Planetary habitability

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"Habitable planet" redirects here. For a list of potentially habitable planets found to date, see List of potentially habitable exoplanets.

Understanding planetary habitability is partly an extrapolation of the conditions on Earth, as this is the only planet known to support life.

Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and maintain environments hospitable to life.[1] Life may be generated directly on a planet or satellite endogenously or be transferred to it from another body, through a hypothetical process known as panspermia.[2] Environments do not need to contain life to be considered habitable nor are accepted habitable zones (HZ) the only areas in which life might arise.[3]

As the existence of

unicellular creatures. Research and theory in this regard is a component of a number of natural sciences, such as astronomy, planetary science and the emerging discipline of astrobiology
.

An absolute requirement for life is an

water worlds could support life.[6][7]

Habitability indicators and

alternative biochemistries and other types of astronomical bodies
.

The idea that planets beyond Earth might host life is an ancient one, though historically it was framed by

robotic spacecraft exploration of other planets and moons within the Solar System has provided critical information on defining habitability criteria and allowed for substantial geophysical comparisons between the Earth and other bodies. The discovery of exoplanets, beginning in the early 1990s[8][9] and accelerating thereafter, has provided further information for the study of possible extraterrestrial life. These findings confirm that the Sun is not unique among stars
in hosting planets and expands the habitability research horizon beyond the Solar System.

History

Earth habitability comparison

While Earth is the only place in the Universe known to harbor life,

planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way.[15][16] 11 billion of these estimated planets may be orbiting Sun-like stars.[17] The nearest such planet may be 12 light-years away, according to the scientists.[15][16] As of June 2021, a total of 59 potentially habitable exoplanets have been found.[18]

In August 2021, a new class of habitable planets, named

ocean planets, which involves "hot, ocean-covered planets with hydrogen-rich atmospheres", has been reported.[19] Hycean planets may soon be studied for biosignatures by terrestrial telescopes as well as space telescopes, such as the James Webb Space Telescope (JWST), which was launched on 25 December 2021.[20]

Suitable star systems

An understanding of planetary habitability begins with the host star.[21] The classical habitable zone (HZ) is defined for surface conditions only; but a metabolism that does not depend on the stellar light can still exist outside the HZ, thriving in the interior of the planet where liquid water is available.[21]

Under the auspices of

Hipparcos Catalogue into a core group of 17,000 potentially habitable stars, and the selection criteria that were used provide a good starting point for understanding which astrophysical factors are necessary for habitable planets.[22] According to research published in August 2015, very large galaxies may be more favorable to the formation and development of habitable planets than smaller galaxies, like the Milky Way galaxy.[23]

However, what makes a planet habitable is a much more complex question than having a planet located at the right distance from its host star so that water can be liquid on its surface: various geophysical and geodynamical aspects, the radiation, and the host star's plasma environment can influence the evolution of planets and life, if it originated.[21] Liquid water is a necessary[24] but not sufficient condition for life as we know it, as habitability is a function of a multitude of environmental parameters.[2]

Spectral class

The

Milky Way galaxy
. "Middle-class" stars of this sort have a number of characteristics considered important to planetary habitability:

K-type stars may be able to support life far longer than the Sun.[28]

Whether fainter late K and M class

Gliese 581 g, in an orbit between these two planets. However, reviews of the discovery have placed the existence of this planet in doubt, and it is listed as "unconfirmed". In September 2012, the discovery of two planets orbiting Gliese 163[30] was announced.[31][32] One of the planets, Gliese 163 c, about 6.9 times the mass of Earth and somewhat hotter, was considered to be within the habitable zone.[31][32]

A recent study suggests that cooler stars that emit more light in the infrared and near infrared may actually host warmer planets with less ice and incidence of snowball states. These wavelengths are absorbed by their planets' ice and greenhouse gases and remain warmer.[33][34]

A 2020 study found that about half of Sun-like stars could host rocky, potentially habitable planets. Specifically, they estimated with that, on average, the nearest habitable zone planet around G and K-type stars is about 6 parsecs away, and there are about 4 rocky planets around G and K-type stars within 10 parsecs (32.6 light years) of the Sun.[35]

A stable habitable zone

Main article: Habitable zone

The habitable zone (HZ) is a shell-shaped region of space surrounding a star in which a planet could maintain liquid water on its surface.[21] The concept was first proposed by astrophysicist Su-Shu Huang in 1959, based on climatic constraints imposed by the host star.[21] After an energy source, liquid water is widely considered the most important ingredient for life, considering how integral it is to all life systems on Earth. However, if life is discovered in the absence of water, the definition of an HZ may have to be greatly expanded.

The inner edge of the HZ is the distance where runaway greenhouse effect vaporize the whole water reservoir and, as a second effect, induce the photodissociation of water vapor and the loss of hydrogen to space. The outer edge of the HZ is the distance from the star where a maximum greenhouse effect fails to keep the surface of the planet above the freezing point, and by CO
2 condensation.[21][3]

A "stable" HZ implies two factors. First, the range of an HZ should not vary greatly over time. All stars increase in luminosity as they age, and a given HZ thus migrates outwards, but if this happens too quickly (for example, with a super-massive star) planets may only have a brief window inside the HZ and a correspondingly smaller chance of developing life. Calculating an HZ range and its long-term movement is never straightforward, as negative feedback loops such as the CNO cycle will tend to offset the increases in luminosity. Assumptions made about atmospheric conditions and geology thus have as great an impact on a putative HZ range as does stellar evolution: the proposed parameters of the Sun's HZ, for example, have fluctuated greatly.[36]

Second, no large-mass body such as a

habitable moons under the right conditions.[37]

Low stellar variation

Main article: Variable star

Changes in

Atmospheres
do mitigate such effects, but their atmosphere might not be retained by planets orbiting variables, because the high-frequency energy buffeting these planets would continually strip them of their protective covering.

The Sun, in this respect as in many others, is relatively benign: the variation between its maximum and minimum energy output is roughly 0.1% over its 11-year

evidence that even minor changes in the Sun's luminosity have had significant effects on the Earth's climate well within the historical era: the Little Ice Age of the mid-second millennium, for instance, may have been caused by a relatively long-term decline in the Sun's luminosity.[38] Thus, a star does not have to be a true variable for differences in luminosity to affect habitability. Of known solar analogs, one that closely resembles the Sun is considered to be 18 Scorpii; unfortunately for the prospects of life existing in its proximity, the only significant difference between the two bodies is the amplitude of the solar cycle, which appears to be much greater for 18 Scorpii.[39]

High metallicity

See also: Metallicity

While the bulk of material in any star is

solar nebula theory of planetary system formation. Any planets that did form around a metal-poor star would probably be low in mass, and thus unfavorable for life. Spectroscopic studies of systems where exoplanets have been found to date confirm the relationship between high metal content and planet formation: "Stars with planets, or at least with planets similar to the ones we are finding today, are clearly more metal rich than stars without planetary companions."[40] This relationship between high metallicity and planet formation also means that habitable systems are more likely to be found around stars of younger generations, since stars that formed early in the universe
's history have low metal content.

