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Artist's impression of the possible appearance of a superhabitable planet. The reddish hue of the masses is due to the color of vegetation..[1]

A Superhabitable Exoplanet is a hypothetical Extra-solar planet, that has more suitable conditions for the emergence and evolution of life than our own planet.[2][3] In recent years, many experts have criticized the criteria used in the search for extraterrestrial life.[4] They believe that the earth does not represent the optimal planetary habitability in various aspects such as the type of star around which it orbits, total area, proportion covered by oceans and average depth of these, intensity field magnetic , geological activity , surface temperature , etc.[5][6] Therefore, there may be exoplanets in the universe that offer better conditions for life, allowing easier evolution and longer timelines.[7]

An extensive report published in January 2014 in the journal Astrobiology entitled Superhabitable Worlds, René Heller and John Armstrong, collects and analyzes many of the studies in the years preceding the issue.[8] Investigations of these astrophysical possible to establish a profile for the superhabitables planets according to stellar type, mass and location in the solar system, among other features.[5] concluded that this kind of planets could be much more common than traditional Earth analogs.[9]

By mid-2015, it has not yet been confirmed exoplanet no possessing all the characteristics of a superhabitable planet. If the atmospheric composition and mass of Kepler-442b which are unknown correspond to a planet of this type, it can be given its location in the habitable zone,[n. 1] type of star and estimated (if it was not detected by the Transit Method size.[11]

Characteristics

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Multiple criteria analysed in Heller and Armstrong's research conclude that a series of basic characteristics, approximate capacity, require Superhabitable exoplanets.[12] From his studies, extracted the planets about 2 Earth masses and 1.3 Earth radii, will have an optimal size for plate tectonics.[13] In addition, its mean mass greater gravitational attraction will occur, assuming an increase in the capture of gases during the planet's formation.[12] It is therefore likely that they have a denser atmosphere that will offer greater concentration of oxygen and greenhouse gases, which in turn raise the average temperature to optimum levels for plant life-about 25℃.[14] It may also influence the relief of the planetary object, making it more regular and decreasing the size of the ocean basins, which will improve the diversity of the most abundant aquatic life in shallow waters.[15]

Other factors to consider are the type of star. K-type stars are less massive and can genuinely have a longer lifespan than Sun-like stars,[16][17] giving more time to life for evolution and a location near the center of the Habitable Zone of the system for longer periods of time.[18][5]

Surface, size and composition

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An exoplanet with a radius of 1.6 R🜨 will be similar to that of Kepler-62e second from the left. At the end of the right is Earth, scaled.

An exoplanet with a volume greater than the earth a relief more complex or a more spacious covered by water liquid surface may be more suitable for life.[19] However, since the volume of a planet tends to be directly related to its mass, the more massive the greater its gravitational pull, which can result in an overly dense atmosphere.[20]

Studies Courtney Dressing equipment, researcher Center for Astrophysics Harvard-Smithsonian (CfA), They indicate that there is a natural limit, set at 1.6R🜨, below which nearly all planets are terrestrial, composed primarily of rock-iron-water mixtures.[n. 2] as Venus and Earth.[22] Generally, objects with a mass below 6 M🜨 are very likely to be of similar composition the Earth.[23] Above this limit, the density of the planets decreases with increasing size, the planet will become a Water World and finally a Gas Giant.[24][25] In addition, most super-Earths high mass may cause them to lack plate tectonics.[13]

Thus, it is expected that any exoplanet similar to Earth's density and a radius under 1.6 R🜨 is more suitable for life.[6] However, other studies indicate that Water Worlds represent a transitional stage between mini neptunes and the terrestrial planets, especially if they belong to Red dwarfs or K Dwarfs whose planets located in the Habitable Zone tend to accumulate a lot more water.[26][27] Although the ocean planets may be habitable, the average depth of the water bodies and the absence of land area away from the concept of superhabitable supported by Heller and Armstrong.[28] Therefore, although slightly more massive than Earth planetary bodies are in principle more suitable for life, oversized get the opposite effect.[28] From a geological perspective, the optimal mass of a planet is located around 2 M🜨, so it must have a radius that keeps the density of the Earth among 1.2 and 1.3R🜨.[29]

