La principal suposición sobre los planetas habitables es que son terrestres. Estos planetas, que se encuentran aproximadamente dentro de un orden de magnitud de la masa de la Tierra, están compuestos principalmente de rocas de silicato y no han acrecido a partir de las capas gaseosas exteriores de hidrógeno y helio que se encuentran en los gigantes gaseosos. No se ha descartado completamente que pueda evolucionar vida en las nubes superiores de los planetas gigantes,[Nota 4]​ aunque se considera poco probable dado que no tienen superficie y su gravedad es enorme.[9]​ Los satélites naturales de los planetas gigantes, por otro lado, son candidatos perfectamente válidos para albergar vida.[10]​.

Al analizar qué ambientes tienen mayor probabilidad de permitir vida, se suele hacer una distinción entre los organismos unicelulares como las bacterias y arqueas, y los organismos complejos como los metazoos (animales). La unicelularidad precede necesariamente a la pluricelularidad en cualquier hipotético árbol de la vida, y donde emergen organismos unicelulares no hay nada que asegure que se desarrollará mayor complejidad que esa.[Nota 5]​ Las características planetarias listadas abajo se consideran generalmente cruciales para la vida, pero en todos los casos los impedimentos a la habitabilidad deben considerarse más severos para los organismos pluricelulares como las plantas y los animales que para la vida unicelular.

Mass

Low-mass planets are poor candidates for life for two reasons. First, its low gravity makes preserving the atmosphere difficult. Constituent molecules are more likely to reach escape velocity and be lost into space when bombarded by solar wind or agitated by a collision. Planets that do not have a thick atmosphere lack the material necessary for primary biochemistry, have little insulation and heat transfer across their surface (for example, Mars "Mars (planet)", with its thin atmosphere, is colder than Earth would be at a similar distance), and less protection from high-frequency radiation and meteoroids. Furthermore, if the atmosphere is less than 0.006 Earth atmospheres, water cannot exist in liquid form because it does not reach the required atmospheric pressure, 4.56 mmHg (608 pascals). The temperature range in which water is liquid is smaller at low pressures, in general.

Second, small planets have small diameters and therefore greater surface-to-volume ratios than their larger cousins. These bodies tend to quickly lose the energy left over after their formation and end up geologically dead, lacking volcanoes, earthquakes and tectonic activity, which provide the surface with materials necessary for life and the atmosphere with temperature moderators such as carbon dioxide. Plate tectonics appears to be particularly crucial, at least on Earth: it not only serves to recycle important minerals and chemical compounds, it also fosters biodiversity by creating continents and increasing environmental complexity and helps create the convective cells necessary to generate the Earth's magnetic field.[11]​.

“Low dough” is a partly relative label; The Earth is considered to have little mass when compared to the gas giants of the solar system, but it is, of all terrestrial bodies, the largest in diameter and mass and also the densest.[Note 6]​ It is large enough to retain an atmosphere with its gravity and for its liquid core to remain a source of heat, driving the diverse geology of the surface (the decay of radioactive elements in a planet's core is another significant component of planetary warming). Mars, in contrast, is almost (or perhaps completely) geologically dead, and has lost much of its atmosphere.[12] Therefore, it would be correct to deduce that the minimum mass limit for habitability lies somewhere between Mars and Earth or Venus. Exceptional circumstances offer exceptional cases: Jupiter's satellite "Jupiter (planet)") Io "Io (satellite)") (smaller than the terrestrial planets) is volcanically active by the gravitational stresses induced by its orbit; neighboring Europa "Europe (satellite)") may have a liquid ocean under a frozen layer also due to the energy created in its orbit around a gas giant; Saturn's satellite "Saturn (planet)") Titan "Titan (satellite)"), on the other hand, has a remote possibility of supporting life, since it retains a thick atmosphere and biochemical reactions are possible in the liquid methane on its surface. These satellites are exceptions, but they demonstrate that mass as a criterion of habitability cannot 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 would otherwise tend to strip it of its atmosphere and bombard living beings with ionized particles. Mass is not the only criterion necessary to produce a magnetic field—the planet must also rotate fast enough to produce a dynamo effect within its core[13]—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 orbital and rotational characteristics on planetary habitability. Orbital eccentricity is the difference between the greatest and smallest distances to the primary object. The greater the eccentricity, the greater the temperature fluctuation on a planet's surface. Although adaptive, living things can only withstand so much variation, especially if the fluctuations exceed both the freezing point and the boiling point of the planet's primary biotic solvent (for example, water on Earth). If, for example, Earth's oceans alternately evaporated and froze, it is difficult to imagine how life as we know it could have evolved. The more complex an organism is, the more sensitive it is to temperatures.[14]​ The Earth's orbit is almost circular, with an eccentricity of less than 0.02; other planets in our system (with the exception of Pluto "Pluto (dwarf planet)") and Mercury "Mercury (planet)") have similarly benign eccentricities.

The data collected on the orbital eccentricity of extrasolar planets has surprised many researchers: 90% have a larger orbital eccentricity than the planets in the solar system, and the average is 0.25.[15] This could easily be the result of sample bias. Planets are often not observed directly, but are inferred from the "wobble" they produce in their star. The greater the eccentricity, the greater the disturbance on the star, and therefore the greater the detectability of the planet.