Planetary characteristics

The moons of some gas giants could potentially be habitable.[41]

Habitability indicators and biosignatures must be interpreted within a planetary and environmental context.[2] Whether a planet will emerge as habitable depends on the sequence of events that led to its formation, which could include the production of organic molecules in molecular clouds and protoplanetary disks, delivery of materials during and after planetary accretion, and the orbital location in the planetary system.[2] The chief assumption about habitable planets is that they are terrestrial. Such planets, roughly within one order of magnitude of Earth mass, are primarily composed of silicate rocks, and have not accreted the gaseous outer layers of hydrogen and helium found on gas giants. The possibility that life could evolve in the cloud tops of giant planets has not been decisively ruled out,[c] though it is considered unlikely, as they have no surface and their gravity is enormous.[44] The natural satellites of giant planets, meanwhile, remain valid candidates for hosting life.[41]

In February 2011 the

list of 1235 extrasolar planet candidates, including 54 that may be in the habitable zone.[45][46] Six of the candidates in this zone are smaller than twice the size of Earth.[45] A more recent study found that one of these candidates (KOI 326.01) is much larger and hotter than first reported.[47] Based on the findings, the Kepler team estimated there to be "at least 50 billion planets in the Milky Way" of which "at least 500 million" are in the habitable zone.[48]

In analyzing which environments are likely to support life, a distinction is usually made between simple, unicellular organisms such as bacteria and archaea and complex metazoans (animals). Unicellularity necessarily precedes multicellularity in any hypothetical tree of life, and where single-celled organisms do emerge there is no assurance that greater complexity will then develop.[d] The planetary characteristics listed below are considered crucial for life generally, but in every case multicellular organisms are more picky than unicellular life.

Mass and size

Mars, with its rarefied atmosphere, is colder than the Earth would be if it were at a similar distance from the Sun.

Low-mass planets are poor candidates for life for two reasons. First, their lesser

atmosphere retention difficult. Constituent molecules are more likely to reach escape velocity and be lost to space when buffeted by solar wind or stirred by collision. Planets without a thick atmosphere lack the matter necessary for primal biochemistry, have little insulation and poor heat transfer across their surfaces (for example, Mars, with its thin atmosphere, is colder than the Earth would be if it were at a similar distance from the Sun), and provide less protection against meteoroids and high-frequency radiation. Further, where an atmosphere is less dense than 0.006 Earth atmospheres, water cannot exist in liquid form as the required atmospheric pressure, 4.56 mm Hg (608 Pa) (0.18 inch Hg
), does not occur. In addition, a lessened pressure reduces the range of temperatures at which water is liquid.

Secondly, smaller planets have smaller

bio-diversity through continent creation and increased environmental complexity and helps create the convective cells necessary to generate Earth's magnetic field.[49]

"Low mass" is partly a relative label: the Earth is low mass when compared to the Solar System's

radioactive elements within a planet's core is the other significant component of planetary heating). Mars, by contrast, is nearly (or perhaps totally) geologically dead and has lost much of its atmosphere.[50] Thus it would be fair to infer that the lower mass limit for habitability lies somewhere between that of Mars and that of Earth or Venus: 0.3 Earth masses has been offered as a rough dividing line for habitable planets.[51] However, a 2008 study by the Harvard-Smithsonian Center for Astrophysics suggests that the dividing line may be higher. Earth may in fact lie on the lower boundary of habitability: if it were any smaller, plate tectonics would be impossible. Venus, which has 85% of Earth's mass, shows no signs of tectonic activity. Conversely, "super-Earths", terrestrial planets with higher masses than Earth, would have higher levels of plate tectonics and thus be firmly placed in the habitable range.[52]

Exceptional circumstances do offer exceptional cases: Jupiter's moon Io (which is smaller than any of the terrestrial planets) is volcanically dynamic because of the gravitational stresses induced by its orbit, and its neighbor Europa may have a liquid ocean or icy slush underneath a frozen shell also due to power generated from orbiting a gas giant.

Saturn's Titan, meanwhile, has an outside chance of harbouring life, as it has retained a thick atmosphere and has liquid methane seas on its surface. Organic-chemical reactions that only require minimum energy are possible in these seas, but whether any living system can be based on such minimal reactions is unclear, and would seem unlikely.[53][54] These satellites are exceptions, but they prove that mass, as a criterion for habitability, cannot necessarily be considered definitive at this stage of our understanding.[55]

A larger planet is likely to have a more massive atmosphere. A combination of higher escape velocity to retain lighter atoms, and extensive outgassing from enhanced plate tectonics may greatly increase the atmospheric pressure and temperature at the surface compared to Earth. The enhanced greenhouse effect of such a heavy atmosphere would tend to suggest that the habitable zone should be further out from the central star for such massive planets.

Finally, a larger planet is likely to have a large iron core. This allows for a

dynamo effect within its core[56]
—but it is a significant component of the process.

The mass of a potentially habitable exoplanet is between 0.1 and 5.0 Earth masses.[18] However it is possible for a habitable world to have a mass as low as 0.0268 Earth Masses.[57] The radius of a potentially habitable exoplanet would range between 0.5 and 1.5 Earth radii.[18]

Orbit and rotation

As with other criteria, stability is the critical consideration in evaluating the effect of orbital and rotational characteristics on planetary habitability.

freezing point and boiling point of the planet's main biotic solvent (e.g., water on Earth). If, for example, Earth's oceans were alternately boiling and freezing solid, it is difficult to imagine life as we know it having evolved. The more complex the organism, the greater the temperature sensitivity.[58] The Earth's orbit is almost perfectly circular, with an eccentricity of less than 0.02; other planets in the Solar System (with the exception of Mercury
) have eccentricities that are similarly benign.

