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Encyclopedia > Planetary habitability
Understanding planetary habitability is partly an extrapolation of the Earth's conditions, as it is the only planet currently known to support life.
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—in particular those 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. Image File history File links Metadata Size of this preview: 599 × 599 pixelsFull resolution (3000 × 3002 pixel, file size: 6. ... Image File history File links Metadata Size of this preview: 599 × 599 pixelsFull resolution (3000 × 3002 pixel, file size: 6. ... This article is about Earth as a planet. ... This article is about the astronomical term. ... This article is about life in general. ... This article is about the astronomical term. ... A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. ... This article is about life in general. ... Green people redirects here. ... In mathematics, extrapolation is the process of constructing new data points outside a discrete set of known data points. ... Sol redirects here. ... This article is about the Solar System. ... Wild-type Caenorhabditis elegans hermaphrodite stained to highlight the nuclei of all cells Multicellular organisms are organisms consisting of more than one cell, and having differentiated cells that perform specialized functions. ... A cluster of Escherichia coli bacteria magnified 10,000 times. ... Planetary science, also known as planetology or planetary astronomy, is the science of planets, or planetary systems, and the solar system. ... The DNA structure might not be the only nucleic acid in the universe capable of supporting life[1] Astrobiology (from Greek: ἀστρο, astro, constellation; βίος, bios, life; and λόγος, logos, knowledge) is the interdisciplinary study of life in space, combining aspects of astronomy, biology and geology. ...


An 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."[1] This article is about the physical quantity. ... ‹ The template below has been proposed for deletion. ... The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks and soils, and the cycles of matter and energy that transport the Earths chemical components in time and space, and their interaction with... Spiral Galaxy ESO 269-57 Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties (luminosity, density, temperature, and chemical composition) of celestial objects such as stars, galaxies, and the interstellar medium, as well as their interactions. ... For other uses, see NASA (disambiguation). ...


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.a 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. For other uses, see Philosophy (disambiguation). ... == Headline text ==cant there be some kind of picture somewhere so i can see by picture???? Physical science is a encompassing term for the branches of natural science, and science, that study non-living systems, in contrast to the biological sciences. ... Space exploration is the physical exploration of outer space, both by human spaceflights and by robotic spacecraft. ... An extrasolar planet, or exoplanet, is a planet beyond the Solar System. ... This article is about the astronomical object. ...

Contents

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.[2] This article is about the astronomical object. ... This article is about the search for extra-terrestrial intelligence. ... Project Phoenix is a SETI project: a search for extraterrestrial intelligence by listening for radio signals. ... This article is being considered for deletion in accordance with Wikipedias deletion policy. ... Jill Cornell Tarter (born 1944) is an American astronomer and the current director of the Center for SETI Research. ... The Catalog of Nearby Habitable Systems (HabCat) is a catalog of star systems which concievably have habitable planets. ... The Hipparcos and Tycho Catalogues (Tycho-1) are the primary products of the European Space Agencys astrometric mission, Hipparcos. ...


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: The photosphere of an astronomical object is the region at which the optical depth becomes one for a photon of wavelength equal to 5000 angstroms. ... Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve along which the majority of stars are located. ... For other uses, see Kelvin (disambiguation). ...

  • 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.[3]b
  • 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.[4]
  • 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. Alternately, a greenhouse effect may render it too hot to support life, while its next-nearest neighbor, Gliese 581 d, may in fact be a more likely candidate for habitability. This article does not cite any references or sources. ... Note: Ultraviolet is also the name of a 1998 UK television miniseries about vampires. ... For other uses, see Ozone (disambiguation). ... Ionisation can be: the process of creating an ion, see ionization a piece of music by Edgar Varèse; see Ionisation (Varèse) This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... Tidal locking makes one side of an astronomical body always face another, like the Moon facing the Earth. ... Red dwarfs constitute the majority of all stars According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type. ... Gliese 581 c (IPA: ) is a super-earth extrasolar planet orbiting the red dwarf star Gliese 581. ... Gliese 581 c, a super-Earth and its star as rendered in Celestia. ... Red dwarfs constitute the majority of all stars According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type. ... Gliese 581 d is an extrasolar planet orbiting the red dwarf star Gliese 581. ...


