Planetary Habitability

The planetary habitability is a measure of the potential of an astronomical body to sustain life . It can be applied to the planets and the moons of the planets.

The only absolute requirement for life is a source of energy . For this reason, it is interesting to determine the habitable zone of different stars , but the notion of planetary habitability implies fulfillment of many other criteria geophysical , geochemical and astrophysicists for an astronomical body is capable of supporting life. As is known the existence of extraterrestrial life , planetary habitability is largely an extrapolation of the conditions of the Earth and the characteristics of the Sun and the solar system that seem favorable for the flourishing of life. Of particular interest is the set of factors that have favored the appearance on Earth of organisms multicellular organisms and not simply celled . Research and theory on this topic are components of planetary science and the emerging discipline of astrobiology .

The idea that other planets could harbor life is very old, but historically has been framed within the philosophy as much as in the physical sciences . The end of the twentieth century experienced two major breakthroughs in this area. Start by exploring robotics and observation of other planets and moons in the solar system have provided information essential to define the criteria for habitability and allowed substantial geophysical comparisons between Earth and other bodies. The discovery of extrasolar planets , which began in 1992 and has exploded since then, was the second milestone. He confirmed that the Sun is not only harboring planets and extended the research horizon beyond habitability of Earth.

Star Systems suitable
Orbit 55 Cancri f within the planetary habitable zone of its star 55 Cancri .

Understanding planetary habitability begins in the stars . Although the bodies may, in general, are similar to Earth are very numerous, it is equally important that the system in which they live is compatible with life. Sponsored by the Phoenix Project of SETI , the scientists Margaret Turnbull and Jill Tarter developed in 2002 the ” HabCat “(or” catalog habitable stellar systems “). The catalog was compiled by screening the nearly 120,000 stars from the Hipparcos catalog to stay with a group of 17,000 “Habstars”, and the selection criteria used provide a good starting point to understand why they are necessary factors for an astrophysical planet habitable.
Spectral type

The spectral type of a star indicates the temperature of its photosphere , which (for main sequence stars ) is correlated with the total mass. Currently considered to be the appropriate spectral range for “Habstars” ranging from “low F” or “G” to “K Medium”. This corresponds to a temperature of just over 7000 K to just over 4000 K, the sun (not coincidentally) is right in the middle of this range, and is classified as a G2 star. The stars of “middle class” as it has a number of features considered important for planetary habitability:

They live at least a few billion years, providing an opportunity for life to evolve. The main sequence stars of type “O”, “B” and “A”, brighter, usually live less than a billion years and in exceptional cases less than 10 million years.
Emit enough ultraviolet radiation high energy occurring atmospheric phenomena such as the formation of ozone , but not so much that the ionization destroy nascent life.
Liquid water can exist on the surface of planets orbiting at a distance that does not produce tidal coupling . (See next section and 3.2).

These stars are neither “hot” nor “too cold” and live long enough for life to have a chance to emerge. This spectral range is between 5 and 10 percent of the stars in the galaxy Milky Way . If the low K type stars and M (” red dwarf “) are also suitable for harboring habitable planets is perhaps the most important open question in the entire field of planetary habitability, since most of the stars fall within that range, this is extensively explained below.
A stable habitable zone
Main article: Zone of habitability .

The habitable zone (HZ) is a theoretical shell surrounding a star, within which any planets have water on its surface liquid. After an energy source, liquid water is considered the most important ingredient for life, considering how essential it is for all living things on Earth. This may reflect the prejudices of a water-dependent species, and if life is discovered in the absence of water (eg, in a solution of ammonia liquid), the notion of ZH would expand much or completely discarded as too restrictive. Note

A ZH “stable” implies two things. First, the range of ZH should not vary much over time. All stars increase of luminosity as they age and move naturally ZH outwards, but if this happens too quickly (for example, a supermassive star), planets would have only a short window in the ZH and therefore less likely develop life. Calculate the range of ZH and long-term movement is never easy, because the feedback loops as negative carbon cycle tend to shift the luminosity increases. The assumptions made ​​about atmospheric conditions and geology have an impact on the HZ range as large as the solar evolution, the proposed parameters for the HZ of the Sun, for example, have fluctuated widely.

