Habitability of a planet

The habitability of a planet is the measurement of the capacity of a astronomical body to develop and accommodate the Vie. This concept can thus be in particular used at the same time for the Planet S and their Natural satellite.

According to the knowledge acquired by the study of terrestrial biology, the elements necessary to the maintenance of the life are a energy source coupled to mobilizable Matière, knowing that various models are proposed in support of the Origins of life. However, the concept of habitability as “possibility of accommodating the life” is intrinsically limited by the comparison to the terrestrial conditions biological, which implies that several other criteria of order Géophysique S, geochemical S and Astrophysique S are respected. Insofar as the existence of a extraterrestrial Vie is unknown, the habitability of a planet is indeed mainly a Extrapolation terrestrial conditions and general characteristics which appear favorable to the development of the life within the Solar system. The Eau liquid is in particular regarded as an element essential to a viable ecosystem. Research in this field thus concerns at the same time the Planétologie and the Astrobiologie.

The idea that planets other than the Ground can accommodate the life is old. During the history, the debate was as much philosophical that scientific. The end of the 20th century was the theater of two major discoveries. First of all, the observation and exploration by probes of planets and satellites of the solar system provided essential information which made it possible to define criteria of habitability and comparisons geophysics between the Ground and the other celestial bodies. In addition, the discovery of planets extrasolaires, which began in 1995 and accelerated since, was second the revolving important one. It confirmed that the Sun is not the only star to shelter planets and widened the field of research on the habitability beyond our solar system.

Suitable solar systems

To define the concept of habitability of a planet starts with the study of stars. The habitability of a planet depends indeed mainly on the characteristics on the planetary system (and thus of star) which shelters it. At the time of the Project Phoenix of the Program SETI, the scientists Margaret Turnbull and Jill Tarter developed the concept of HabCat (for Catalog of livable stellar systems) in 2002. The catalog was made up by extracting them: the 120000 stars closest to the Earth of the Catalog Hipparcos. Then, a more precise selection made it possible to insulate: 17000 HabStars . The choice of the criteria was a good starting point to include/understand which characteristics Astrophysique S are necessary to accommodate livable planets.

Spectral class

The spectral class of a star indicates the Température Photosphère, which for stars of the principal Séquence is related to their mass. It is currently estimated that the spectral field adapted for stars likely to accommodate systems sheltering the life ( HabStars ) goes from the beginning of the class “F” or “G” until “semi K”. That corresponds to the temperatures active of a little more than 7000 K to a little more than 4000 K. the Sun, star of the G2 class, is intentionally in the middle of this field. The stars of this type have a certain number of characteristics which are important from the point of view of the habitability of planets:

• They burn at least a few billion years, which leaves sufficient time to the life to develop. The stars of the principal sequence more luminous, like those of the classes “O”, “B” and “have”, in general burn less than one billion years and in certain cases less than 10 million years , .

• They emit sufficient Ultraviolet rays high-frequency to initiate important reactions in the atmosphere, such as the formation of ozone, but not too, because that would destroy the life.

• liquid water can exist on the planet surface orbiting at a distance which does not involve a synchronous Rotation (see the following section and 3.1.).

These stars are neither “too hot”, nor “too cold”, and burn sufficiently a long time so that the life has a chance to appear there. This type of stars probably constitutes 5 to 10% of stars of our galaxy. On the other hand, the question of knowing if less luminous stars, i.e. those between the end of the class K and the class M (dwarf reds), are also likely to accommodate livable planets remains open. It is however extremely important because the majority of stars are of this type.

Stable livable zone

See also: livable Zone

The livable zone (HZ in English) is a theoretical field near star in which all the planets present could have liquid water at their surface. After an energy source, liquid water is regarded as the most important element for the life, mainly because of the part which she plays on Earth. It is possible that is only the reflection of a skew due to the dependence with the water of the terrestrial species. So forms of life were discovered on planets whose water is absent (for example, in a solution of Ammoniac), the concept of zone livable should be deeply revised, and even entirely isolated because too restrictive.