The movement of a planet around its axis of rotation must also meet certain criteria for life to have a chance to evolve. A first assumption is that the planet must have moderate seasons. If there is little or no axial inclination (or obliquity) relative to the perpendicular of the ecliptic, there will be no seasons and therefore a major stimulant of biosphere dynamics will disappear. The planet would also be much colder than it would be if it had a significant tilt: when the most intense radiation always falls within a few degrees of the equator, the warm climate cannot overtake the polar climate and the planet's climate ends up dominated by the colder polar climate systems.

On the other hand, if a planet is radically tilted, the seasons will be extreme and make it more difficult for the biosphere to achieve homeostasis. Although during the Quaternary the Earth had a greater axial tilt that coincided with a reduction in polar ice, warmer temperatures and less seasonal variation, scientists do not know if this trend would have continued indefinitely with a greater axis tilt. (See Global glaciation).

The exact effects of these changes can only be modeled by computer today, and studies show that even extreme tilts of up to 85 degrees do not absolutely rule out life, "as long as they do not occupy continental surfaces that seasonally experience the highest temperature."[16] Not only the mean axial tilt must be considered, 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 much shorter periodicity, would induce climatic changes such as variations in the severity of the seasons.

Geochemistry

It is generally assumed that any extraterrestrial life that may exist will be based on the same fundamental chemistry as terrestrial life, since the four elements essential for life, carbon, hydrogen, oxygen and nitrogen, are also the most common reactive chemical elements in the universe. In fact, simple biogenic compounds, such as amino acids, have been found in meteorites and in interstellar space. These four elements make up 96% of the total biomass "Biomass (ecology)" on Earth. Carbon has an unparalleled ability to bond with itself and form varied and intricate structures, making it the ideal material for the complex mechanisms that form living cells. Hydrogen and oxygen, in the form of water, make up the solvent in which biological processes take place and in which the first reactions that led to the emergence of life took place. The energy released in the formation of powerful covalent bonds between carbon and oxygen, available when oxidizing organic compounds, is the fuel for all complex living beings. These four elements serve to build amino acids, which are the building blocks of proteins, the substance of living tissue.

Relative abundance in space is not always reflected in abundance on the planets; For example, of the four vital elements, only oxygen exists in abundance in the Earth's crust.[18] This can be explained in part by the fact that many of these elements, such as hydrogen and nitrogen, along with their more basic compounds, such as carbon dioxide, carbon monoxide, methane, ammonia, and water, are gases at temperate temperatures. In the warm region close to the Sun, these volatile compounds could not have played a significant role in the geological formation of the planets. Instead, they were captured in gaseous form beneath the young crusts, which were mostly made up of non-volatile rock compounds such as silicon dioxide (a compound of silicon and oxygen that accounts for the relative abundance of oxygen). The release of volatile compounds through the first volcanoes would have contributed to the formation of the planets' atmosphere. Miller's experiments demonstrated that amino acids can be formed in a primordial atmosphere by synthesis of simple compounds.[19]​.

Despite this, volcanic gas release cannot explain the amount of water in Earth's oceans.[20] The vast majority of the water, and arguably carbon, needed for life had to come from the outer solar system, far from solar heat where it could remain solid. Comets that impacted Earth in the early years of the solar system would have deposited vast amounts of water, plus the other volatile compounds necessary for life (including amino acids), on the young Earth, providing the ignition spark for the evolution of life.

Therefore, although there is reason to suspect that the four "vital elements" are available everywhere, it is likely that a habitable system also requires a long-term supply of orbiting bodies to seed the inner planets. Without comets it is possible that the life we ​​know would not exist on Earth. There is also the possibility that elements other than those essential on Earth are those that provide a biochemical basis for life in other places; see hypothetical biochemistry.

Para determinar la viabilidad de la vida extraterrestre, durante mucho tiempo los astrónomos han centrado su atención en las estrellas parecidas al Sol. Sin embargo, han empezado a explorar la posibilidad de que la vida se pueda formar en sistemas muy distintos al sistema solar.

Binary systems

Typical estimates suggest more than 50% of star systems are binary systems. This may be due in part to sample bias, as bright, massive stars tend to belong to binary systems and are the easiest to observe and catalogue; Another more precise analysis has suggested that the most common stars, which are less bright, do not usually have a companion and that therefore up to two-thirds of all star systems are solitary.[21]​.

The separation between stars in a binary system ranges from less than one astronomical unit (AU, the distance between the Earth and the Sun) to several hundred. In the latter case, the gravitational effects will be negligible on a planet orbiting one of the stars, and its planetary habitability will not be disrupted unless the orbit is very eccentric (see Nemesis, for example). However, when the separation is significantly smaller, a stable orbit may be impossible. If a planet's distance from its primary star is greater than one-fifth of the minimum distance the other star approaches, orbital stability is not guaranteed.[22] The mere fact that planets can form in binary systems has long been unclear, as gravitational forces could interfere with planet formation. Theoretical work by Alan Boss") at the Carnegie Institute has shown that gas giants can form around binary systems in the same way they do with solitary stars.[23]​.

A study of Alpha Centauri, the closest star system to the Sun, suggests that binary systems should not be ruled out in the search for habitable planets. Centauri A and B are separated by 11 AU at closest approach (23 AU on average), and both may have stable habitable zones. A study of the long-term orbital stability of simulated planets in this system shows that planets located approximately three AU from either star can remain stable (i.e., the semimajor axis deviates by less than 5%). A conservative estimate of the ZH of Centauri A places it at 1.2 or 1.3 AU and that of Centauri B at 0.73 or 0.74 AU, well into the stable region in both cases.[24]​.

