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Understanding planetary habitability is partly an extrapolation of the
Earth's conditions, as it is the only
planet currently known to support
life.
'Planetary habitability' is the measure of a
planet's or a
natural satellite's potential to develop and sustain
life. As the existence of
life beyond Earth is currently uncertain, planetary habitability is largely an
extrapolation of conditions on Earth and the characteristics of the
Sun and
solar system which appear favorable to life's flourishing; of particular interest are factors that have sustained complex,
multicellular organisms and not just simpler,
unicellular creatures. Research and theory in this regard is a component of
planetary science and the emerging discipline of
astrobiology.
The only absolute requirement for life is an
energy source, but the notion of planetary habitability implies that many other
geophysical,
geochemical, and
astrophysical criteria must be met before an astronomical body can support life. In its astrobiology roadmap,
NASA has defined the principal habitability criteria as "extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism."
[ Goal 1: Understand the nature and distribution of habitable environments in the Universe ]
The idea that planets beyond Earth might host life is an ancient one, though historically it was framed by
philosophy as much as
physical science. The late 20th century saw two breakthroughs in the field. The observation and robotic
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
extrasolar planets, beginning in the early 1990s and accelerating thereafter, has provided further information for the study of possible extraterrestrial life. Most importantly, it confirmed that the Sun is not unique among
stars in hosting planets and expanded habitability research horizon beyond our own solar system.
Suitable star systems
An understanding of planetary habitability begins with
stars. While bodies that are generally Earth-like may be plentiful, it is just as important that their larger system be agreeable to life. Under the auspices of
SETI's
Project Phoenix, scientists
Margaret Turnbull and
Jill Tarter developed the "
HabCat" (or Catalogue of Habitable Stellar Systems) in 2002. The catalogue was formed by winnowing the nearly 120,000 stars of the larger
Hipparcos Catalogue into a core group of 17,000 "HabStars," and the selection criteria that were used provide a good starting point for understanding which astrophysical factors are necessary to habitable planets.
[1]
Spectral class
The spectral class of a star indicates its
photospheric temperature, which (for
main-sequence stars) correlates to overall mass. The appropriate spectral range for "HabStars" is presently considered to be "early F" or "G", to "mid-K". This corresponds to temperatures of a little more than 7,000
K down to a little more than 4,000 K; the Sun, a G2 star, is well within these bounds. "Middle-class" stars of this sort have a number of characteristics considered important to planetary habitability:
★ They live at least a few billion years, allowing life a chance to evolve. More
luminous main-sequence stars of the "O," "B," and "A" classes usually live less than a billion years and in exceptional cases less than 10 million.
[2]
★ They emit enough high-frequency
ultraviolet radiation to trigger important atmospheric dynamics such as
ozone formation, but not so much that
ionisation destroys incipient life.
[3]
★ Liquid water may exist on the surface of planets orbiting them at a distance that does not induce
tidal lock (see
next section and
3.2).
This spectral range likely accounts for between 5 and 10% of stars in the local Milky Way galaxy. Whether fainter late K and M class
red dwarf stars are also suitable hosts for habitable planets is perhaps the most important open question in the entire field of planetary habitability;
Gliese 581 c, a "super-earth," has been found orbiting in the habitable zone of a
red dwarf and may possess liquid water.
A stable habitable zone
Main articles: Habitable zone
The habitable zone (HZ) is a theoretical shell surrounding a star in which any planets present would have liquid
water on their surfaces. After an energy source, liquid water is considered the most important ingredient for life, considering how integral it is to all life-systems on Earth. This may reflect the bias of a water-dependent species, and if life is discovered in the absence of water (for example, in a liquid-
ammonia solution), the notion of an HZ may have to be greatly expanded or else discarded altogether as too restricting.
A "stable" HZ denotes 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 naturally 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 weaker chance to develop life. Calculating an HZ range and its long-term movement is never straightforward, given that negative
feedback loops such as the
carbon 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 Solar evolution; the proposed parameters of the Sun's HZ, for example, have fluctuated greatly.
