Rare Earth hypothesis

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In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life on Earth (and, subsequently, intelligence) required an improbable combination of astrophysical and geological events and circumstances. The hypothesis argues that complex extraterrestrial life is a very improbable phenomenon and likely to be extremely rare. 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.

An alternative view point was argued by Carl Sagan and Frank Drake, among others. It holds that Earth is a typical rocky planet in a typical planetary system, located in a non-exceptional region of a common barred-spiral galaxy. Given the principle of mediocrity (also called the Copernican principle), it is probable that the universe teems with complex life. Ward and Brownlee argue to the contrary: that 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.

On 4 November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars and red dwarf stars within the Milky Way Galaxy.^[1][2] 11 billion of these estimated planets may be orbiting sun-like stars.^[3] The nearest such planet may be 12 light-years away, according to the scientists.^[1][2] With the closest found at 16 light-years (Gliese 832 c). Nonetheless, 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?"^[4]

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 ensure 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^[5]) 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.^[6]
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.

Item #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 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.^[7] 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.^[8] 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.^[9] 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.^[10] Some researchers have suggested that several mass extinctions do correspond with previous crossings of the spiral arms.^[11]
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.^[12] 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 even rarer. The Milky Way black hole appears to be just right.^[13]

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.^[14] 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 (H
2O), carbon dioxide (CO
2), 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 -as of June 2013- only 400 parts per million of CO
2, these small amounts suffice to raise the average surface temperature of the Earth by about 40 °C from what it would otherwise be,^[15] 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 CO
2 contributing between 9% and 26% of the effect.^[16]

Rocky planets must orbit within the habitable zone for life to form. Although the habitable zone of such hot stars as Sirius or Vega is wide:

1. Rocky planets that form too close to the star to lie within the habitable zone cannot sustain life; however, life could arise on a moon of a gas giant. Hot stars also emit much more ultraviolet radiation that ionizes any planetary atmosphere.
2. Hot stars, as mentioned above, may become red giants before advanced life evolves on their planets.

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

Small red dwarf stars conversely have small habitable zones wherein planets are in tidal lock—one side always faces the star and becomes very hot and the other always faces away and becomes very cold—and are also at increased risk of solar flares (see Aurelia) that would tend to ionize the atmosphere and be otherwise inimical to complex life. Rare Earth proponents argue that life therefore cannot arise in such systems and that only central stars that range from F7 to K1 stars are hospitable. Such stars are rare: G type stars such as the Sun (between the hotter F and cooler K) comprise only 9%^[17] of the hydrogen-burning stars in the Milky Way. However, some exobiologists have suggested that stars outside this range may give rise to life under the right circumstances; this possibility is a central point of contention to the theory because these late-K and M category stars make up about 82% of all hydrogen-burning stars.^[17]

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

Such aged stars 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 completed their red giant phase. Stars that become red giants expand into or overheat the habitable zones of their youth and middle age (though theoretically planets at a much greater distance may become habitable).

An energy output that varies with the lifetime of the star will very likely prevent life (e.g., as Cepheid variables). A sudden decrease, even if brief, may freeze the water of orbiting planets, and a significant increase may evaporate them and cause a greenhouse effect that may prevent the oceans from reforming.

Life without complex chemistry is unknown. Such chemistry requires metals, namely elements other than hydrogen or helium and thereby suggests that a planetary system rich in metals is a necessity for life. The only known mechanism for creating and dispersing metals is a supernova explosion. The absorption spectrum of a star reveals the presence of metals within, 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 that formed when the universe was young, stars in most galaxies other than large spirals, and stars in the outer regions of all galaxies. Metal-rich central stars capable of supporting complex life are therefore believed to be most common in the quiet suburbs of the larger spiral galaxies—where radiation also happens to be weak.^[18]

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.^[19]

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.^[20]

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.^[21]

A terrestrial planet of the right size

It is argued that life requires terrestrial planets like Earth and as gas giants lack such a surface, that complex life cannot arise there.^[22]

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.^[23]

Great American Interchange on Earth, around ~ 3.5 to 3 Ma

With plate tectonics

Rare Earth proponents argue that plate tectonics is essential for the emergence and sustenance of complex life.^[24] 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.^[25]

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.^[26]

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^[27] 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^[28] 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.

