Rare Earth hypothesis

The rare Earth hypothesis asserts that the emergence of complex multicellular life (metazoa) on Earth required an extremely unlikely combination of astrophysical and geological circumstances. Hence 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 recent advocates include Carl Sagan and Frank Drake. The principle of mediocrity maintains that the Earth is a typical planet in a typical planetary system, located in an unexceptional region of a large but conventional spiral galaxy. Ward and Brownlee argue to the contrary: 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, bearing complex 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 exist, why aren't they obvious?"

The main objection to the rare Earth hypothesis is that it is based on a single observation, namely, the existence of the Earth and its lifeforms. Current scientific instrumentation and search techniques such as Doppler shift are unable to routinely detect extrasolar planets that are of equivalent size and mass to Earth. Extrasolar planets that have been detected are predominantly of Jupiter-mass or larger, thus, their effect on their parent star's motion can be more easily detected. Planets the size and mass of Earth cannot be detected using this technique. Therefore, as of 2006, there is no data on which to base a conclusion about the rarity of Earth-like planets in the Milky Way galaxy or the prevalence of complex life. Although the hypothesis discusses a number of conditions that appear to favor development of life under "Earth-like" conditions, until the scientific community both discovers other Earth-like worlds (or formally demonstrates the lack of them), no conclusions about the rare Earth hypothesis' validity can be drawn. An example of a problem like the rare-Earth hypothesis which has recently been resolved in favor of the principle of mediocrity involves the status of the planet Pluto in the solar system. For many years, Pluto was considered rare and unusual among the solar system's planets, being of similar size to the inner terrestrial planets like Mercury, Venus, Earth and Mars but located in a higly inclined and eccentric orbit in the outer part of the solar system which is dominated by the gas giant planets Jupiter, Saturn, Uranus and Neptune. Only when more sensitive detection techniques were used, such as telescope-mounted charged coupled devices, was it discovered that Pluto and its moon Charon are in fact somewhat typical members of a large group of trans-Neptunian objects, such as Quaoar, which may number in the thousands. Continual improvements in detection instrumentation along with larger telescopes may help to address the question of the existence of other Earth-like worlds in the galaxy.

Complex life is probably very rare

The Rare Earth hypothesis argues, contra the principle of mediocrity, 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.

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 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, regions it calls "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:

The metal content of stars declines. Because metals are considered necessary to the formation of terrestrial planets, the likelihood of life would appear decrease with increasing distance from the galactic center.
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. For this reason as well, the early universe, are regions where the stellar density is high and supernovae not rare, such as galactic inner regions, globular clusters, and the spiral arms of spiral galaxies, are considered unfit for life according to the Rare Earth hypothesis.
Gravitational perturbation of planets and planetesimals by nearby stars becomes less likely as stars density decrease. Hence the further a planet lies from the galactic center, the less that it will be struck by a large bolide. A sufficiently large impact may extinguish all complex life on a planet.

Such considerations suggest that the galactic habitable zone maybe ring-shaped, with the galactic center and outer reaches remaining uninhabitable.

While a planetary system may enjoy a location favorable to complex life, it must also maintain that location for a span of time sufficient 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. The optimal orbit as suggested in Rare Earth is nearly circular orbit about the center of its galaxy, with an orbital velocity equal to the rotational velocity of the spiral arms so that the star will only gradually, if at all, drift into a spiral. The required synchronization of the orbital velocity of a central star with the rotational velocity of the galaxy containing it can occur only within a fairly narrow range of distances from the galactic center.

The orbit of the Sun around the center of the Milky Way is almost perfectly circular, with a period of 226 Ma, one closely matching the rotational period of the galaxy. The Sun's orbit is so perfect that it has remained clear of the spiral arms of the Milky Way over its entire 4.6 Ga lifetime. (However, Karen Masters of the Harvard Smithsonian Center for Astrophysics states that the orbit of the Sun takes it through a spiral arm approximately every 100 million years.)[1] Such close matches are possible only for stars located in a narrow ring about the galactic center, making up the galactic habitable zone. According to Guillermo Gonzalez, at most 5% of stars in the Milky Way lie within the galactic habitable zone [2]. Lineweaver et al (2004) calculated that the galactic habitable zone is an annular ring 7 to 9 kiloparsecs in diameter, including no more than 10% of the stars in the Milky Way. Based on conservative estimates of the number of stars in the galaxy, this could represent something like 20 to 40 billion stars.

A central star of the right character

The central star must be of appropriate size. Large stars emit much ultraviolet radiation, which likely 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 radiation. 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.

Small stars, on the other hand, have habitable zones with a small radius. This 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. Rare Earth suggests that this rules out the possibility of life in such systems, though research has suggested habitability may exist under the right circumstances.

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 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 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 a 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 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 and metals are not particularly common. 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, in which case it is unlikely that such orbits will remain predictable and nearly circular for the length 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. Hence 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 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.

Circumstellar 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:

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.
Hot stars 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).