A stable habitable zone

Main article: 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.c It has been suggested that Goldilocks phenomenon be merged into this article or section. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... For other uses, see Ammonia (disambiguation). ...

A range of theoretical habitable zones with stars of different mass (our solar system at centre). Not to scale.
A range of theoretical habitable zones with stars of different mass (our solar system at centre). Not to scale.

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.[5] Image File history File links Habitable_zone-en. ... Image File history File links Habitable_zone-en. ... This article is about the Solar System. ... For other uses, including Audio feedback, see Feedback (disambiguation) In cybernetics and control theory, feedback is a process whereby some proportion or in general, function, of the output signal of a system is passed (fed back) to the input. ... For the thermonuclear reaction involving carbon that helps power stars, see CNO cycle. ...


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.[6] For other uses, see Venus (disambiguation). ... Adjectives: Martian Atmosphere Surface pressure: 0. ...


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. An extrasolar planet, or exoplanet, is a planet beyond the Solar System. ...


Low stellar variation

Main article: 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. Most stars are of almost constant luminosity. ... This article does not cite any references or sources. ... For other uses, see Temperature (disambiguation). ... This article is about electromagnetic radiation. ... In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz... Atmosphere is the general name for a layer of gases that may surround a material body of sufficient mass. ...


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.[7] 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; 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.[8] It has been suggested that this article or section be merged with Schwabe-Wolf cycle. ... It has been suggested that this article or section be merged with Schwabe-Wolf cycle. ... The Little Ice Age (LIA) was a period of cooling occurring after a warmer era known as the Medieval climate optimum. ... This article does not cite any references or sources. ... 18 Scorpii is a star located some 45. ...


High metallicity

Main article: 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."[9] 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. The globular cluster M80. ... This article is about the chemistry of hydrogen. ... General Name, symbol, number helium, He, 2 Chemical series noble gases Group, period, block 18, 1, s Appearance colorless Standard atomic weight 4. ... In astronomy and cosmology, the term metal or metal-rich is used to refer to any element heavier than hydrogen and helium. ... A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ... This article or section does not cite any references or sources. ... An artists concept of a protoplanetary disc. ... Animation of the dispersion of light as it travels through a triangular prism. ... Infrared Image of a possible extrasolar planet (lower left) in the Constellation Taurus, taken by the Hubble Space Telescope. ... For other uses, see Universe (disambiguation). ...


Planetary characteristics

The moons of some gas giants could potentially be habitable.
The moons of some gas giants could potentially be habitable.[10]

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,d though it is considered unlikely given that they have no surface and their gravity is enormous.[11] The natural satellites of giant planets, meanwhile, remain perfectly valid candidates for hosting life.[10] Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... The inner planets, Mercury, Venus, Earth, and Mars, their sizes to scale. ... In chemistry, a silicate is a compound containing an anion in which one or more central silicon atoms are surrounded by electronegative ligands. ... This article is about the chemistry of hydrogen. ... General Name, symbol, number helium, He, 2 Chemical series noble gases Group, period, block 18, 1, s Appearance colorless Standard atomic weight 4. ... This article is about the rock band. ...


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.e 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. Phyla Actinobacteria Aquificae Chlamydiae Bacteroidetes/Chlorobi Chloroflexi Chrysiogenetes Cyanobacteria Deferribacteres Deinococcus-Thermus Dictyoglomi Fibrobacteres/Acidobacteria Firmicutes Fusobacteria Gemmatimonadetes Lentisphaerae Nitrospirae Planctomycetes Proteobacteria Spirochaetes Thermodesulfobacteria Thermomicrobia Thermotogae Verrucomicrobia Bacteria (singular: bacterium) are unicellular microorganisms. ... Phyla Crenarchaeota Euryarchaeota Korarchaeota Nanoarchaeota ARMAN The Archaea (pronounced ) are a group of prokaryotic and single-celled microorganisms. ...

Mars, with its thin atmosphere, is colder than Earth would be at a similar distance from the Sun.
Mars, with its thin atmosphere, is colder than Earth would be at a similar distance from the Sun.

