulnerable to bolides. If a planet's gravitational strength substantially exceeds the Earth's, it cannot form mountains and continents; such a planet would probably be covered with an ocean.

Large moon

The Moon is atypically large and close. A large moon acts like a gyroscope to stabilize the tilt of the earth's axis, which is otherwise chaotic. Moreover, this tilt cannot be too extreme (relative to the orbital plane), or else the seasonal climate patterns become too extreme. If the tilt were too large or too small, the result could also be disastrous; Earth's tilt is "just right".

A large moon increases the likelihood of plate tectonics. So, once a planet forms within the habitable zone, it may still need to form as a double planet. For example, in the Earth's case, a Mars-sized body might be required to impact it (as postulated by the Giant impact theory). Without this impact, plate tectonics might not be able to develop because the continental crust would cover the entire planet, leaving no room for oceanic crust; it is currently not known whether the organization of the large scale mantle convection needed to drive plate tectonics could develop even in the absence of crustal inhomogeneity.

Magnetic field

A magnetosphere protects the biosphere from solar wind and cosmic rays, which are harmful to life. The magnetosphere results from a massive conductive core acting as a dynamo. The Earth owes its magnetic field to a core of iron and radioactive elements. The decay of the latter keeps the iron in a molten state. If complex life can exist only on the surface of a planet surrounded by a magnetosphere, then complex life requires a planet whose interior contains radioactive elements. Moreover, these must have half lives long enough (e.g., uranium 238, thorium 232) to allow sufficient time for complex life to evolve. Such elements are rather rare in the universe. As the universe grows older, the frequency of the sort of supernova that produces radioactive elements with long half lives is believed to decline. Hence these elements are fated to grow ever rarer as the universe grows older. Hence the possibility that there is an upper bound to the age of a universe capable of supporting complex life.

The unusually massive iron core that generates the Earth's magnetosphere may have resulted from the merger of the proto-Earth's smaller core with the core of an impacting body. A collision event of this nature, whether or not required for a powerful magnetosphere, is not as improbable as may seem. Recent work by Edward Belbruno and J. Richard Gott has suggested that a suitable impact body could form in a planet's trojan points (L₄ or L₅).

Plate tectonics

This is the most original part of Ward and Brownlee's analysis (however this section owes much to Webb 2002: 180-84). They argue that in order for a rocky planet to support animal life, its crust must experience plate tectonics. That is, the lithosphere must consist of large crustal plates that along certain margins, are continuously created from liquid matter carried from the deep interior in convection cells. Along other margins, called subduction zones, these crustal plates are reabsorbed into the planet's interior.

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 occuring deep in the planet's interior. Continents must also be made up of lighter rocks that "float" on underlying denser rock (in the case of the Earth, basalt).

The reasons why plate tectonics help animal life include the following. Plate tectonics:

• Enable the magnetosphere;
• Create and alter dry land via continental drift;
• Regulate the temperature of the atmosphere.

By drawing heat from the interior to the surface, plate tectonics assures that if a planet has a core of molten iron, that core keeps moving. That motion means that the core of the earth acts like a dynamo, generating a magnetic field.

It is difficult to imagine how an acquatic species would smelt and shape metal ores or manipulate electricity (sea water is a fair conductor thanks to its dissolved minerals). Hence it is likely that intelligent life with technology can only evolve on dry land; plate tectonics assures that a planet with ample water also has dry land. More generally, a planet with mountains, islands, and continents gives rise to more microclimates and evolutionary niches, which present evolution with more challenges. Hence plate tectonics promote biodiversity.

If the atmosphere contains too few greenhouse gases, the planet slides into a permanent ice age. Too much greenhouse gas, and the temperature becomes first too high for complex life (many proteins denature at temperatures well short of the boiling point of water), and eventually the oceans turn to water vapor. The primary greenhouse gas in the Earth's atmosphere is carbon dioxide, CO₂. It appears that plate tectonics play an important role in a complex feedback system (for details, see Ward and Brownlee) that stabilizes the Earth's temperature. Atmospheric CO₂ combines with rainwater to form dilute carbonic acid. This acid interacts with surface rocks to form calcium carbonate, CaCO₃, which is eventually deposited on the ocean bottom and carried into the Earth's interior at subduction zones via continental drift. Thus CO₂ is removed from the atmosphere. The high temperatures and pressures within the Earth's mantle transform CaCO₃ into CO₂ and CaO. This subterranean CO₂ is eventually returned to the atmosphere via vulcanism.

Feedback occurs because a rise in atmospheric CO₂ results in higher temperatures via the greenhouse effect, and more rainfall, and more acid rainwater. Hence the rate at which CO₂ is removed from the atmosphere rises. When atmospheric CO₂ falls, the rate at which it is removed from the atmosphere declines. Plate tectonics exposes and buries rocks in a way that automatically regulates the CO₂ content of the atmosphere. The result has been an Earth with a more or less steady surface temperature, even though the sun's energy output is believed to be about 25% greater now than it was when the Earth was young. Absent this recycling of atmospheric carbon, the expected lifetime of the biosphere is not expected to exceed a few million years.

