Rare Earth hypothesis

In

According to the hypothesis, complex extraterrestrial life is an improbable phenomenon and likely to be rare throughout the universe as a whole. 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, both faculty members at the University of Washington.

In the 1970s and 1980s,

There is no reliable or reproducible evidence that

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 might still 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 that may evolve on such planets, which would solve the Fermi paradox: "If extraterrestrial aliens are common, why aren't they obvious?"^{[citation needed]}

The right location in the right kind of galaxy

Rare Earth suggests that much of the known universe, including large parts of our galaxy, are "dead zones" unable to support complex life. Those parts of a galaxy where complex life is possible make up the galactic habitable zone, which is primarily characterized by distance from the Galactic Center.

1. As that distance increases, star metallicity declines. Metals (which in astronomy refers to all elements other than hydrogen and helium) are necessary for 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 as distance increases. Thus the early universe, and present-day galactic regions where stellar density is high and supernovae are common, will be dead zones.^[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 which could extinguish all complex life on a planet.

Item #1 rules out the outermost reaches of a galaxy; #2 and #3 rule out galactic inner regions. Hence a galaxy's habitable zone may be a relatively narrow ring of adequate conditions sandwiched between its uninhabitable center and outer reaches.

Also, a habitable planetary system must maintain its favorable location long enough for complex life to evolve. A star with an eccentric (elliptical or hyperbolic) galactic orbit will pass through some spiral arms, unfavorable regions of high star density; thus a life-bearing star must have a galactic orbit that is nearly circular, with a close synchronization between the orbital velocity of the star and of the spiral arms. This further restricts the galactic habitable zone within a fairly narrow range of distances from the Galactic Center. Lineweaver et al. calculate this zone to be a ring 7 to 9 kiloparsecs in radius, including no more than 10% of the stars in the Milky Way,^[6] about 20 to 40 billion stars. Gonzalez et al.^[7] would halve these numbers; they estimate that at most 5% of stars in the Milky Way fall within the galactic habitable zone.

Approximately 77% of observed galaxies are spiral,^[8] two-thirds of all spiral galaxies are barred, and more than half, like the Milky Way, exhibit multiple arms.^[9] According to Rare Earth, our own galaxy is unusually quiet and dim (see below), representing just 7% of its kind.^[10] Even so, this would still represent more than 200 billion galaxies in the known universe.

Our galaxy also appears unusually favorable in suffering fewer collisions with other galaxies over the last 10 billion years, which can cause more supernovae and other disturbances.^[11] Also, the Milky Way's central black hole seems to have neither too much nor too little activity.^[12]

The orbit of the Sun around the center of the Milky Way is indeed almost perfectly circular, with

The terrestrial example suggests that complex life requires liquid water, the maintenance of which requires an orbital distance neither too close nor too far from the central star, another scale of habitable zone or Goldilocks principle.^[15] The habitable zone varies with the star's type and age.

For advanced life, the star must also be highly stable, which is typical of middle star life, about 4.6 billion years old. Proper metallicity and size are also important to stability. The Sun has a low (0.1%) luminosity variation. To date, no solar twin star, with an exact match of the Sun's luminosity variation, has been found, though some come close. The star must also have no stellar companions, as in binary systems, which would disrupt the orbits of any planets. Estimates suggest 50% or more of all star systems are binary.^{[16][17][18][19]} The habitable zone for a main sequence star very gradually moves out over its lifespan until the star becomes a white dwarf and the habitable zone vanishes.

The liquid water and other gases available in the habitable zone bring the benefit of the greenhouse effect. Even though the Earth's atmosphere contains a water vapor concentration from 0% (in arid regions) to 4% (in rainforest and ocean regions) and – as of November 2022 – only 417.2 parts per million of CO₂,^[20] these small amounts suffice to raise the average surface temperature by about 40 °C,^[21] with the dominant contribution being due to water vapor.

Rocky planets must orbit within the habitable zone for life to form. Although the habitable zone of such hot stars as

Conversely, small

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 much greater distances may then become habitable).