Another factor inherent to the surface habitability that can improve fitness for life on Earth is the distribution of masses. In the past, supercontinents and Pangea could have vast deserts inside because of the far distance from the sea.[30] By contrast, the separate continents and islands have a higher amount of vegetation and biodiversity.[31][6]

The average depth of the oceans also affect the habitability of a planet. The shallow areas of the sea, given the amount of light they receive, usually more comfortable for aquatic species, so it is likely that exoplanets with a lower average depth more suitable for life.[28][32] The more massive than Earth exoplanets tend to have a more regular surface effect of gravity, which can mean a shallower ocean basins.[33] On the other hand, the planets with less water than Earth are less likely to present a greenhouse wild if they are in the inner ends of the habitable zone and is less likely suffering from a global glaciation if they belong to the external border.[34]

Geology

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Plate tectonics, in combination with the presence of large bodies of water on a planet, is able to maintain high levels of CO2 constants.[35][36][37] Este proceso parece ser habitual en los planetas telúricos geológicamente activos con una velocidad de rotación significativa.[38] The more massive a planetary body, the longer endure its internal heat, a major factor contributing to plate tectonics.[13] However, excessively high mass can also slow plate tectonics because of increased pressure and viscosity of the mantle, which hinders the sliding of the lithosphere.[13] esearch suggests that plate tectonics peaks of activity in bodies with between 1 and 5M🜨, with the optimum mass approximately 2M🜨.[29]

If the geological activity is not strong enough to generate a lot of greenhouse gases that increase global temperatures above the freezing point of water, the planet could experience a permanent Ice Age, unless the process is offset by a or by an intense internal heat source such as tidal heating or stellar irradiation.[39]

Another favorable feature to life on planets more massive than Earth lies in its potential to develop greater magnetosphere that protects the planet from cosmic radiation efficiently and especially the stellar winds factor (Especially around Red dwarfs).[40] Less massive bodies and those with a slow rotation, or are tidally locked have weak or no magnetic field that over time can result in the loss of a significant portion its atmosphere, especially hydrogen by hydrodynamic escape.[13]

The climate of a warmer and wetter terrestrial exoplanet may be similar to the tropical regions of Earth. In the picture, mangrove in Cambodia.

Temperature and Climate

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The thermal suitability of a planet for life is determined by its temperature equilibrium that is, which correspond to the Earth in its place and the stellar flux.[41] Throughout its history, the Earth has undergone major changes in temperature for long periods, such as the Ice age during Cryogenic and global warming that could contribute to Permian-Triassic extinction event.[42][43] Even today shows significant temperature fluctuations depending on the latitude and seasons. It is possible that planets with more dense than Earth's atmosphere, a more dispersed distribution of its land area and / or lesser tilt of its axis have a lower temperature range and less pronounced seasons.[41] In this case, the native species would not be adapted to regular radical changes of temperature and due to this life on the planet may be more diverse.[41] The thermoregulatory effect of the sea may involve a moderate temperature range in ocean planets located in the Habitable zone of its parent star.[44]

The optimum temperature for life is unknown, although it appears that on Earth animal diversity has been greater in warmer seasons.[45] It is possible therefore that exoplanets with slightly higher than average temperatures on Earth are more suitable for life.[41] However, recent studies indicate that the Earth is in the inner edge of the habitable zone of the Solar system,[46] Which can harm long-term livability as the stars increase their brightness over time.[47][48] Therefore, Superhabitable exoplanets must be warmer than Earth and, in turn, orbit closeser to the center of the system's Habitable zone.[49][18] This would be possible if its atmosphere was thicker and / or had a higher concentration of greenhouse gases. [50][51]