Red dwarf systems

Determining the habitability of a red dwarf can help determine how common life is in the universe, since red dwarfs make up 70 to 90 percent of all stars in the galaxy. Brown dwarfs are probably more numerous than red dwarfs. However, they are not usually classified as stars, and could never support life as we know it, since the little heat they emit disappears quickly.

For many years, astronomers have dismissed red dwarfs as a potential abode for life. Their small size (from 0.1 to 0.6 solar masses) means that their nuclear reactions occur at an exceptionally slow rate, and they emit very little light (from 3% to 0.01% of that produced by the Sun). Any planet orbiting a red dwarf would have to be very close to its star to reach a surface temperature similar to that of Earth; from 0.3 AU (just inside Mercury's orbit "Mercury (planet)") for a star like Lacaille 8760 to 0.032 AU for a star like Proxima Centauri (such a world would have a 6.3-day year).[25] At these distances, the star's gravity would cause tidal coupling. The day side of the planet would forever point towards the star, while the night side would always point in the opposite direction. The only way potential life could avoid hell or 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, preventing photosynthesis.

This pessimism has been softened by research. Studies by Robert Harbele and Manoj Joshi of NASA's Ames Research Center in California have shown that a planet's atmosphere (assuming it was composed of the greenhouse gases CO and H O) would need to be only 100 mb, 10% of Earth's atmosphere, for heat to be effectively transferred to the night side.[26] This is well within the levels required for photosynthesis, although water would still be frozen on the night side. night for some of its models. Martin Heath of Greenwich Community College) has shown that sea water could also circulate without freezing if ocean basins were deep enough to allow free flow beneath the nightside ice sheet. Further research—including a study of the amount of photosynthetically active radiation—suggests that orbitally coupled planets in red dwarf systems would be habitable at least for higher plants.[27]

The drawback of tidal coupling can disappear if we consider the possibility that the planet has a satellite or consider the satellite itself as a candidate for habitability.

• - If the habitability of the planet is studied, the satellite could have produced the coupling of the planet's rotation with its own movement around it, preventing the planet from always showing the same face to the star. In the solar system, an example is found in Pluto "Pluto (dwarf planet)"), which rotates on itself in the same period (6.4 days) that it takes its satellite Charon "Charon (satellite)") to complete one revolution.

• - If the habitability of the satellite is studied, it is found that most of the satellites in the solar system (including the Moon) rotate always showing the same face to the planet and some of them do so in periods that are suitable for habitability. However, no satellite in the solar system is large enough to be considered habitable.

However, size is not the only factor that can make a red dwarf incompatible with life. On a planet orbiting a red dwarf, photosynthesis would be impossible on the night side, since it would never see the sun. On the daytime side, since the sun would never rise or set, the areas under the shadow of a mountain would remain that way forever. Known photosynthesis would be complicated by the fact that a red dwarf produces most of its radiation in the infrared, and on Earth this process depends on visible light. There are several positive aspects to this scenario. For example, many terrestrial ecosystems rely on chemosynthesis rather than photosynthesis, something that would be possible in a red dwarf system. A static position of the sun eliminates the need for plants to turn their leaves towards it, deal with changes in the sun/shade pattern, or have to switch during the night from photosynthesis to stored energy. In the absence of a day-night cycle, including weak morning and evening light, there will be much more energy available at a certain radiation level.

Red dwarfs are much more variable and violent than their older, more stable cousins. They are often covered in sunspots that can dim their light by up to 40% for months at a time, while other times they emit giant flares that can double their brightness in a matter of minutes.[28] This variation would be very harmful to life, although it could also stimulate evolution by increasing mutation rates and rapidly changing climatic conditions.

However, red dwarfs have a big advantage over other stars in terms of habitability for life: they live a long time. It took humanity 4.5 billion years to appear on Earth, and life as we know it will have suitable conditions for about 500 million more years.[29] Red dwarfs, on the other hand, can live for billions of years, because their nuclear reactions are much slower than those of larger stars, which means that life could have more time to evolve and survive. Furthermore, although the probability of finding a planet in the habitable zone of a particular red dwarf is small, the total amount of habitable zone around all red dwarfs together is equal to the total amount around Sun-like stars, given their ubiquity.[30]​.

"Good Jupiters"

"Good Jupiters" are giant gas planets, like Jupiter "Jupiter (planet)"), that orbit their stars in circular orbits far enough from the HZ not to disturb it but close enough to "protect" closer orbiting terrestrial planets in two ways. First, they help stabilize the orbits, and therefore the climates, of the inner planets. Second, they keep the inner solar system relatively free of comets and asteroids that could cause devastating impacts.[31] Jupiter orbits the Sun at about five times the distance from Earth to the Sun. This is about the distance at which we should expect to find good Jupiters elsewhere. Jupiter's "gatekeeper" role was spectacularly illustrated in 1994, when comet Shoemaker-Levy 9 hit the giant; If Jovian gravity had not captured the comet, it could have entered the inner solar system.

Early in the history of the solar system, Jupiter played a somewhat opposite role: it increased the eccentricity of the asteroid belt's orbit and allowed many objects to cross Earth's orbit and provide the planet with important compounds. Before the Earth reached half its current mass, icy bodies in the region of Jupiter and Saturn and small bodies in the primordial asteroid belt provided water to the Earth through gravitational dispersion from Jupiter and, to a lesser extent, from Saturn "Saturn (planet)").[32] Thus, while today the gas giants are gentle protectors, they were once suppliers of material critical for habitability.