[4]
Secondly, no large-mass body such as a gas giant should be present in or relatively close to the HZ, thus disrupting the formation of Earth-like bodies. The mass of the asteroid belt, for example, appears to have been unable to accrete into a planet due to orbital resonances with Jupiter; if the giant had appeared in the region that is now between the orbits of
Venus and
Mars, Earth would almost certainly not have developed its present form. This is somewhat ameliorated by suggestions that a gas giant inside the HZ might have habitable moons under the right conditions.
[5]
It was once assumed that the inner-rock planets, outer-gas giants pattern observable in the solar system was likely to be observed elsewhere. Discoveries of
extrasolar planets may have overturned this notion, as numerous Jupiter-sized bodies have been found in close orbit about their primary, disrupting potential HZs. However, present data for extrasolar planets is likely to be skewed towards these types (large planets in close orbits) because they are far easier to identify; thus, it remains to be seen which type of solar system is the norm, or indeed if there is one.
Low stellar variation
Main articles: Stellar variation
Changes in
luminosity are common to all stars, but the severity of such fluctuations covers a broad range. Most stars are relatively stable, but a significant minority of variable stars often experience sudden and intense increases in luminosity and consequently the amount of energy radiated toward bodies in orbit. These are considered poor candidates for hosting life-bearing planets as their unpredictability and energy output changes would negatively impact organisms. Most obviously, living things adapted to a particular
temperature range would likely be unable to survive too great a temperature deviation. Further, upswings in luminosity are generally accompanied by massive doses of
gamma ray and
X-ray radiation which might prove lethal.
Atmospheres do mitigate such effects (an absolute increase of 100% in the Sun's luminosity would not necessarily mean a 100% absolute temperature increase on Earth), but atmosphere retention might not occur on planets orbiting variables, because the high-frequency energy buffeting these bodies would continually strip them of their protective covering.
The Sun, as in much else, is benign in terms of this danger: the variation between solar max and minimum is roughly 0.1% over its 11-year
solar cycle. There is strong (though not undisputed)
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.
[6] Thus, a star does not have to be a true variable for differences in luminosity to affect habitability. Of known "
solar analogs," the one that most closely resembles the Sun is considered to be
18 Scorpii; interestingly (and 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.
[7]
High metallicity
Main articles: Metallicity
While the bulk of material in any star is
hydrogen and
helium, there is a great variation in the amount of heavier elements (
metals) stars contain. A high proportion of metals in a star correlates to the amount of heavy material initially available in
protoplanetary disks. A low amount of metal significantly decreases the probability that planets will have formed around that star, under the
solar nebula theory of
planetary systems formation. Any planets that did form around a metal-poor star would likely 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."
[8] High metallicity also places a requirement for youth on hab-stars: stars formed early in the
universe's history have low metal content and a correspondingly lesser likelihood of having planetary companions.
Planetary characteristics
The moons of some gas giants could potentially be habitable.
[9]
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. That life could evolve in the cloud tops of giant planets has not been decisively ruled out, though it is considered unlikely given that they have no surface and their gravity is enormous.
[10] The natural satellites of giant planets, meanwhile, remain perfectly valid candidates for hosting life.
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 this will lead to greater complexity. The planetary characteristics listed below are considered crucial for life generally, but in every case habitability impediments should be considered greater for multicellular organisms such as plants and animals versus unicellular life.
Mars, with its thin atmosphere, is colder than Earth would be at a similar distance from the Sun.
Mass
Low-mass planets are poor candidates for life for two reasons. First, their lesser
gravity makes
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 at similar distance) and lesser protection against high-frequency
radiation and
meteoroids. Further, where an atmosphere is less than 0.006 Earth atmospheres water cannot exist in liquid form as the required
atmospheric pressure, 4.56
mmHg (608 Pa) (0.18
inHG), does not occur . The temperature range at which water is liquid is smaller at low pressures generally.
