Rare Earth hypothesis

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The Rare Earth hypothesis is a hypothesis in planetary astronomy and astrobiology which argues that the emergence of complex multicellular life (metazoa) on Earth required an extremely unlikely combination of astrophysical and geological events and circumstances.

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, contains a good introduction to the hypothesis

The rare earth hypothesis is the contrary of the principle of mediocrity (also called the Copernican principle), whose best known recent advocates include Carl Sagan and Frank Drake. The principle of mediocrity maintains that the universe is probably teeming with advanced life: the Earth is a typical rocky planet in a typical planetary system, located in an unexceptional region of a large but conventional barred-spiral galaxy. 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 probably extremely rare. If so, the Earth could be the only place in the Milky Way, and perhaps even in the entire universe, featuring complex life, though this is debated. Richgard Dawkins disagrees.Richard Dawkins, The God Delusion

(….)The anthropic alternative to the design hypothesis is statistical. Scientists invoke the magic of large numbers. It has been estimated that there are between 100 billion and 30 billion planets in out galaxy, and about 100 billion galaxies in the universe. Knocking a few noughts off for reasons of ordinary prudence, a billion billion is a conservative estimate of the number of available planets in the universe. Now suppose the origin of life, the spontaneous arising of something equivalent to DNA, really was quite a staggeringly improbable event. Suppose it was so improbable as to occur on only one in a billion planets. (….) But here we are talking about odds of one in a billion. And yet … even with such absurdly long odds, life will still have arisen on a billion planets – of which Earth of course, is one.

The conclusion is so surprising I’ll say it again. If the odds of life originating spontaneously on a planet were a billion to one against, nevertheless that stupefyingly improbable event would still happen on a billion planets. The chance of finding any one of those billion life-bearing planets recalls the proverbial needle in a haystack. But we don’t have to go out of our way to find a needle because (back a to the anthropic princlple) any beings capable of looking must necessarily be sitting on one of these prodigiously rare needles before they even start the search. |Richard Dawkins| The God Delusion

Contents 1 Many-Worlds quantum mechanics 2 Complex life is considered very rare 2.1 The galactic habitable zone 2.2 A central star of the right character 2.3 Planetary system 2.4 Size of planet 2.5 Large moon 2.6 Magnetic field 2.7 Plate tectonics 2.8 Chemistry of the atmosphere 2.8.1 Global glaciation 2.8.2 Bolide impacts 2.8.3 Inertial interchange event 2.8.4 Difficulties imagining life outside one’s own system 3 Microbes may be common 4 Rare Earth equation 5 Advocates 6 See also 7 References 8 External links

Many-Worlds quantum mechanics

Many-worlds interpretation cannot easily explain the apparently Fine-tuned universe. In most versions of many worlds the physical constants of all the “many worlds” are the same. Many worlds interpretation can, however explain the apparent improbability of a planet like Earth existing. See Rare Earth hypothesis. If the Many worlds interpretation is true there are so many copies of our universe that the existence of at least one planet like Earth is not surprising.

According to many-worlds literally every single quantum event which happens anywhere in the universe leads to a new universe splitting off. Similarly any quantum event which happens in any new universe leads to leads to a yet another new universe splitting off. The number of new universes is uncountable and multiplies exponentially.

The Big bang is believed to have started 15 billion years ago. The Solar System formed about 5 billion years ago. During the 10 billion years between the formation of the Universe and the formation of the Solar System uncountably many copies of the universe developed.

The development of at least one Solar System among all these copies of the universe is unsurprising. Hugh Ross’s analysis of the probability of Earth existing is controversial but even if Hugh Ross is right the existence of the Solar System and Earth is unsurprising.

Similarly during the 5 billion years ago since the Solar System and the Earth developed it is unsurprising that at least one copy of the Earth developed to form intelligent life.

If complex life can evolve only on an Earth-like planet, then the Rare Earth hypothesis solves the Fermi paradox (Webb 2002): "If extraterrestrial aliens are common, why aren't they obvious?"

Complex life is considered very rare

The Rare Earth hypothesis argues that the emergence of complex life required 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.

