Planetary habitability

Planetary habitability is the measure of an astronomical body's potential to develop and sustain life. It may be applied both to planets and to the natural satellites of planets.

The only absolute requirement for life is an energy source (usually but not necessarily solar energy), but the notion of planetary habitability implies that many other geophysical, geochemical, and astrophysical criteria must be met before an astronomical body is able to support life. As the existence of life beyond Earth is currently unknown, 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 is the set of factors that has sustained complex, multicellular animals and not merely unicellular organisms on this planet. Research and theory in this regard is a component of planetary science and the emerging discipline of astrobiology.

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 ¹ . The late 20th century saw two breakthroughs in the field. To begin with, 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 1995 and accelerating thereafter—was the second milestone. It confirmed that the Sun is not unique in hosting planets and expanded the habitability research horizon beyond our own solar system.

PAGE Contents 1 Suitable star systems 1.1 Spectral class 1.2 A stable habitable zone 1.3 Low stellar variation 1.4 High metallicity 1.5 Binary systems 2 Planetary characteristics 2.1 Mass 2.2 Orbit and rotation 2.3 Geochemistry 3 Other considerations 3.1 Gaia 3.2 The habitability of red dwarf planetary systems 3.3 "Good Jupiters" 3.4 The galactic neighborhood 4 See also 5 Notes 6 References 6.1 Primary 6.2 Secondary 7 General reading 8 External links

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.

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 7,000K down to a little more than 4,000K; the Sun (not coincidentally) is directly in the middle of these bounds, classified as a G2 star. "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]  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 3.1).

These stars are neither "too hot" nor "too cold" and live long enough that life has a chance to begin. This spectral range likely accounts for between 5 and 10 percent 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 given that the majority of stars fall within this range; this is discussed extensively below.

A range of theoretical habitable zones with stars of different mass (our solar system in middle) - a stable 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.

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. If, for example, Jupiter had appeared in the region that is now between the orbits of Venus and Earth, the two smaller planets would almost certainly not have formed. It was once assumed that the inner-rock planets, outer-gas giants pattern observable in the solar system was likely to be the norm elsewhere, but discoveries of extrasolar planets have overturned this notion. Numerous Jupiter-sized bodies have been found in close orbit about their primary, disrupting potential HZs. Present data for extrasolar planets is likely skewed towards large planets in close eccentric orbits because they are far easier to identify; it remains to be seen which type of solar system is the norm.

Low 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 percent in the Sun's luminosity would not necessarily mean a 100 percent absolute temperature increase on Earth), but atmosphere retention might not occur on planets orbiting variables, because the high-frequency energy buffetting 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 percent 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 ^[5] . Thus, a star does not have to be a true variable for differences in luminosity to affect habitability. Of known "solar twins," 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 on 18 Scorpii.

High 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 ^[7] ." 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 companion.

Binary systems

Current estimates suggest that at least half of all stars are in a binary system ^[8] , which further complicates a delineation of habitability. 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  ^[9] . 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 ^[10] .

Alpha Centauri, the nearest star system to the Sun, underscores the fact 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 percent). 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.

Planetary characteristics

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 ⁴ , though it is considered unlikely given that they have no surface and in most, their gravity is enormous ^[12] . The natural satellites of giant planets, meanwhile, remain perfectly valid candidates for hosting life ^[13] .

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 ⁶ . 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. 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.

"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 ⁵ . It is large enough (along with Venus) 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. Mars, by contrast, is nearly (or perhaps totally) geologically dead and has lost much of its atmosphere ^[14] . Thus, it would be fair to infer that the lower mass limit for habitability lies somewhere between Mars and Earth-Venus. 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 energy created 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 ^[15] — but 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 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 (i.e., water). If, for example, Earth's oceans were alternately boiling off into space and freezing solid, it is difficult to imagine life as we know it having evolved. Fortunately, 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 Pluto and to a lesser extent Mercury) have eccentricities that are similarly benign. Data collected on the orbital eccentricities of extrasolar planets has surprised most researchers and reduced the expected extraterrestrial possibilities for life: 90% have an orbital eccentricity greater than that found within the solar system, and the average is fully 0.25, although this could very easily be the result of sample bias due to increased star 'wobble' caused by the planet's eccentricity.

A planet's movement around its rotational axis must also meet certain criteria if life is to have the opportunity to evolve.

• The day-night cycle must not be 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 must have moderate seasons. If there is little axial tilt (relative to the perpendicular to the ecliptic), seasons will not occur and a main stimulant to biospheric dynamism will disappear; such planets will generally be colder than they would be with a tilt. If a planet is radically tilted seasons will be extreme and make it more difficult for a biosphere to achieve homeostasis. 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] ."
• The rotational "wobble" must not be pronounced. Precession on Earth occurs over a 23 000 year cycle; if this period were radically shorter or if the wobble were more extreme, drastic climatic changes would again affect habitability.

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 ⁷ .

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 by mass make up over 96 percent 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 contained in the powerful covalent bond between carbon and hydrogen, released from the breakdown of carbohydrates, 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.

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 most basic 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. The possibility also remains that other elements beyond those necessary on Earth will provide a biochemical basis for life elsewhere; see alternative biochemistry.

Other considerations

Gaia

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. In other words, an effect of "survival of the fittest" is that 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. Proponents of Gaia hypothesize that once life takes hold on a planet, it is very likely that life will remain on the planet in some form over a geological time scale.

The habitability of red dwarf planetary systems

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 percent 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 (such a world would have a year lasting just 6.3 days) for a star like Proxima Centauri ^[22] . 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 need only be 15% thicker than Earth's for the star's heat to be effectively carried to the night side (see Aurelia). This is well within the levels required for photosynthesis, though water would still remain frozen on the dark side in some of their models ^[23] . 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. Thus, a planet with deep enough sea basins and a thick enough atmosphere could, at least potentially, harbour life in a red dwarf system.

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, making photosynthesis difficult. Photosynthesis as we understand it would be further 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.

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. Such variation would be very damaging for life, though it might also stimulate evolution by increasing mutation rates and rapidly shifting climatic conditions.

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 ^[24] . 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 ^[25] .

"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 ^[26] . 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 ^[27] . Thus, while the gas giants are now helpful protectors, they were once suppliers of critical habitability material.

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 ^[28] . Well away from the galactic center, it avoids various dangers:

• It is not in a globular cluster.
• It is not near an active gamma ray source.
• It is not near the black hole which is believed to lie at the middle of the galaxy.
• The circular orbit of the Sun around the galactic centre keeps out of the way of the galaxy's spiral arms where intense radiation and gravitation may lead to disruption.

Relative loneliness is ultimately what a life-bearing system needs. If Sol were crowded amongst other systems, neighbours might disrupt the stability of various orbiting bodies (not least Oort cloud and Kuiper Belt objects, which can bring catastrophe if knocked into the inner solar system). Close neighbors also increase the likelihood of being fatally close to supernova explosions and pulsars.

See also

• Astrobiology
• Astrophysics
• Class M planet
• Definition of a planet
• Drake equation
• Extraterrestrial life
• Fermi paradox
• Origin of life
• Planetary science
• Rare Earth hypothesis
• Solar System
• Solar twin

Notes