Ice ages, by covering much of a planet's rocks and by reducing rainfall, interfere with this self-regulating process. Fortunately, ice interferes neither with plate tectonics nor with the vulcanism to which they give rise. Greenhouse gases emitted by volcanoes probably ended the two known episodes of global ice ages on Earth.

While plate tectonics appear to have helped complex life to evolve on Earth, how essential plate tectonics are for complex life in general, and the rarity of planets with plate tectonics, are both not well understood at present. The only object in the solar system other than the Earth believed to experience plate tectonics is Europa, one of the Galilean moons of Jupiter.

Chemistry of the atmosphere

There must be enough atmospheric carbon dioxide and crustal carbon (in the form of carbonate compounds) to enable carbon-based biochemistry to emerge, but not so much carbon as to give rise to a runaway greenhouse effect. Atmospheric oxygen is necessary to support the metabolism of animals and hence intelligent life. Hence something like photosynthesis has to evolve to shift the atmosphere from a reducing one to an oxidizing one. But too much oxygen means that plants spontaneously ignite, making plant life impossible.

Central stars invariably emit ultraviolet (UV) radiation. UV radiation whose wavelength falls in the range of 260-90 nm is efficiently absorbed by nucleic acids and proteins, and hence is lethal for all forms of terrestrial life. Fortunately, ozone efficiently absorbs UV radiation in the range 200-300 nm, and atmospheric oxygen is the building block for ozone. Hence a planet with complex life living on dry land must have an ozone layer in its upper atmosphere. Oxygen first appears in the atmosphere when UV radiation in the range 100-200 nm breaks water down into its atomic components. Once there is enough of an ozone layer to permit photosynthetic microbes to evolve on a planet's surface, the oxygen content of the atmosphere gradually rises through photosynthesis. The oxygen content of the Earth's atmosphere is believed to have reached its present (or even higher) level during the Cambrian era, and hence may have been a necessary condition for the Cambrian explosion.

Even if conditions on a planet's surface allow water in the liquid phase, we cannot conclude that there will in fact be any water present. The inner planets in our solar system were formed with little water. The water in the Earth's oceans is believed to be the result of a comet bombardment that ended about 3.9Ba ago. The oceans play a crucial role in moderating the seasonal swings in the Earth's temperature. The high specific heat of water enables oceans to warm slowly during the summer and then to give up their summer heat over the following winter. Too much water, on the other hand, leads to a planet with little or no land, and hence no weathering mechanism for regulating the carbon dioxide content of the atmosphere.

The mysterious Cambrian explosion

3.8 Ba ago, the late heavy bombardment ended, marking the end of the Hadean eon. Only then could life begin to evolve. Over the next 3.2Ba, there is no evidence of life more complex than the protists, other than a few worm tracks; if there were proto-nematodes or other small soft bodied organisms, they left no fossils. The earliest unambiguous fossil evidence of multicellular life is the Ediacaran fauna, about 580Ma years old. 542Ma ago came the Cambrian explosion, in which representatives of all currently extant (and some now extinct) animal phyla suddenly appeared. Just how or why the Cambrian explosion came about is still not understood. There is evidence of an associated surge in the oxygen content of the atmosphere, making the Earth better disposed to large animal life. 850M to 635Ma ago, the Earth was also in the grip of several successive ice ages that covered all (or nearly all) of its surface. It is an open question what role, if any, these ice ages played in triggering the emergence of complex life. It remains possible that the Cambrian explosion resulted from exceedingly rare circumstances.

Impact frequency and evolution

Life has to be given a chance to evolve. Frequent impacts form large bolides, while not incompatible with the emergence and survival of microbes, make it unlikely that complex life will emerge. Rare bolide impacts put complex life at risk of extinction. The theory of Punctuated equilibrium argues that:

1. Once a planet has an ecosystem whose niches are all filled, the rate of evolutionary change drops considerably.
2. On Earth, the time required for evolution to fill all niches (to reach equilibrium) has been relatively short compared to geological time.

The fossil record is thought to show that a stable ecology has been reached on Earth several times, first just after the Cambrian Explosion. A small number of mass-extinction events may be required to give evolution the chance to explore radical new approaches to the challenges of the environment rather than becoming stuck in a suboptimal local maximum (Suboptimal means "the likelihood that human-like intelligence will eventually emerge is not at a maximum."). The K-T extinction, for example, removed dinosaurs from the ecology and allowed other types of animals (such as mammals) to fill their niches in new ways.