Plate tectonics and as a result continental drift and the creation of separate land masses would create diversified ecosystems which is thought to have promoted the diversification of species, and that diversity is one of the strongest defenses against extinction.^[29]

An example of species diversification and later competition on Earth's continents is the Great American Interchange. This was the result of the tectonically induced connection between North & Middle America with the South American continent, at around 3.5 to 3 Ma. The previously undisturbed fauna of South America could evolve in their own way for about 30 million years, since Antarctica separated. Many species were subsequently wiped out in mainly South America by competing Northern American animals.

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.^[27] 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.^{[citation needed]} 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.^[30]

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.^[31]

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

If a giant impact is the only 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, some argue that life usually remains simple bacteria. Biochemist Nick Lane argues that simple cells (prokaryotes) emerged soon after Earth's formation, but almost half the planet's life had passed before they evolved into complex ones (eukaryotes) and because all complex life has a common origin, this event can only have happened once. In his view, prokaryotes lack the cellular architecture to evolve into eukaryotes because a bacterium expanded up to eukaryotic proportions would have tens of thousands of times less energy available; two billion years ago, one simple cell incorporated itself into another, multiplied, and evolved into mitochondria that supplied the vast increase in available energy that enabled the evolution of complex life. If this incorporation occurred only once in four billion years or is otherwise unlikely, then life on most planets remains simple.^[32] An alternative view that mitochondria evolution was environment triggered, and that mitochondria containing organisms appear very soon after first traces of oxygen appear in Earth`s atmosphere.^[33]

The right time in evolution

Paleogeography of the Earth (~260 Ma)
The Siberian Traps, exterminating 95% of Earth's Permian life formed in the green lowlands of paleo-Siberia, farthest north on the map

 

Timeline of evolution; writing exists only since 0.000218 % back in Earths history

While life on Earth is regarded to have spawned relatively early in the planet's history, the evolution to complex organs took around 800 million years^[34] Civilizations on Earth have existed for ~10,000 years and radio communication with space is not older than 80 years. Relative to the age of our solar system (~4.57 Ga) this is a tiny age span, an age span where extreme climatic variations, super volcanoes or large meteorite impacts were absent. The fragility of intelligent or evolving to intelligent life to factors that have nearly all of the living higher species on the planet is very big. The Permian-Triassic mass extinction, attributed to have been caused by widespread and continuous volcanic eruptions in an area the size of Western Europe; the Siberian Traps, around 251.2 Ma ago, had 95% of the species extinct. This event happened in a time when the Earths paleocontinents were joined in Pangaea.

More complex life on Earth started some 542 million years ago with the Cambrian Explosion. In the time span since this period the Earth has been hit by a number of larger meteorite impacts of which only one is considered to have caused a mass extinction; the Chicxulub impact at the Cretaceous–Paleogene boundary (~65.5 Ma) on the Yucatán peninsula in Mexico.

If intelligent extraterrestrial civilizations did exist and with such an intelligence level that they could make contact with distant Earth, they would have to live in the same time span in evolution. The nearest Earth-like planets are around 11.9 light years away; probable planets as Tau Ceti e and f around the star Tau Ceti in the constellation of Cetus, a star considered to be 5.8 Ga; 1.23 billion years older than the Sun.

Under the assumption that both the explosion of life and the development of civilization were to be relative to the planet's age, they would have spawned 723 Ma and 12.691 ka, respectively. The time between the life explosion if that had existed on an exoplanet and the dawn of civilizations is thus very large and the time between civilization and radio signals evenly so.

The risk of intelligent-life destruction is not a Drake equation factor; in the 33 million years since the Eocene-Oligocene extinction event there have been no major mass extinctions.

The chance of bigger impacts in the time span of evolution to intelligent life depends on the amount of shielding by larger bodies, such as our system's Jupiter or the Moon. The chance of a large impact and resulting mass extinction happening in a multi-planetary "protected" system is, however, impossible to predict.

Rare Earth equation

The following discussion is adapted from Cramer.^[35] The Rare Earth equation is Ward and Brownlee's riposte to the Drake equation. It calculates , the number of Earth-like planets in the Milky Way having complex life forms, as:

^[36]

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.
• 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 = 1 is a likely upper bound.

We assume . 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, could be as small as 0 or 1. Ward and Brownlee do not actually calculate the value of , 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.

