# Rare Earth hypothesis

Are planets that support complex life, such as Earth, rare?

In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life (metazoa) on Earth (and, as follows, intelligence) required an improbable combination of astrophysical and geological events and circumstances. The hypothesis argues that complex extraterrestrial life requires an Earth-like planet with similar circumstance and that few if any such planets exist. The term "Rare Earth" originates from Rare Earth: Why Complex Life Is Uncommon in the Universe (2000), a book by Peter Ward, a geologist and paleontologist, and Donald E. Brownlee, an astronomer and astrobiologist.

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

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

## Rare Earth's requirements for complex life

The Rare Earth hypothesis argues that the emergence of complex life requires a host of fortuitous circumstances. A number of such circumstances are set out below under the following headings: galactic habitable zone, a central star and planetary system having the requisite character, the circumstellar habitable zone, a right sized terrestrial planet, the advantage of a gas giant guardian and large satellite, conditions needed to assure the planet has a magnetosphere and plate tectonics, the chemistry of the lithosphere, atmosphere, and oceans, the role of "evolutionary pumps" such as massive glaciation and rare bolide impacts, and whatever led to the still mysterious Cambrian explosion of animal phyla. The emergence of intelligent life may have required yet other rare events.

In order for a small rocky planet to support complex life, Ward and Brownlee argue, the values of several variables must fall within narrow ranges. The universe is so vast that it could contain many Earth-like planets. But if such planets exist, they are likely to be separated from each other by many thousands of light years. Such distances may preclude communication among any intelligent species evolving on such planets, which would solve the Fermi paradox.

### The right location in the right kind of galaxy

The dense centre of galaxies such as NGC 7331 (often referred to as a "twin" of the Milky Way[3])) have high levels of radiation which are dangerous to complex life

Rare Earth suggests that much of the known universe, including large parts of our galaxy, cannot support complex life; Ward and Brownlee refer to such regions as "dead zones." Those parts of a galaxy where complex life is possible make up the galactic habitable zone. This zone is primarily a function of distance from the galactic center. As that distance increases:

1. Star metallicity declines. Metals (which in astronomy means all elements other than hydrogen and helium) are necessary to the formation of terrestrial planets.
2. The X-ray and gamma ray radiation from the black hole at the galactic center, and from nearby neutron stars, becomes less intense. Radiation of this nature is considered dangerous to complex life, hence the Rare Earth hypothesis predicts that the early universe, and galactic regions where stellar density is high and supernovae are common, will be unfit for the development of complex life.[4]
3. Gravitational perturbation of planets and planetesimals by nearby stars becomes less likely as the density of stars decreases. Hence the further a planet lies from the galactic center or a spiral arm, the less likely it is to be struck by a large bolide. A sufficiently large impact may extinguish all complex life on a planet.

(1) rules out the outer reaches of a galaxy; (2) and (3) rule out galactic inner regions, globular clusters,[citation needed] and the spiral arms of spiral galaxies.[citation needed] These arms are not physical objects, but regions of a galaxy characterized by a higher rate of star formation, moving very slowly through the galaxy in a wave-like manner. As one moves from the center of a galaxy to its furthest extremity, the ability to support life rises then falls. Hence the galactic habitable zone may be ring-shaped, sandwiched between its uninhabitable center and outer reaches.

While a planetary system may enjoy a location favorable to complex life, it must also maintain that location for a span of time sufficiently long for complex life to evolve. Hence a central star with a galactic orbit that steers clear of galactic regions where radiation levels are high, such as the galactic center and the spiral arms, would appear most favourable. If the central star's galactic orbit is eccentric (elliptic or hyperbolic), it will pass through some spiral arms, but if the orbit is a near perfect circle and the orbital velocity equals the "rotational" velocity of the spiral arms, the star will drift into a spiral arm region only gradually—if at all. Therefore Rare Earth proponents conclude that a life-bearing star must have a galactic orbit that is nearly circular about the center of its galaxy. The required synchronization of the orbital velocity of a central star with the wave velocity of the spiral arms can occur only within a fairly narrow range of distances from the galactic center. This region is termed the "galactic habitable zone". Lineweaver et al.[5] calculate that the galactic habitable zone is a ring 7 to 9 kiloparsecs in diameter, that includes no more than 10% of the stars in the Milky Way.[6] Based on conservative estimates of the total number of stars in the galaxy, this could represent something like 20 to 40 billion stars. Gonzalez, et al.[7] would halve these numbers; he estimates that at most 5% of stars in the Milky Way fall in the galactic habitable zone.