An energy output that varies with the lifetime of the star will 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 it and cause a greenhouse effect that prevents the oceans from reforming.

All known life requires the complex chemistry of

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, rocky inner planets and massive outer gas giants.^[24] Without the protection of such "celestial vacuum cleaner" planets with strong gravitational pulls, other planets would be subject to more frequent catastrophic asteroid collisions.

Observations of exoplanets have shown that arrangements of planets similar to the Solar System are rare. Most planetary systems have super-Earths, several times larger than Earth, close to their star, whereas the Solar System's inner region has only a few small rocky planets and none inside Mercury's orbit. Only 10% of stars have giant planets similar to Jupiter and Saturn, and those few rarely have stable, nearly circular orbits distant from their star. Konstantin Batygin and colleagues argue that these features can be explained if, early in the history of the Solar System, Jupiter and Saturn drifted towards the Sun, sending showers of planetesimals towards the super-Earths which sent them spiralling into the Sun, and ferrying icy building blocks into the terrestrial region of the Solar System which provided the building blocks for the rocky planets. The two giant planets then drifted out again to their present positions. In the view of Batygin and his colleagues: "The concatenation of chance events required for this delicate choreography suggest that small, Earth-like rocky planets – and perhaps life itself – could be rare throughout the cosmos."^[25]

A continuously stable orbit

Rare Earth proponents argue that a gas giant also must not be too close to a body where life is developing. Close placement of one or more gas giants could disrupt the orbit of a potential life-bearing planet, either directly or by drifting into the habitable zone.

Newtonian dynamics can produce

The Rare Earth hypothesis argues that life requires terrestrial planets like Earth, and since gas giants lack such a surface, that complex life cannot arise there.^[29]

A planet that is too small cannot maintain much atmosphere, rendering its surface temperature low and variable and oceans 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 may be brief or entirely absent. A planet that is too large will retain too dense an atmosphere, like Venus. Although Venus is similar in size and mass to Earth, its surface atmospheric pressure is 92 times that of Earth, and its surface temperature is 735 K (462 °C; 863 °F). The early Earth once had a similar atmosphere, but may have lost it in the giant impact event which formed the Moon.^[30]

Plate tectonics

Rare Earth proponents argue that

Plate tectonics depend on the right chemical composition and a long-lasting source of heat from radioactive decay. Continents must be made of less dense felsic rocks that "float" on underlying denser mafic rock. Taylor^[33] emphasizes that tectonic subduction zones require the lubrication of oceans of water. Plate tectonics also provide a means of biochemical cycling.^[34]

Plate tectonics and, as a result, continental drift and the creation of separate landmasses would create diversified ecosystems and biodiversity, one of the strongest defenses against extinction.^[35] An example of species diversification and later competition on Earth's continents is the Great American Interchange. North and Middle America drifted into South America at around 3.5 to 3 Ma. The fauna of South America had already evolved separately for about 30 million years, since Antarctica separated, but, after the merger, many species were wiped out, mainly in South America, by competing North 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 only relatively tiny satellites which are probably captured asteroids (Mars). After Charon, the Moon is also the largest natural satellite in the Solar System relative to the size of its parent body, being 27% the size of Earth.^[36]

The

A large satellite also increases the likelihood of plate tectonics through the effect of tidal forces on the planet's crust.^{[citation needed]} 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 if the crust had a uniform composition. A further theory indicates that such a large moon may also contribute to maintaining a planet's magnetic shield by continually acting upon a metallic planetary core as dynamo, thus protecting the surface of the planet from charged particles and cosmic rays, and helping to ensure the atmosphere is not stripped over time by solar winds.^{[citation needed]}

An atmosphere

A terrestrial planet must be the right size, like Earth and Venus, in order to retain an atmosphere. On Earth, once the giant impact of

Regardless of whether planets with similar physical attributes to the Earth are rare or not, some argue that life tends not to evolve into anything more complex than simple bacteria without being provoked by rare and specific circumstances. Biochemist

