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LECTURE 8: THE RARE EARTH HYPOTHESIS

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

The rare earth hypothesis is the contrary of the principle of mediocrity (also called the Copernican principle), advocated by Carl Sagan and Frank Drake, among others.^[1] The principle of mediocrity concludes that the Earth is a typical rocky planet in a typical planetary system, located in an unexceptional region of a common barred-spiral galaxy. Hence it is probable that the universe teems with complex life. Ward and Brownlee argue to the contrary: planets, planetary systems, and galactic regions that are as friendly to complex life as are the Earth, the solar system, and our region of the Milky Way are probably very rare.

By concluding that complex life is uncommon, the Rare Earth hypothesis is a possible solution to the Fermi paradox: "If extraterrestrial aliens are common, why aren't they obvious?"^[2]

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.

In order for a small rocky planet to support complex life, Ward and Brownlee argue, the values of several variables must fall within narrow ranges. The universe is so vast that it could contain 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.

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 predicts that the early universe, and likewise regions in the galaxy at present where the stellar density is high and supernovae common, will be unfit for the development of complex life.^[4]
3. Gravitational perturbation of planets and planetesimals by nearby stars becomes less likely as the density of stars decreases. Hence the further a planet lies from the galactic center, 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 (eliptical or hyperbolic), it will pass through some spiral arms, but if the orbit is a near perfect circle and the orbital velocity equals the "rotational" velocity of the spiral arms, the star will drift into a spiral arm region only gradually, if at all. Therefore Rare Earth proponents conclude that a life-bearing star must have a galactic orbit that is nearly circular about the center of its galaxy. The required synchronization of the orbital velocity of a central star with the wave velocity of the spiral arms can occur only within a fairly narrow range of distances from the galactic center. This region is termed the "galactic habitable zone". Lineweaver et al ^[5] calculate that the galactic habitable zone is an annular ring 7 to 9 kiloparsecs in diameter, that includes no more than 10% of the stars in the Milky Way.^[6] Based on conservative estimates of the total number of stars in the galaxy, this could represent something like 20 to 40 billion stars. Gonzalez et al ^[7] would halve these numbers; he estimates that at most 5% of stars in the Milky Way fall in the galactic habitable zone.

The orbit of the Sun around the center of the Milky Way is indeed almost perfectly circular, with a period of 226 Ma, one closely matching the rotational period of the galaxy. However Karen Masters calculates that the orbit of the Sun takes it through a major 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.^[8]

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 radiation. Hence the planets orbiting the large, hot or binary stars believed to give rise to supernovae do not exist long enough to allow complex life to evolve. An advanced technology capable of Interstellar travel may migrate to any star system including a hot blue star.^{[citation needed]}

The terrestrial example suggests complex life requires water in the liquid state and its planet must therefore be at an appropriate distance. This is the core of the notion of the habitable zone or Goldilocks Principle ^[9]. The habitable zone forms a ring around the central star. If a planet orbits its sun too closely or too far away, the surface temperature is incompatible with water being liquid (though sub-surface water, as suggested for Europa, Enceladus, and Ceres, may be possible at varying locations^[10]). Kasting et al (1993) estimate that the habitable zone for the Sun ranges from 0.95 to 1.15 astronomical units.^[11]

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 387 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 ^[12].

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

1. Given that rocky planets were (at the time Rare Earth was written) thought to form closer to their central stars, the planet 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).

The energy output of a star over its lifespan should only change very gradually; variable stars such as Cepheid variables, for instance, are highly unlikely to support life. If the central star's energy output suddenly decreases, even for a relatively short while, the planet's water may freeze. Conversely, if the central star's energy output 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.^[13]

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.

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 was thought to protect the inner rocky planets from asteroid bombardment. Recent computer simulations suggest that the situation may be more complex than this, however, with gas giants both protecting from and contributing to asteroid bombardment; Jupiter appears to cause about as many asteroid impacts as it prevents and replacing Jupiter with a Saturn-sized body would actually increase the bombardment.^[14]

A gas giant must not be too close to a body upon which life is developing, unless that body is one of its moons. Close placement of gas giant(s) could disrupt the orbit of a potential life-bearing planet, either directly or by drifting into the habitable zone.

Newtonian dynamics can produce chaotic planetary orbits, especially in a system having large planets at high orbital eccentricity ^[15]

The need for stable orbits rules out planetary systems resembling those that have been discovered in recent years (extra-solar systems consisting of large planets with close orbits, known as hot Jupiters). It is believed that hot Jupiters formed much further from their parent stars than they are now, and have migrated inwards to their current orbits. In the process, they would have catastrophically disrupted the orbits of any planets in the habitable zone. ^[16]

A planetary system capable of supporting complex life may need to include at least one large outer planet. 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.

A planet that is too small cannot hold much of an atmosphere. Hence the surface temperature becomes more variable and the average temperature drops. Substantial and long-lasting oceans become impossible. A small planet will also tend to have a rough surface, with large mountains and deep canyons. The core will cool faster, and plate tectonics will either not last as long as they would on a larger planet or may not occur at all.^[17]

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

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

—Michael Meyer, ^[18]

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

The 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).