Courtesy: NASA/JPL-Caltech Sunset on Mars as imaged by Mars Pathfinder This is a close-up of the sunset on Sol 24 as seen by the Imager for Mars Pathfinder. ... Courtesy: NASA/JPL-Caltech Sunset on Mars as imaged by Mars Pathfinder This is a close-up of the sunset on Sol 24 as seen by the Imager for Mars Pathfinder. ... Adjectives: Martian Atmosphere Surface pressure: 0. ...

Mass

Low-mass planets are poor candidates for life for two reasons. First, their lesser gravity makes atmosphere retention difficult, although global magnetic fields can play a large role in atmospheric retention. Constituent molecules are more likely to reach escape velocity and be lost to space when buffeted by solar wind or stirred by collision. Planets without a thick atmosphere lack the matter necessary for primal biochemistry, have little insulation and poor heat transfer across their surfaces (for example, Mars with its thin atmosphere is colder than the Earth would be if it were at a similar distance) 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. Gravity is a force of attraction that acts between bodies that have mass. ... Atmosphere is the general name for a layer of gases that may surround a material body of sufficient mass. ... 3D (left and center) and 2D (right) representations of the terpenoid molecule atisane. ... Space Shuttle Atlantis launches on mission STS-71. ... The plasma in the solar wind meeting the heliopause The solar wind is a stream of charged particles (i. ... Biochemistry (from Greek: , bios, life and Egyptian kēme, earth[1]) is the study of the chemical processes in living organisms. ... In thermal physics, heat transfer is the passage of thermal energy from a hot to a colder body. ... Adjectives: Martian Atmosphere Surface pressure: 0. ... For other uses, see Radiation (disambiguation). ... “Meteor” redirects here. ... Atmospheric pressure is the pressure at any given point in the Earths atmosphere. ... The torr (symbol: Torr) or millimeter of mercury (mmHg) is a non-SI unit of pressure. ... Pressure is the application of force to a surface, and the concentration of that force in a given area. ...


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. The existance of tidel forces from nearby objects such as natural moons can convert rotational energy into vast amounts of internal heating which can be seen to play an important alternate internal heating role in moons in our own solar system such as Io. 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.[12] DIAMETER is a computer networking protocol for AAA (Authentication, Authorization and Accounting). ... This article includes a list of works cited but its sources remain unclear because it lacks in-text citations. ... Cleveland Volcano in the Aleutian Islands of Alaska photographed from the International Space Station For other uses, see Volcano (disambiguation). ... This article is about the natural seismic phenomenon. ... The tectonic plates of the world were mapped in the second half of the 20th century. ... Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. ... Rainforests are the most biodiverse ecosystem on earth Biodiversity or biological diversity is the diversity of life. ... For the indie-pop band, see The Magnetic Fields. ...


"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.f 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.[13] 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.[14] 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. This article does not cite any references or sources. ... Radioactive decay is the set of various processes by which unstable atomic nuclei (nuclides) emit subatomic particles. ... For other uses, see Jupiter (disambiguation). ... Atmosphere Surface pressure: trace Composition: 90% sulfur dioxide Io (eye-oe, IPA: , Greek Ῑώ) is the innermost of the four Galilean moons of Jupiter and, with a diameter of 3,642 kilometers, is the fourth largest moon in the Solar System. ... Apparent magnitude: 5. ... This article is about the planet. ... Titan (, from Ancient Greek Τῑτάν) or Saturn VI is the largest moon of Saturn and the only moon known to have a dense atmosphere. ...


Finally, a larger planet is likely to have a large iron core. This allows for a magnetic field to protect the planet from its stellar wind, which otherwise would tend to strip away 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 it is a significant component of the process. For the indie-pop band, see The Magnetic Fields. ... A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. ... Dynamo, or Dinamo, may refer to: Dynamo, an electrical generator Dynamo (sports society) of the Soviet Union Operation Dynamo, the 1940 mass evacuation at Dunkirk Dynamo, the rock band based in Belfast Dynamo theory, a theory relating to magnetic fields of celestial bodies Dynamo Open Air, annual heavy metal music...


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.[16] 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. (This page refers to eccitricity in astrodynamics. ... Freezing point can refer to several things: For the chemistry term, see Melting point. ... Italic text This article is about the boiling point of liquids. ... This article is about the planet. ...