Habitability is also influenced by the architecture of the planetary system around a star. The evolution and stability of these systems are determined by gravitational dynamics, which drive the orbital evolution of terrestrial planets. Data collected on the orbital eccentricities of extrasolar planets has surprised most researchers: 90% have an orbital eccentricity greater than that found within the Solar System, and the average is fully 0.25.[59] This means that the vast majority of planets have highly eccentric orbits and of these, even if their average distance from their star is deemed to be within the HZ, they nonetheless would be spending only a small portion of their time within the zone.

A planet's movement around its rotational axis must also meet certain criteria if life is to have the opportunity to evolve. A first assumption is that the planet should have moderate seasons. If there is little or no axial tilt (or obliquity) relative to the perpendicular of the ecliptic, seasons will not occur and a main stimulant to biospheric dynamism will disappear. The planet would also be colder than it would be with a significant tilt: when the greatest intensity of radiation is always within a few degrees of the equator, warm weather cannot move poleward and a planet's climate becomes dominated by colder polar weather systems.

If a planet is radically tilted, seasons will be extreme and make it more difficult for a biosphere to achieve homeostasis. The axial tilt of the Earth is higher now (in the Quaternary) than it has been in the past, coinciding with reduced polar ice, warmer temperatures and less seasonal variation. Scientists do not know whether this trend will continue indefinitely with further increases in axial tilt (see Snowball Earth).

The exact effects of these changes can only be computer modelled at present, and studies have shown that even extreme tilts of up to 85 degrees do not absolutely preclude life "provided it does not occupy continental surfaces plagued seasonally by the highest temperature."[60] Not only the mean axial tilt, but also its variation over time must be considered. The Earth's tilt varies between 21.5 and 24.5 degrees over 41,000 years. A more drastic variation, or a much shorter periodicity, would induce climatic effects such as variations in seasonal severity.

Other orbital considerations include:

  • The planet should rotate relatively quickly so that the day-night cycle is not overlong. If a day takes years, the temperature differential between the day and night side will be pronounced, and problems similar to those noted with extreme orbital eccentricity will come to the fore.
  • The planet also should rotate quickly enough so that a magnetic dynamo may be started in its iron core to produce a magnetic field.
  • Change in the direction of the axis rotation (precession) should not be pronounced. In itself, precession need not affect habitability as it changes the direction of the tilt, not its degree. However, precession tends to accentuate variations caused by other orbital deviations; see Milankovitch cycles. Precession on Earth occurs over a 26,000-year cycle.

The Earth's Moon appears to play a crucial role in moderating the Earth's climate by stabilising the axial tilt. It has been suggested that a chaotic tilt may be a "deal-breaker" in terms of habitability—i.e. a satellite the size of the Moon is not only helpful but required to produce stability.[61] This position remains controversial.[f]

In the case of the Earth, the sole Moon is sufficiently massive and orbits so as to significantly contribute to

ocean tides, which in turn aids the dynamic churning of Earth's large liquid water oceans. These lunar forces not only help ensure that the oceans do not stagnate, but also play a critical role in Earth's dynamic climate.[62][63]

Geology

Geological cross section of Earth
A visualization showing a simple model of Earth's magnetic field

Concentrations of

emission spectra of stars could be used to identify those which are more likely to host habitable Earth-like planets. As of 2020, radionuclides are thought to be produced by rare stellar processes such as neutron star mergers.[64][65]

Additional geological characteristics may be essential or major factors in the habitability of natural celestial bodies – including some that may shape the body's heat and magnetic field. Some of these are unknown or not well understood and being investigated by

planetary scientists, geochemists and others.[66][additional citation(s) needed
]

Geochemistry

Further information: Geochemistry

It is generally assumed that any extraterrestrial life that might exist will be based on the same fundamental

amino acids, which in turn are the building blocks of proteins, the substance of living tissue. In addition, neither sulfur (required for the building of proteins) nor phosphorus (needed for the formation of DNA, RNA, and the adenosine phosphates essential to metabolism
) are rare.

Relative abundance in space does not always mirror differentiated abundance within planets; of the four life elements, for instance, only

atmospheres. The Miller–Urey experiment showed that, with the application of energy, simple inorganic compounds exposed to a primordial atmosphere can react to synthesize amino acids.[69]

Even so,

origin of life
.

Thus, while there is reason to suspect that the four "life elements" ought to be readily available elsewhere, a habitable system probably also requires a supply of long-term orbiting bodies to seed inner planets. Without comets there is a possibility that life as we know it would not exist on Earth.

Microenvironments and extremophiles

The Atacama Desert in South America provides an analog to Mars and an ideal environment to study the boundary between sterility and habitability.

One important qualification to habitability criteria is that only a tiny portion of a planet is required to support life, a so-called Goldilocks Edge or Great Prebiotic Spot. Astrobiologists often concern themselves with "micro-environments", noting that "we lack a fundamental understanding of how evolutionary forces, such as

hydrothermal vents
.

The discovery of life in extreme conditions has complicated definitions of habitability, but also generated much excitement amongst researchers in greatly broadening the known range of conditions under which life can persist. For example, a planet that might otherwise be unable to support an atmosphere given the solar conditions in its vicinity, might be able to do so within a deep shadowed rift or volcanic cave.[72] Similarly, craterous terrain might offer a refuge for primitive life. The Lawn Hill crater has been studied as an astrobiological analog, with researchers suggesting rapid sediment infill created a protected microenvironment for microbial organisms; similar conditions may have occurred over the geological history of Mars.[73]

Earth environments that cannot support life are still instructive to astrobiologists in defining the limits of what organisms can endure. The heart of the

Viking landings on Mars in the 1970s; no DNA could be recovered from two soil samples, and incubation experiments were also negative for biosignatures.[75]

Ecological factors

The two current ecological approaches for predicting the potential habitability use 19 or 20 environmental factors, with emphasis on water availability, temperature, presence of nutrients, an energy source, and protection from solar ultraviolet and galactic cosmic radiation.[76][77]

Some habitability factors[77]
Water  · Activity of liquid water
 · Past or future liquid (ice) inventories
 · Salinity, pH, and Eh of available water
Chemical environment Nutrients:
 · C, H, N, O, P, S, essential metals, essential micronutrients
 ·
Heavy metals
(e.g. Zn, Ni, Cu, Cr, As, Cd, etc.; some are essential, but toxic at high levels)
 · Globally distributed oxidizing soils
Energy for metabolism Solar (surface and near-surface only)
Geochemical (subsurface)
 · Oxidants
 · Reductants
 · Redox gradients
Conducive
physical conditions		  · Temperature
 · Extreme diurnal temperature fluctuations
 · Low pressure (is there a low-pressure threshold for terrestrial
solar particle events (long-term accumulated effects)
 · Solar UV-induced volatile oxidants, e.g. O 2−, O−, H2O2, O3
 · Climate and its variability (geography, seasons, diurnal, and eventually, obliquity variations)
 · Substrate (soil processes, rock microenvironments, dust composition, shielding)
 · High CO2 concentrations in the global atmosphere
 · Transport (aeolian
, ground water flow, surface water, glacial)

Classification terminology

The Habitable Exoplanets Catalog[78] uses estimated surface temperature range to classify exoplanets:

  • hypopsychroplanets - very cold (<−50 °C)
  • psychroplanets - cold (<−50 to 0 °C)
  • mesoplanets - medium temperature (0–50 °C; not to be confused with the other definition of mesoplanets)
  • thermoplanets - hot (50-100 °C)
  • hyperthermoplanets - (> 100 °C)

Mesoplanets would be ideal for complex life, whereas hypopsychroplanets and hyperthermoplanets might only support extremophilic life.