A magnetosphere is considered important, as without it "solar" radiation from the planet's star would impact the surface and harm life's chemistry. It is believed that life on Earth originated in caves,[1][ round Hydrothermal vents on the ocean floor[2] or in similar areas away from the Earth's surface. Early life allegedly derived energy from chemical reactions within caves or from Geothermal sources. Photosynthesis evolved much later. At the time of the origin of life the sun gave out less heat than now. Life may have originated under a frozen ocean. In that case solar radiation , the composition of the atmosphere etc would have had less significance. [3] If life on another planet evolved away from harmful stellar radiation life would have thousands of millions of years to evolve protection against this radiation. Life may evolve a robust chemistry or it may evolve ways of shielding sensitive chemicals like DNA from radiation. As protection gradually evolved life could gradually exploit potential ecosystems increasingly close to the planetary surface. Later the planetary surface could be exploited. The aparent need for plate tectonics is given below. There is not obvious reason why sentient, intelligent life could not evolve in an ocean though weather systems would be very much more vigorous on a planet where the surface is composed entirely of ocean.

The advantages of a large satellite need to be explained. Life did not evolve in tidal regions. In any case solar tides also exist. Other factors could kick start evolution on other planets. Glaciations are not the only possible "evolutionary pumps"

In order for a small rocky planet to support complex life, the values of hundreds of variables must fall within narrow ranges as asserted by Hugh Ross. The universe is 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 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. The metal content of stars declines, and 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 and quasars, becomes less intense. Radiation of this nature is considered dangerous to complex life. Hence the Rare Earth hypothesis deems unfit for life in the early universe, and regions where the stellar density is high and supernovae not rare. (If life originated in sheltered places like caves life could possibly evolve protection from radiation. Evolving protection against supernovae would be more problematic as these are sudden events and life cannot adapt gradually to them.)
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, 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 (egg-shaped), 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. A planetary system may form in the metal rich part of a galaxy but have an elliptic orbit and regularly move out of this region to the metal poor regions this would not prevent life. 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 (2004) calculate that the galactic habitable zone is an annular ring 7 to 9 kiloparsecs in diameter, that includes no more than 10% of the stars in the Milky Way. Based on conservative estimates of the total number of stars in the galaxy, this could represent something like 20 to 40 billion stars. Gonzalez (2001) 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, one closely matching the rotational period of the galaxy. However, Masters (2002) calculates that the orbit of the Sun takes it through a spiral arm approximately every 100 million years. In contrast, the Rare Earth hypothesis predicts that the Sun, since its formation, should have passed through no spiral arm at all.

A central star of the right character

It is generally accepted by exobiologists that the central star for a life-bearing planet must be of appropriate size. Large stars emit much ultraviolet radiation, which precludes life other than underground microbes. Large stars also exist for millions, not billions, of years, after which they explode as supernovae. A supernova remnant becomes a neutron star or black hole, giving off high energy x-ray and Gamma rays. Hence the planets orbiting the large hot or binary stars believed to give rise to supernovae do not live long enough to allow their planets to evolve complex life.

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 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 (though sub-surface water, as suggested for Europa, may be possible at varying locations). 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 40ºC from what it would otherwise be (Ward and Brownlee 2000: 18).

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 tend to form closer to their central stars, the planet likely 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).

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 lock rules out axial rotation; hence one side of a planet will 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 K and M category stars are estimated to make up 90% of all stars.

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 5% of the 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). Red Giant stars are changing rapidly. No one area will remain potentially habitable sufficiently long for life to evolve there.

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. The oceans froze here. The event is called, Snowball Earth. It seems this glaciation helped kick start evolution of complex life. Conversely, if the central star's energy output temporarily 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, helium, and lithium. 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.

If a star is poor in metals, any associated planetary system is likely poor in metals as well. 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.

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. 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 to be riddled with comets and asteroids which inevitably collide with planets. Such collisions, known as bolide impacts, can be highly disruptive for complex life. Hence bolide impacts must be rare (but nonexistent is not necessarily 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 bolides that would have otherwise hit Earth since the end of the late heavy bombardment about 3.8 Ga. 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 either to plunge into its central star or to be ejected into interstellar space.

Size of planet

(Lissauer 1999, as summarized by Conway Morris 2003: 92; also see Comins 1993). A planet that is too small cannot hold much of an atmosphere. Hence the surface temperature becomes more variable and the average temperature drops. Water will either freeze, boil away, or decompose under the action of UV radiation; in any event, 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.