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.[17] 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. This article is about rotation as a movement of a physical body. ... In astronomy, axial tilt is the inclination angle of a planets rotational axis in relation to a perpendicular to its orbital plane. ... The plane of the ecliptic is well seen in this picture from the 1994 lunar prospecting Clementine spacecraft. ...


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). Homeostasis is the property of either an open system or a closed system, especially a living organism, which regulates its internal environment so as to maintain a stable, constant condition. ... The Quaternary Period is the geologic time period from the end of the Pliocene Epoch roughly 1. ... This article is about the geological formation. ... One computer simulation of conditions during the Snowball Earth period. ...


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."[18] 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.[19] This position remains controversial.g Precession redirects here. ... Milankovitch cycles are the collective effect of changes in the Earths movements upon its climate, named after Serbian civil engineer and mathematician Milutin Milanković. The eccentricity, axial tilt, and precession of the Earths orbit vary in several patterns, resulting in 100,000 year ice age cycles of the... This article is about Earth as a planet. ...


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, required for the building of proteins, nor phosphorus, needed for the formation of DNA and RNA and the adenosine phosphates essential to metabolism, are rare. For other uses, see Carbon (disambiguation). ... This article is about the chemistry of hydrogen. ... This article is about the chemical element and its most stable form, or dioxygen. ... General Name, symbol, number nitrogen, N, 7 Chemical series nonmetals Group, period, block 15, 2, p Appearance colorless gas Standard atomic weight 14. ... This article is about the class of chemicals. ... Willamette Meteorite A meteorite is a natural object originating in outer space that survives an impact with the Earths surface without being destroyed. ... Interstellar Space was one of the last albums recorded before the death of John Coltrane in 1967. ... For the use of the term in ecology, see Biomass (ecology). ... Covalent redirects here. ... A representation of the 3D structure of myoglobin showing coloured alpha helices. ... This article is about the chemical element. ... General Name, symbol, number phosphorus, P, 15 Chemical series nonmetals Group, period, block 15, 3, p Appearance waxy white/ red/ black/ colorless Standard atomic weight 30. ... The structure of part of a DNA double helix Deoxyribonucleic acid, or DNA, is a nucleic acid molecule that contains the genetic instructions used in the development and functioning of all known living organisms. ... For other uses, see RNA (disambiguation). ...


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.[20] 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.[21] Earth cutaway from core to exosphere. ... This article is about the chemistry of hydrogen. ... General Name, symbol, number nitrogen, N, 7 Chemical series nonmetals Group, period, block 15, 2, p Appearance colorless gas Standard atomic weight 14. ... Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. ... R-phrases , , , , S-phrases , , , , Flash point Flammable gas Related Compounds Related oxides carbon dioxide; carbon suboxide; dicarbon monoxide; carbon trioxide Supplementary data page Structure and properties n, εr, etc. ... Methane is a chemical compound with the molecular formula . ... For other uses, see Ammonia (disambiguation). ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... The chemical compound silicon dioxide, also known as silica, is the oxide of silicon, chemical formula SiO2. ... Not to be confused with Silicone. ... Outgassing (sometimes called Offgassing, particularly when in reference to indoor air quality) is the slow release of a gas that was trapped, frozen, absorbed or adsorbed in some material. ... Atmosphere is the general name for a layer of gases that may surround a material body of sufficient mass. ... The experiment The Miller-Urey experiment (or Urey-Miller experiment) was an experiment that simulated hypothetical conditions present on the early Earth and tested for the occurrence of chemical evolution. ... This article is about the class of chemicals. ...


Even so, volcanic outgassing could not have accounted for the amount of water in Earth's oceans.[22] 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. Comet Hale-Bopp Comet West For other uses, see Comet (disambiguation). ...


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.[23] For the band of the same name, see: Binary Star (band) Hubble image of the Sirius binary system, in which Sirius B can be clearly distinguished (lower left). ...


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.[24] 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.[25] The astronomical unit (AU or au or a. ... Nemesis is a hypothetical red dwarf star or brown dwarf, orbiting the Sun at a distance of about 50,000 to 100,000 AU, somewhat beyond the Oort cloud. ... Alan P. Boss is an American astrophysicist. ... The Carnegie Institution of Washington (CIW) is a foundation established by Andrew Carnegie in 1902 to support scientific research. ...