A livable zone “stable” presents two characteristics. Firstly, its localization must little vary during time. The luminosity of stars increases with their age and a given livable zone deviates from star progressively. If this migration is too fast (for example, for a super-massive star), the planets are in the livable zone only for one very short duration, which reduces the probability considerably that the life develops to with it. To determine the livable zone and its displacement during the life of star is very difficult: Rétroaction S, such as those due to the Cycle of carbon tend to compensate for the impact of the increase in the luminosity. Thus, just as the evolution of star, the assumptions made on the atmospheric conditions and the geology of planet have a very great influence on the calculation of a livable zone. Thus, the parameters suggested to calculate the livable zone of the Sun strongly varied while this concept developed.

Then, no body of important mass such as a giant gas planet must be present in the livable zone or near this one: its presence could prevent the telluric planet formation. If, for example, Jupiter had appeared in the area which is currently between the orbits of Venus and the Earth, those could probably not have been formed. So at a certain time, the scientists supposed that the combination telluric planet on the interior orbits - gas giant planets on the external orbits were the standard, the recent discoveries of planets extrasolaires contradicted this assumption. Many giant gas planets were found on orbits close to their star, destroying any potential livable zone. The current data on the planets extrasolaires are probably skewed because the large planets having orbits eccentric and close to star are easier to find than the others. To date, it was not possible yet to determine which type of solar system is most current.

Weak variation of luminosity

See also: variable Star

All the stars know variations of luminosity, but the amplitude of these fluctuations is very different from one star to another. The majority of stars are relatively stable, but a significant minority of them is variable and present often the sudden ones and intense increases in luminosity. Consequently, the quantity of radiative energy that the bodies in orbit receive knows of abrupt variations. These last are thus of bad candidates to accommodate planets able to accommodate the life insofar as the strong changes of radiant energy fluxes would have an negative impact on the survival of the organizations. For example, of the living beings adapted to a particular temperature range would have probably evil to survive important temperature variations. Moreover, the starts of luminosity are generally accompanied by the emission by massive amounts by gamma rays and x-rays, radiations which could be lethal. The atmosphere of planets could mitigate such effects (an increase of 100  % of the solar luminosity does not imply necessarily an increase in 100  % of the temperature on Earth), but it is also possible that such planets are not able to retain their atmosphere because strong radiations striking it with repetition could disperse it.

The Sun does not know this type of variations: during the solar Cycle, the difference between the luminosities minimum and maximum is of approximately 0,1  %. There exist important evidence (and disputed) that the changes of luminosity of the Sun, although minors, had a significant effect on the terrestrial climate during the historical period. The Petit Ice Age could be caused by the reduction in the solar luminosity over one long life. Thus, it is not necessary that a star is a variable star so that its changes of luminosity affect the habitability. Among the solar similar known, that which most strongly resembles the sun is 18 Scorpii. The great difference between two stars is the amplitude of the solar cycle which is much larger on 18 Scorpii, which decreases the probability considerably that the life can develop on its orbit.

High Metallicity

See also: Métallicité

If the most abundant elements in a star are always the Hydrogène and the Hélium, there exists a great variation in the quantity of metal elements (in astronomy, one calls “metal” or qualifies “metal” any element heavier than helium) than they contain. A high proportion of metals in a star corresponds to the quantity of heavy elements present in the initial Disque protoplanétaire. According to the theory of formation of the planetary systems within the solar nebulas, a small quantity of metals in star decreases the probability significantly that planets are formed around. Any planet being formed around a star low in metals is probably of low mass, and consequently would be unfavourable with the life. Spectroscopic studies of systems in which exoplanètes were found confirm the relation between a metal high rate and the planet formation: “the stars with planets, or at least with planets similar to those which we currently find, are clearly richer in metals than stars without planetary companion”. The influence of the metallicity is discriminating as for the potential age of livable stars: the stars formed at the beginning of the history of the universe have metal low levels and a corresponding probability to accommodate planetary companions.