If a planet's size is such that its gravitational field substantially exceeds the Earth's, it will attract more bolides to itself. The stronger the gravitational field, the harder it is for mountains and continents to form. In the limit, such a planet would probably be covered with an ocean, in which case the lack of exposed rocks would rule out the feedback mechanism, described below, regulating atmospheric CO₂.

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 captured asteroids (Mars).
• As a fraction of its planet, it is much larger than any other satellite in the Solar System. The ex-dwarf planet Pluto's Charon would have been an exception, but it is no longer classified as a moon.

The giant impact theory hypothesizes that the Moon results from the impact of a Mars-sized body with the very young Earth. This giant impact also gave the Earth its axis tilt and velocity of rotation (Taylor 1998). 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 though other stimuli to evolution may well exist. In this view, the Earth's tilt is "just right". A large satellite can also act as a gyroscope, stabilising the planet's tilt; without this effect the tilt will be chaotic, presumably also causing difficulties for developing life forms.

If the Earth had no Moon, the ocean tides resulting solely from the Sun's gravity would be very modest. A large satellite gives rise to substantial tides and the resulting tidal pools, which are likely to have been an important locus for the evolution of complex life. Other theories suggest that life originated in caves or volcanic vents where tides would have no effect. 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.

If a giant impact is the only known 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).^[1]

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 planetary core made of molten iron, acting as a dynamo. The iron is molten because of heat given off by the decay of radioactive elements. 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 elements must have half lives long enough (e.g., uranium 238, thorium 232, and potassium 40) to sustain the magnetosphere for a time span long enough for complex life to evolve. Such elements are relatively rare in the universe. As the universe grows older, the frequency of the sort of supernovae 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 there is possibly 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 that of an impacting body. This impacting body could have been the one that, under the giant impact theory (see above), gave rise to the Moon.

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 fluid 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 occurring deep in the planet's interior. Continents must also be made up of less dense granitic rocks that "float" on underlying more dense basaltic rock. Taylor (1998) 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.

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. At present, it is not known whether the organization of the large scale mantle convection needed to drive plate tectonics could develop in the absence of crustal inhomogeneity.

The reasons why convection-driven plate tectonics promotes the development of complex life include the following. Plate tectonics:

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

By drawing heat from the interior to the surface, convection driven 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.

If the atmosphere contains too few greenhouse gases, the planet slides into a permanent ice age. Too much greenhouse gas, and the temperature becomes 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. 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 volcanism.

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 feedback process.

It is difficult to imagine how an aquatic species would smelt and shape metal ores or manipulate electricity (sea water is a fair electrical conductor thanks to its dissolved minerals). Hence it is likely that intelligent life with technology can only evolve on dry land. Intelligent ocean based life may develop different technologies, which we have not developed. 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.

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 now is the Galilean moon Europa.

Chemistry of the atmosphere

Carbon-based biochemistry clearly requires a large supply of atmospheric carbon dioxide and crustal carbon (in the form of carbonate compounds); however large amounts of carbon would give rise to a runaway greenhouse effect. Atmospheric oxygen is necessary to support the metabolism of Earthly animals and hence intelligent life. Hence something like photosynthesis had to evolve to shift the atmosphere from a reducing one to an oxidizing one.

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 appeared in the atmosphere when UV radiation in the range 100-200 nm broke water down into its atomic components. Once there was enough of an ozone layer to permit photosynthetic microbes to evolve on the planet's surface, the oxygen content of the atmosphere gradually rose through photosynthesis, and is believed to have reached its present (or even higher) level during the Cambrian era. Hence an atmosphere sufficiently rich in oxygen 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. Much of the water in the oceans is believed to have been brought to Earth by the icy asteroid impacts during the early bombardment phase about 4.5 Ga. 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.

Global glaciation

The evolution of life on Earth includes two very important and unexpected leaps, the emergence of:

1. Unicellular eukaryotes characterized by organelles, such as chromosomes, nuclei, and mitochondria;
2. Multicellular life with specialized biological tissues and organs, especially animals with calcified shells and skeletons, capable of leaving a clear fossil record.

The earliest unambiguous fossil evidence of multicellular life is the Ediacaran biota, about 580 Ma. Hence the better part of 2 billion years elapsed between the first and the second leaps. Meanwhile, only about 400 million years were required in order for the first multicellular animals (sponges and Ediacaran biota) to evolve into dinosaurs.