The orbit of the Sun around the center of the Milky Way is indeed almost perfectly circular, with a period of 226 Ma (1 Ma = 1 million years), one closely matching the rotational period of the galaxy. While the Rare Earth hypothesis predicts that the Sun should rarely, if ever, have passed through a spiral arm since its formation, astronomer Karen Masters has calculated that the orbit of the Sun takes it through a major spiral arm approximately every 100 million years.[8] Some researchers have suggested that several mass extinctions do correspond with previous crossings of the spiral arms.[9]

Andromeda and the Milky Way have a similar mass, but whereas Andromeda is a typical spiral galaxy the Milky Way is unusually quiet and dim. It appears to have suffered fewer collisions with other galaxies over the last 10 billion years, and its peaceful history may have made it more hospitable to complex life than galaxies which have suffered more collisions, and consequently more supernovae and other disturbances.[10] The level of activity of the black hole at the centre of the Milky Way may also be important: too much or too little and the conditions for life may be rare. The Milky Way black hole appears to be just right.[11]

### Orbiting at the right distance from the right type of star

The terrestrial example suggests that complex life requires water in the liquid state, and a central star's planet must therefore be at an appropriate distance. This is the core of the notion of the habitable zone or Goldilocks Principle.[12] 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.

The habitable zone varies with the type and age of the central star. The habitable zone for a main sequence star very gradually moves out over time until the star becomes a white dwarf, at which time the habitable zone vanishes. The habitable zone is closely connected to the greenhouse warming afforded by atmospheric water vapor (H2O), carbon dioxide (CO2), and/or other greenhouse gases. Even though the Earth's atmosphere contains a water vapor concentration from 0% (in arid regions) to 4% (in rain forest and ocean regions) and only 387 parts per million of CO2, these small amounts suffice to raise the average surface temperature of the Earth by about 40 °C from what it would otherwise be,[13] with the dominant contribution being due to water vapor, which together with clouds makes up between 66% and 85% of Earth's greenhouse effect, with CO2 contributing between 9% and 26% of the effect.[14]

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

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

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

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

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

According to Rare Earth, globular clusters are unlikely to support life.

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

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

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

### Enough time elapsed since the big bang for evolution to occur

The Geneticists Sharov and Gordon have analysed the complexity of living organisms using the evolutionary equivalent of Moores's law. The method was used to extrapolate backwards the complexity of nucleotides as RNA precursors to explain development of RNA molecules that are necessary to explain complexity observed today. This method expands the definition of "life" to include said precursors as well as the RNA molecules themselves. If their calculations are correct and "life" considered under their definitions, then it has taken about 10 billion (9.7 ±2.5 billion years) years rather than 5 billion years for life to evolve to its present form.

Their conclusions, if correct, imply that it is statistically probable that the Earth was seeded by simple bacterial life that had taken about five billion years to evolve rather than developing it on its own. Survival of bacteria in extremely harsh environments, including extremely radiation-tolerant species such as Thermococcus gammatolerans, makes such survival and panspermia plausible.

The original Drake equation for guesstimating the number of civilizations in our galaxy may be wrong, as we conclude that intelligent life like us has just begun appearing in our universe. The Drake equation is a steady state model, and we may be at the beginning of a pulse of civilization. Emergence of civilizations is a non-ergodic process, and some parameters of the equation are therefore time-dependent. Because the cosmic transport of life is most likely limited to prokaryotes, young planets have not had enough time to develop intelligent life. Another time-dependent process is the probability of interstellar transfer of bacteria, which we expect to have become more frequent as the total pool of bacteria in the galaxy increased with time. There are many modifications of the Drake equation, but if civilizations have just begun to appear, any version is of limited use. The answer to the Fermi paradox may be that we are amongst the first, if not the only so far, civilization to emerge in our galaxy. The “Rare Earth” hypothesis need not be invoked. The linking of civilization to the lifetime of a particular star, such as our Sun , is also not necessary. [17][18]

The data imply that the Earth is among the first planets to develop intelligent life, on a cosmological timescale. Under these assumptions it is unlikely that there are civilizations more than ten million years more advanced than humanity as this implies that said civilization was originally seeded in an earlier time when interstellar seeding of life was less common.