Second, there must be no massive body as a gas giant within or relatively close to the HZ, interfering with the formation of bodies like the Earth. The mass of the asteroid belt, for example, it appears that he was unable to form a planet by accretion due to orbital resonances with Jupiter, if the giant had appeared in the region that is now between the orbits of Venus and Mars , almost Earth security would not have developed its present form. This is offset somewhat by the signs that a gas giant in the ZH, under certain conditions, could have moons.

Before it was assumed that the pattern of inner rocky planets and outer gas giants observable in the solar system was the norm everywhere, but discoveries of extrasolar planets have thrown this idea. There have been numerous Jupiter-sized bodies orbiting close to its parent star, thwarting the potential ZHS. It is likely that the current data of extrasolar planets are biased towards large planets with eccentric orbits and small, because they are much easier to identify, still remains unknown what kind of solar system is the norm.
Low variation stellar
Main article: Variation stellar .

Changes in brightness are common in all the stars, but the magnitude of these fluctuations covers a wide range. Most of the stars are relatively stable, but a significant minority of variable stars often experienced intense surges of light, and therefore energy radiated into orbiting bodies. These stars are considered poor candidates for harboring habitable planets, as their unpredictability and changes in energy emissions would negatively impact on organisms. As a result more evident living adapted to a temperature particularly likely to be unable to survive a temperature change too large. Moreover, increases in brightness are usually accompanied by massive doses of gamma rays and X-rays that can be lethal. The atmospheres mitigate such effects (an absolute increase of 100 percent of the sun’s luminosity does not necessarily mean an increase of 100 percent of the absolute temperature of the Earth), but protection may not be given the atmospheres on planets orbiting variable stars, since the high frequency energy that hits these bodies continually deprive them of their protective cover.

The Sun, as in almost everything and is kind on this danger: the variation between the maximum and solar minimum is only 0.1 percent, over the solar cycle of 11 years. There is strong evidence (though not undisputed) that small changes in the brightness of the Sun have had significant effects on the Earth’s climate within historical time, the Little Ice Age in the middle of the second millennium, for example, could have its cause a long-term decline of the luminosity of the Sun Thus, a star need not be a true variable star that differences in brightness affect habitability. Of the ” twin sun “known, is considered the most similar to the Sun is 18 Scorpii , is interesting (and unfortunate for the chances of life existing in its proximity) that 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
Main article: Metallicity .

Although the bulk of the material of any star is hydrogen and helium , there is great variation in the amount of heavy elements which it contains. A large proportion of metals in a star is correlated with the amount of heavy material available in protoplanetary disk . A low amount of metal significantly decreases the likelihood that planets are formed around a star, according to the theory of solar nebula formation of planetary systems . Any planets forming around a star with little metal will probably very low mass, and therefore will not be favorable for life. To date, studies spectroscopic systems in which it has found a exoplanet confirm the relationship between high metal content and planet formation, “the stars with planets, planets or at least similar to those found today in day, are clearly more metal rich than stars without planetary company “. The high metallicity also establishes a requirement for Habstars youth: the stars formed early in the history of the universe has a low metal content and corresponding less likely to have planetary companions.
Features planetary

The main assumption about habitable planets is that they are terrestrial . These planets, which lie approximately within an order of magnitude of the mass of earth, rock mainly composed of silicate and have accreted from gaseous outer layers of hydrogen and helium that are in the gas giants . Not completely ruled life can evolve in the cloud tops of the giant planets, ​​although it is considered unlikely because no surface and its gravity is enormous. The natural satellites of giant planets, other hand, are perfectly valid candidate to support life.

By analyzing which environments are more likely to support life, is often a distinction between single-celled organisms such as bacteria and archaea , and complex organisms such as metazoans (animals). The unicelularidad necessarily precedes multicellularity in any hypothetical tree of life, and unicellular organisms emerge where there is nothing to ensure that they develop more complex than that. The planetary features listed below are generally considered crucial for life, but in all cases the habitability impediments should be considered more severe for multicellular organisms like plants and animals for unicellular life.
Mars , with its atmosphere, is colder than the Earth would be if it were at a similar distance from the Sun.