Secondly, smaller planets have smaller
diameters and thus higher surface-to-volume ratios than their larger cousins. Such bodies tend to lose the energy left over from their formation quickly and end up
geologically dead, lacking the
volcanoes,
earthquakes and
tectonic activity which supply the surface with life-sustaining material and the atmosphere with temperature moderators like
carbon dioxide. Plate tectonics appear particularly crucial, at least on Earth: not only does the process recycle important chemicals and minerals, it also fosters
bio-diversity through continent creation and increased environmental complexity and helps create the convective cells necessary to generate Earth's
magnetic field.
[11]
"Low mass" is partly a relative label; the Earth is considered low mass when compared to the Solar System's
gas giants, but it is the largest, by diameter and mass, and densest of all terrestrial bodies. It is large enough to retain an atmosphere through gravity alone and large enough that its molten core remains a heat engine, driving the diverse geology of the surface (the decay of
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.
[12] Thus, it would be fair to infer that the lower mass limit for habitability lies somewhere between Mars and Earth-Venus; 0.3 Earth masses has been offered as a rough dividing line for habitable planets.
[13] Exceptional circumstances do offer exceptional cases:
Jupiter's moon
Io (smaller than the terrestrial planets) is volcanically dynamic because of the gravitational stresses induced by its orbit; neighbouring
Europa may have a liquid ocean underneath a frozen shell due also to power generated in its orbiting a gas giant;
Saturn's
Titan, meanwhile, has an outside chance of harbouring life as it has retained a thick atmosphere and bio-chemical reactions are possible in liquid methane on its surface. These satellites are exceptions, but they prove that mass as a habitability criterion cannot be considered definitive.
Finally, a larger planet is likely to have a large iron core. This allows for a
magnetic field to
protect the planet from the
solar wind, which otherwise tends to strip away the planetary atmosphere and to bombard living things with ionised particles. Mass is not the only criterion for producing a magnetic field — as the planet must also rotate fast enough to produce a
dynamo effect within its core
[14]— but it is a significant component of the process.
Orbit and rotation
As with other criteria, stability is the critical consideration in determining the effect of orbital and rotational characteristics on planetary habitability.
Orbital eccentricity is the difference between a planet's closest and farthest approach to its primary. The greater the eccentricity the greater the temperature fluctuation on a planet's surface. Although they are adaptive, living organisms can only stand so much variation, particularly if the fluctuations overlap both the
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.
[15] The Earth's orbit is almost wholly circular, with an eccentricity of less than 0.02; other planets in our solar system (with the exception of
Mercury) have eccentricities that are similarly benign.
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.
[16] This is a potential barrier to habitability, but it is unclear exactly how much orbital eccentricity would cause a critical problem.
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, meanwhile, seasons will be extreme and make it more difficult for a biosphere to achieve
homeostasis. Although during the
Quaternary higher axial tilt of the Earth coincides with reduced polar
ice, warmer temperatures and ''less'' seasonal variation, scientists do not know whether this trend would 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."
[17] 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.
★ 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.
[18] This position remains controversial.
Geochemistry
It is generally assumed that any extraterrestrial life that might exist will be based on the same fundamental chemistry as found on Earth, as the four elements most vital for life,
carbon,
hydrogen,
oxygen, and
nitrogen, are also the most common chemically reactive elements in the universe. Indeed, simple biogenic compounds, such as
amino acids, have been found in
meteorites and in
interstellar space. These four elements together comprise over 96% of Earth's collective
biomass. Carbon has an unparalleled ability to bond with itself and to form a massive array of intricate and varied structures, making it an ideal material for the complex mechanisms that form living cells. Hydrogen and oxygen, in the form of water, compose the solvent in which biological processes take place and in which the first reactions occurred that led to life's emergence. The energy released in the formation of powerful
covalent bonds between carbon and oxygen, available by oxidizing organic compounds, is the fuel of all complex lifeforms. These four elements together make up amino acids, which in turn are the building blocks of
proteins, the substance of living tissue. In addition, neither
sulfur nor
phosphorus (the latter almost exclusively as phosphate ion in
DNA and
RNA in genetic material, and the adenosine phosphates essential to metabolism), both essential to all known life on earth, is rare.
Relative abundance in space does not always mirror differentiated abundance within planets; of the four life elements, for instance, only oxygen is present in any abundance in the Earth's
crust.