The evolution and persistence of sexual reproduction is another mystery in biology. The purpose of sexual reproduction is unclear, as in many organisms it has a 50% cost (fitness disadvantage) in relation to asexual reproduction.^[52] Mating types (types of gametes, according to their compatibility) may have arisen as a result of anisogamy (gamete dimorphism), or the male and female sexes may have evolved before anisogamy.^[53][54] It is also unknown why most sexual organisms use a binary mating system,^[55] and why some organisms have gamete dimorphism. Charles Darwin was the first to suggest that sexual selection drives speciation; without it, complex life would probably not have evolved.

The right time in evolutionary history

While life on Earth is regarded to have spawned relatively early in the planet's history, the evolution from multicellular to intelligent organisms took around 800 million years.

The following discussion is adapted from Cramer.[57] The Rare Earth equation is Ward and Brownlee's riposte to the Drake equation. It calculates ${\displaystyle N}$, the number of Earth-like planets in the Milky Way having complex life forms, as:

${\displaystyle N=N^{*}\cdot n_{e}\cdot f_{g}\cdot f_{p}\cdot f_{pm}\cdot f_{i}\cdot f_{c}\cdot f_{l}\cdot f_{m}\cdot f_{j}\cdot f_{me}}$^[58]

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, with little information about the number of small stars. N* is at least 100 billion, and may be as high as 500 billion, if there are many low visibility stars.
• ${\displaystyle n_{e}}$ is the average number of planets in a star's habitable zone. This zone is fairly narrow, being 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, ${\displaystyle n_{e}}$=1 is a likely upper bound.

We assume ${\displaystyle N^{*}\cdot n_{e}=5\cdot 10^{11}}$. The Rare Earth hypothesis can then be viewed as asserting that the product of the other nine Rare Earth equation factors listed below, which are all fractions, is no greater than 10⁻¹⁰ and could plausibly be as small as 10⁻¹². In the latter case, ${\displaystyle N}$ could be as small as 0 or 1. Ward and Brownlee do not actually calculate the value of ${\displaystyle N}$, 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.

• ${\displaystyle f_{g}}$ is the fraction of stars in the galactic habitable zone (Ward, Brownlee, and Gonzalez estimate this factor as 0.1^[7]).
• ${\displaystyle f_{p}}$ is the fraction of stars in the Milky Way with planets.
• ${\displaystyle f_{pm}}$ is the fraction of planets that are rocky ("metallic") rather than gaseous.
• ${\displaystyle f_{i}}$ is the fraction of habitable planets where microbial life arises. Ward and Brownlee believe this fraction is unlikely to be small.
• ${\displaystyle f_{c}}$ 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 small.
• ${\displaystyle f_{l}}$ 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.
• ${\displaystyle f_{m}}$ 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.
• ${\displaystyle f_{j}}$ is the fraction of planetary systems with large Jovian planets. This fraction could be large.
• ${\displaystyle f_{me}}$ 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.^{[citation needed]}

The Rare Earth equation, unlike the

• Being one of a handful of 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^{[citation needed]} 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

Writers who support the Rare Earth hypothesis:

• Stuart Ross Taylor,^[33]
  a specialist on the Solar System, firmly believed in the hypothesis. Taylor concludes that the Solar System is probably unusual, because it resulted from so many chance factors and events.
• Stephen Webb,[60] 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^{[clarification needed]}
• paleontologist, endorses the Rare Earth hypothesis in chapter 5 of his Life's Solution: Inevitable Humans in a Lonely Universe,^[61] and cites Ward and Brownlee's book with approval.^[62]
• 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 thorough study of the anthropic principle and of how the laws of physics are peculiarly suited to enable the emergence of complexity in nature.^[63]
• Singularitarian, argues in his 2005 book The Singularity Is Near that the coming Singularity
  requires that Earth be the first planet on which sapient, 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 Alone in the Universe: Why our planet is unique (2011).^[64]
• astrophysicist who proposed a narrow habitable zone based on climate studies, edited the influential 1982 book Extraterrestrials: Where are They and authored one of its chapters "Atmospheric Evolution, the Drake Equation and DNA: Sparse Life in an Infinite Universe".^[65]
• Marc J. Defant, professor of geochemistry and volcanology, elaborated on several aspects of the rare Earth hypothesis in his TEDx talk entitled: Why We are Alone in the Galaxy.[66] He also wrote in his book in 1998: "I do not believe that we were the destined outcome of evolution. In fact, we are probably the result of an incredible number of chance circumstances (one example is the meteorite impact at the end of the Cretaceous which probably destroyed the dinosaurs and led to mammal domination). The coincidental nature of our evolution should be clear from this book. I might even contend that so many "coincidences" had to take place during the history of the universe, that intelligent life on this planet may be the only life in our universe. I do not mean to suggest that we must have been "created." I mean to say that maybe there is not as much chance of finding life in our galaxy or universe as some would have us believe. We may be it."^[67]
• Brian Cox, physicist and popular science celebrity confesses his support for the hypothesis in his 2014 BBC production of the Human Universe.
• Richard Dawkins, evolutionary biologist, notes the Fermi paradox in his book, The Greatest Show on Earth, while discussing how life first evolved on Earth. Although we do not yet know the precise process for how life first began on Earth, Dawkins's view is that it is an implausible theory (i.e., improbable) given we have not encountered any evidence for life existing elsewhere in the universe. He concludes that life is probably very rare throughout the universe.^[68]

Criticism

Cases against the Rare Earth hypothesis take various forms.

The hypothesis appears anthropocentric

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

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.^[71]

Critics also argue that there is a link between the Rare Earth hypothesis and the unscientific idea of intelligent design.^[72]

Exoplanets around main sequence stars are being discovered in large numbers

An increasing number of

Current technology limits the testing of important Rare Earth criteria: surface water, tectonic plates, a large moon and biosignatures are currently undetectable. Though planets the size of Earth are difficult to detect and classify, scientists now think that rocky planets are common around Sun-like stars.^[74] The Earth Similarity Index (ESI) of mass, radius and temperature provides a means of measurement, but falls short of the full Rare Earth criteria.^[75][76]

Rocky planets orbiting within habitable zones may not be rare

Some argue that Rare Earth's estimates of rocky planets in habitable zones (${\displaystyle n_{e}}$ in the Rare Earth equation) are too restrictive.

Uncertainty over Jupiter's role

The requirement for a system to have a

Jovian planet

as protector (Rare Earth equation factor ${\displaystyle f_{j}}$) has been challenged, affecting the number of proposed extinction events (Rare Earth equation factor ${\displaystyle f_{me}}$). Kasting's 2001 review of Rare Earth questions whether a Jupiter protector has any bearing on the incidence of complex life.

Plate tectonics may not be unique to Earth or a requirement for complex life

Ward and Brownlee argue that for complex life to evolve (Rare Earth equation factor ${\displaystyle f_{c}}$),

Recent evidence also points to similar activity either having occurred or continuing to occur elsewhere. The geology of Pluto, for example, described by Ward and Brownlee as "without mountains or volcanoes ... devoid of volcanic activity",^[23] has since been found to be quite the contrary, with a geologically active surface possessing organic molecules^[91] and mountain ranges^[92] like Tenzing Montes and Hillary Montes comparable in relative size to those of Earth, and observations suggest the involvement of endogenic processes.^[93] Plate tectonics has been suggested as a hypothesis for the Martian dichotomy, and in 2012 geologist An Yin put forward evidence for active plate tectonics on Mars.^[94] Europa has long been suspected to have plate tectonics^[95] and in 2014 NASA announced evidence of active subduction.^[96] Like Europa, analysis of the surface of Jupiter's largest moon Ganymede strike-strip faulting and surface materials of possible endogenic origin suggests that plate tectonics has also taken place there.^{[97] [98]} In 2017, scientists studying the geology of Charon confirmed that icy plate tectonics also operated on Pluto's largest moon.^[99] Since 2017 several studies of the geodynamics of Venus have also found that, contrary to the view that the lithosphere of Venus is static, it is actually being deformed via active processes similar to plate tectonics, though with less subduction, implying that geodynamics are not a rare occurrence in Earth sized bodies.^[100][101]