### With the right arrangement of planets

According to Rare Earth, without the presence of the massive gas giant Jupiter (fifth planet from the Sun and the largest) complex life on Earth would not have arisen.

Rare Earth proponents argue that 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 outer gas giants.[citation needed]

In addition, Rare Earth proponents have argued that the arrangement of the Solar System is not only rare, but optimal as the large mass and gravitational attraction of the gas giants provide protection for the inner rocky planets from Small Solar System body impacts and asteroid bombardment.

### A continuously stable orbit

Rare Earth argues that 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.[19]

The need for stable orbits rules out stars with systems of planets that contain large planets with orbits close to the host star (called "hot Jupiters"). It is believed that hot Jupiters formed much further from their parent stars than they are now, and have migrated inwards to their current orbits. In the process, they would have catastrophically disrupted the orbits of any planets in the habitable zone.[20]

### A terrestrial planet of the right size

It is argued that life requires terrestrial planets like Earth[21] and as gas giants lack such a surface, that complex life cannot arise there.[citation needed]

A planet that is too small cannot hold much of an atmosphere. Hence the surface temperature becomes more variable and the average temperature drops. Substantial and long-lasting oceans become impossible. A small planet will also tend to have a rough surface, with large mountains and deep canyons. The core will cool faster, and plate tectonics will either not last as long as they would on a larger planet or may not occur at all.[22]

### With plate tectonics

Rare Earth proponents argue that plate tectonics is essential for the emergence and sustenance of complex life.[23] Ward & Brownlee assert that biodiversity, global temperature regulation, carbon cycle and the magnetic field of the Earth that make it habitable for complex terrestrial life all depend on plate tectonics.[24]

Ward & Brownlee contend that the lack of mountain chains elsewhere in the Solar System is direct evidence that Earth is the only body with plate tectonics and as such the only body capable of supporting life.[25]

Plate tectonics is dependent on chemical composition and a long-lasting source of heat in the form of radioactive decay occurring deep in the planet's interior. Continents must also be made up of less dense felsic rocks that "float" on underlying denser mafic rock. Taylor[26] 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.

Ward & Brownlee and others such as Tilman Spohn of the German Space Research Centre Institute of Planetary Research[27] argue that plate tectonics provides a means of biochemical cycling which promotes complex life on Earth and that water is required to lubricate planetary plates.

### A large moon

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

The giant impact theory hypothesizes that the Moon resulted from the impact of a Mars-sized body, Theia, with the very young Earth. This giant impact also gave the Earth its axis tilt and velocity of rotation.[26] Rapid rotation reduces the daily variation in temperature and makes photosynthesis viable.[citation needed] The Rare Earth hypothesis further argues that the axis tilt cannot be too large or too small (relative to the orbital plane). A planet with a large tilt will experience extreme seasonal variations in climate, unfriendly to complex life. A planet with little or no tilt will lack the stimulus to evolution that climate variation provides. In this view, the Earth's tilt is "just right". The gravity of a large satellite also stabilizes the planet's tilt; without this effect the variation in tilt would be chaotic, probably making complex life forms on land impossible.[28]

If the Earth had no Moon, the ocean tides resulting solely from the Sun's gravity would be only half that of the lunar tides. A large satellite gives rise to tidal pools, which may be essential for the formation of complex life, though this is far from certain.[29]

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.[citation needed] 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 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.