The role of gas giants in the habitability of a planet has been questioned in recent years. In 2008, Horner and Jones demonstrated through computer simulations that Jupiter's gravitational effect has possibly caused more impacts on Earth than it has prevented.[33]​.

In contrast, Jupiter-sized bodies that orbit too close to but not within the habitable zone (as in 47 Ursae Majoris), or have a highly elliptical orbit that crosses the habitable zone (as in 16 Cygni B), will make the existence of a terrestrial planet in the system very difficult. See explanation of a stable habitable zone above.

The galactic neighborhood

Scientists have also considered the possibility that certain areas of galaxies (galactic habitable zones) are more suitable for life than others; The solar system in which we live, in the Orion Arm, at the edge of the Milky Way galaxy, is considered to be in a favorable point for life:[34]​.

• - It is not in a globular cluster, where the density of stars is hostile to life, given the excessive radiation and gravitational disturbances. Furthermore, globular clusters are composed mainly of old stars, probably with few metals.

• - It is not near an active source of gamma rays.

• - It is not close to the galactic core, where again stellar density increases the amount of ionizing radiation (for example, from magnetars and supernovae). It is also believed that there is a supermassive black hole in the center of the galaxy, which can be dangerous to any nearby body.

• - The Sun's circular orbit around the galactic center keeps it outside the spiral arms, where again the intense radiation and gravity could be incompatible with life.[35]​.

Therefore, what a system suitable for life needs is relative solitude. If the Sun were immersed in a crowd of systems, the probability of being fatally close to a dangerous radiation source would increase significantly. What's more, close neighbors could disrupt the stability of several orbital bodies such as objects in the Oort cloud and the Kuiper Belt, which could cause catastrophe if they enter the inner solar system.

Although a star crowd is disadvantageous for habitability, so is extreme isolation. A star as rich in metals as the Sun would not have formed in the outermost regions of the Milky Way, given the decline in the relative abundance of metals and the general absence of star formation. Therefore, a "suburban" situation, such as that enjoyed by our solar system, is preferable to the center of the galaxy or the most distant areas.[36]​.

Impact of life on habitability

An interesting addition to the factors that foster the emergence of life is the notion that life itself, once formed, becomes a factor of habitability in its own right. An important example on Earth was the production of oxygen by ancient cyanobacteria, and later by photosynthetic plants, resulting in a radical change in the composition of the Earth's atmosphere. This oxygen would turn out to be essential for the respiration of later animal species.

This interaction between later life and habitability has been studied in several ways. The Gaia hypothesis, a type of scientific model of the geobiosphere founded by Sir James Lovelock in 1975, states that life as a whole fosters and sustains adequate conditions for itself, helping to create a planetary environment suitable for its continuity; In its most dramatic version, the Gaia hypothesis suggests that planetary systems behave like a type of organism. The most successful life forms change the composition of air, water and soil in ways that ensure their continued existence, a controversial extension of the accepted laws of ecology.

The consequence of biota revealing coordinated foresight is questioned as unscientific and unfalsifiable. However, many mainstream researchers have reached similar conclusions without necessarily accepting Lovelock's teleology. David Grinspoon") has suggested a "living worlds hypothesis", whereby our understanding of what constitutes habitability cannot be separated from the life already existing on a planet. Furthermore, planets that are geologically and meteorologically alive are much more likely to be biologically alive, and "a planet and its life will co-evolve".[37]

In their book The privileged planet, published in 2004, Guillermo González&action=edit&redlink=1 "Guillermo González (physicist) (not yet written)") and Jay Richards") study the possible relationship between the habitability of a planet and its suitability for observing the rest of the universe. This idea of a "privileged" position for life on Earth is questioned for its philosophical implications, especially the violation of the principle Copernican.

• - Habitability of natural satellites.

• - Analogue to Earth.

• - Astrobiology.

• - Astrophysics.

• - Hypothetical biochemistry.

• - Definition of planet.

• - Drake equation.

• - Gliese 667 Cc.

• - Origin of life.

• - Fermi paradox.

• - Superhabitable planet.

• - Solar system.

• - Terraforming.

• - Extraterrestrial life.

• - Living area.

• - Biosignature.

• - Neocatastrophism.

• - Panspermia.

• - Cohen, Jack and Ian Stewart. Evolving the Alien: The Science of Extraterrestrial Life, Ebury Press, 2002. ISBN 0-09-187927-2.

• - Dole, Stephen H. Habitable Planets for Man, American Elsevier Pub. Co, 1970. ISBN 0-444-00092-5.

• - Fogg, Martyn J.), ed. «Terraforming» (entire special issue) Journal of the British Interplanetary Society, April 1991.

• - Fogg, Martyn J. Terraforming: Engineering Planetary Environments, SAE International, 1995. ISBN 1-56091-609-5.

• - Gonzalez, Guillermo and Richards, Jay W. The Privileged Planet, Regnery, 2004. ISBN 0-89526-065-4.

• - González, Guillermo; Richards, Jay Wesley. The privileged planet: how our place in the cosmos is designed for discovery, Ediciones Palabra, 2006, ISBN 978-84-8239-989-8.

• - 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.

• - Research articles by Alan Boss.

• - David Darling Encyclopedia.

• - Astrobiology of general interest.

• - Research articles by James Kasting.

• - Margaret Turnbull's HabCat files.

• - Sol Station.

• - Terraforming information pages by Martyn J. Fogg.