Star

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HZ position of some of the most similar and average surface temperature exoplanets.[52][n. 3]

The star's type largely determines the conditions present in a system.[53][54] The most massive stars O, B and A have a very short life cycle, quickly leaving the main sequence.[55][56] In addition, O and B type star's produce an effect photoevaporation which prevents the formation of planets around the star.[57][58]

On the opposite side, the less massive types M and K, are by far the most common and long-lived of the universe, but its potential for harboring life is still under study.[53][58] Its low lumosity reduces the size of the Habitable zone, which are exposed to ultraviolet radiation outbreaks that occur frequently, especially during the first billion year of these stars.[16] Within short orbit can also plunge the Tidal locking of the planet, which always present the same hemisphere to the Star-known as day hemisphere .[59][58] Even if it were possible the existence of life in a system of this type, it is unlikely that any exoplanet belonging to a Red dwarf would be considered superhabitable.[53]

Dismissing both ends, systems based on K, are best homes for life.[16][58] K-type stars allow the formation of planets around them, K-type stars have a long life expectancy and provide a stable and free habitable zone of the effects of excessive proximity to its star.[58] G-type stars, such as the Sun, have a greater habitability area, but his life is considerably shorter than those of type K.[16] Furthermore, the radiation is high enough to allow complex life without the existence of a ozone.[16] In contrast, the Type K remain on the main sequence for periods up to three times those of type G.[60] They are also the most stable and habitable zone does not move very much during his lifetime, so a terrestrial analog located in a star-K may be habitable for almost all the main sequence.[16] Furthermore, the low radiation may facilitate the development of complex life without the existence of an ozone.[16][61][62]

Orbit and rotation

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Artistic impression of an Earth analog. Some Superhabitable planets could have a similar appearance and may not have important differences with Earth.

Tidal locking on planets anchored by its star may not be an important factor for life as provided with an atmosphere dense enough to distribute heat between day and night hemispheres.[63][64]

Experts have not reached a consensus about what is the optimal rotation speed for a planet, but it should not be too high nor too slow. In last instance, the latter case can cause some problems similar to those observed in Venus, which completes one rotation every 243 Earth days and as a result, can not generate an Earth-like magnetic field.[65][66] The Rare Earth hypothesis adds the need for a sizeable natural satellite to balance the planetary axis, but this theory has received significant criticism in most of their arguments and recent research suggests it may be preferable absence of a satellite.[67][68]

The orbit of a Superhabitable exoplanet must be in the Habitable zone of its system.[69] Beyond this consideration, there is no consensus on the effect it can have a greater orbital eccentricity in Earth analogs:[50][70] it is possible that thermal fluctuations resulting from obvious differences in the distance to the star in the aphelion and perihelion are detrimental to life;[50] on the other hand a moderate but greater eccentricity than the Earth can serve as protection from ice or events of wild global greenhouse emissions.[2][71][72]

Atmosphere

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There are no solid arguments to ensure that the Earth's atmosphere have an optimal composition for life.[50] Assuming that no multicellular organisms can be fully anaerobic and the presence of a significant amount of oxygen in the atmosphere is considered essential for exoplanets to develop complex life forms, the percentage of oxygen relative to the total atmosphere appears to limit the maximum size that can be have some form of life (a higher concentration animal for example). More oxygen allows greater diversity and influences the amplitude of metabolic networks.[73][50] On Earth, during the period when Coal was first formed, atmospheric oxygen levels were up to 35%, which coincided with the periods of greatest biodiversity on Earth.[74]

While less dense atmospheres than Earth offer less protection against high energy cosmic radiation and therefore carry a greater temperature difference between day and night and between the equatorial and polar regions as a poor distribution of rainfall, a denser atmosphere can get the opposite effect.[51][50] The air density should be higher in more massive planets, which reinforces the hypothesis that super-Earths can cause superhabitable conditions to occur.[50]

Age and Metallicity

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The first stars appeared in the universe were Metal free stars, which probably prevented planet formation.