### An evolutionary trigger for complex life

Regardless of whether planets with similar physical attributes to the Earth are rare or not, there are arguments that whenever life does emerge it is unlikely to develop beyond simple bacteria. Biochemist Nick Lane argues that simple cells (prokaryotes) emerged soon after earth's formation, but it took almost half the planet's life before they evolved into complex ones (eukaryotes), and as all complex life has a common origin this can have happened only once. In his view, prokaryotes do not have the cellular architecture to evolve into eukaryotes, because if a bacterium is expanded up to eukaryotic proportions it would have tens of thousands of times less energy available. Two billion years ago this problem was solved when one simple cell became incorporated in another one, and the cell within multiplied and evolved into mitochondria, supplying the vast increase in energy available which made the evolution of complex life possible. The fact that this occurred only once in four billion years suggests that it may be a freak occurrence which very rarely occurs, and life on most planets does not evolve beyond simple cells.[30]

## Rare Earth equation

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

$N = N^* \cdot n_e \cdot f_g \cdot f_p \cdot f_{pm} \cdot f_i \cdot f_c \cdot f_l \cdot f_m \cdot f_j \cdot f_{me}$[32]

where:

• 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.

We assume $N^* \cdot n_e = 5\cdot10^{11}$. The Rare Earth hypothesis can then be viewed as asserting that the product of the other nine Rare Earth equation factors listed below, which are all fractions, is no greater than 10−10 and could plausibly be as small as 10−12. In the latter case, $N$ could be as small as 0 or 1. Ward and Brownlee do not actually calculate the value of $N$, because the numerical values of quite a few of the factors below can only be conjectured. They cannot be estimated simply because we have but one data point: the Earth, a rocky planet orbiting a G2 star in a quiet suburb of a large barred spiral galaxy, and the home of the only intelligent species we know, namely ourselves.

• $f_g$ 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. Ward 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.
• $f_l$ is the fraction of the total lifespan of a planet during which complex life is present. 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 (Ward and Brownlee are not evolutionary biologists). Barrow and Tipler[33] review the consensus among such biologists that the evolutionary path from primitive Cambrian chordates, e.g. Pikaia to Homo sapiens, was a highly improbable event. For example, the large brains of humans have marked adaptive disadvantages, requiring as they do an expensive metabolism, a long gestation period, and a childhood lasting more than 25% of the average total life span. Other improbable features of humans include:

• Being the only extant bipedal land (non-avian) vertebrate.[dubious ] 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. Only recently did humans acquire anything like their current scientific and technological sophistication.

Authors that advocate the Rare Earth hypothesis:

• Stuart Ross Taylor,[26] 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.
• Stephen Webb,[2] 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, a paleontologist, endorses the Rare Earth hypothesis in chapter 5 of his Life's Solution: Inevitable Humans in a Lonely Universe,[34] and cites Ward and Brownlee's book with approval.[35] His main purpose, however, is to argue that if a planet does harbour life, intelligent beings something like humans are inevitable.[36]
• 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 The Anthropic Cosmological Principle, 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.
• John Gribbin, a prolific science writer, defends the hypothesis in a book devoted to it called Alone in the Universe: Why our planet is unique.[37]
• Guillermo Gonzalez, astrophysicist who coined the term Galactic Habitable Zone uses the hypothesis in his book The Privileged Planet to promote the concept of intelligent design[38]
• Michael H. Hart, astrophysicist who proposed a very narrow habitable zone based on climate studies edited the influential book "Extraterrestrials: Where are They" and authored "Atmospheric Evolution, the Drake Equation and DNA: Sparse Life in an Infinite Universe"[39]

## Criticism

Cases against the Rare Earth Hypothesis take various forms.

### Exoplanets with Earth-like properties are being discovered

An increasing number of extrasolar planet discoveries are being made with 888 now known.[40] Based on these discoveries, and tools such as the Kepler space telescope scientists are better equipped to estimate the frequency of Earth-like planets and increasingly these studies show results more in favour of the Copernican principle than the Rare Earth Hypothesis. In 2011 NASA's Jet Propulsion Laboratory (JPL) calculated that about "1.4 to 2.7 percent" of all sun-like stars are expected to have earthlike planets "within the habitable zones of their stars". This means there are "two billion" of them in our own Milky Way galaxy alone and assuming that all galaxies have a similar number as the Milky Way, in the 50 billion galaxies in the observable universe there may be as many as a sextillion.[41]

NASA and the SETI Institute now categorise Earth like planets using an Earth Similarity Index (ESI) based on mass, radius and temperature.[42][43]

Among the exoplanets documented, however, none have been found that meet some important Rare Earth hypothesis criterion such as surface water, tectonic plates or a large moon. However, such measurements are beyond the limits of current technology. In addition, of the 133 multiplanetary systems discovered, few have found to bear a resemblance to the planetary arrangement of the Solar System, though again, the current methodology for detecting planets does not favor detecting Solar System like planetary bodies as most exoplanets currently being discovered are much larger than Earth.