La principal suposición sobre los planetas habitables es que son terrestres. Estos planetas, que se encuentran aproximadamente dentro de un orden de magnitud de la masa de la Tierra, están compuestos principalmente de rocas de silicato y no han acrecido a partir de las capas gaseosas exteriores de hidrógeno y helio que se encuentran en los gigantes gaseosos. No se ha descartado completamente que pueda evolucionar vida en las nubes superiores de los planetas gigantes,[Nota 4]​ aunque se considera poco probable dado que no tienen superficie y su gravedad es enorme.[9]​ Los satélites naturales de los planetas gigantes, por otro lado, son candidatos perfectamente válidos para albergar vida.[10]​.

Al analizar qué ambientes tienen mayor probabilidad de permitir vida, se suele hacer una distinción entre los organismos unicelulares como las bacterias y arqueas, y los organismos complejos como los metazoos (animales). La unicelularidad precede necesariamente a la pluricelularidad en cualquier hipotético árbol de la vida, y donde emergen organismos unicelulares no hay nada que asegure que se desarrollará mayor complejidad que esa.[Nota 5]​ Las características planetarias listadas abajo se consideran generalmente cruciales para la vida, pero en todos los casos los impedimentos a la habitabilidad deben considerarse más severos para los organismos pluricelulares como las plantas y los animales que para la vida unicelular.

Mass

Low-mass planets are poor candidates for life for two reasons. First, its low gravity makes preserving the atmosphere difficult. Constituent molecules are more likely to reach escape velocity and be lost into space when bombarded by solar wind or agitated by a collision. Planets that do not have a thick atmosphere lack the material necessary for primary biochemistry, have little insulation and heat transfer across their surface (for example, Mars "Mars (planet)", with its thin atmosphere, is colder than Earth would be at a similar distance), and less protection from high-frequency radiation and meteoroids. Furthermore, if the atmosphere is less than 0.006 Earth atmospheres, water cannot exist in liquid form because it does not reach the required atmospheric pressure, 4.56 mmHg (608 pascals). The temperature range in which water is liquid is smaller at low pressures, in general.

Second, small planets have small diameters and therefore greater surface-to-volume ratios than their larger cousins. These bodies tend to quickly lose the energy left over after their formation and end up geologically dead, lacking volcanoes, earthquakes and tectonic activity, which provide the surface with materials necessary for life and the atmosphere with temperature moderators such as carbon dioxide. Plate tectonics appears to be particularly crucial, at least on Earth: it not only serves to recycle important minerals and chemical compounds, it also fosters biodiversity by creating continents and increasing environmental complexity and helps create the convective cells necessary to generate the Earth's magnetic field.[11]​.

“Low dough” is a partly relative label; The Earth is considered to have little mass when compared to the gas giants of the solar system, but it is, of all terrestrial bodies, the largest in diameter and mass and also the densest.[Note 6]​ It is large enough to retain an atmosphere with its gravity and for its liquid core to remain a source of heat, driving the diverse geology of the surface (the decay of radioactive elements in a planet's core is another significant component of planetary warming). Mars, in contrast, is almost (or perhaps completely) geologically dead, and has lost much of its atmosphere.[12] Therefore, it would be correct to deduce that the minimum mass limit for habitability lies somewhere between Mars and Earth or Venus. Exceptional circumstances offer exceptional cases: Jupiter's satellite "Jupiter (planet)") Io "Io (satellite)") (smaller than the terrestrial planets) is volcanically active by the gravitational stresses induced by its orbit; neighboring Europa "Europe (satellite)") may have a liquid ocean under a frozen layer also due to the energy created in its orbit around a gas giant; Saturn's satellite "Saturn (planet)") Titan "Titan (satellite)"), on the other hand, has a remote possibility of supporting life, since it retains a thick atmosphere and biochemical reactions are possible in the liquid methane on its surface. These satellites are exceptions, but they demonstrate that mass as a criterion of habitability cannot 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 would otherwise tend to strip it of its atmosphere and bombard living beings with ionized particles. Mass is not the only criterion necessary to produce a magnetic field—the planet must also rotate fast enough to produce a dynamo effect within its core[13]—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 orbital and rotational characteristics on planetary habitability. Orbital eccentricity is the difference between the greatest and smallest distances to the primary object. The greater the eccentricity, the greater the temperature fluctuation on a planet's surface. Although adaptive, living things can only withstand so much variation, especially if the fluctuations exceed both the freezing point and the boiling point of the planet's primary biotic solvent (for example, water on Earth). If, for example, Earth's oceans alternately evaporated and froze, it is difficult to imagine how life as we know it could have evolved. The more complex an organism is, the more sensitive it is to temperatures.[14]​ The Earth's orbit is almost circular, with an eccentricity of less than 0.02; other planets in our system (with the exception of Pluto "Pluto (dwarf planet)") and Mercury "Mercury (planet)") have similarly benign eccentricities.

The data collected on the orbital eccentricity of extrasolar planets has surprised many researchers: 90% have a larger orbital eccentricity than the planets in the solar system, and the average is 0.25.[15] This could easily be the result of sample bias. Planets are often not observed directly, but are inferred from the "wobble" they produce in their star. The greater the eccentricity, the greater the disturbance on the star, and therefore the greater the detectability of the planet.