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.[26] Alpha Centauri (α Cen / α Centauri, also known as Rigil Kentaurus), is the brightest star system in the southern constellation of Centaurus. ... The semi-major axis of an ellipse In geometry, the term semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolae. ...


Red dwarf systems

Main article: Habitability of 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.
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 to 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. Determining the habitability of red dwarf systems could help reveal how likely extraterrestrial life is to exist, as red dwarfs make up a majority of all the stars in the galaxy. ... Image File history File links Relative_star_sizes. ... Image File history File links Relative_star_sizes. ... The photosphere of an astronomical object is the region at which the optical depth becomes one for a photon of wavelength equal to 5000 angstroms. ... Tidal locking makes one side of an astronomical body always face another, like the Moon facing the Earth. ... Aurelia and Blue Moon are two fictional/hypothetical satellites on which extraterrestrial life could evolve. ... Red dwarfs constitute the majority of all stars According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool star, of the main sequence, either late K or M spectral type. ... This brown dwarf (smaller object) orbits the star Gliese 229, which is located in the constellation Lepus about 19 light years from Earth. ...


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[27] (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. In nuclear physics, a nuclear reaction is a process in which two nuclei or nuclear particles collide to produce products different from the initial particles. ... This article is about the planet. ... Lacaille 8760 is a red dwarf star in the constellation Microscopium, the microscope. ... Look up Au, au in Wiktionary, the free dictionary. ... Proxima Centauri (Latin proximus, -a, -um: meaning next to or nearest to)[4] is a red dwarf star that is likely a part of the Alpha Centauri star system and is the nearest star to the Sun at a distance of 4. ... Tidal locking makes one side of an astronomical body always face another, like the Moon facing the Earth. ... assimilation. ...


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 included 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.[28] 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.[29] For other uses, see NASA (disambiguation). ... Aerial View of Moffett Field and NASA Ames Research Center. ... Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. ... H2O and HOH redirect here. ... A millibar (mb) is 1/1000th of a bar, a unit for measurement of pressure. ... Greenwich Community College is a post-secondary educational institution located near London, UK. External links School homepage Category: ...


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. For other uses, see Infrared (disambiguation). ... Chemosynthesis is the biological conversion of 1 or more carbon molecules (usually carbon dioxide or methane) and nutrients into organic matter using the oxidation of inorganic molecules (e. ...


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.[30] 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 lifecycle 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.[31] 400 year sunspot history A sunspot is a region on the Suns surface (photosphere) that is marked by a lower temperature than its surroundings, and intense magnetic activity. ...


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.[32] 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.[33] 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, and its star, Gliese 581, is indeed a red dwarf. Gliese 581 c, a super-Earth and its star as rendered in Celestia. ... It has been suggested that Goldilocks phenomenon be merged into this article or section. ... Gliese 581 c (IPA: ) is a super-earth extrasolar planet orbiting the red dwarf star Gliese 581. ... Gliese 581 (IPA: ) is an M2. ...


Other considerations

Alternative biochemistry

While most investigations of extraterrestrial life start with the assumption that advanced lifeforms must have similar requirements for life as on Earth, the hypothesis of alternative biochemistry suggests possibility of lifeforms 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 are 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. Alternative biochemistry is the speculative biochemistry of alien life forms that differ radically from those on Earth. ... Evolving the Alien: The Science of Extraterrestrial Life (2002, second edition published as What Does a Martian Look Like? The Science of Extraterrestrial Life) is a book about xenobiology by biologist Jack Cohen and mathematician Ian Stewart. ... A biologist is a scientist devoted to and producing results in biology through the study of organisms. ... See: Jack Cohen, biologist and special effects consultant. ... Leonhard Euler, considered one of the greatest mathematicians of all time A mathematician is a person whose primary area of study and research is the field of mathematics. ... Ian Stewart, FRS (b. ... The DNA structure might not be the only nucleic acid in the universe capable of supporting life[1] Astrobiology (from Greek: ἀστρο, astro, constellation; βίος, bios, life; and λόγος, logos, knowledge) is the interdisciplinary study of life in space, combining aspects of astronomy, biology and geology. ... The Rare Earth hypothesis is a hypothesis in planetary astronomy and astrobiology which argues that the emergence of complex multicellular life (metazoa) on Earth required an extremely unlikely combination of astrophysical and geological events and circumstances. ...