Curiously, both of these evolutionary transitions came hard on the heels of extended periods of glaciation so extensive that it is suspected that the earth was covered with ice, either entirely or over all but a narrow band about the Equator. This much ice cover would have raised the Earth's albedo to such an extent that the Earth's average temperature may have fallen to about -50°C. The thick ice covering almost all oceans ruled out any interactions between the oceans and the atmosphere. The continents were either covered with ice, or consisted of bare rock devoid of life. This scenario has been named Snowball Earth.

During such periods of catastrophic glaciation, life probably retreated to a narrow band near the equator, and to places warmed by tectonic activity, such as hydrothermal vents on the ocean floor, and volcanoes. Fortunately, glaciation interferes neither with plate tectonics nor with the resulting vulcanism. A hypothesized eventual rise in vulcanism increased atmospheric levels of greenhouse gases, which led to a dramatic increase in temperature and the end of the two apparent snowball earth episodes.

The first Snowball Earth episode, the Huronian glaciation, began about 2.4 Ga, shortly after the appearance of the oldest known eukaryotic unicellular organisms. The second episode, the Cryogenian period, lasted from 850 Ma to 635 Ma, ending about 50 Ma before the emergence of the Ediacaran biota. It is an open question what role, if any, these ice ages played in triggering the emergence of complex life. In any event, when the glaciation ended, life eventually sprang back with renewed vigor and diversity. The Cambrian explosion began 542 Ma, in which representatives of all currently extant (and some now extinct) animal phyla suddenly appear in the fossil record. Just how or why the Cambrian explosion came about is still not understood, but it is likely to have resulted from one or more "evolutionary pumps."

More modest glaciations are also associated with rapid evolutionary change. The rapid evolution of hominids, which culminated in the appearance of homo sapiens about 200 ka, coincides with the oscillating Quarternary ice age that began about 1.5 Ma. Moreover, the agricultural revolution, when homo sapiens emerged as an aggressive discoverer of technology, began shortly after the last glacial retreat, around 12 ka.

Bolide impacts

The impact of a sufficiently massive asteroid or comet can act as an evolutionary pump. The evolution of complex life requires long periods of tranquility. Frequent impacts from large bolides, while not incompatible with the emergence and survival of microbes, make it unlikely that complex life will emerge and survive. Rare bolide impacts, however, while making many forms of complex life extinct, on balance appear to act as evolutionary pumps. 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 remain trapped in a suboptimal local maximum. By "suboptimal" is meant "the likelihood that human-like intelligence will eventually emerge is not at a maximum."

A case in point is the asteroid impact that created the Chicxulub Crater, believed to have triggered the Cretaceous-Tertiary extinction event, when an estimated 70% of extant metazoans species, including all dinosaurs, became extinct. By removing dinosaurs from all niches they happened to fill, the K-T extinction opened the way for mammals to become large and take their place.

Inertial interchange event

There is ample evidence that the rate of continental drift during the Cambrian explosion was unusually high. In fact, continents moved from Arctic to equatorial locations, and vice versa, in 15 million years or less. Kirschvink et al (1997) have proposed the following controversial explanation: a 90° change in the Earth's axis of rotation resulting from an imbalance in the distribution of continental masses relative to the axis. The result was huge changes in climates, ocean currents, and so on, occurring in a very short time and affecting the entire Earth. They named their explanation the "inertial interchange event." This scenario is not yet received science, but if such an event took place then it is a very unlikely occurrence, and if such an event was required for the evolution of animal life more complex than sponges and coral reefs, then we have yet another reason why complex life will be rare in the universe.

Difficulties imagining life outside one’s own system

Humans are subject to Carbon Chauvinism and also terrestrial chauvinism. The only life scientists have up to now studied is terrestrial, carbon-based life. Scientists cannot currently work out the details of how non-terrestrial life may or may not be able to function. It is possible to speculate that life may have evolved in a red-dwarf stellar system. That life may be based on reactive chemicals able to derive energy from the red-dwarf star. Sunlight from our sun may well break up such chemicals. Alien theologians on such a planet may consider the Solar System unsuitable for life and may believer that a Designer has made their red-dwarf sun especially for life. If life has evolved on a planet without the earth’s strong magnetosphere that life may be able to derive energy from cosmic rays and high energy “solar” or stellar radiation which is blocked by the earth’s magnetosphere. Life may even depend on this energy. Alien theologians on such a planet may consider the earth unsuitable for life because terrestrial life could not access this energy.