The movement of a planet around its axis of rotation must also meet certain criteria for life to have a chance to evolve. A first assumption is that the planet must have moderate seasons. If there is little or no axial inclination (or obliquity) relative to the perpendicular of the ecliptic, there will be no seasons and therefore a major stimulant of biosphere dynamics will disappear. The planet would also be much colder than it would be if it had a significant tilt: when the most intense radiation always falls within a few degrees of the equator, the warm climate cannot overtake the polar climate and the planet's climate ends up dominated by the colder polar climate systems.

On the other hand, if a planet is radically tilted, the seasons will be extreme and make it more difficult for the biosphere to achieve homeostasis. Although during the Quaternary the Earth had a greater axial tilt that coincided with a reduction in polar ice, warmer temperatures and less seasonal variation, scientists do not know if this trend would have continued indefinitely with a greater axis tilt. (See Global glaciation).

The exact effects of these changes can only be modeled by computer today, and studies show that even extreme tilts of up to 85 degrees do not absolutely rule out life, "as long as they do not occupy continental surfaces that seasonally experience the highest temperature."[16] Not only the mean axial tilt must be considered, 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 much shorter periodicity, would induce climatic changes such as variations in the severity of the seasons.

Geochemistry

It is generally assumed that any extraterrestrial life that may exist will be based on the same fundamental chemistry as terrestrial life, since the four elements essential for life, carbon, hydrogen, oxygen and nitrogen, are also the most common reactive chemical elements in the universe. In fact, simple biogenic compounds, such as amino acids, have been found in meteorites and in interstellar space. These four elements make up 96% of the total biomass "Biomass (ecology)" on Earth. Carbon has an unparalleled ability to bond with itself and form varied and intricate structures, making it the ideal material for the complex mechanisms that form living cells. Hydrogen and oxygen, in the form of water, make up the solvent in which biological processes take place and in which the first reactions that led to the emergence of life took place. The energy released in the formation of powerful covalent bonds between carbon and oxygen, available when oxidizing organic compounds, is the fuel for all complex living beings. These four elements serve to build amino acids, which are the building blocks of proteins, the substance of living tissue.

Relative abundance in space is not always reflected in abundance on the planets; For example, of the four vital elements, only oxygen exists in abundance in the Earth's crust.[18] This can be explained in part by the fact that many of these elements, such as hydrogen and nitrogen, along with their more basic compounds, such as carbon dioxide, carbon monoxide, methane, ammonia, and water, are gases at temperate temperatures. In the warm region close to the Sun, these volatile compounds could not have played a significant role in the geological formation of the planets. Instead, they were captured in gaseous form beneath the young crusts, which were mostly made up of non-volatile rock compounds such as silicon dioxide (a compound of silicon and oxygen that accounts for the relative abundance of oxygen). The release of volatile compounds through the first volcanoes would have contributed to the formation of the planets' atmosphere. Miller's experiments demonstrated that amino acids can be formed in a primordial atmosphere by synthesis of simple compounds.[19]​.

Despite this, volcanic gas release cannot explain the amount of water in Earth's oceans.[20] The vast majority of the water, and arguably carbon, needed for life had to come from the outer solar system, far from solar heat where it could remain solid. Comets that impacted Earth in the early years of the solar system would have deposited vast amounts of water, plus the other volatile compounds necessary for life (including amino acids), on the young Earth, providing the ignition spark for the evolution of life.

Therefore, although there is reason to suspect that the four "vital elements" are available everywhere, it is likely that a habitable system also requires a long-term supply of orbiting bodies to seed the inner planets. Without comets it is possible that the life we ​​know would not exist on Earth. There is also the possibility that elements other than those essential on Earth are those that provide a biochemical basis for life in other places; see hypothetical biochemistry.

Para determinar la viabilidad de la vida extraterrestre, durante mucho tiempo los astrónomos han centrado su atención en las estrellas parecidas al Sol. Sin embargo, han empezado a explorar la posibilidad de que la vida se pueda formar en sistemas muy distintos al sistema solar.

Binary systems

Typical estimates suggest more than 50% of star systems are binary systems. This may be due in part to sample bias, as bright, massive stars tend to belong to binary systems and are the easiest to observe and catalogue; Another more precise analysis has suggested that the most common stars, which are less bright, do not usually have a companion and that therefore up to two-thirds of all star systems are solitary.[21]​.

The separation between stars in a binary system ranges from less than one astronomical unit (AU, the distance between the Earth and the Sun) to several hundred. In the latter case, the gravitational effects will be negligible on a planet orbiting one of the stars, and its planetary habitability will not be disrupted unless the orbit is very eccentric (see Nemesis, for example). However, when the separation is significantly smaller, a stable orbit may be impossible. If a planet's distance from its primary star is greater than one-fifth of the minimum distance the other star approaches, orbital stability is not guaranteed.[22] The mere fact that planets can form in binary systems has long been unclear, as gravitational forces could interfere with planet formation. Theoretical work by Alan Boss") at the Carnegie Institute has shown that gas giants can form around binary systems in the same way they do with solitary stars.[23]​.

A study of Alpha Centauri, the closest star system to the Sun, suggests that binary systems should not be ruled out in the search for habitable planets. Centauri A and B are separated by 11 AU at closest approach (23 AU on average), and both may have stable habitable zones. A study of the long-term orbital stability of simulated planets in this system shows that planets located approximately three AU from either star can remain stable (i.e., the semimajor axis deviates by less than 5%). A conservative estimate of the ZH of Centauri A places it at 1.2 or 1.3 AU and that of Centauri B at 0.73 or 0.74 AU, well into the stable region in both cases.[24]​.