Binary systems

The current estimates suggest that at least half of stars are in binary systems, which seriously complicates the delimitation of the concept of habitability. The distance between two stars of a binary system lies between a astronomical Unité and a few hundreds. If separation between two stars is large, the gravitational influence of second star on a planet turning around first star will be negligible: its habitability is not modified unless the orbit is strongly eccentric (see Hypothèse of Némésis for example). However, when the two stars are brought closer, the planet could not have a stable orbit. If the distance between a planet and its principal star exceeds a fifth of the minimal distance between two stars, the orbital stability of planet is not guaranteed. It is not sure that the planets can be formed in a binary system because the gravitational forces could obstruct the formation of planets. Theoretical work of Alan Boss of the Carnegie Institute showed that gas giants can be formed around stars of binary systems in a way similar to their formation around solitary stars.

Alpha of the Centaur, the star nearest to the Sun, underlines the fact that the binary stars should not be systematically isolated at the time of the search for livable planets. Centauri has and B have a minimal distance from 11  UA (23  UA on average) and both should have stable livable zones. A simulation of the orbital stability in the long run of planets in this system shows that planets with approximately 3  UA of one of two stars can remain stable (i.e. the Equatorial radius deviates of less than 5%). The livable zone of Centauri has would be at least of 1,2 with 1,3  UA and that of Centauri B of 0,73 with 0,74  UA.

Planetary characteristics

The principal assumption made on livable planets is that they are telluric. Such planets, whose mass would be of the same order of magnitude that of the Earth, are mainly made up of Silicate S and did not preserve external gas layers of Hydrogène and Hélium like gas planets. A form of life which would lie in the roadbases of clouds of the gas giants is not excluded, although it is regarded as improbable being given the absence of surface and gigantic gravity . On the other hand, the natural satellites such planets could accommodate the life very well.

During the analysis of the environments probably able to accommodate the life, one distinguishes the unicellular organizations in general such as the Bactérie S and the Archaea of the forms of life Animal be, more complex. The unicellularity precedes necessarily the multicellularity in all Phylogenetic tree hypothetical and the appearance of unicellular organizations does not involve necessarily the appearance of more complex forms of life. The planetary characteristics listed low are regarded as essential for the life, but in all the cases, the conditions of habitability of a planet will be more restrictive for the multicellular organizations such as the plants and the animals that for the unicellular life.

Mass

The planets of low mass would be bad candidates to accommodate the life for two reasons. First of all, them weaker Gravité returns to them thinner atmosphere. The constituent molecules of the life have a probability much more raised to reach the Escape velocity and to be ejected in space when they are propelled by the solar wind or a collision. The planets of which the atmosphere is not thick would not have sufficient matter for the initial Biochimie, have little insulation and a bad Transfer of heat through their surface (for example, Mars with its fine atmosphere, is colder than would have been to it the Earth at the same distance) and less protection against radiations high-frequency and the Météoroïde S. Moreover, the smaller planets have a smaller diameter and thus of larger ratios surface-volume than their cousins of big size. Such bodies tend to see their energy escaping much more quickly after their formation and thus have little geological activity. They do not have a Volcan S, of earthquakes and tectonic activity which provides to surface elements supporting the life and to the atmosphere of the molecules controlling the temperature (like the Carbon dioxide).