[19] This can be partly explained by the fact that many of these elements, such as
hydrogen and
nitrogen, along with their simplest and most common compounds, such as
carbon dioxide,
carbon monoxide,
methane,
ammonia, and
water, are gaseous at warm temperatures. In the hot region close to the Sun, these volatile compounds could not have played a significant role in the planets' geological formation. Instead, they were trapped as gases underneath the newly formed crusts, which were largely made of rocky, involatile compounds such as
silica (a compound of
silicon and oxygen, accounting for oxygen's relative abundance).
Outgassing of volatile compounds through the first volcanoes would have contributed to the formation of the planets'
atmospheres. The
Miller experiments showed that, with the application of energy,
amino acids can form from the synthesis of the simple compounds within a primordial atmosphere.
[20]
Even so, volcanic outgassing could not have accounted for the amount of water in Earth's oceans.
[21] The vast majority of the water, and arguably of the carbon, necessary for life must have come from the outer solar system, away from the Sun's heat, where it could remain solid.
Comets impacting with the Earth in the Solar system's early years would have deposited vast amounts of water, along with the other volatile compounds life requires (including amino acids) onto the early Earth, providing a kick-start to the evolution of life.
Thus, while there is reason to suspect that the four "life elements" ought be readily available elsewhere, a habitable system likely 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.
Alternative star systems
In determining the feasibility of extraterrestrial life, astronomers had long focused their attention on stars like our own Sun. However, they have begun to explore the possibility that life might form in systems very unlike our own.
Binary 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 more common, fainter, stars are usually singular and that up to two thirds of all stellar systems are therefore solitary.
[17]
The separation between stars in a binary may range from less than one
astronomical unit (AU, the Earth-Sun distance) to several hundred. In latter instances, the gravitational effects will be negligible on a planet orbiting an otherwise suitable star and habitability potential will not be disrupted unless the orbit is highly eccentric (see
Nemesis, for example). However, where the separation is significantly less, a stable orbit may be impossible. If a planet’s distance to its primary exceeds about one fifth of the closest approach of the other star, orbital stability is not guaranteed.
[23] Whether planets might form in binaries at all had long been unclear, given that gravitational forces might interfere with planet formation. Theoretical work by
Alan Boss at the
Carnegie Institute has shown that gas giants can form around stars in binary systems much as they do around solitary stars.
[17]
One study of
Alpha Centauri, the nearest star system to the Sun, suggested that binaries need not be discounted in the search for habitable planets. Centauri A and B have an 11 AU distance at closest approach (23 AU mean), and both should have stable habitable zones. A study of long-term orbital stability for simulated planets within the system shows that planets within approximately three AU of either star may remain stable (i.e. the
semi-major axis deviating by less than 5%). The HZ 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.
[25]
Red dwarf systems
Relative star sizes and
photospheric temperatures. Any planet around a red dwarf such as the one shown here would have to huddle close to achieve Earth-like temperatures, likely inducing
tidal lock. See
Aurelia.
Determining the habitability of
red dwarf stars could help determine how common life in the universe is, as red dwarfs make up between 70 and 90% of all the stars in the galaxy.
Brown dwarfs are likely more numerous than red dwarfs. However, they are not generally classified as stars, and could never support life as we understand it, since what little heat they emit quickly disappears.
Astronomers for many years ruled out red dwarfs as potential abodes for life. Their small size (from 0.1 to 0.6 solar masses) means that their
nuclear reactions proceed exceptionally slowly, and they emit very little light (from 3% of that produced by the Sun to as little as 0.01%). Any planet in orbit around a red dwarf would have to huddle very close to its parent star to attain Earth-like surface temperatures; from 0.3 AU (just inside the orbit of
Mercury) for a star like
Lacaille 8760, to as little as 0.032
AU for a star like
Proxima Centauri[26] (such a world would have a year lasting just 6.3 days). At those distances, the star's gravity would cause
tidal lock. The daylight side of the planet would eternally face the star, while the night-time side would always face away from it. The only way potential life could avoid either an inferno or a deep freeze would be if the planet had an atmosphere thick enough to transfer the star's heat from the day side to the night side. It was long assumed that such a thick atmosphere would prevent sunlight from reaching the surface in the first place, preventing
photosynthesis.