Red dwarf systems

Determining the habitability of a red dwarf can help determine how common life is in the universe, since red dwarfs make up 70 to 90 percent of all stars in the galaxy. Brown dwarfs are probably more numerous than red dwarfs. However, they are not usually classified as stars, and could never support life as we know it, since the little heat they emit disappears quickly.

For many years, astronomers have dismissed red dwarfs as a potential abode for life. Their small size (from 0.1 to 0.6 solar masses) means that their nuclear reactions occur at an exceptionally slow rate, and they emit very little light (from 3% to 0.01% of that produced by the Sun). Any planet orbiting a red dwarf would have to be very close to its star to reach a surface temperature similar to that of Earth; from 0.3 AU (just inside Mercury's orbit "Mercury (planet)") for a star like Lacaille 8760 to 0.032 AU for a star like Proxima Centauri (such a world would have a 6.3-day year).[25] At these distances, the star's gravity would cause tidal coupling. The day side of the planet would forever point towards the star, while the night side would always point in the opposite direction. The only way potential life could avoid hell or 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, preventing photosynthesis.

This pessimism has been softened by research. Studies by Robert Harbele and Manoj Joshi of NASA's Ames Research Center in California have shown that a planet's atmosphere (assuming it was composed of the greenhouse gases CO and H O) would need to be only 100 mb, 10% of Earth's atmosphere, for heat to be effectively transferred to the night side.[26] This is well within the levels required for photosynthesis, although water would still be frozen on the night side. night for some of its models. Martin Heath of Greenwich Community College) has shown that sea water could also circulate without freezing if ocean basins were deep enough to allow free flow beneath the nightside ice sheet. Further research—including a study of the amount of photosynthetically active radiation—suggests that orbitally coupled planets in red dwarf systems would be habitable at least for higher plants.[27]

The drawback of tidal coupling can disappear if we consider the possibility that the planet has a satellite or consider the satellite itself as a candidate for habitability.

• - If the habitability of the planet is studied, the satellite could have produced the coupling of the planet's rotation with its own movement around it, preventing the planet from always showing the same face to the star. In the solar system, an example is found in Pluto "Pluto (dwarf planet)"), which rotates on itself in the same period (6.4 days) that it takes its satellite Charon "Charon (satellite)") to complete one revolution.

• - If the habitability of the satellite is studied, it is found that most of the satellites in the solar system (including the Moon) rotate always showing the same face to the planet and some of them do so in periods that are suitable for habitability. However, no satellite in the solar system is large enough to be considered habitable.

However, size is not the only factor that can make a red dwarf incompatible with life. On a planet orbiting a red dwarf, photosynthesis would be impossible on the night side, since it would never see the sun. On the daytime side, since the sun would never rise or set, the areas under the shadow of a mountain would remain that way forever. Known photosynthesis would be complicated by the fact that a red dwarf produces most of its radiation in the infrared, and on Earth this process depends on visible light. There are several positive aspects to this scenario. For example, many terrestrial ecosystems rely on chemosynthesis rather than photosynthesis, something that would be possible in a red dwarf system. A static position of the sun eliminates the need for plants to turn their leaves towards it, deal with changes in the sun/shade pattern, or have to switch during the night from photosynthesis to stored energy. In the absence of a day-night cycle, including weak morning and evening light, there will be much more energy available at a certain radiation level.

Red dwarfs are much more variable and violent than their older, more stable cousins. They are often covered in sunspots that can dim their light by up to 40% for months at a time, while other times they emit giant flares that can double their brightness in a matter of minutes.[28] This variation would be very harmful to life, although it could also stimulate evolution by increasing mutation rates and rapidly changing climatic conditions.

However, red dwarfs have a big advantage over other stars in terms of habitability for life: they live a long time. It took humanity 4.5 billion years to appear on Earth, and life as we know it will have suitable conditions for about 500 million more years.[29] Red dwarfs, on the other hand, can live for billions of years, because their nuclear reactions are much slower than those of larger stars, which means that life could have more time to evolve and survive. Furthermore, although the probability of finding a planet in the habitable zone of a particular red dwarf is small, the total amount of habitable zone around all red dwarfs together is equal to the total amount around Sun-like stars, given their ubiquity.[30]​.

"Good Jupiters"

"Good Jupiters" are giant gas planets, like Jupiter "Jupiter (planet)"), that orbit their stars in circular orbits far enough from the HZ not to disturb it but close enough to "protect" closer orbiting terrestrial planets in two ways. First, they help stabilize the orbits, and therefore the climates, of the inner planets. Second, they keep the inner solar system relatively free of comets and asteroids that could cause devastating impacts.[31] Jupiter orbits the Sun at about five times the distance from Earth to the Sun. This is about the distance at which we should expect to find good Jupiters elsewhere. Jupiter's "gatekeeper" role was spectacularly illustrated in 1994, when comet Shoemaker-Levy 9 hit the giant; If Jovian gravity had not captured the comet, it could have entered the inner solar system.

Early in the history of the solar system, Jupiter played a somewhat opposite role: it increased the eccentricity of the asteroid belt's orbit and allowed many objects to cross Earth's orbit and provide the planet with important compounds. Before the Earth reached half its current mass, icy bodies in the region of Jupiter and Saturn and small bodies in the primordial asteroid belt provided water to the Earth through gravitational dispersion from Jupiter and, to a lesser extent, from Saturn "Saturn (planet)").[32] Thus, while today the gas giants are gentle protectors, they were once suppliers of material critical for habitability.