The " term; weak masse" is only relative: the Earth is considered of low mass when it is compared with giant planets of the solar system, but is largest, most massive and densest of telluric planets. It is sufficiently large so that its gravity retains its atmosphere and so that its liquid core continues to remain active and hot, thus generating a geological activity on the surface (the Désintégration of the elements Radioactifs in the middle of planet is the other source of heat of planets). March, on the contrary, is almost (or perhaps completely) inactive and lost the major part of its atmosphere. Thus, it seems that the minimal mass of a planet so that it is livable locates some share between that of Mars and that of the Earth (or Venus). However, this rule can admit exceptions: Io, a Jupiter satellite smaller than telluric planets, has a volcanic activity because of the constraints generated by the gravitational influence jovienne. Its neighbor, Europe, could shelter a liquid ocean under her surface frozen because of the energy created by the field of gravitation jovien. For a different reason, one of the moons of Saturn, Titan, is of unquestionable interest: it preserved a thick atmosphere and the biochemical reactions are possible in the Méthane liquid on its surface. These satellites are exceptions, but they prove that the mass should not be regarded as discriminating in terms of habitability.

Lastly, a large planet will probably have a large core made up of iron. This last created a magnetic field which protects planet from the Solar wind, which in its absence would tend to disperse the planetary atmosphere and to bombard ionized particles the living beings. The mass is not the only element to be taken into account to determine the existence of a magnetic field. The planet must also have a sufficiently fast rotation movement to produce a $dynamo effect within its core.

Orbits and rotation

As for other criteria, stability would be paramount for the orbits and the rotation of a planet so that it is livable. Larger orbital Excentricité is the , larger is the fluctuation of the temperature on the surface of planet. Although they adapt, the living organisms cannot support a too great variation, in particular if this one recovers at the same time the Point boiling and the melting Point of principal biotic solvent of planet (on Earth, water). If, for example, the oceans of our planet vaporized in space and froze in turn, it is difficult to imagine there that the life such as one knows it could have evolved/moved. The orbit of the Earth is almost circular, its eccentricity being of less than 0,02. The another planets of our solar system (except for Pluto and with a less measurement of Mercury) have similar eccentricities. The data collected on the eccentricities of the planets extrasolaires have surprised the majority of the researchers: 90  % have eccentricities larger than those of planets of the solar systems, the average being of 0,25. This characteristic could be due to a simple skew of observation because a strong eccentricity increases the oscillation of star and thus facilitates the detection of planet.

The movement of a planet around its Axis of rotation must undoubtedly respect certain characteristics so that the life is likely to evolve/move.

• the cycle day-night should not be too long. If one day takes years (terrestrial), the difference in temperature between the enlightened part and the part sinks will be high and the problems are similar to those of a very great orbital eccentricity.
• the planet must have moderate seasons. If there is a little axial slope (compared to the perpendicular with the ecliptic ), the seasons will not be marked and one of the principal stimulants of the dynamism of the Biosphère disappears; such planets will be in general colder than if they had been tilted. If a planet has a strong inclination, the seasons will be extreme and the Homéostasie of the biosphere will have evil to be exerted. At present, the exact effects of these changes can only be simulated: studies showed that even extreme slopes (until 85°) could be compatible with the life provided that " it does not occupy the exposed continents each season at the temperature more élevée".
• the oscillation must remain weak. The cycle of Précession of the Earth lasts: 23000 years. If it were much shorter or if the oscillation were more important, of the important climate changes would affect the habitability largely.

The Moon seems to play a crucial role in the regulation of the terrestrial climate by stabilizing the slope of the axis of rotation. It was suggested that a planet whose slope would have a chaotic movement, could not accommodate the life: a satellite of the size of the moon could be not only useful, but even essential to allow the habitability. This thesis is however discussed.

Geochemistry

It is estimated in general that all extraterrestrial Vie should be based on same chemistry as that of the Earth because the four most important elements for the terrestrial life (the Carbone, the Hydrogène, the Oxygène and the Azote) are also the four chemical elements reactive most abundant in the universe. Indeed, of the simple pre-biotic molecules, such as the Amino-acid , was found in Météorite S and the interstellar Espace. In mass, these four elements constitute approximately 96  % of the terrestrial biomass. The carbon atoms have an incomparable capacity to establish chemical bonds between them and to form great complex structures, which makes them ideal to be the base of the complex mechanisms which constitute the living beings. Water, made up of oxygen and hydrogen, constitutes the solvent in which the biological processes and the initial reactions leading to the appearance of the life occurred. Energy coming from the covalent bond between the atoms from carbon and those of hydrogen released by the dissociation of the Carbohydrate and other organic molecules, is the containing hydrocarbon one of all complex forms of life. These four elements join to form amino-acids, which them even constitute proteins, essential components of the living organisms.