However, a large moon and planetary arrangements resembling the solar system are not necessarily important for the development of life in a system (see other reasons below).

### Oxygen is not a requirement for multicellular life

Ward & Brownlee assert that there is irrefutable evidence that oxygen is a necessary ingredient for animal life.[44] However this argument has since been disproven by the discovery of anaerobic metazoa. Three multicellular species including Spinoloricus nov. sp. discovered in the hypersaline anoxic L'Atalante basin at the bottom of the Mediterranean Sea in 2010, appear to metabolise with hydrogen instead of oxygen, lacking mitochondria and using hydrogenosomes instead.[45][46]

### Anthropic reasoning

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.

According to David Darling, the Rare Earth hypothesis is neither hypothesis nor prediction, but merely a description of how life arose on Earth.[47] 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.[48]

Critics also point to a link between the Rare Earth Hypothesis and the creationist ideas of intelligent design.[49]

### Alternative habitats for complex Life

Complex life may exist elsewhere in environments similar to those found around black smokers on Earth.

Rare Earth proponents argue that simple life may be common, though complex life requires specific environmental conditions to arise. It has been argued by some that complex life may exist in diverse habitats, including some beyond the Solar System's habitable zone and on bodies that are not planets where both water and an active energy source may exist. For example, theorised sub-surface water habitats may exist on Europa and Enceladus where tidal heating may be a potential energy source.[50][51] Some theories on the origin of life on Earth point to complex life evolving in such environments before arising on the surface.

### Uncertainty over Jupiter's role

Recent computer simulations on Jupiter's role as guardian of the terrestrial planets has shown that while the gas giant's mass appears to provide increased protection against asteroids, the total effect on all orbital bodies within the Solar System is unclear.[52][53]

Some studies[which?] show that Jupiter has caused more impacts on Earth than it has prevented.[53] Such models would appear to invalidate the argument that Jupiter-like planets as necessary protectors.[citation needed] The role of Jupiter, has since been revised by the Nice model.[how?]

### Necessity of tectonics

The discovery of tectonic features and mountain chains on other Solar System objects (including mountain ranges on the moon, Montes on Mars and Montes on Venus and Montes on Titan including a massive new feature discovered on the moon in 2006), many of which have been formed by geological folds and faults counter Ward & Brownlee's minor argument that such geological features are unique to Earth. There is growing evidence of tectonic like activity on Mars,[54] Venus[55] and Titan.[56]

Many Rare Earth proponents argue that the Earth's plate tectonics would likely not exist if not for the tidal forces of the moon. However there is currently no proof of the hypothesis that the moon's tidal influence initiated Earth's plate tectonics. Additionally, there is strong evidence that plate tectonics existed on Mars which does not currently have a large companion.[57]

NASA scientists Hartman and McKay argue that plate tectonics may in fact slow the rise of oxygenation (and thus stymie complex life rather than promote it).[58]

Possible biogeochemical cycles have recently been detected on Europa, Titan and Mars.

### Giant Impacts may not be rare nor necessary for rotational speed

Recent work by Edward Belbruno and J. Richard Gott of Princeton University suggests that Giant Impacts such as those that form the moon can indeed form in planetary trojan points (L4 or L5 Lagrangian point) which means that similar circumstances may occur in other planetary systems.[59]

Although the Giant Impact Theory posits that the impact forming the Moon increased Earth's rotational speed to make a day about 5 hours long, the Moon has slowly "[[3]]" much of this speed to reduce Earth's solar day since then to about 24 hours. The length of the day is still increasing as the Moon decelerates Earth's rotation. In 100 million years Earth's solar day will be roughly 24 hours 38 minutes, in 1 billion 30 hours 23 minutes. Larger secondary bodies, due to larger tidal forces, would decelerate their primaries faster, which could increase the solar day of a planet in all other respects like earth to over 120 hours within a few billion years. This long solar day would make effective heat dissipation for organisms in the tropics and subtropics extremely difficult in a similar manner to tidal locking to a red dwarf star.