• is the fraction of stars in the galactic habitable zone (Ward, Brownlee, and Gonzalez estimate this factor as 0.1^[9]).
• is the fraction of stars in the Milky Way with planets.
• is the fraction of planets that are rocky ("metallic") rather than gaseous.
• is the fraction of habitable planets where microbial life arises. Ward and Brownlee believe this fraction is unlikely to be small.
• 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.
• 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.
• 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.
• is the fraction of planetary systems with large Jovian planets. This fraction could be large.
• 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^[37] 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 – discuss]} 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.

Advocates

Authors that advocate the Rare Earth hypothesis:

• Stuart Ross Taylor,^[27] 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,^[4] 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,^[38] and cites Ward and Brownlee's book with approval.^[39] His main purpose, however, is to argue that if a planet does harbour life, intelligent beings something like humans are inevitable.^[40]
• 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.^[41]
• 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^[42]
• 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"^[43]

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 3,548 candidate planets now known as of August 2013. These discoveries and such tools as the Kepler space telescope aid estimating the frequency of Earth-like planets. Because life has not been found on other planets, and because the Copernican principle states that life should be common on these other Earth-like planets if the Copernican principle is true, the more Earth-like planets that are found without life increases the strength of the Rare Earth Hypothesis. In 2013 a study that was published in the journal
Proceedings of the National Academy of Sciences calculated that about "one in five" of all sun-like stars are expected to have earthlike planets "within the habitable zones of their stars"; 8.8 billion of them therefore exist in the Milky Way galaxy alone.^[44]

NASA and the SETI Institute now categorise Earth like planets with an Earth Similarity Index (ESI) of mass, radius and temperature.^[45][46]

Current technology limits the testing of important Rare Earth Criteria: surface water, tectonic plates, or a large moon, are currently undetectable, and few of the 146 documented exasolar systems have been found to resemble ours because Earth-sized planets are difficult to detect. However, a large moon and planetary arrangements that resemble that of 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

Multicellular life, e.g., anaerobic metazoa, can exist without oxygen (despite Ward & Brownlee's now disproven contrary assertion ^[47]). 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 instead using hydrogenosomes.^[48][49]

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.^[50] 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.^[51]

Critics also point to a link between the Rare Earth Hypothesis and the creationist ideas of intelligent design.^[52]

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. Some argue that complex life may exist in such diverse habitats as those beyond the Solar System's habitable zone and on non-planetary bodies where both water and an active energy source may exist. For example, sub-surface water habitats that are warmed by tidal heating may exist on Europa and Enceladus.^[53][54] Some theories on the origin of life on Earth indicate that complex life evolved in such environments before arising on the surface.

Uncertainty over Jupiter's role

The assertion that Jupiter's mass guards the terrestrial planets from impacts has been challenged.
Since Rare Earth, the 2005 Nice model and 2007 Nice 2 model have provided computer modelling of planetary formation. A study by Horner & Jones (2008) using computer simulation found that while the total effect on all orbital bodies within the Solar System is unclear, Jupiter has caused more impacts on Earth than it has prevented.^[55]

Necessity of tectonics

Ward & Brownlee argue that tectonics is necessary to support biogeochemical cycles required for intelligent life and that such geological features are unique to Earth and that such processes do not occur elsewhere citing the lack of any observable orogenic evidence. However recent evidence points to similar activity either having occurred or continuing to occur on other terrestrial objects including Mars,^[56] Venus,^[57] Titan,^[58][59] Europa,^[60] Enceladus^[61] and the Moon.^[59] Several more natural satellites exhibit similar processes though that may have different mechanisms.

Many Rare Earth proponents argue that the Earth's plate tectonics would probably not exist if not for the tidal forces of the moon. However the hypothesis that the moon's tidal influence initiated Earth's plate tectonics remains unproven. Additionally, strong evidence suggests that plate tectonics existed on Mars, which does not currently have a large companion.^[62]

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).^[63] Computer modelling by Tilman Spohn in 2014 found that plate tectonics on Earth may have arisen from the effects of complex life's emergence, rather than the other way around as the Rare Earth might suggest. The action of lichens on rock may have contributed to the formation of subduction zones in the presence of water.^[64]

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 formed 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.^[65]
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 "stolen" much of this speed to reduce Earth's solar day since then to about 24 hours and continues to do so: 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 would exert proportionally larger tidal forces that would in turn decelerate their primaries faster and potentially 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.