Relative abundances of the various elements in space are not always similar to their values on planets. For example, of the four above-mentioned elements, only oxygen is present in great quantity in the Earth's crust. That can be partly explained by the fact why many of these elements, such as hydrogen and nitrogen, as well as other simple molecules, such as the Carbon dioxide, the Carbon monoxide, the Méthane, the Ammoniac and water, are gas with the high temperatures. In the hot areas near the sun, these volatile molecules did not play a great part in the geological formation of planets. They were indeed trapped in a gas state under the crusts lately made up. Those are mainly made up of nonvolatile molecules in rock forms, like the Silice (a molecule made up of Silicium and Oxygène whose great abundance in the earth's crust explains that of oxygen). The degasification of the volatile molecules by the first Volcan S would have contributed to the formation of the atmosphere of planets. The Expérience of Miller-Urey showed that with a contribution of energy, the amino-acids could be synthesized starting from molecules simple present in the primary atmosphere.

Even thus, volcanic degasification cannot explain the quantity of water of the terrestrial oceans. The major part of water necessary to the life, and perhaps of carbon, undoubtedly came from the external solar system where, moved away from the heat of the sun, it could remain solid. The comets being crushed on the Earth at the beginning of the solar system would have deposited great quantities of water there, as well as the other volatile molecules which the life needs (of which amino-acids). That would have allowed the fast appearance of the life on Earth.

Thus, although it is probable that the four principal elements are present in other places, a livable system would need a continuous contribution of body in orbit in order to provide in elements interior planets. It is possible that the life such as we know it on Earth would not exist without comets. It is however possible that other elements can be used as building blocks for forms of life based on a different chemistry.

Other considerations

Habitability of the systems around dwarf the reds

To determine the habitability of the dwarf reds could help to determine if the life is current in the universe. Indeed, dwarf the reds constitute between 70% and 90% of stars of our galaxy. The brown dwarf are probably more numerous than dwarf the reds. However, they are not regarded as stars and it is probable that they cannot lodge the life, at least such as we know it, because they emit only very little heat.

During years, the astronomers drew aside dwarf the reds of the potentially livable systems. Their small size (between 0,1 and 0,6 solar mass) corresponds to nuclear reactions extremely slow: they emit very little light (between 0,01 and 3% of that of the sun). Any planet orbits about it around dwarf a red should be very close to its star-host to have a temperature of surface comparable with that of the Earth: of 0,3  UA (slightly less than Mercury) for a star like Lacaille 8760 with 0,032  UA (the year of such a planet would last six days terrestrial) for a star like Proxima of the Centaur. At these distances, the gravity of star generates a synchronous Rotation. A half of planet would all the time be lit, while the other would be it never. The only possibility so that a potential life is not subjected to a heat or a freezing cold is the case where this planet would have a sufficiently thick atmosphere to transfer heat on the side lit towards the dark side. For a long time, it was supposed that such a thick atmosphere would prevent the light of star from reaching surface, returning the impossible Photosynthèse.

The recent discovered ones however tend to dispute this point of view. Studies undertaken by Robert Haberle and Manoj Joshi of the Ames Research Center of NASA showed that the atmosphere of a planet around dwarf a red would need only to be 15  % thicker than that of the Earth to allow the heat of star to diffuse itself on the face never enlightened. Water would remain cold on this face in some their models. This margin is in addition completely compatible with photosynthesis. Martin Heath of the Greenwich Community College showed that the sea water could also circulate without freezing entirely on the side in the shade if the oceans were sufficiently deep on this face to allow a free movement of water under the layer of ice located at surface. Thus, a planet with suitable oceans and an atmosphere orbits around dwarf a red, could about it, at least in theory, to accommodate the life.