This pessimism has been tempered by research. Studies by Robert Haberle and Manoj Joshi of
NASA's
Ames Research Center in California have shown that a planet's atmosphere (assuming it comprised greenhouse gases
CO2 and
H2O) need only be 100
mbs, or 10% of Earth's atmosphere, for the star's heat to be effectively carried to the night side.
[27] This is well within the levels required for photosynthesis, though water would still remain frozen on the dark side in some of their models. Martin Heath of
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 photosynthetically active radiation—suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants.
[28]
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.
[29] 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. However, a tidally locked planet rotates only very slowly, and so cannot produce a geodynamo at its core. However, the violent flaring period of a red dwarf's lifecyle 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 tidelock, 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.
[30]
There is, however, one major advantage that red dwarfs have over other stars as abodes for life: they live a long time. It took 4.5 billion years before humanity appeared on Earth, and life as we know it will see suitable conditions for as little as half a billion years more.
[17] 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 both would have longer to evolve and longer to survive. Further, while the odds of finding a planet in the habitable zone around any specific red dwarf are slim, the total amount of habitable zone around all red dwarfs combined is equal to the total amount around sun-like stars given their ubiquity.
[32] The first possibly rocky extrasolar planet with the mass of a
Super-Earth that was found at the "warm" edge of the
habitable zone of its star is
Gliese 581 c, whereby
Gliese 581 is indeed a red dwarf.
Other considerations
Alternative biochemistry
While most investigations of extraterrestrial life start with the assumption that advanced lifeforms must have similar requirement for life as on Earth, the hypothesis of
alternative biochemistry suggests possibility of lifeform evolving around a metabolism mechanism different from what is known on Earth. There is a possibility that other elements beyond those necessary on Earth will provide a biochemical basis for life elsewhere. The idea of biochemical cycles that is not carbon-based nor involving water has been explored mostly in fiction. As discussed in ''
Evolving the Alien''
biologist Jack Cohen and
mathematician Ian Stewart argue
astrobiology based on the
Rare Earth hypothesis is restrictive and unimaginative. They argue that Earth-like planets may be very rare, but non carbon-based complex life could possibly emerge in other environments.
"Good Jupiters"
"Good Jupiters" are gas giant planets, like the solar system's Jupiter, that orbit their stars in circular orbits far enough away from the HZ to not 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 solar system relatively free of comets and asteroids that could cause devastating impacts.
[33] 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; had Jovian gravity not captured the comet, it may well have entered the inner solar system.
Early in the Solar System's history, Jupiter played a somewhat contrary role: it increased the eccentricity of
asteroid belt orbits and enabled many to cross Earth's orbit and supply the planet with important volatiles. 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.
[34] 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 Earthlike planet to exist in the system. See discussion of a
stable habitable zone above.
The galactic neighborhood
Scientists have also considered the possibility that particular areas of galaxies (
galactic habitable zones) are better suited to life than others; the solar system in which we live, in the
Orion Spur, on the Milky Way galaxy's edge is considered to be in a life-favorable spot:
[35]
★ It is not in a
globular cluster where immense star densities are inimical to life, given excessive radiation and gravitational disturbance. Globular clusters are also primarily composed of older, likely metal-poor, stars.
★ It is not near an active
gamma ray source.
★ It is not near the galactic center where once again star densities increase the likelihood of ionizing radiation (e.g., from
magnetars and
supernovae). A
supermassive black hole is also believed to lie at the middle of the galaxy which might prove a danger to any nearby bodies.
★ The circular orbit of the Sun around the galactic centre keeps it out of the way of the galaxy's spiral arms where once more intense radiation and gravitation may lead to disruption.
[36]
Thus, relative loneliness 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 neighbours 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 likely 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 our Solar System enjoys, is preferable to a Galaxy's center or farthest reaches.
[37]
Life's impact on habitability
An interesting 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 oxygen by ancient
cyanobacteria, and eventually photosynthesizing plants, leading to a radical change in the composition of Earth’s atmosphere. This oxygen would prove fundamental to the respiration of later animal species.