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 ^[19]. Rapid rotation reduces the daily variation in temperature and makes photosynthesis viable. The Rare Earth hypothesis further argues that the axis tilt cannot be too large or too small (relative to the orbital plane). A planet with a large tilt will experience extreme seasonal variations in climate, unfriendly to complex life. A planet with little or no tilt will lack the stimulus to evolution that climate variation provides. In this view, the Earth's tilt is "just right". 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 tidal pools, which may be essential for the formation of complex life.^[20]

A large satellite also increases the likelihood of plate tectonics through the effect of tidal forces on the planet's crust. The impact that formed the Moon may also have initiated plate tectonics, without which the continental crust would cover the entire planet, leaving no room for oceanic crust. It is possible that the large scale mantle convection needed to drive plate tectonics could not have emerged in the absence of crustal inhomogeneity. However, plate tectonics existed on Mars in past, without such a mechanism to initiate it. ^[21]

If a giant impact is the only way for a rocky inner planet to acquire a large satellite, any planet in the circumstellar habitable zone will need to form as a double planet in order that there be an impacting object sufficiently massive to give rise in due course to a large satellite. An impacting object of this nature is not necessarily improbable. Recent work by Edward Belbruno and J. Richard Gott of Princeton University suggests that a suitable impacting body could form in a planet's trojan points (L₄ or L₅).^[22]

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 ^[23] 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.

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 ^[24] 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.^[25]

The following discussion is adapted from Cramer ^[26]. The Rare Earth equation is Ward and Brownlee's riposte to the Drake equation. It calculates N, the number of Earth-like planets in the Milky Way having complex life forms.

• 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.

The Rare Earth hypothesis can then be viewed as asserting that the product of the other nine Rare Earth equation factors listed below, which are all fractions, is no greater than 10^-10 and could plausibly be as small as 10^-12. In the latter case, N could be as small as 0 or 1. Ward and Brownlee do not actually calculate the value of N, because the numerical values of quite a few of the factors below can only be conjectured. They cannot be estimated simply because we have but one data point: the Earth, a rocky planet orbiting a G2 star in a quiet suburb of a large barred spiral galaxy, and the home of the only intelligent species we know, namely ourselves.

• f_g is the fraction of stars in the galactic habitable zone (Ward, Brownlee, and Gonzalez estimate this factor as 0.1 ^[7]).
• f_p is the fraction of stars in the Milky Way with planets.
• 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. Ware and Brownlee believe this fraction is unlikely to be small.
• f_c is the fraction of planets where complex life evolves. For 80% of the time since microbial life first appeared on the Earth, there was only bacterial life. Hence Ward and Brownlee argue that this fraction may be very small.
• 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. Ward and Brownlee argue that the low number of such events the Earth has experienced since the Cambrian explosion may be unusual, in which case this fraction would be small.

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 ^[28] 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 (non-avian) 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.

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

• Taylor ^[29], 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 ^[30], 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 ^[31], a paleontologist, mainly argues that evolution is convergent. Morris devotes chapter 5 to the Rare Earth hypothesis, citing Rare Earth 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.
• Ray Kurzweil, a computer pioneer and self-proclaimed Singularitarian, argues in The Singularity Is Near that the coming 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.

Criticisms of the Rare earth Hypothesis take various forms.

Central to the Rare Earth hypothesis is the following claim about evolutionary biology: while microbes of some sort could well be common in the universe, complex life is unlikely to be. Yet to date, the only evolutionary biologist to speak to the hypothesis at any length is Simon Conway Morris (2003). The hypothesis concludes, more or less, that complex life is rare because it can evolve only on the surface of an Earth-like planet or on a suitable satellite of a planet. Some biologists, such as Jack Cohen, believe this assumption too restrictive and unimaginative; they see it as a form of circular reasoning (see Alternative biochemistry, a speculative biochemistry of alien life forms). Earth-like planets may indeed be very rare, but non carbon-based complex life could possibly emerge in other environments.^[32]

According to Darling, the Rare Earth hypothesis is neither hypothesis nor prediction, but merely a description of how life arose on Earth.^[33] 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."

There are very many planets. This inevitably will include some Earth-like planets.

(….)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 300 billion planets in our 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 to the anthropic principle) 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

Astronomers, Cosmologists and other Physicists also criticize the Rare Earth hypothesis. If the suggestions below are true other places in the universe or multiverse are likely to feature complex life or inevitably feature complex life.

The Many worlds interpretation would yield at least one earth-like planet in some time-lines or worlds. In this context a time line means a world or parallel universe. See Multiverses (Ellis, Koechner and Stoeger sense) This applies even in a universe where the probability of forming even a single one were low.

The Big bang is believed to have roughly started 13.6 billion years ago. The Solar System formed about 4.6 billion years ago. During the 9 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 should be expected. Similarly, during the 4.6 billion years ago since the Solar System and the Earth developed, copies of the Earth have been increasing exponentially. It is unsurprising that at least one copy of the Earth developed to form intelligent life.

If the Rare Earth hypothesis, and the Many worlds interpretation are both true, then intelligent observers are guaranteed even if the Rare Earth Equation is close to zero. But, such Earths would almost always be alone, answering the Fermi Paradox. The Rare Earth hypothesis may not be true as explained above.

The Many worlds interpretation alone cannot so easily explain the apparently Fine-tuned universe. In most versions of Many worlds the physical constants of all the “many worlds” are the same.

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