The HEC uses the following terms to classify exoplanets in terms of mass, from least to greatest: asteroidan, mercurian, subterran, terran, superterran, neptunian, and jovian.

Alternative star systems

In determining the feasibility of extraterrestrial life, astronomers had long focused their attention on stars like the Sun. However, since planetary systems that resemble the Solar System are proving to be rare, they have begun to explore the possibility that life might form in systems very unlike the Sun's.[79][80]

It is believed that

Kepler Space Telescope.[82]

Binary systems

Main article: Habitability of binary star systems

Typical estimates often suggest that 50% or more of all stellar systems are binary systems. This may be partly sample bias, as massive and bright stars tend to be in binaries and these are most easily observed and catalogued; a more precise analysis has suggested that the more common fainter stars are usually singular, and that up to two thirds of all stellar systems are therefore solitary.[83]

The separation between stars in a binary may range from less than one

Carnegie Institution has shown that gas giants can form around stars in binary systems much as they do around solitary stars.[85]

One study of

semi-major axis deviating by less than 5% during 32 000 binary periods). The continuous habitable zone (CHZ for 4.5 billion years) for Centauri A is conservatively estimated at 1.2 to 1.3 AU and Centauri B at 0.73 to 0.74—well within the stable region in both cases.[86]

Red dwarf systems

Main article: Habitability of red dwarf systems
Aurelia. Credit: MPIA/V. Joergens.

M-type stars also considered possible hosts of habitable exoplanets, even those with flares such as Proxima b. Determining the habitability of red dwarf stars could help determine how common life in the universe might be, as red dwarfs make up between 70 and 90% of all the stars in the galaxy. However, it is important to bear in mind that flare stars could greatly reduce the habitability of exoplanets by eroding their atmosphere.[87]

Size

Astronomers for many years ruled out red dwarfs as potential abodes for life. Their small size (from 0.08 to 0.45 solar masses) means that their

habitable moon, which would be locked to the planet instead of the star, allowing a more even distribution of radiation over the moon. It was long assumed that such a thick atmosphere would prevent sunlight from reaching the surface in the first place, preventing photosynthesis
.

Greenwich Community College, has shown that seawater, too, could be effectively circulated without freezing solid if the ocean basins were deep enough to allow free flow beneath the night side's ice cap. Further research—including a consideration of the amount of photosynthetically active radiation—suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants.[90]

Other factors limiting habitability

Size is not the only factor in making red dwarfs potentially unsuitable for life, however. On a red dwarf planet, photosynthesis on the night side would be impossible, since it would never see the sun. On the day side, because the sun does not rise or set, areas in the shadows of mountains would remain so forever. Photosynthesis as we understand it would be complicated by the fact that a red dwarf produces most of its radiation in the infrared, and on the Earth the process depends on visible light. There are potential positives to this scenario. Numerous terrestrial ecosystems rely on chemosynthesis rather than photosynthesis, for instance, which would be possible in a red dwarf system. A static primary star position removes the need for plants to steer leaves toward the sun, deal with changing shade/sun patterns, or change from photosynthesis to stored energy during night. Because of the lack of a day-night cycle, including the weak light of morning and evening, far more energy would be available at a given radiation level.

Red dwarfs are far more variable and violent than their more stable, larger cousins. Often they are covered in starspots that can dim their emitted light by up to 40% for months at a time, while at other times they emit gigantic flares that can double their brightness in a matter of minutes.[91] Such variation would be very damaging for life, as it would not only destroy any complex organic molecules that could possibly form biological precursors, but also because it would blow off sizeable portions of the planet's atmosphere.

For a planet around a red dwarf star to support life, it would require a rapidly rotating magnetic field to protect it from the flares. A tidally locked planet rotates only very slowly, and so cannot produce a geodynamo at its core. The violent flaring period of a red dwarf's life cycle is estimated to only last roughly the first 1.2 billion years of its existence. If a planet forms far away from a red dwarf so as to avoid tidal locking, and then migrates into the star's habitable zone after this turbulent initial period, it is possible that life may have a chance to develop.

stellar flare, showing that it is a flare star.[93]

Longevity and ubiquity

Red dwarfs have one advantage over other stars as abodes for life: far greater longevity. It took 4.5 billion years before humanity appeared on Earth, and life as we know it will see suitable conditions for 1[94] to 2.3[95] billion years more. Red dwarfs, by contrast, could live for trillions of years because their nuclear reactions are far slower than those of larger stars, meaning that life would have longer to evolve and survive.

While the likelihood of finding a planet in the habitable zone around any specific red dwarf is slight, the total amount of habitable zone around all red dwarfs combined is equal to the total amount around Sun-like stars given their ubiquity.[96] Furthermore, this total amount of habitable zone will last longer, because red dwarf stars live for hundreds of billions of years or even longer on the main sequence.[97] However, combined with the above disadvantages, it is more likely that red dwarf stars would remain habitable longer to microbes, while the shorter-lived yellow dwarf stars, like the Sun, would remain habitable longer to animals.