The habitable zone of a cool central star is narrow and lies close to the star, reducing the likelihood that the zone includes a rocky planet. Any planet lying within the habitable zone would be vulnerable to 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

(Lissauer 1999, as summarized by 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 termperature 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 and have an atmosphere like that of a gas giant. 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. A world covered with an ocean is probably a world in which complex life cannot master technology.

Large moon

The Earth's moon is doubly unusual:

The other rocky planets in the Solar System either have no satellites (Mercury and Venus), or have tiny satellites that are captured asteroids (Mars).
The Moon as a fraction of its planet is much larger than any other satellite in the Solar System. It is also atypically close.

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 axis tilt cannot be too large (relative to the orbital plane); otherwise the seasonal climate patterns become too extreme. If the tilt is too small, there is too little seasonal variation in climate, and that variation may have stimulated the diversity and complexity of complex life. Earth's tilt appears "just right". A large satellite also stabilizes the axis tilt of its planet by acting as a gyroscope; otherwise the axis is chaotic.

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 serious tides and the ensuing tidal pools, which are likely to have been an important locus for the evolution of complex life. 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.

If a giant impact is the only way a rocky inner planet can 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 to a large satellite in due course. However, an impact of this nature is not as improbable as may seem. Recent work by Edward Belbruno and J. Richard Gott suggests that a suitable impact body could form in a planet's trojan points (L₄ or L₅).

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.

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 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. 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 acquatic 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; 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

The balance of this entry will often touch on the natural history of the Earth during the Precambrian era and the Cambrian explosion. For an introduction to what is presently known about the flora and fauna of that era, and about the chemistry of its atmosphere and oceans, see Knoll (2003).

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, 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.

Evolutionary "pumps"

Even if all of these above conditions are met, complex life does not necessarily evolve. There is no evidence whatsoever of life until 3.8 Ga, when the late heavy bombardment ended, marking the end of the Hadean eon. Over the next 3.2 Ga, there is no evidence, other than a few possible worm tracks, of life more complex than the protists; if there were proto-nematodes or other small soft bodied organisms, they left no fossils.

The terrestrial fossil record is thought to show that a complex ecosystem, consisting of many niches, each filled, has been attained several times, the first being just after the Cambrian Explosion. The theory of Punctuated equilibrium argues that:

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

An "evolutionary pump" is any mass extinction event that results in many empty ecological niches, thereby speeding up evolution. Such events, which can place all of a planet's complex life at risk, include a sudden change in the energy put out by the central star, a collapse of the magnetosphere, a sudden change in a planet's spin rate or axial tilt, a nearby supernova, gamma ray bursts anywhere in the galaxy (perhaps resulting from merging neutron stars), and any rapid and drastic change in climate or ocean chemistry. Rare Earth focuses on two candidate evolutionary pumps, global glaciation, and bolide impacts.

Global glaciation

The evolution of life on Earth included some surprising leaps. Two very important ones are:

The appearance of unicellular eukaryotes characterized by organelles, such as chromosomes, nuclei, and mitochondria;
The appearance of multicellular life with specialized 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 Ga elapsed between the first and the second leaps, a good deal longer than the time it took for the first multicellular animals (sponges and Ediacaran biota) to become as highly evolved as 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 all but a narrow band about the Equator. The extensive ice would have raised the Earth's albedo to such an extent that the average temperature may have fallen to about -50°C. The thick ice covering almost all oceans meant that interactions between the oceans and the atmosphere largely ceased. 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. Hence greenhouse gases emitted by volcanoes probably ended the two apparent snowball earth episodes by a dramatic increase in temperature.

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 (for an intriguing candidate explanation, see [3]), but it is likely to have resulted from one or more “evolutionary pumps”.

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 discover 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. A case in point is the asteroid impact that created the Chicxulub Crater, which probably triggered the Cretaceous-Tertiary extinction event, when an estimated 70% of extant metazoans species, including all dinosaurs, became extinct.

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 becoming stuck in a suboptimal local maximum (Suboptimal means "the likelihood that human-like intelligence will eventually emerge is not at a maximum."). For example, by removing dinosaurs from all niches they happened to occupy, 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 and 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.

Microbes may be common

Complex life does not include 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 bacteria, thriving 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, and hydrothermal vents on the floor. These bacteria, now assigned to the new domain Archaea, require an anoxic environment and an ambient temperature exceeding 80°C and thrive in temperatures exceeding 100°C. They need no sunlight. 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 dated about 3.5 Ga; 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.

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 result.

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 $N$ , the number of Earth-like planets in the Milky Way having complex life forms, as:

$N=N^{*}\times n_{e}\times f_{g}\times f_{p}\times f_{pm}\times f_{i}\times f_{c}\times f_{l}\times f_{m}\times f_{j}\times f_{me}$ .