Evolution must succeed

Even if all of these above conditions are met, complex life does not necessarily evolve. The evolution of life on Earth included some surprising leaps; two worth mentioning are that from prokaryotes to eukaryotes characterized by organelles, and the appearance of multicellular life with specialized tissues and organs, especially complex life with calcified shells and skeletons leaving an ample fossil record. The better part of two Ba elapsed between the first and the second leaps, a good deal longer than the time it took for primitive complex life (sponges and Ediacaran fauna).

Any of several types of disasters can completely extinguish all life on a planet. These include a nearby supernova (complex life is not likely to be possible on a planet orbiting the large hot or binary stars that are believed to give rise to supernovas), a massive asteroid impact (such as the one that probably caused the extinction of dinosaurs and 70% of all other life-forms present at the KT boundary), global ice ages, other drastic changes of climate or ocean chemistry; and so on.

Microbes may be common

Complex life does not include microbes. The Rare Earth hypothesis does not preclude microbial life being far more common than complex life. This part of the hypothesis builds on the discovery, since 1980 or so, of extremophilic bacteria, bacteria thriving in locations characterized by unusual heat, cold, darkness, pressure, salinity, or acidity. Examples of such locations include rocks several kilometers under the surface of the Earth, and geothermal vents on the ocean bottom. These bacteria, now assigned to a new domain, Archaea, need no sunlight, and require an anoxic environment and an ambient temperature exceeding 80 C. They thrive in temperatures exceeding 100 C. Such conditions could well have been common in the oceans of the young Earth. Archaea have also been found in deep Antarctic ice cores.

Evidence of bacteria has been found in rocks about 3.5Ba old; hence bacteria did not take very long to evolve, once the surface of the earth cooled enough to allow life. The findings in his section suggest that microbial life can emerge fairly quickly in a much broader range of environments than those compatible with complex life. Hence the universe could well teem with simple microbes. Under the Rare Earth hypothesis, only eukaryotic, complex, animal, and intelligent life are rare, in that order.

Concurring voices

In their extensive study of the anthropic principle, Barrow and Tipler (1986: 3.2, 8.7, 9) reach conclusions similar to the rare Earth hypothesis. So does Webb (2002), whose launching point is the Fermi paradox. The paleontologist Simon Conway Morris (2003), cites War and Brownlee (2000) with approval, and agrees that the Earth could well be the only planet in the Milky Way harboring complex life. Given a planet as friendly to life as the Earth is, however, Conway Morris sees the emergence of intelligent life as more probable than Ward and Brownlee do.

Criticism

The most controversial part of the rare Earth hypothesis is its assumption that the evolution of complex life can only take place on the surface of an Earth-like planet. Such planets may indeed be very rare, but complex life could possibly emerge in other environments. Some biologists, such as Jack Cohen, believe that this assumption is too restrictive and unimaginative and amounts to circular reasoning (see Alternative biochemistry). For a detailed critique of the rare Earth hypothesis, see Cohen and Ian Stewart's 2002 book Evolving the Alien: The Science of Extraterrestrial Life.

Other issues with the Rare Earth theory have also fallen under attack:

• Much of its evidence is contested. For example, while the giant impact theory of the Moon's origin has fair support, it is far from universally accepted.
• It relies on the improbability of its evidence, when much of it merely seems improbable. Taking into account the size of the universe, the extremely long time spans of astronomical time, and alternate ways for similar circumstances to arise, there may be a much larger number of Earth-like planets than this evidence suggests.
• It may understate the ability of intelligent life to adapt its environment. One intelligent space-faring race might be able to colonize many otherwise uninhabitable planets for very long periods of time (though they would need a habitable planet from which to arise).

See also

• The mediocrity principle and cosmic pluralism are the antithesis of rare Earth hypothesis.
• Planetary habitability
• Fine-tuned universe

References

• John D. Barrow and Frank J. Tipler, 1986. The Anthropic Cosmological Principle. Oxford Univ. Press.
• Simon Conway Morris, 2003. Life's Solution. Cambridge Univ. Press.
• Cohen, Jack, and Ian Stewart, 2002. Evolving the Alien: The Science of Extraterrestrial Life. Ebury Press. ISBN 0091879272.
• James Kasting, Whitmire, D. P., and Reynolds, R. T., 1993, "Habitable zones around main sequence stars," Icarus 101: 108-28.
• Ward, Peter D., and Brownlee, Donald, 2000. Rare Earth: Why Complex Life is Uncommon in the Universe. Copernicus Books (Springer Verlag). ISBN 0387987010.
• 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).

External links

• Home page of Rare Earth.
• Period of Earth's Galactic Orbit
• Astrobiology Article
• Reviews of Rare Earth:
  • Athena Andreadis, PhD in molecular biology
  • Kendrick Frazier, Skeptical Inquirer editor
  • Tal Cohen, PhD student in computer science