This interaction between life and subsequent habitability has been explored in various ways. The
Gaia hypothesis, a class of scientific models of the geo-biosphere pioneered by Sir
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; at its most dramatic Gaia suggests that planetary systems behave as a kind of organism. The most successful life forms change the composition of the
air,
water, and
soil in ways that make their continued existence more certain—a controversial extension of the accepted laws of
ecology.
The implication that biota reveal concerted foresight would be challenged as unscientific and unfalsifiable. More mainstream researchers have arrived at related conclusions, however, without necessarily accepting the
teleology implied by Lovelock.
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."
[38]
In their 2004 book "The Privileged Planet", Guillermo Gonzalez and Jay Richards explore the possible link between the habitability of a planet and its suitability for observing the rest of the universe. This idea of a "privileged" position for Earth life is disputed because of its philosophical implications, especially what some consider a violation of the
Copernican principle.
Micro-Environments
Another complicating factor in the search of planetary habitability criteria is that only a tiny portion of a planet needs to be habitable to support some life. Astrobiologists often concern themselves with "microenvironments" noting that "we lack a fundamental understanding of how evolutionary forces, such as mutation, selection, and genetic drift, operate in microorganisms that act on and respond to changing microenvironments."
[39]
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.
[40] Carl Sagan explored examples of this concept in the solar system itself, considering the possibility of organisms that are always airborne within the high atmosphere of Jupiter in a 1976 paper, despite the fact that Jupiter's surface itself is obviously uninhabitable.
[41]
See also
★
Aurelia and Blue Moon
★
Class M planet
★
Definition of planet
★
Drake equation
★
Extraterrestrial life
★
Fermi paradox
★
Origin of life
★
Planetary science
★
Rare Earth hypothesis
★
Solar System
★
Space colonization
★
Terraforming
★
Alien Planet
★
Liquid water beyond Earth
Notes
This article is an analysis of planetary habitability from the perspective of contemporary physical science. A historical viewpoint on the possibility of habitable planets can be found at
Beliefs in extraterrestrial life and
Cosmic pluralism.
For a discussion of the probability of alien life see the
Drake Equation and
Fermi Paradox. Habitable planets are also a staple of fiction; see
Planets in science fiction.
Life appears to
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 a small fraction of "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 timeframes 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.
That
Europa and to a lesser extent
Titan (respectively, 3.5 and 8
astronomical units outside our Sun’s putative habitable zone) are considered prime extraterrestrial possibilities underscores the problematic nature of the HZ criterion. In secondary and tertiary descriptions of habitability it is often stated that habitable planets ''must'' be within the HZ—this remains to be proven.
In ''
Evolving the Alien'',
Jack Cohen and
Ian Stewart evaluate plausible scenarios in which life might form in the cloudtops of Jovian planets. Similarly,
Carl Sagan suggested that the clouds of
Venus might host life.
There is an emerging consensus that single-celled microorganisms 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 likely 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 mya but more than 3 billion years after life itself appeared. That Earth life remained unicellular for so long underscores that the decisive step toward complex organisms need not necessarily occur.
Interestingly, there is a "mass-gap" in our solar system between Earth and the two smallest gas giants,
Uranus and
Neptune, which are 13 and 17 Earth-masses. This is likely coincidence as there is no geophysical barrier to the formation of intermediary bodies (see for instance
OGLE-2005-BLG-390Lb) and we should expect to find planets throughout the galaxy between two and twelve Earth-masses. If the star system is otherwise favourable, such planets would be good candidates for life as they would be large enough to remain internally dynamic and atmosphere retentive over billions of years but not so large as to accrete the gaseous shell which limits the possibility of life formation.
According to prevailing theory, the formation of the Moon commenced when a Mars-sized body struck the Earth in a glancing collision late in its formation, and the ejected material coalesced and fell into orbit (see
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. Turnbull, Margaret C., and Jill C. Tarter. "Target selection for SETI: A catalog of nearby habitable stellar systems," ''The Astrophysical Journal Supplement Series'', 145: 181–198, March 2003. (Link). ''Habitability criteria defined—the foundational source for this article.''