F-type star systems

Main article: Habitability of F-type main-sequence star systems

G-type star systems

Main article: Habitability of yellow dwarf systems

G-type stars would allow to host the exoplanets most similar to Earth, that is, Earth-like planets.[98]

K-type star systems

Main article: Habitability of K-type main-sequence star systems

K-type stars would provide the necessary conditions for

super habitable exoplanets, which are exoplanets that could be more habitable than Earth.[99]

Massive stars

Recent research suggests that very large stars, greater than ~100 solar masses, could have planetary systems consisting of hundreds of Mercury-sized planets within the habitable zone. Such systems could also contain

brown dwarfs and low-mass stars (~0.1–0.3 solar masses).[100] However the very short lifespans of stars of more than a few solar masses would scarcely allow time for a planet to cool, let alone the time needed for a stable biosphere to develop. Massive stars are thus eliminated as possible abodes for life.[101]

However, a massive-star system could be a progenitor of life in another way – the supernova explosion of the massive star in the central part of the system. This supernova will disperse heavier elements throughout its vicinity, created during the phase when the massive star has moved off of the main sequence, and the systems of the potential low-mass stars (which are still on the main sequence) within the former massive-star system may be enriched with the relatively large supply of the heavy elements so close to a supernova explosion. However, this states nothing about what types of planets would form as a result of the supernova material, or what their habitability potential would be.

Neutron stars

Main article: Habitability of neutron star systems

Post-main sequence stars

Main article: Red giant § Prospects for habitability

Four classes of habitable planets based on water

In a review of the factors which are important for the evolution of habitable Earth-sized planets, Lammer et al. proposed a classification of four water-dependent habitat types:[21][102]

Class I habitats are planetary bodies on which stellar and geophysical conditions allow liquid water to be available at the surface, along with sunlight, so that complex multicellular organisms may originate.

Class II habitats include bodies which initially enjoy Earth-like conditions, but do not keep their ability to sustain liquid water on their surface due to stellar or geophysical conditions. Mars, and possibly Venus are examples of this class where complex life forms may not develop.

Class III habitats are planetary bodies where liquid water oceans exist below the surface, where they can interact directly with a silicate-rich core.

Such a situation can be expected on water-rich planets located too far from their star to allow surface liquid water, but on which subsurface water is in liquid form because of the
Enceladus. In such worlds, not only is light not available as an energy source, but the organic material brought by meteorites (thought to have been necessary to start life in some scenarios) may not easily reach the liquid water. If a planet can only harbor life below its surface, the biosphere
would not likely modify the whole planetary environment in an observable way, thus, detecting its presence on an exoplanet would be extremely difficult.

Class IV habitats have liquid water layers between two ice layers, or liquids above ice.

If the water layer is thick enough, water at its base will be in solid phase (ice polymorphs) because of the high pressure. Ganymede and Callisto are likely examples of this class. Their oceans are thought to be enclosed between thick ice layers. In such conditions, the emergence of even simple life forms may be very difficult because the necessary ingredients for life will likely be completely diluted.

The galactic neighborhood

Along with the characteristics of planets and their star systems, the wider galactic environment may also impact habitability. Scientists considered the possibility that particular areas of galaxies (galactic habitable zones) are better suited to life than others; the Solar System, in the Orion Arm, on the Milky Way galaxy's edge is considered to be in a life-favorable spot:[103]

Thus, relative isolation is ultimately what a life-bearing system needs. If the Sun were crowded amongst other systems, the chance of being fatally close to dangerous radiation sources would increase significantly. Further, close neighbors might disrupt the stability of various orbiting bodies such as Oort cloud and Kuiper belt objects, which can bring catastrophe if knocked into the inner Solar System.

While stellar crowding proves disadvantageous to habitability, so too does extreme isolation. A star as metal-rich as the Sun would probably not have formed in the very outermost regions of the Milky Way given a decline in the relative abundance of metals and a general lack of star formation. Thus, a "suburban" location, such as the Solar System enjoys, is preferable to a Galaxy's center or farthest reaches.[105]

Other considerations

Alternative biochemistries

Main article: Hypothetical types of biochemistry

While most investigations of extraterrestrial life start with the assumption that advanced life-forms must have similar requirements for life as on Earth, the hypothesis of

hydrocarbons are sometimes suggested as alternative solvents to water. The astrobiologist Dirk Schulze-Makuch and other scientists have proposed a Planet Habitability Index whose criteria include "potential for holding a liquid solvent" that is not necessarily restricted to water.[106][107]

More speculative ideas have focused on bodies altogether different from Earth-like planets. Astronomer Frank Drake, a well-known proponent of the search for extraterrestrial life, imagined life on a neutron star: submicroscopic "nuclear molecules" combining to form creatures with a life cycle millions of times quicker than Earth life.[108] Called "imaginative and tongue-in-cheek", the idea gave rise to science fiction depictions.[109] Carl Sagan, another optimist with regards to extraterrestrial life, considered the possibility of organisms that are always airborne within the high atmosphere of Jupiter in a 1976 paper.[42][43] Cohen and Stewart also envisioned life in both a solar environment and in the atmosphere of a gas giant.

"Good Jupiters"

"Good Jupiters" are gas giants, like the Solar System's Jupiter, that orbit their stars in circular orbits far enough away from the habitable zone not to disturb it but close enough to "protect" terrestrial planets in closer orbit in two critical ways. First, they help to stabilize the orbits, and thereby the climates of the inner planets. Second, they keep the inner stellar system relatively free of comets and asteroids that could cause devastating impacts.[110] Jupiter orbits the Sun at about five times the distance between the Earth and the Sun. This is the rough distance we should expect to find good Jupiters elsewhere. Jupiter's "caretaker" role was dramatically illustrated in 1994 when Comet Shoemaker–Levy 9 impacted the giant.[according to whom?]

However, the evidence is not quite so clear. Research has shown that Jupiter's role in determining the rate at which objects hit Earth is significantly more complicated than once thought.[111][112][113][114]

The role of Jupiter in the early history of the Solar System is somewhat better established, and the source of significantly less debate.[115][116] Early in the Solar System's history, Jupiter is accepted as having played an important role in the hydration of our planet: it increased the eccentricity of asteroid belt orbits and enabled many to cross Earth's orbit and supply the planet with important volatiles such as water and carbon dioxide. Before Earth reached half its present mass, icy bodies from the Jupiter–Saturn region and small bodies from the primordial asteroid belt supplied water to the Earth due to the gravitational scattering of Jupiter and, to a lesser extent, Saturn.[117] Thus, while the gas giants are now helpful protectors, they were once suppliers of critical habitability material.