Also, Mars has not known to have undergone a massive impact since its formation and it has a solar day nearly equal in length to the Earth. This is because a large portion of rotational speed arises from initial accumulation of planetary dust into planets. [60]

## Notes

1. ^ Ward & Brownlee 2000, pp. xxi–xxiii
2. ^ a b Webb 2002
3. ^ 1 Morphology of Our Galaxy's 'Twin' Spitzer Space Telescope, Jet Propulsion Laboratory, NASA.
4. ^ Ward & Brownlee 2000, pp. 27–29
5. ^ Lineweaver, Charles H.; Fenner, Yeshe; Gibson, Brad K. (2004). "The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way" (PDF). Science 303 (5654): 59–62. arXiv:astro-ph/0401024. Bibcode:2004Sci...303...59L. doi:10.1126/science.1092322. PMID 14704421.
6. ^ Ward & Brownlee 2000, p. 32
7. ^ a b Gonzalez, Brownlee & Ward 2001
8. ^ How often does the Sun pass through a spiral arm in the Milky Way?, Karen Masters, Curious About Astronomy
9. ^ Dartnell 2007, p. 75
10. ^ "Sibling Rivalry". New Scientist. 31 March 2012.
11. ^ Scharf, 2012
12. ^ Hart, M.H. (January 1979). "Habitable Zones Around Main Sequence Stars". Icarus 37 (1): 351–7. Bibcode:1979Icar...37..351H. doi:10.1016/0019-1035(79)90141-6.
13. ^ Ward & Brownlee 2000, p. 18
14. ^ Schmidt, Gavin (6 April 2005). "Water vapour: feedback or forcing?". RealClimate.
15. ^ a b [1] The One Hundred Nearest Star Systems, Research Consortium on Nearby Stars.
16. ^ Ward & Brownlee 2000, pp. 15–33
17. ^ Alexei A. Sharov, Richard Gordon Life Before Earth Cornel University Library arXiv:1304.3381 [physics.gen-ph]
18. ^ The Physics arXiv BlogLaw and the Origin of Life MIT Technology review
19. ^ Hinse, T.C. "Chaos and Planet-Particle Dynamics within the Habitable Zone of Extrasolar Planetary Systems (A qualitative numerical stability study)" (PDF). Niels Bohr Institute. Retrieved 2007-10-31. "Main simulation results observed: [1] The presence of high-order mean-motion resonances for large values of giant planet eccentricity [2] Chaos dominated dynamics within the habitable zone(s) at large values of giant planet mass."
20. ^ "Once you realize that most of the known extrasolar planets have highly eccentric orbits (like the planets in Upsilon Andromedae), you begin to wonder if there might be something special about our solar system" (UCBerkeleyNews quoting Extra solar planetary researcher Eric Ford.) Sanders, Robert (13 April 2005). "Wayward planet knocks extrasolar planets for a loop". Retrieved 2007-10-31.
21. ^ pg 220 Ward & Brownlee
22. ^ Lissauer 1999, as summarized by Conway Morris 2003, p. 92; also see Comins 1993
23. ^ pg 191. Rare Earth: Why Complex Life is Uncommon in the Universe. By Peter D. Ward, Donald Brownlee
24. ^ pg 194. Rare Earth: Why Complex Life is Uncommon in the Universe. By Peter D. Ward, Donald Brownlee
25. ^ pg 200. Rare Earth: Why Complex Life is Uncommon in the Universe. By Peter D. Ward, Donald Brownlee
26. ^ a b c Taylor 1998
27. ^ http://www.space.com/4076-plate-tectonics-essential-alien-life.html
28. ^ Dartnell 2007, pp. 69–70
29. ^ A formal description of the hypothesis is given in: Lathe, Richard (March 2004). "Fast tidal cycling and the origin of life". Icarus 168 (1): 18–22. Bibcode:2004Icar..168...18L. doi:10.1016/j.icarus.2003.10.018. "tidal cycling, resembling the polymerase chain reaction (PCR) mechanism, could only replicate and amplify DNA-like polymers. This mechanism suggests constraints on the evolution of extra-terrestrial life." It is taught less formally here: Schombert, James. "Origin of Life". University of Oregon. Retrieved 2007-10-31. "with the vastness of the Earth's oceans it is statistically very improbable that these early proteins would ever link up. The solution is that the huge tides from the Moon produced inland tidal pools, which would fill and evaporate on a regular basis to produce high concentrations of amino acids".