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.
$n_{e}$ 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 $n_{e}$ = 1 is a likely upper bound.
Hence we assume $N^{*}\times n_{e}=5\times 10^{11}$ . The rare Earth hypothesis can then be viewed as asserting that the product of the other nine Rare Earth equation factors listed below, 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 spiral galaxy, and the home of the only intelligent species we know, namely ourselves.
$f_{g}$ is the fraction of stars in the galactic habitable zone. 0.1 at most.
$f_{p}$ is the fraction of stars in the Milky Way with planets. All known extrasolar planets orbit metal-rich stars, suggesting that planets may be peculiar to metal-rich stars.
$f_{pm}$ is the fraction of planets that are rocky ("metallic") rather than gaseous.
$f_{i}$ is the fraction of habitable planets where microbial life arises. W&B believe this fraction is unlikely to be small.
$f_{c}$ is the fraction of planets where complex life evolve. 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 climactic and geological events.
$f_{l}$ 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.
$f_{m}$ 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.
$f_{j}$ is the fraction of planetary systems with large Jovian planets. This fraction could be large.
$f_{me}$ 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 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.

The Rare Earth hypothesis is perhaps little more than the contention that a properly specified Drake equation predicts that we may be the only intelligent species in the Milky Way with a fair grasp of technology. We may even be alone in the universe.

Concurring voices

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.
Simon Conway Morris (2003), a paleontologist, mainly argue 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.

Dissenting Voices

A humorous summary of objections to the Rare Earth hypothesis is perhaps the rhetorical witticism said of extraterrestrial intelligent life: "Is absence of evidence evidence of absence?"

The main objection to the Rare Earth hypothesis is that it is based on a single observation, namely the Earth and its lifeforms. Current scientific instrumentation and search techniques, such as Doppler shift ,are unable to routinely detect extrasolar planets whose size and mass are similar to that of the Earth. Extrasolar planets detected to date are predominantly of Jupiter-mass or larger, thus, rendering detectable their effect on the motion of their parent star. Planets the size and mass of Earth cannot be detected using this technique. Therefore, as of 2006, there is no data on which to base a conclusion about the rarity of Earth-like planets in the Milky Way, or the prevalence of complex life. Although the hypothesis discusses a number of conditions that appear to favor the emergence and development of life under "Earth-like" conditions, until the scientific community either discovers other Earth-like worlds, or formally demonstrates the absence thereof, no conclusions can be reached about the validity of the hypothesis.

An example of a situation similar to the rare Earth hypothesis, except that it has recently been resolved in favor of the principle of mediocrity, is the status of the planet Pluto. For many years, Pluto was considered rare and unusual among the solar system's planets, being of similar size to the inner terrestrial planets like Mercury, Venus, Earth and Mars but located in a higly inclined and eccentric orbit in the outer part of the solar system which is dominated by gas giants. Only when more sensitive detection techniques were used, such as telescope-mounted charged coupled devices, was it discovered that Pluto and its moon Charon are in fact somewhat typical members of a large group of trans-Neptunian objects, such as Quaoar, which may number in the thousands. Continual improvements in detection instrumentation, along with larger telescopes, may help address the rarity of Earth-like worlds in the Milky Way.

Central to the Rare Earth hypothesis is the claim that while microbes of some sort could well be common in the universe, complex life is unlikely to be. At bottom, this conclusion pertains to evolutionary biology. 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 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). Earth-like planets may indeed be very rare, but complex life could possibly emerge in other environments. For a detailed critique of the Rare Earth hypothesis, see Cohen and Ian Stewart (2002).

Darling (2001) says that the Rare Earth hypothesis is neither hypothesis nor prediction, but merely a description of how life arose on Earth. Moreover, 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."

Other aspects of the Rare Earth hypothesis have come under attack. It:

Is partly based on contested evidence. For example, while the giant impact theory of the Moon's origin has fair support, it is not universally accepted. Another example is the argument based on star metallicity. Tau Ceti has a metallicity estimated to lie between 22% and 70% of the Sun's, yet has recently been found to have more than ten times the cometary and asteroidal material that the Sun has, which suggests that it is very likely that Tau Ceti has terrestrial planets.
Assumes the improbability of many situations about which we have almost no hard evidence. Taking into account the size of the universe, the extremely long span of astronomical time, and the many possible ways life-friendly circumstances could arise, there may be more Earth-like planets than the Rare Earth hypothesis allows.
Ignores the possibility that intelligent life could adapt its environment. An intelligent space-faring species could, over a very long span of time, gradually transform and colonize many initially uninhabitable planets. Admittedly, such a species would need, at the outset, a habitable planet on which to evolve.
Ignores the possibility that an intelligent space-faring species could, over a sufficiently long period of time, disseminate highly adaptable lifeforms into all environments capable of supporting them. See Directed panspermia.

References

John D. Barrow and Frank J. Tipler, 1986. The Anthropic Cosmological Principle. Oxford Univ. Press.
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 0091879272.
Cramer, John G., 2000, "The 'Rare Earth' Hypothesis," Analog Science Fiction & Fact Magazine (September 2000).
Darling, David, 2001. Life Everywhere: The Maverick Science of Astrobiology. Basic Books/Perseus.
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.
Taylor, Stuart Ross, 1998. Destiny or Chance: Our Solar System and Its Place in the Cosmos. Cambridge Univ. Press.
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.
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."