In contrast, Jupiter-sized bodies that orbit too close to the habitable zone but not in it (as in 47 Ursae Majoris), or have a highly elliptical orbit that crosses the habitable zone (like 16 Cygni B) make it very difficult for an independent Earth-like planet to exist in the system. See the discussion of a stable habitable zone above. However, during the process of migrating into a habitable zone, a Jupiter-size planet may capture a terrestrial planet as a moon. Even if such a planet is initially loosely bound and following a strongly inclined orbit, gravitational interactions with the star can stabilize the new moon into a close, circular orbit that is coplanar with the planet's orbit around the star.[118]

Life's impact on habitability

A supplement to the factors that support life's emergence is the notion that life itself, once formed, becomes a habitability factor in its own right. An important Earth example was the production of molecular oxygen gas (O
2) by ancient cyanobacteria, and eventually photosynthesizing plants, leading to a radical change in the composition of Earth's atmosphere. This environmental change is called the Great Oxidation Event. This oxygen proved fundamental to the respiration of later animal species. The Gaia hypothesis, a scientific model of the geo-biosphere pioneered by James Lovelock in 1975, argues that life as a whole fosters and maintains suitable conditions for itself by helping to create a planetary environment suitable for its continuity. Similarly, David Grinspoon has suggested a "living worlds hypothesis" in which our understanding of what constitutes habitability cannot be separated from life already extant on a planet. Planets that are geologically and meteorologically alive are much more likely to be biologically alive as well and "a planet and its life will co-evolve."[119] This is the basis of Earth system science.

The role of chance

In 2020, a computer simulation of the evolution of planetary climates over 3 billion years suggested that feedbacks are a necessary but not a sufficient condition for preventing planets from ever becoming too hot or cold for life, and that chance also plays a crucial role.[120][121] Related considerations include yet unknown factors influencing the thermal habitability of planets such as "feedback mechanism (or mechanisms) that prevents the climate ever wandering to fatal temperatures".[122]

See also

Notes

  1. Planets in science fiction
    .
  2. have emerged
    on Earth approximately 500 million years after the planet's formation. "A" class stars (which shine for between 600 million and 1.2 billion years) and the very latest of the "B" class stars (which shine 10+ million to 600 million) fall within this window. At least theoretically life could emerge in such systems but it would almost certainly not reach a sophisticated level given these time-frames and the fact that increases in luminosity would occur quite rapidly. Life around "O" class stars is exceptionally unlikely, as they shine for less than ten million years.
  3. Jack Cohen and Ian Stewart evaluate plausible scenarios in which life might form in the cloud-tops of Jovian planets. Similarly, Carl Sagan suggested that the clouds of Jupiter might host life.[42][43]
  4. ^ There is an emerging consensus that single-celled micro-organisms may in fact be common in the universe, especially since Earth's extremophiles flourish in environments that were once considered hostile to life. The potential occurrence of complex multi-celled life remains much more controversial. In their work Rare Earth: Why Complex Life Is Uncommon in the Universe, Peter Ward and Donald Brownlee argue that microbial life is probably widespread while complex life is very rare and perhaps even unique to Earth. Current knowledge of Earth's history partly buttresses this theory: multi-celled organisms are believed to have emerged at the time of the Cambrian explosion close to 600 million years ago, but more than 3 billion years after life first appeared. That Earth life remained unicellular for so long underscores that the decisive step toward complex organisms need not necessarily occur.
  5. ^ There is a "mass-gap" in the Solar System between Earth and the two smallest gas giants, Uranus and Neptune, which are 13 and 17 Earth masses. This is probably just chance, as there is no geophysical barrier to the formation of intermediate bodies (see for instance OGLE-2005-BLG-390Lb and Super-Earth) and we should expect to find planets throughout the galaxy between two and twelve Earth masses. If the star system is otherwise favorable, such planets would be good candidates for life as they would be large enough to remain internally dynamic and to retain an atmosphere for billions of years but not so large as to accrete a gaseous shell which limits the possibility of life formation.
  6. giant impact hypothesis
    ). In Rare Earth Ward and Brownlee emphasize that such impacts ought to be rare, reducing the probability of other Earth-Moon type systems and hence the probability of other habitable planets. Other moon formation processes are possible, however, and the proposition that a planet may be habitable in the absence of a moon has not been disproven.