30. ^ Lane, 2012
31. ^ Cramer 2000
32. ^ Ward & Brownlee 2000, pp. 271–5
33. ^ Barrow, John D.; Tipler, Frank J. (19 May 1988). The Anthropic Cosmological Principle. foreword by John A. Wheeler. Oxford: Oxford University Press. ISBN 9780192821478. LC 87-28148. Retrieved 31 December 2009. Section 3.2
34. ^ Conway Morris 2003, Ch. 5
35. ^ Conway Morris, 2003, p. 344, n. 1
36. ^ Conway Morris, 2003, p. xv
37. ^ Gribbin 2011
38. ^ The Measurability of the Universe––a Record of the Creator’s Design By Guillermo Gonzalez, Facts for Faith Issue 4, 2000.
39. ^ Extraterrestrials: Where are They? 2nd ed., Eds. Ben Zuckerman and Michael H. Hart (Cambridge: Press Syndicate of the University of Cambridge, 1995), 153.
40. ^ Schneider, Jean. "Interactive Extra-solar Planets Catalog". The Extrasolar Planets Encyclopedia.
41. ^ Choi, Charles Q. (21 March 2011). "New Estimate for Alien Earths: 2 Billion in Our Galaxy Alone". Space.com. Retrieved 2011-04-24.
42. ^ http://www.wired.co.uk/news/archive/2011-11/21/exoplanet-indices
43. ^ Stuart Gary New approach in search for alien life ABC Online. November 22, 2011
44. ^ Ward & Brownlee 2000, p. 217
45. ^ Oxygen-Free Animals Discovered-A First, National Geographic news
46. ^ Danovaro R, Dell'anno A, Pusceddu A, Gambi C, Heiner I, Kristensen RM (April 2010). "The first metazoa living in permanently anoxic conditions". BMC Biology 8 (1): 30. doi:10.1186/1741-7007-8-30. PMC 2907586. PMID 20370908.
47. ^ Darling, David (2001). Life Everywhere: The Maverick Science of Astrobiology. Basic Books/Perseus. ISBN 0-585-41822-5.
48. ^ Darling 2001, p. 103
49. ^ Frazier, Kendrick. 'Was the 'Rare Earth' Hypothesis Influenced by a Creationist?' The Skeptical Inquirer. November 1, 200
50. ^ Reynolds, R.T.; McKay, C.P.; Kasting, J.F. (1987). "Europa, Tidally Heated Oceans, and Habitable Zones Around Giant Planets". Advances in Space Research 7 (5): 125–132. Bibcode:1987AdSpR...7..125R. doi:10.1016/0273-1177(87)90364-4.
51. ^ For a detailed critique of the Rare Earth hypothesis along these lines, see Cohen & Stewart 2002.
52. ^ Horner, J.; Jones, B.W. (2009). "Jupiter – friend or foe? II: the Centaurs". arXiv:0903.3305 [astro-ph.EP].
53. ^ a b Horner, J.; Jones, B.W. (2008). "Jupiter – friend or foe? I: the asteroids" (PDF). International Journal of Astrobiology 7 (3&4): 251–261. arXiv:0806.2795. Bibcode:2008IJAsB...7..251H. doi:10.1017/S1473550408004187.
54. ^ UCLA scientist discovers plate tectonics on Mars By Stuart Wolpert August 09, 2012
55. ^ Plate tectonics on Venus Dr. Richard Ghail
56. ^ Massive Mountain Range Imaged on Saturn's Moon Titan NASA 12.12.06
57. ^ "New Map Provides More Evidence Mars Once Like Earth". 10 December 2005.
58. ^ Hartman H, McKay CP "Oxygenic photosynthesis and the oxidation state of Mars." Planet Space Sci. 1995 Jan-Feb;43(1-2):123-8.
59. ^ Belbruno, E.; J. Richard Gott III (2005). "Where Did The Moon Come From?". The Astronomical Journal 129 (3): 1724–45. arXiv:astro-ph/0405372. Bibcode:2005AJ....129.1724B. doi:10.1086/427539.