2. Star tables
3. Ultraviolet radiation from F and K stars and implications for planetary habitability, , James F., Kasting, Origins of Life and Evolution of Biospheres, 1997
4. Habitable Zones Around Main Sequence Stars, , James F., Kasting, Icarus, 1993
5. Habitable moons around extrasolar giant planets, , Darren M., Williams, Nature, 1997
6. The Little Ice Age
7. 18 Scorpii
8. Confirming the Metal-Rich Nature of Stars with Giant Planets
9. An interview with Dr. Darren Williams
10. Could there be life in the outer solar system?
11. Ward, Peter and Donald Brownlee. '','' pp. 191–220, Springer, 2000.
12. The Heat History of the Earth
13. High-resolution simulations of the final assembly of Earth-like planets 2: water delivery and planetary habitability, , Sean N., Raymond, Astrobiology, 2007
14. Magnetic Field of the Earth
15. ''Rare Earth'', pp. 122–123.
16. Elusive Earths
17.
18. Stabilization of the earth's obliquity by the moon, , J., Lasker, Nature, 1993
19. Elements, biological abundance
20. How did chemisty and oceans produce this?
21. How did the Earth Get to Look Like This?
22.
23. Stars and Habitable Planets
24.
25. The stability of planets in the Alpha Centauri system, , Paul A., Wiegert, The Astronomical Journal, 1997
26. Habitable zones of stars
27. Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability, , M. M., Joshi, Icarus,
28. Habitability of Planets Around Red Dwarf Stars, , Martin J., Heath, Origins of Life and Evolution of the Biosphere, 1999
29. Red, willing and able
30. AstronomyCast episode 40: American Astronomical Society Meeting, May 2007 Cain, Fraser; and Gay, Pamela
31.
32. M Dwarfs: The Search for Life is On, Interview with Todd Henry .
33. Coming Soon: "Good" Jupiters
34. The occurrence of Jovian planets and the habitability of planetary systems, , Jonathan I., Lunine, Proceedings of the National Academy of Sciences,
35. Galactic Habitable Zones
36. ''Rare Earth'', pp. 26–29.
37. Dark Threat, , Bruce, Dorminey, Astronomy, 2005
38. The Living Worlds Hypothesis
39. Understand the evolutionary mechanisms and environmental limits of life
40. Cave Dwellers: ET Might Lurk in Dark Places
41. Jupiter, life on
Further reading
★
Abstracts from the Astrobiology Science Conference 2004.
★ Cohen, Jack and Ian Stewart. ''Evolving the Alien: The Science of Extraterrestrial Life,'' Ebury Press, 2002. ISBN 0-09-187927-2
★
Habitable Planets for Man, , Stephen H., Dole, American Elsevier Publishing Co., 1970, ISBN 0-444-00092-5
★
Fogg, Martyn J., ed. "Terraforming" (entire special issue) ''Journal of the British Interplanetary Society'', April 1991
★ Fogg, Martyn J. ''Terraforming: Engineering Planetary Environments'', SAE International, 1995. ISBN 1-56091-609-5
★ Gonzalez, Guillermo and Richards, Jay W. ''The Privileged Planet,'' Regnery, 2004. ISBN 0-89526-065-4
★ Grinspoon, David. ''Lonely Planets: The Natural Philosophy of Alien Life,'' HarperCollins, 2004.
★ Lovelock, James. ''Gaia: A New Look at Life on Earth.'' ISBN 0-19-286218-9
★ Schmidt, Stanley and Robert Zubrin, eds. ''Islands in the Sky'', Wiley, 1996. ISBN 0-471-13561-5
★ Ward, Peter and Donald Brownlee. ''Rare Earth: Why Complex Life is Uncommon in the Universe,'' Springer, 2000. ISBN 0-387-98701-0
External links
★
Alan Boss research papers
★
David Darling encyclopedia
★
General interest astrobiology
★
James Kasting research papers
★
Margaret Turnbull HabCat related files
★
Sol Station
★
Martyn J. Fogg Terraforming Information Pages
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