The role of gas giants in the habitability of a planet has been questioned in recent years. In 2008, Horner and Jones demonstrated through computer simulations that Jupiter's gravitational effect has possibly caused more impacts on Earth than it has prevented.[33]​.

In contrast, Jupiter-sized bodies that orbit too close to but not within the habitable zone (as in 47 Ursae Majoris), or have a highly elliptical orbit that crosses the habitable zone (as in 16 Cygni B), will make the existence of a terrestrial planet in the system very difficult. See explanation of a stable habitable zone above.

The galactic neighborhood

Scientists have also considered the possibility that certain areas of galaxies (galactic habitable zones) are more suitable for life than others; The solar system in which we live, in the Orion Arm, at the edge of the Milky Way galaxy, is considered to be in a favorable point for life:[34]​.

• - It is not in a globular cluster, where the density of stars is hostile to life, given the excessive radiation and gravitational disturbances. Furthermore, globular clusters are composed mainly of old stars, probably with few metals.

• - It is not near an active source of gamma rays.

• - It is not close to the galactic core, where again stellar density increases the amount of ionizing radiation (for example, from magnetars and supernovae). It is also believed that there is a supermassive black hole in the center of the galaxy, which can be dangerous to any nearby body.

• - The Sun's circular orbit around the galactic center keeps it outside the spiral arms, where again the intense radiation and gravity could be incompatible with life.[35]​.

Therefore, what a system suitable for life needs is relative solitude. If the Sun were immersed in a crowd of systems, the probability of being fatally close to a dangerous radiation source would increase significantly. What's more, close neighbors could disrupt the stability of several orbital bodies such as objects in the Oort cloud and the Kuiper Belt, which could cause catastrophe if they enter the inner solar system.

Although a star crowd is disadvantageous for habitability, so is extreme isolation. A star as rich in metals as the Sun would not have formed in the outermost regions of the Milky Way, given the decline in the relative abundance of metals and the general absence of star formation. Therefore, a "suburban" situation, such as that enjoyed by our solar system, is preferable to the center of the galaxy or the most distant areas.[36]​.

Impact of life on habitability

An interesting addition to the factors that foster the emergence of life is the notion that life itself, once formed, becomes a factor of habitability in its own right. An important example on Earth was the production of oxygen by ancient cyanobacteria, and later by photosynthetic plants, resulting in a radical change in the composition of the Earth's atmosphere. This oxygen would turn out to be essential for the respiration of later animal species.

This interaction between later life and habitability has been studied in several ways. The Gaia hypothesis, a type of scientific model of the geobiosphere founded by Sir James Lovelock in 1975, states that life as a whole fosters and sustains adequate conditions for itself, helping to create a planetary environment suitable for its continuity; In its most dramatic version, the Gaia hypothesis suggests that planetary systems behave like a type of organism. The most successful life forms change the composition of air, water and soil in ways that ensure their continued existence, a controversial extension of the accepted laws of ecology.

The consequence of biota revealing coordinated foresight is questioned as unscientific and unfalsifiable. However, many mainstream researchers have reached similar conclusions without necessarily accepting Lovelock's teleology. David Grinspoon") has suggested a "living worlds hypothesis", whereby our understanding of what constitutes habitability cannot be separated from the life already existing on a planet. Furthermore, planets that are geologically and meteorologically alive are much more likely to be biologically alive, and "a planet and its life will co-evolve".[37]

In their book The privileged planet, published in 2004, Guillermo González&action=edit&redlink=1 "Guillermo González (physicist) (not yet written)") and Jay Richards") study the possible relationship between the habitability of a planet and its suitability for observing the rest of the universe. This idea of a "privileged" position for life on Earth is questioned for its philosophical implications, especially the violation of the principle Copernican.

• - Habitability of natural satellites.

• - Analogue to Earth.

• - Astrobiology.

• - Astrophysics.

• - Hypothetical biochemistry.

• - Definition of planet.

• - Drake equation.

• - Gliese 667 Cc.

• - Origin of life.

• - Fermi paradox.

• - Superhabitable planet.

• - Solar system.

• - Terraforming.

• - Extraterrestrial life.

• - Living area.

• - Biosignature.

• - Neocatastrophism.

• - Panspermia.

• - Cohen, Jack and Ian Stewart. Evolving the Alien: The Science of Extraterrestrial Life, Ebury Press, 2002. ISBN 0-09-187927-2.

• - Dole, Stephen H. Habitable Planets for Man, American Elsevier Pub. Co, 1970. ISBN 0-444-00092-5.

• - Fogg, Martyn J.), ed. «Terraforming» (entire special issue) Journal of the British Interplanetary Society, April 1991.

• - Fogg, Martyn J. Terraforming: Engineering Planetary Environments, SAE International, 1995. ISBN 1-56091-609-5.

• - Gonzalez, Guillermo and Richards, Jay W. The Privileged Planet, Regnery, 2004. ISBN 0-89526-065-4.

• - González, Guillermo; Richards, Jay Wesley. The privileged planet: how our place in the cosmos is designed for discovery, Ediciones Palabra, 2006, ISBN 978-84-8239-989-8.

• - 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.

• - Research articles by Alan Boss.

• - David Darling Encyclopedia.

• - Astrobiology of general interest.

• - Research articles by James Kasting.

• - Margaret Turnbull's HabCat files.

• - Sol Station.

• - Terraforming information pages by Martyn J. Fogg.