"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.[34] 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. Hubble Space Telescope image of Comet Shoemaker-Levy 9, taken on May 17, 1994. ...


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.[35] Thus, while the gas giants are now helpful protectors, they were once suppliers of critical habitability material. For other uses, see Asteroid (disambiguation). ... This article is about the planet. ...


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. 47 Ursae Majoris (abbreviated 47 UMa) is a 5th magnitude yellow dwarf star in the constellation of Ursa Major. ... 16 Cygni is a double star system 69. ...


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:[36] It has been suggested that Goldilocks phenomenon be merged into this article or section. ... Observed structure of the Milky Ways spiral arms The Orion Arm or Local Arm (labeled 0) is a minor, spiral arm of the Milky Way galaxy. ...

  • 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.[37]

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. The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars. ... This article is about electromagnetic radiation. ... Artists conception of a magnetar, with magnetic field lines A magnetar is a neutron star with an extremely strong magnetic field, the decay of which powers the emission of copious amounts of high-energy electromagnetic radiation, particularly X-rays and gamma-rays. ... Remnant of Keplers Supernova, SN 1604. ... Top: artists conception of a supermassive black hole tearing apart a star. ... Artists rendering of the Oort cloud and the Kuiper Belt. ... The Kuiper belt, derived from data from the Minor Planet Center. ...


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.[38]


Life's impact on habitability

A supplement to the factors that support life's emergence is the notion that life itself, once formed, becomes a habitability factor in its own right. An important Earth example was the production of 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. Orders The taxonomy is currently under revision. ...


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. For other uses, see Gaia. ... Dr. James Ephraim Lovelock, CH, CBE, FRS (born 26 July 1919) is an independent scientist, author, researcher, environmentalist, and futurologist who lives in Cornwall, in the south west of Great Britain. ... For other uses, see Atmosphere (disambiguation). ... The movement of water around, over, and through the Earth is called the water cycle, a key process of the hydrosphere. ... Loess field in Germany Surface-water-gley developed in glacial till, Northern Ireland For the American hard rock band, see SOiL. For the System of a Down song, see Soil (song). ... For the journal, see Ecology (journal). ...


The implication that biota reveal concerted foresight could 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."[39] Teleology (Greek: telos: end, purpose) is the philosophical study of design, purpose, directive principle, or finality in nature or human creations. ...


In their 2004 book The Privileged Planet, astronomer Guillermo Gonzalez and philosopher 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. The Privileged Planet: How Our Place in the Cosmos is Designed for Discovery is a book by Guillermo Gonzalez and Jay Richards which attempts to use scientific evidence to show that the Earth isnt a result of natural processes, and that it was optimized for life and learning. ... For other uses, see Guillermo Gonzalez. ... Jay Wesley Richards is Director of Acton Media and a Research Fellow at the Acton Institute,[1] and Program Director of the Discovery Institutes Center for Science and Culture (CSC), which has as its primary role the advocacy of intelligent design. ... In cosmology, the Copernican principle, named after Nicolaus Copernicus, states [1] More recently, the principle is generalised to the relativistic concept that humans are not privileged observers of the universe. ...


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."[40]


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.[41] 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.[42] Insert non-formatted text here Carl Edward Sagan (November 9, 1934 – December 20, 1996) was an American astronomer and astrobiologist and a highly successful popularizer of astronomy, astrophysics, and other natural sciences. ...