Kasting suggests that there is nothing unusual about the occurrence of plate tectonics in large rocky planets and liquid water on the surface as most should generate internal heat even without the assistance of radioactive elements.^[83] Studies by Valencia^[102] and Cowan^[103] suggest that plate tectonics may be inevitable for terrestrial planets Earth-sized or larger, that is, Super-Earths, which are now known to be more common in planetary systems.^[104]

Free oxygen may be neither rare nor a prerequisite for multicellular life

The hypothesis that

${\displaystyle f_{c}}$

Ward and Brownlee ask "whether oxygenation, and hence the rise of animals, would ever have occurred on a world where there were no continents to erode".^[105] Extraterrestrial free oxygen has recently been detected around other solid objects, including Mercury,^[106] Venus,^[107] Mars,^[108] Jupiter's four Galilean moons,^[109] Saturn's moons Enceladus,^[110] Dione^[111][112] and Rhea^[113] and even the atmosphere of a comet.^[114] This has led scientists to speculate whether processes other than photosynthesis could be capable of generating an environment rich in free oxygen. Wordsworth (2014) concludes that oxygen generated other than through photodissociation may be likely on Earth-like exoplanets, and could actually lead to false positive detections of life.^[115] Narita (2015) suggests photocatalysis by titanium dioxide as a geochemical mechanism for producing oxygen atmospheres.^[116]

Since Ward & Brownlee's assertion that "there is irrefutable evidence that oxygen is a necessary ingredient for animal life",

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).[125] 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.^[126] Kasting argues that if oxygenation caused the Cambrian explosion then any planet with oxygen producing photosynthesis should have complex life.^[127]

A magnetosphere may not be rare or a requirement

The importance of Earth's magnetic field to the development of complex life has been disputed. The origin of Earth's magnetic field remains a mystery^[128] though the presence of a magnetosphere appears to be relatively common for larger planetary mass objects as all Solar System planets larger than Earth possess one.^[129] There is increasing evidence of present or past magnetic activity in terrestrial bodies such as the Moon, Ganymede, Mercury and Mars.^[130] Without sufficient measurement present studies rely heavily on modelling methods developed in 2006 by Olson & Christensen to predict field strength.^[131] Using a sample of 496 planets such models predict Kepler-186f to be one of few of Earth size that would support a magnetosphere (though such a field around this planet has not currently been confirmed).^[131] However current recent empirical evidence points to the occurrence of much larger and more powerful fields than those found in our Solar System, some of which cannot be explained by these models.^[132][133]

Kasting argues that the atmosphere provides sufficient protection against cosmic rays even during times of magnetic pole reversal and atmosphere loss by sputtering.

The assertion that the Moon's stabilization of Earth's obliquity and spin is a requirement for complex life has been questioned. Kasting argues that a moonless Earth would still possess habitats with climates suitable for complex life and questions whether the spin rate of a moonless Earth can be predicted.

Many Rare Earth proponents argue that the Earth's plate tectonics would probably not exist if not for the tidal forces of the Moon or the impact of Theia (prolonging mantle effects).^[137][138] The hypothesis that the Moon's tidal influence initiated or sustained Earth's plate tectonics remains unproven, though at least one study implies a temporal correlation to the formation of the Moon.^[139] Evidence for the past existence of plate tectonics on planets like Mars^[140] which may never have had a large moon would counter this argument, although plate tectonics may fade anyway before a moon is relevant to life.^[137][138] Kasting argues that a large moon is not required to initiate plate tectonics.^[83]

Complex life may arise in alternative habitats

Rare Earth proponents argue that simple life may be common, though complex life requires specific environmental conditions to arise. Critics consider life could arise on a