References

  1. ^ a b Dyches, Preston; Chou, Felcia (7 April 2015). "The Solar System and Beyond is Awash in Water". NASA. Archived from the original on 10 April 2015. Retrieved 8 April 2015.
  2. ^ a b c d e NASA (October 2015), NASA Astrobiology Strategy (PDF)[permanent dead link]
  3. ^
    S2CID 206546351
    .
  4. doi:10.1016/0921-8009(95)00048-8
    .
  5. ^ "Goal 1: Understand the nature and distribution of habitable environments in the Universe". Astrobiology: Roadmap. NASA. Archived from the original on 17 January 2011. Retrieved 11 August 2007.
  6. EurekAlert
    . Retrieved 1 September 2018.
  7. S2CID 46991835
    .
  8. ^ Wolszczan, A.; Frail, D. A. (9 January 1992). "A planetary system around the millisecond pulsar PSR1257 + 12". Nature. 355 (6356): 145–147.
    S2CID 4260368
    .
  9. S2CID 19621191
    .
  10. ^ Graham, Robert W. (February 1990). "NASA Technical Memorandum 102363 – Extraterrestrial Life in the Universe" (PDF). NASA. Lewis Research Center, Ohio. Retrieved 7 July 2014.
  11. ISBN 978-1-4020-8836-0
    .
  12. ISBN 978-3-527-40660-9
    .
  13. ^ Davies, Paul (18 November 2013). "Are We Alone in the Universe?". The New York Times. Retrieved 20 November 2013.
  14. ^ Overbye, Dennis (6 January 2015). "As Ranks of Goldilocks Planets Grow, Astronomers Consider What's Next". The New York Times. Retrieved 6 January 2015.
  15. ^ a b Overbye, Dennis (4 November 2013). "Far-Off Planets Like the Earth Dot the Galaxy". The New York Times. Retrieved 5 November 2013.
  16. ^
    PMID 24191033
    .
  17. ^ Khan, Amina (4 November 2013). "Milky Way may host billions of Earth-size planets". Los Angeles Times. Retrieved 5 November 2013.
  18. ^ a b c "The Habitable Exoplanets Catalog – Planetary Habitability Laboratory @ UPR Arecibo". phl.upr.edu. Retrieved 19 August 2021.
  19. ^ University of Cambridge (25 August 2021). "New class of habitable exoplanets represent a big step forward in the search for life". Phys.org. Retrieved 25 August 2021.
  20. ^ Staff (27 August 2021). "Alien life could be living on big 'Hycean' exoplanets". BBC News. Retrieved 27 August 2021.
  21. ^
    S2CID 123220355. Archived from the original
    (PDF) on 2 June 2016. Retrieved 3 May 2016.
  22. S2CID 14734094. Archived from the original
    (PDF) on 22 February 2006. Habitability criteria defined—the foundational source for this article.
  23. ^ Choi, Charles Q. (21 August 2015). "Giant Galaxies May Be Better Cradles for Habitable Planets". Space.com. Retrieved 24 August 2015.
  24. OCLC 1247136170.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  25. ^ "Star tables". California State University, Los Angeles. Archived from the original on 14 June 2008. Retrieved 12 August 2010.
  26. S2CID 9685420
    .
  27. ^ "Light Absorption for Photosynthesis" (Graphic with references). phy-astr.gus.edu. Georgia State University. Retrieved 1 May 2018. It is evident from these absorption and output plots that only the red and blue ends of the visible part of the electromagnetic spectrum are used by plants in photosynthesis. The reflection and transmission of the middle of the spectrum gives the leaves their green visual color.
  28. ^ Guinan, Edward; Cuntz, Manfred (10 August 2009). "The violent youth of solar proxies steer course of genesis of life". International Astronomical Union. Retrieved 27 August 2009.
  29. ^ "Gliese 581: one planet might indeed be habitable" (Press release). Astronomy & Astrophysics. 13 December 2007. Retrieved 7 April 2008.
  30. ^ Staff (20 September 2012). "LHS 188 – High proper-motion Star". Centre de données astronomiques de Strasbourg (Strasbourg astronomical Data Center). Retrieved 20 September 2012.
  31. ^ a b Méndez, Abel (29 August 2012). "A Hot Potential Habitable Exoplanet around Gliese 163". University of Puerto Rico at Arecibo (Planetary Habitability Laboratory). Archived from the original on 21 October 2019. Retrieved 20 September 2012.
  32. ^ a b Redd, Nola Taylor (20 September 2012). "Newfound Alien Planet a Top Contender to Host Life". Space.com. Retrieved 20 September 2012.
  33. ^ "Planets May Keep Warmer in a Cool Star System". Redorbit. 19 July 2013.
  34. PMID 23855332
    .
  35. ^ Center, By Frank Tavares NASA's Ames Research. "About Half of Sun-Like Stars Could Host Rocky, Potentially Habitable Planets". Exoplanet Exploration: Planets Beyond our Solar System. Retrieved 14 December 2020.
  36. PMID 11536936. Archived from the original
    (PDF) on 18 March 2009. Retrieved 6 August 2007.
  37. S2CID 4233894
    .
  38. ^ "The Little Ice Age". Department of Atmospheric Science. University of Washington. Archived from the original on 9 February 2012. Retrieved 11 May 2007.
  39. ^ "18 Scorpii". solstation.com. Sol Company. Retrieved 11 May 2007.
  40. University of Colorado. Archived from the original
    (PDF) on 15 April 2012. Retrieved 11 August 2007.
  41. ^ a b "An interview with Dr. Darren Williams". Astrobiology: The Living Universe. 2000. Archived from the original on 28 August 2007. Retrieved 5 August 2007.
  42. ^
    hdl:2060/19760019038
    .
  43. ^
    Darling, David. "Jupiter, life on". The Encyclopedia of Astrobiology, Astronomy, and Spaceflight. Archived from the original
    on 10 February 2012. Retrieved 6 August 2007.
  44. ^ "Could there be life in the outer solar system?". Millennium Mathematics Project, Videoconferences for Schools. University of Cambridge. 2002. Retrieved 5 August 2007.
  45. ^
    S2CID 15233153
    .
  46. ^ "NASA Finds Earth-size Planet Candidates in Habitable Zone, Six Planet System". NASA. 2 February 2011. Archived from the original on 29 April 2011. Retrieved 2 February 2011.
  47. Discover Magazine. Archived from the original
    on 15 March 2014. Retrieved 9 March 2011.
  48. ^ Borenstein, Seth (19 February 2011). "Cosmic census finds crowd of planets in our galaxy". Associated Press. Retrieved 14 January 2024.
  49. ^ Ward, pp. 191–220
  50. ^ "The Heat History of the Earth". Geolab. James Madison University. Retrieved 11 May 2007.
  51. S2CID 10257401
    .
  52. ^ "Earth: A Borderline Planet for Life?". Harvard-Smithsonian Center for Astrophysics. 2008. Retrieved 4 June 2008.
  53. ^ "Titan Most Likely Non-Habitable, Astrobiologists Say | Sci.News". Sci.News: Breaking Science News. 20 February 2024. Retrieved 19 April 2024.
  54. PMID 38306187
    .
  55. ^ "Most liveable alien worlds ranked". Science & Environment. BBC News. 23 November 2011. Retrieved 16 August 2017.
  56. ^ Nave, C. R. "Magnetic Field of the Earth". HyperPhysics. Georgia State University. Retrieved 11 May 2007.
  57. S2CID 195584241
    .
  58. ^ Ward, pp. 122–123.
  59. ^ Bortman, Henry (22 June 2005). "Elusive Earths". Astrobiology Magazine. Archived from the original on 19 June 2021. Retrieved 8 June 2020.{{cite web}}: CS1 maint: unfit URL (link)