See also

Aurelia and Blue Moon are two fictional/hypothetical satellites on which extraterrestrial life could evolve. ... The Class M planet Earth. ... The Darwin Mission is a proposed European Space Agency (ESA) program designed to directly detect Earth-like planets orbiting nearby stars, and search for evidence of life on these planets. ... Photograph of the planet Neptune and its moon Triton, taken by Voyager 2 as it entered the outer solar system. ... The Drake equation (rarely also called the Green Bank equation or the Sagan equation) is a famous result in the speculative fields of exobiology, astrosociobiology and the search for extraterrestrial intelligence. ... Green people redirects here. ... A graphical representation of the Arecibo message - Humanitys first attempt to use radio waves to communicate its existence to alien civilizations The Fermi paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for or contact with... For the definition, see Life. ... Planetary science, also known as planetology or planetary astronomy, is the science of planets, or planetary systems, and the solar system. ... The Rare Earth hypothesis is a hypothesis in planetary astronomy and astrobiology which argues that the emergence of complex multicellular life (metazoa) on Earth required an extremely unlikely combination of astrophysical and geological events and circumstances. ... This article is about the Solar System. ... Artists conception of a space habitat called the Stanford torus, by Don Davis Space colonization (also called space settlement, space humanization, space habitation, etc. ... Artists conception of a terraformed Mars in four stages of development. ... Terrestrial Planet Finder - Infrared interferometer concept The Terrestrial Planet Finder (TPF) is a plan by NASA for a telescope system that would be capable of detecting extrasolar terrestrial planets. ... Alien Planet is a two-hour special on Discovery Channel about two Internationally built robot probes investigating for alien life on the fictional planet Darwin IV. It was based on the book Expedition, by sci-fi/fantasy artist and writer Wayne Douglas Barlowe, who was also executive producer on the... Extraterrestrial liquid water, the presence of water in its liquid state, is a subject of wide interest because it is a commonly suggested prerequisite for the emergence of extraterrestrial life. ...

Notes

  • Note b: 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.
  • Note c: 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.
  • Note e: 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.
  • Note f: 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.
  • Note g: 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.

Green people redirects here. ... Cosmic pluralism or the plurality of worlds describes the belief in numerous other worlds beyond the Earth which harbour extraterrestrial life. ... The Drake equation (rarely also called the Green Bank equation or the Sagan equation) is a famous result in the speculative fields of exobiology, astrosociobiology and the search for extraterrestrial intelligence. ... A graphical representation of the Arecibo message - Humanitys first attempt to use radio waves to communicate its existence to alien civilizations The Fermi paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for or contact with... Planets in science fiction are fictional planets that appear in various media, especially those of the science fiction genre, as story-settings or depicted locations. ... Life on Earth  â€¢  â€¢  | Axis scale: millions of years ago. ... Apparent magnitude: 5. ... Titan (, from Ancient Greek Τῑτάν) or Saturn VI is the largest moon of Saturn and the only moon known to have a dense atmosphere. ... The astronomical unit (AU or au or a. ... Evolving the Alien: The Science of Extraterrestrial Life (2002, second edition published as What Does a Martian Look Like? The Science of Extraterrestrial Life) is a book about xenobiology by biologist Jack Cohen and mathematician Ian Stewart. ... See: Jack Cohen, biologist and special effects consultant. ... Ian Stewart, FRS (b. ... Insert non-formatted text here Carl Edward Sagan (November 9, 1934 – December 20, 1996) was an American astronomer and astrobiologist and a highly successful popularizer of astronomy, astrophysics, and other natural sciences. ... For other uses, see Jupiter (disambiguation). ... An extremophile is an organism, usually unicellular, which thrives in or requires extreme conditions that would exceed optimal conditions for growth and reproduction in the majority of mesophilic terrestrial organisms. ... The Rare Earth hypothesis is a hypothesis in planetary astronomy and astrobiology which argues that the emergence of complex multicellular life (metazoa) on Earth required an extremely unlikely combination of astrophysical and geological events and circumstances. ... Peter D. Ward is a paleontologist and professor of Biology and of Earth and Space Sciences at the University of Washington. ... The Cambrian explosion is the geologically kukko sudden appearance in the fossil record of the ancestors of familiar animals, starting about 542 million years ago (Mya). ... For other uses, see Uranus (disambiguation). ... For other uses, see Neptune (disambiguation). ... OGLE-2005-BLG-390Lb is a super-Earth extrasolar planet orbiting the star OGLE-2005-BLG-390L, which is situated 21,500 ± 3,300 light years away from Earth, near the center of the Milky Way galaxy. ... The Big Splash redirects here. ...