The low-mass planets are poor candidates for life for two reasons. First, its low gravity makes conserve atmosphere difficult. The molecules constituents are more likely to achieve escape velocity and be lost in space when they are bombarded by solar wind or agitated by a collision. The planets do not have a thick atmosphere lack the necessary material for biochemical primary, have little insulation and low heat transfer between the surface (eg, Mars , with its thin atmosphere, is colder than the Earth would a similar distance) and less protection against radiation of high frequency and meteoroids . Furthermore, if the atmosphere is less than 0.006 atmospheres ground, can not exist in liquid water for not reaching the atmospheric pressure required, 4.56 mmHg (608 Pascals). The temperature range in which the water is liquid is smaller at low pressures, generally.

Second, the small planets have diameters smaller and therefore greater surface / volume ratio than their larger cousins. These bodies tend to quickly lose energy left over after formation and end geologically dead, lacking volcanoes , earthquakes and tectonic activity , which provide the necessary materials to surface life and moderators of temperature atmosphere as carbon dioxide . Plate tectonics seems particularly crucial, at least on Earth: not only serves to recycle important minerals and chemicals, also promotes biodiversity continents creating and increasing environmental complexity and helps create convective cells necessary to generate the magnetic field land .

“Low mass” is a section on etiquette, it is considered that the Earth has little mass when compared to the gas giants of the solar system, but it is, of all terrestrial bodies, the largest diameter and mass and also the most dense. It is large enough to hold an atmosphere with its severity and its liquid core remains a source of heat, driving the diverse geology of the area (the breakdown of the elements radioactive in the core of a planet is another significant component of global warming). Mars, in contrast, is almost (or perhaps totally) geologically dead, and has lost much of its atmosphere. Therefore, it would be correct to conclude that the minimum mass limit for habitability is somewhere between Mars and Earth or Venus. Exceptional circumstances offer exceptional: Moon Jupiter Io (smaller than the terrestrial planets) is volcanically active gravitational stresses induced by its orbit, the nearby Europa may have a liquid ocean under a frozen layer due also to the energy created in orbit around a gas giant, the moon of Saturn, Titan , on the other hand, has an outside chance of harboring life as it retains a thick atmosphere and biochemical reactions are possible in liquid methane on its surface. These satellites are exceptions, but show that the mass of habitability criterion can not be considered definitive.

Finally, a large planet is likely to have a large iron core. This allows the existence of a magnetic field that protects the planet from the solar wind , which otherwise tend to strip it of its atmosphere and bombard living with ionized particles. The mass is not the only criteria needed to produce a magnetic field – the planet must also rotate fast enough to produce an effect of dynamo in its core – but it is a significant component of the process.
Orbit and rotation

As with the other criteria, stability is the critical consideration in determining the effect of the orbital and rotational characteristics of planetary habitability. The orbital eccentricity is the difference between the largest and smallest distances to the parent object. The greater the eccentricity, the greater the fluctuation of the temperature on the surface of a planet. Although adaptive living things can only support some variation, especially if fluctuations exceed both the freezing point and the boiling point of the solvent planet main biotic (eg, water on Earth). If, for example, the Earth’s oceans will evaporate and freeze alternately, it is difficult to imagine how life might have evolved as we know it. The more complex an organism, the more sensitive to temperatures. The Earth’s orbit is almost circular, with an eccentricity of less than 0.02, other planets in our system (except Pluto and Mercury ) have equally benign eccentricities.

Data collected on the orbital eccentricity of extrasolar planets has surprised many researchers: 90% have orbital eccentricity larger than the planets of the solar system, and the mean is 0.25. This could be easily result of a bias in the sample. Often the planets are not observed directly, but is inferred from the “wobble” in the star-producing. The greater the eccentricity, the greater the disturbance on the star, and therefore greater detectability of the planet.

The motion of a planet around its axis of rotation must also meet certain criteria for life to have a chance to evolve. The first assumption is that the planet must be moderate seasons. If there is little or no axial tilt (or obliquity) relative to the perpendicular to the ecliptic , and stations will not disappear so main stimulant of the dynamics of the biosphere. The planet should be much colder than it would if it had a significant tilt: when more intense radiation falls always within a few degrees of Ecuador, the warm weather can not beat the polar and global climate systems dominated by just polar climate, colder.