  60. Penn State University
    . 25 August 2003. Retrieved 11 May 2007.
  61. S2CID 4233758
    .
  62. ^ Dorminey, Bruce (29 April 2009). "Without the Moon, Would There Be Life on Earth?". scientificamerican.com. Scientific American. Retrieved 1 May 2018. Europa must have big tides, so it's my favorite for microbial life," says Max Bernstein, an astrochemist and program scientist at NASA Headquarters in Washington, D.C. "Europa is considered by many as the best place to find life in the solar system.
  63. ^ File:Tidalwaves1.gif
  64. ^ Woo, Marcus. "Stellar Smashups May Fuel Planetary Habitability, Study Suggests". Scientific American. Retrieved 9 December 2020.
  65. S2CID 226289878
    .
  66. ^ "The existence of a magnetic field beyond 3.5 billion years ago is still up for debate". phys.org. Retrieved 28 December 2020.
  67. ^ "Organic Molecule, Amino Acid-Like, Found in Constellation Sagittarius". ScienceDaily. 2008. Retrieved 20 December 2008.
  68. Darling, David. "Elements, biological abundance"
    . The Encyclopedia of Astrobiology, Astronomy, and Spaceflight. Retrieved 11 May 2007.
  69. ^ "How did chemistry and oceans produce this?". The Electronic Universe Project. University of Oregon. Retrieved 11 May 2007.
  70. ^ "How did the Earth Get to Look Like This?". The Electronic Universe Project. University of Oregon. Retrieved 11 May 2007.
  71. ^ "Understand the evolutionary mechanisms and environmental limits of life". Astrobiology: Roadmap. NASA. September 2003. Archived from the original on 26 January 2011. Retrieved 6 August 2007.
  72. ^ Hart, Stephen (17 June 2003). "Cave Dwellers: ET Might Lurk in Dark Places". Space.com. Archived from the original on 20 June 2003. Retrieved 6 August 2007.
  73. S2CID 20466013
    .
  74. ^ McKay, Christopher (June 2002). "Too Dry for Life: The Atacama Desert and Mars" (PDF). Ames Research Center. NASA. Archived from the original (PDF) on 26 August 2009. Retrieved 26 August 2009.
  75. S2CID 18220447
    .
  76. doi:10.1016/j.pss.2012.07.026
    .
  77. ^
    PMID 17067257
    , retrieved 6 June 2013
  78. ^ "PHL's Exoplanets Catalog - Planetary Habitability Laboratory @ UPR Arecibo". Archived from the original on 21 May 2019. Retrieved 1 December 2022.
  79. ^ "Does life exist outside of the solar system? | Center for Astrophysics | Harvard & Smithsonian". www.cfa.harvard.edu. Retrieved 19 April 2024.
  80. ^ Billings, Lee (1 June 2023). "We Live in the Rarest Type of Planetary System". Scientific American. Retrieved 19 April 2024.
  81. ^ "Habitable Exoplanet Observatory (HabEx)". www.jpl.nasa.gov. Retrieved 31 March 2020.
  82. ^ Center, By Frank Tavares NASA's Ames Research. "About Half of Sun-Like Stars Could Host Rocky, Potentially Habitable Planets". Exoplanet Exploration: Planets Beyond our Solar System. Retrieved 19 November 2020.
  83. Harvard-Smithsonian Center for Astrophysics. 30 January 2006. Archived from the original
    on 13 August 2007. Retrieved 5 June 2007.
  84. ^ "Stars and Habitable Planets". solstation.com. Sol Company. Archived from the original on 28 June 2011. Retrieved 5 June 2007.
  85. Carnegie Institution. Archived from the original
    on 15 May 2011. Retrieved 5 June 2007.
  86. S2CID 18969130
    .
  87. ^ "Habitable Exoplanet Observatory (HabEx)". www.jpl.nasa.gov. Retrieved 31 March 2020.
  88. ^ "Habitable zones of stars". NASA Specialized Center of Research and Training in Exobiology. University of Southern California, San Diego. Archived from the original on 21 November 2000. Retrieved 11 May 2007.
  89. doi:10.1006/icar.1997.5793. Archived from the original
    (PDF) on 14 August 2011. Retrieved 4 April 2011.
  90. S2CID 12329736
    . Retrieved 11 August 2007.
  91. ^ Croswell, Ken (27 January 2001). "Red, willing and able" (www.kencroswell.com/reddwarflife.html Full reprint). New Scientist. Retrieved 5 August 2007.
  92. ^ Cain, Fraser; Gay, Pamela (2007). "AstronomyCast episode 40: American Astronomical Society Meeting, May 2007". Universe Today. Retrieved 17 June 2007.
  93. ^ Croswell, Ken (November 2005). "A Flare for Barnard's Star". Astronomy Magazine. Kalmbach Publishing Co. Retrieved 10 August 2006.
  94. ^ Hines, Sandra (13 January 2003). "'The end of the world' has already begun, UW scientists say" (Press release). University of Washington. Retrieved 5 June 2007.
  95. PMID 19487662
    . Retrieved 19 July 2009.
  96. ^ "M Dwarfs: The Search for Life is On, Interview with Todd Henry". Astrobiology Magazine. 29 August 2005. Archived from the original on 3 June 2011. Retrieved 5 August 2007.{{cite web}}: CS1 maint: unfit URL (link)
  97. ^ Cain, Fraser (4 February 2009). "Red Dwarf Stars". Universe Today.
  98. PMID 24191033
    .
  99. S2CID 1824897
    .
  100. S2CID 119255193
    .
  101. ^ Stellar mass#Age
  102. S2CID 118534798
    .
  103. ^ Mullen, Leslie (18 May 2001). "Galactic Habitable Zones". Astrobiology Magazine. Archived from the original on 3 June 2011. Retrieved 5 August 2007.{{cite web}}: CS1 maint: unfit URL (link)
  104. ^ Ward, pp. 26–29.
  105. Bibcode:2005Ast....33g..40D
    .
  106. ^ Alan Boyle (22 November 2011). "Which alien worlds are most livable?". NBC News. Retrieved 20 March 2015.
  107. ^ Dirk Schulze-Makuch; et al. (December 2011). "A Two-Tiered Approach to Assessing the Habitability of Exoplanets".
    PMID 22017274
    .
  108. ^ Drake, Frank (1973). "Life on a Neutron Star". Astronomy. 1 (5): 5.
  109. Darling, David. "Neutron star, life on"
    . The Encyclopedia of Astrobiology, Astronomy, and Spaceflight. Retrieved 5 September 2009.
  110. ^ Bortman, Henry (29 September 2004). "Coming Soon: "Good" Jupiters". Astrobiology Magazine. Archived from the original on 8 February 2012. Retrieved 5 August 2007.{{cite web}}: CS1 maint: unfit URL (link)
  111. doi:10.1111/j.1468-4004.2010.51616.x
    .
  112. S2CID 8870726
    .
  113. S2CID 8032181
    .
  114. S2CID 1103987
    .
  115. PMID 25831540
    .
  116. PMID 25825762
    .
  117. PMID 11158551
    .
  118. S2CID 118574839
  119. ^ "The Living Worlds Hypothesis". Astrobiology Magazine. 22 September 2005. Archived from the original on 3 June 2011. Retrieved 6 August 2007.{{cite web}}: CS1 maint: unfit URL (link)
  120. ^ "Chance played a major role in keeping Earth fit for life". phys.org. Retrieved 17 January 2021.
  121. S2CID 228086341. Available under CC BY 4.0
    .
  122. ^ Tyrrell, Toby (19 January 2021). "Earth has stayed habitable for billions of years – exactly how lucky did we get?". The Conversation. Retrieved 30 January 2021.

Bibliography

  • Ward, Peter; Brownlee, Donald (2000). Rare Earth: Why Complex Life is Uncommon in the Universe. Springer.
    ISBN 978-0-387-98701-9
    .

Further reading

External links

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