Note: the author of this section is a social scientist. This may need review by physical & biological scientists.

Microbes may be common

Multicellular life does not include most microbes. The Rare Earth hypothesis permits microbial life to be far more common than complex life. This part of the hypothesis builds on the discovery, since 1980 or so, of extremophilic microorganisms that thrive in unusually hot, cold, dark, high pressure, salty, or acid conditions. Examples of such locations include rocks several kilometers under the surface of the Earth, hydrothermal vents on the ocean floor, and deep in Antarctic ice, as revealed by drilled ice cores. Some of these organisms are prokaryotes called Archaea that can gain energy from inorganic chemical reactions and do not require sunlight. Some of these extremophile Archaea also require an ambient temperature exceeding 80°C, and thrive in temperatures exceeding 100°C. Such conditions could well have been common deep in the oceans of the young Earth.

Evidence of single-celled microorganisms has been found in rocks dated about 3.5 Ga; hence these forms of life did not take very long to evolve, once the Earth's surface had cooled sufficiently. The domain Archaea suggests 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, multicellular, animal, and intelligent life are rare, in that order.

Rare Earth equation

The well-known Drake equation estimating the number of planets in the Milky Way harboring intelligent life, was set out some years before the scientific community became fully aware of many of the factors described above and now believed important. 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 product of all factors.

The following discussion is adapted from Cramer (2000). The Rare Earth equation is Ward and Brownlee's (W&B) 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:

<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>.

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.
• <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.

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^-10 and could plausibly be as small as 10^-12. 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 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. 0.1 at most.
• <math>f_p</math> is the fraction of stars in the Milky Way with planets.
• <math>f_{pm}</math> is the fraction of planets that are rocky ("metallic") rather than gaseous.
• <math>f_i</math> is the fraction of habitable planets where microbial life arises. W&B believe this fraction is unlikely to be small.
• <math>f_c</math> is the fraction of planets where complex life evolves. For 80% of the time since microbial life first appeared on the Earth, there was only bacterial life. Hence W&B argue that this fraction may be very small. Moreover, the Cambrian Explosion, when complex life really got off the ground, may have been triggered by extraordinary climatic and geological events.
• <math>f_l</math> is the fraction of the total lifespan of a planet during which complex life is present. This fraction cannot be high because complex life takes so long to evolve. 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.
• <math>f_m</math> is the fraction of habitable planets with a large moon. If the giant impact theory of the Moon's origin is correct, this fraction is small.
• <math>f_j</math> is the fraction of planetary systems with large Jovian planets. This fraction could be large.
• <math>f_{me}</math> is the fraction of planets with a sufficiently low number of extinction events. W&B 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. Such a low number again requires a very stable planetary system, with outer planets having nearly circular orbits, no gravitational perturbations from passing stars, and no nearby supernovas, quasars, or gamma ray bursts.

The Rare Earth equation, unlike the Drake equation, does not factor the probability that complex life evolves into intelligent life that discovers technology. (Keep in mind that Ward and Brownlee are not evolutionary biologists.) Barrow and Tipler (1986: 3.2) 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:

• Being the only extant bipedal land vertebrate. Combined with an unusual eye-hand coordination, this permits dextrous manipulations of the physical environment with the hands;
• 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;
• The capability of formulating abstractions to a degree permitting the invention of mathematics, and the discovery of science and technology. Keep in mind how recently humans acquired anything like their current scientific and technological sophistication.