On the other hand, if a planet is radically inclined, stations will be extreme and will make it harder to reach the biosphere homeostasis . Although during the Quaternary Earth had greater axial tilt which coincided with reduced ice polar, warmer temperatures and less seasonal variation, scientists do not know whether this trend would have continued indefinitely greater axial tilt. (See global glaciation ).

The exact effects of these changes can only be modeled by computer today, and studies show that even extreme slopes up to 85 degrees not absolutely rule out life, “provided they do not occupy continental surfaces suffering seasonally higher temperature.” should be considered not only mean axial tilt, but also its variation over time. The Earth’s tilt varies between 21.5 and 24.5 degrees in 41,000 years. A more drastic variation or a periodicity much shorter induce climate changes as variations in the severity of the stations.

Other orbital considerations are:

The planet must rotate relatively fast for the day-night cycle is not too long. If one day last year, the temperature differential between the day side and the night side will be pronounced, and appear similar problems of extreme orbital eccentricity.
Changes in the rotation axis direction ( precession ) should not be delivered. By itself, the precession does not affect the habitability, as it changes the direction of the tilt, not its degree. However, the precession tends to accentuate the variations caused by other orbital deviations, see Orbital variations . On Earth, the precession has a cycle of 23,000 years.

Earth’s moon appears to play a crucial role in moderating the Earth’s climate by stabilizing the axial tilt. It has been suggested that a chaotic tilt can be fatal for habitability-ie, a satellite the size of the Moon not only aid but a requirement to produce stability. There is controversy on this point.

It is generally assumed that any extraterrestrial life that may exist will be based on the same fundamental chemistry that terrestrial life as the four essential elements for life, carbon , hydrogen , oxygen and nitrogen are also the most common elements of chemical reagents universe. In fact, we have found simple biogenic compounds such as amino acids in meteorites and in interstellar space . These four elements constitute 96 percent of the biomass total Earth. Carbon has an unparalleled ability to bond with itself and form varied and intricate structures, making it the ideal material for forming the complex mechanisms of living cells. The hydrogen and oxygen into water, comprising the solvent in which biological processes take place and in which the initial reaction occurred leading to the emergence of life. The energy released in the formation of strong covalent bonds between carbon and oxygen available to oxidize organic compounds, is the fuel of all living complex. These four elements are used to construct amino acids which are the building blocks of proteins , the substance of living tissue.

The relative abundance in space is not always reflected in an abundance of planets, for example, of the four vital elements, only oxygen is present in abundance in the earth’s crust . This can be partly explained by the fact Many of these elements, such as hydrogen and nitrogen together with their most basic compounds such as carbon dioxide , the carbon monoxide , the methane , the ammonia and the water , are gaseous at warm temperatures. In the warm region near the Sun, these volatiles could not have played a significant role in the geological formation of the planets. In contrast, were captured in gaseous form under the young bark, which largely consisted of non-volatile compounds like rocky silicon dioxide (a compound of silicon and oxygen that realizes the relative abundance of oxygen). The release of volatiles through the first volcanoes have contributed to the formation of the atmosphere of the planets. The Miller experiments showed that can form amino acids in an atmosphere mainly by synthesis of the single compounds.

However, the release of volcanic gases can not explain the amount of water in Earth’s oceans. The vast majority of the water, and carbon arguably, necessary for life had to come from outer solar system, solar heat away from where it could remain solid. The comet that hit the Earth in the early solar system would have deposited vast amounts of water, and other volatile compounds necessary for life (including amino acids) on the early Earth, providing the ignition spark for the evolution life.

Thus, although there is reason to suspect that the four “vital elements” are available anywhere, it is likely that a living system also need a long-term supply of orbiting bodies that sow the inner planets. No comets may life as we know it would not exist on Earth. There is also the possibility that other essential elements beyond those on Earth are those that provide a biochemical basis for life elsewhere; see biochemical hypothetical .

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