   #copyright

Planetary habitability

2007 Schools Wikipedia Selection. Related subjects: The Planets

   Understanding planetary habitability is partly an extrapolation of the
   Earth's conditions, as it is the only planet currently known to harbor
   life.
   Enlarge
   Understanding planetary habitability is partly an extrapolation of the
   Earth's conditions, as it is the only planet currently known to harbour
   life.

   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 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 . 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 the early 1990s 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.

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 7,000 K down to a little more than 4,000 K; 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  .
     * 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 .
     * 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).

   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).
   Enlarge
   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. 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 .

   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 to be 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 .
   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
   for 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 ." 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 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 . 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.
   Enlarge
   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 .

   "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 . 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 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 — 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 . 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 . This could very easily be the result of sample bias. Often
   planets are not observed directly, but rather are inferred based on the
   "wobble" they cause on their parent star—the greater the eccentricity
   the greater the perturbance in the star, and thus, the greater the
   detectability of the planet.

   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 ."
   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 23 000 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 . 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
   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 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.

   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 . 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 .

   Even so, volcanic outgassing could not have accounted for the amount of
   water in Earth's oceans . 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.

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 .

   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  . 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 .

   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
   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 .

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.
   Enlarge
   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.

   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 for a star like Proxima
   Centauri (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 Centre in California have shown
   that a planet's atmosphere (assuming it were compromised of greenhouse
   gases CO[2] and H[2]O) need only be 100 mbs, or 10% of Earth's
   atmosphere, for the star's heat to be effectively carried to the night
   side . 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 .

   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 . 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 .
   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 .

Other considerations

"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 . 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 . 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 neighbourhood

   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 :
     * 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 centre 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 .

   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 centre or farthest reaches .

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 ."

   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 the violation of the Copernican
   principle.

Suggested reading

     * Cohen, Jack and Ian Stewart. Evolving the Alien: The Science of
       Extraterrestrial Life, Ebury Press, 2002. ISBN 0-09-187927-2
     * Doyle, Stephen H. Habitable Planets for Man, American Elsevier Pub.
       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

   Retrieved from " http://en.wikipedia.org/wiki/Planetary_habitability"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