References

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For other uses, see NASA (disambiguation). ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 223rd day of the year (224th in leap years) in the Gregorian calendar. ... The California State University (CSU) is one of three public higher education systems in the state of California, the other two being the University of California system and the California Community College System. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 131st day of the year (132nd in leap years) in the Gregorian calendar. ... James F. Kasting (b. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 220th day of the year (221st in leap years) in the Gregorian calendar. ... James F. Kasting (b. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 218th day of the year (219th in leap years) in the Gregorian calendar. ... A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 218th day of the year (219th in leap years) in the Gregorian calendar. ... The University of Washington, founded in 1861, is a public research university in Seattle, Washington. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 131st day of the year (132nd in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 131st day of the year (132nd in leap years) in the Gregorian calendar. ... The University of Colorado at Boulder (CU-Boulder, UCB officially[3]; Colorado and CU colloquially) is the flagship university of the University of Colorado System in Boulder, Colorado. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 223rd day of the year (224th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 217th day of the year (218th in leap years) in the Gregorian calendar. ... The University of Cambridge (often Cambridge University), located in Cambridge, England, is the second-oldest university in the English-speaking world and has a reputation as one of the worlds most prestigious universities. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 217th day of the year (218th in leap years) in the Gregorian calendar. ... JMU redirects here. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 131st day of the year (132nd in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 218th day of the year (219th in leap years) in the Gregorian calendar. ... 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Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 131st day of the year (132nd in leap years) in the Gregorian calendar. ... Nature is one of the oldest and most reputable general-purpose scientific journals, first published on November 4, 1869. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 223rd day of the year (224th in leap years) in the Gregorian calendar. ... David Darling David Darling (b. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 131st day of the year (132nd in leap years) in the Gregorian calendar. ... The University of Oregon is a public university located in Eugene, Oregon. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 131st day of the year (132nd in leap years) in the Gregorian calendar. ... The University of Oregon is a public university located in Eugene, Oregon. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 131st day of the year (132nd in leap years) in the Gregorian calendar. ... The Harvard-Smithsonian Center for Astrophysics (CfA) is located in Cambridge, Massachusetts. ... For information on Wikipedia press releases, see Wikipedia:Press releases. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 156th day of the year (157th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 156th day of the year (157th in leap years) in the Gregorian calendar. ... The Carnegie Institution of Washington (CIW) is a foundation established by Andrew Carnegie in 1902 to support scientific research. ... For information on Wikipedia press releases, see Wikipedia:Press releases. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 156th day of the year (157th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 223rd day of the year (224th in leap years) in the Gregorian calendar. ... The Trojan Shrine, better known as Tommy Trojan located in the center of University of Southern California campus. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 131st day of the year (132nd in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 223rd day of the year (224th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 223rd day of the year (224th in leap years) in the Gregorian calendar. ... New Scientist is a weekly international science magazine covering recent developments in science and technology for a general English-speaking audience. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 217th day of the year (218th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 168th day of the year (169th in leap years) in the Gregorian calendar. ... The University of Washington, founded in 1861, is a public research university in Seattle, Washington. ... For information on Wikipedia press releases, see Wikipedia:Press releases. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 156th day of the year (157th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 217th day of the year (218th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 217th day of the year (218th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 223rd day of the year (224th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 217th day of the year (218th in leap years) in the Gregorian calendar. ... For other uses, see Astronomy (disambiguation). ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 218th day of the year (219th in leap years) in the Gregorian calendar. ... For other uses, see NASA (disambiguation). ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 218th day of the year (219th in leap years) in the Gregorian calendar. ... Space. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 218th day of the year (219th in leap years) in the Gregorian calendar. ... David Darling David Darling (b. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 218th day of the year (219th in leap years) in the Gregorian calendar. ...

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
  • Dole, Stephen H. (1970). Habitable Planets for Man, 2nd edition, American Elsevier Publishing Co.. ISBN 0-444-00092-5. Retrieved on 2007-03-11. 
  • 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

Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 70th day of the year (71st in leap years) in the Gregorian calendar. ... Artists conception of a terraformed Mars in three stages of development. ...

External links

Image File history File links PlanetaryHabitability. ... Image File history File links Sound-icon. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 22nd day of the year in the Gregorian calendar. ...


 
 

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