The size is however not the only criterion making the presence of life improbable around dwarf the reds. A planet around dwarf a red would be enlightened only on one side and thus photosynthesis would be impossible on more half of its surface (the night side and zones in the shade on the enlightened face). Moreover, radiations of dwarf a red are mainly in the infra-red whereas on Earth, photosynthesis uses the visible light.

The dwarf reds are much more variable and violent that their larger and more stable cousins. They are often covered with sunspots which can decrease the light emitted by star until 40  % during a few months while at other gigantic periods solar eruptions doubles its brightness in a few minutes. Such variations would strongly damage the life, although it is possible that they would stimulate the evolution species by increasing the rate of change and by modifying the climate quickly.

Dwarf the reds however have a major advantage on other stars like host systems of the life: they burn very a long time. Humanity appeared on Earth 4,5 billion years after the formation of our planet and the life such as we know it would have adequate conditions around our star for only five hundred million additional years. On the contrary, dwarf the reds can burn Billion S years because the nuclear reactions of which they are the seat are much slower than those of largest stars. The life would thus lay out to with it of more than time to develop and evolve/move. Moreover, even if the probability of finding a planet in the livable zone around dwarf a red is weak, the full number of livable zones around dwarf the reds is equal to that of stars similar to the Sun being given their great number.

“Good Jupiters”

The Bons Jupiters are gas giant planets, like planet of the solar system Jupiter, which turn around their star on circular, sufficiently far from the livable zone not to have a disturbing effect, but sufficiently close orbits “to protect” telluric planets located on interior orbits. First of all, they stabilize the orbits of these planets, and consequently their climate. Moreover, they contribute to limit the number of Comet S and Astéroïde S which could cause impacts devastators on a planet sheltering the life. Jupiter turns around the sun at a distance approximately five times larger than the Earth does it. It is at a comparable distance that the scientists hope to find " Jupiters" goods; around other stars. The role of Jupiter was clarified in 1994 when the Comet Shoemaker-Levy 9 was crushed there. If gravity jovienne had not captured comet, this one could very well have entered the interior solar system.

At the first ages of the solar system, Jupiter had an opposite role: it contributed to increase the eccentricity of the orbits of the objects of the Ceinture of asteroids. A great number of them were crushed on the Earth and provided him great quantity of volatile elements. Before the Earth does not reach half of its current mass, the frozen bodies of the area around Jupiter and Saturne and small bodies of the initial belt of asteroids brought water on Earth because of the gravitational disturbances their trajectories generated by Jupiter and Saturne. Thus, whereas the gas giants are from now on the useful protective ones, they were important by allowing the essential matter contribution the habitability.

Galactic vicinity

The scientists as put forth the assumption as certain zones of the galaxy ( livable galactic zones ) allow better than others the existence of the life. The solar system in which we live, in the Bras of Orion, on a side of the Milky Way is regarded as being a favorable site. Very far away from the galactic Center, it avoids certain dangers:

• It is not in a globular Amas.
• It is not near an active source of Gamma rays.
• It is far away from the supermassif Black hole commonly associated with Sagittarius A*.
• the circular orbit of the sun around the galactic center does not make it meet one of the spiral arms of the galaxy, where intense radiations and the gravitation would disturb any form of life considerably.

A relative insulation is ultimately that whose system where the life is present needs. If the solar system were surrounded by many close systems, those could disturb the stability of the orbits of the objects of the solar system (in particular objects of the Nuage of Oort and the Ceinture of Kuiper which could have catastrophic consequences if they were deviated towards the interior of the solar system). Nearby close relations increase also the possibility of being in the fatal zone of a Pulsar or an explosion of Supernova.

See too

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