Advocates

Books that advocate the Rare Earth hypothesis, listed in order of increasing difficulty, include:

• Taylor (1998), 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.
• Webb (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.
• Ray Kurzweil, a computer pioneer and self-proclaimed Singularitarian, argues in The Singularity Is Near that the coming Technological 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 Singularity and expanded to harness the full computational capacity of the physical universe. Many-worlds interpretation of quantum mechanics suggests there may be uncountably many copies of the universe and of the Earth unable to communicate or to influence each other. Humans may be the first advanced technology in our (copy/copies) of the universe. See below.
• Simon Conway Morris (2003), a paleontologist, mainly argues that evolution is convergent. Morris devotes chapter 5 to the Rare Earth hypothesis, citing Ward and Brownlee (2000) with approval. Yet while Morris agrees that the Earth could well be the only planet in the Milky Way harboring complex life, he sees the evolution of complex life into intelligent life as fairly probable, contra Ernst Mayr's views as reported in section 3.2 of the following reference.
• John D. Barrow and Frank J. Tipler (1986. 3.2, 8.7, 9), cosmologists, 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, 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. This book is beginning to date, because when it was written much of what is discussed in this entry was either unknown or insufficiently canonical.

See also

References

• John D. Barrow and Frank J. Tipler, 1986. The Anthropic Cosmological Principle. Oxford Univ. Press.
• Cirkovic, Milan M., and Bradbury, Robert J., 2006, "Galactic Gradients, Postbiological Evolution, and the Apparent Failure of SETI," New Astronomy, vol. 11, pp. 628-639.
• Comins, Neil F., 1993. What if the moon didn't exist? Voyages to Earths that might have been. HarperCollins.
• Simon Conway Morris, 2003. Life's Solution. Cambridge Univ. Press. See chpt. 5; many references.
• Cohen, Jack, and Ian Stewart, 2004 (2002). What Does a Martian Look Like: The Science of Extraterrestrial Life. Ebury Press. ISBN 0-09-187927-2.
• Cramer, John G., 2000, "The 'Rare Earth' Hypothesis," Analog Science Fiction & Fact Magazine (September 2000).
• Guillermo Gonzalez, Brownlee, Donald, and Ward, Peter, 2001, "The Galactic Habitable Zone: Galactic Chemical Evolution," Icarus 152: 185-200.
• James Kasting, Whitmire, D. P., and Reynolds, R. T., 1993, "Habitable zones around main sequence stars," Icarus 101: 108-28.
• Kirschvink, Joseph L., Robert L. Ripperdan, and David A. Evans, 1997, "Evidence for a Large-Scale Reorganization of Early Cambrian Continental Masses by Inertial Interchange True Polar Wander," Science 277: 541-45.
• Knoll, Andrew H., 2003. Life on a Young Planet: The First Three Billion Years of Evolution on Earth. Princeton Univ. Press.
• Lineweaver, Charles H., Fenner, Yeshe, and Gibson, Brad K., 2004, "The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way," Science 303: 59-62.
• Lissauer, 1999, "How common are habitable planets?" Nature 402: C11-14.
• Ross, Hugh, 1993, "Some of the parameters of the galaxy-sun-earth-moon system necessary for advanced life" in The Creator and the Cosmos, 2nd ed. Colorado Springs CO: NavPress, pp.113-114.
• Taylor, Stuart Ross, 1998. Destiny or Chance: Our Solar System and Its Place in the Cosmos. Cambridge Univ. Press.
• Frank J. Tipler, 2003, "Intelligent Life in Cosmology," International Journal of Astrobiology 2: 141-48.
• Ward, Peter D., and Brownlee, Donald, 2000. Rare Earth: Why Complex Life is Uncommon in the Universe. Copernicus Books (Springer Verlag). ISBN 0-387-98701-0.
• 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).
• Origins: How Life Began
• Undersea World Points to Possible Origin of Life, Maybe Even ET
• Possible Sites for the Origin of Life

This page uses content from Wikipedia. The original article was at Rare Earth hypothesis. The list of authors can be seen in the page history. The text of this Wikinfo article is available under the GNU Free Documentation License and the Creative Commons Attribution-Share Alike 3.0 license. English | Română | edit

External links

• Home page of Rare Earth.
• Reviews of Rare Earth:
  • Athena Andreadis, PhD in molecular biology.
  • Kendrick Frazier, editor, Skeptical Inquirer.
  • Tal Cohen, PhD student in computer science.
• "Galactic Habitable Zone," Astrobiology Magazine, May 18, 2001.
• Solstation.com: "Stars and Habitable Planets."
• Recer, Paul, "Radio astronomers measure sun's orbit around Milky Way," Associated Press, June 1, 1999.
• Caltech press release: "Cambrian explosion may have been caused by Earth 'losing its balance' 500 Ma."
• On the Galactic Habitable Zone

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