From one point of view biologically, older planets than Earth may have greater biodiversity, since native species have had more time to evolve, adapting and stabilizing environmental conditions to sustain a suitable environment for life that can benefit their descendants.[17]

The Habitable zone of a Planetary system moves away from the star over time, with increasing luminosity.[16] Stars less massive than the Sun take longer to leave the main sequence and its stellar evolution is much slower.[75] As a result, a habitable planet belonging to a K-type star can maintain its condition for billions of years before crossing the internal border of the Habitable zone.[47] Therefore, it is expected that planets orbiting a K-type star coming of age to the universe offer a better scenario for life.[16]

However, for years it has questioned the potential for finding life on older systems, the apparent relationship between stellar metallicity and planet formation.[76] The number of metallic items in the universe has grown steadily since its inception, so the oldest known stars have a lower metallicity to 10% of the Sun.[77] The first exoplanetary discoveries, mostly gas giants orbiting very close to their stars or Hot Jupiters, suggesting that the planets were rare in systems with low metallicity, which invited suspicion of a time limit on the appearance of the first objects landmass.[78] Later the Kepler telescope's observations have allowed the experts find that this relationship is much more restrictive in systems with Hot Jupiters and terrestrial planets could form in much lower metallicity star, to some extent. These results were officially announced by an international team of astronomers led by Lars Buchhave, the Niels Bohr Institute at the University of Copenhagen, in the 220th meeting of the American Astronomical Society.[77]

In his presentation, they indicated that there should be a time limit for the appearance of the first terrestrial planets. It is believed, at the expense of new observations that the first Earth-mass objects should appear sometime between 7 and 12 million years.[77] Given the greater stability of the orange dwarfs (K type) from the Sun (G type) and longer life expectancy, it is possible that habitable exoplanets belonging to an orange dwarf that is within the Habitable zone, to provide better life for the evolutionary margin granted to local species stage.[16]

Summary

[edit]
A size comparison and Artists impression of a Superhabitable exoplanet (1.34 R🜨) to the Earth (right).

Despite the scarcity of information available, theories seen in the preceding paragraphs invited to draw up a profile of the prototype superhabitable planet.[12] Even if part of the seen points are still under discussion, in others it does seem to be some consensus. Thus, some of the typical features of a planet could be superhabitable:[12]

  • Mass: approximately 2M🜨.
  • Radius: To maintain a similar terrestrial density, its radius should be between 1,2 and 1,3R🜨.
  • Oceans: Percentage of surface area covered by oceans should be Earth-like, but more distributed without large continuous land masses.
  • Distance: Shorter distance from the center of the habitable zone of the system than Earth.
  • Temperature: Average surface temperature slightly above that of Earth (14℃).[79]
  • Star and Age: Belonging to an intermediate K-type star with an older age than the Sun (4.568 Billion years) but lower than 7 Billion years.
  • Satellites: No large Satellites.
  • Atmosphere: Somewhat denser than the Earth and with a higher concentration of oxygen atmosphere.

No exoplanet whose existence has been confirmed that meets all requirements. After updating the database of exoplanets NASA of July 23, 2015, the one that comes closest is Kepler-442b, belonging to an orange dwarf, with a radius of 1.34R🜨 and a mass of 2.34M🜨, but with an estimated surface temperature -2.65℃ it becomes a psychroplanet Considering similar to that of the Earth's atmosphere.[80] It is possible that their larger size has given it a higher atmospheric density and this, coupled with an increased presence of greenhouse gases, suppose an actual temperature equal to or greater than the Earth's.[n. 4] In this case, it could be a superhabitable planet. At the moment, although it is the fourth confirmed exoplanet in terms of ESI (84%), is the one most likely to harbor some kind of life.[82]

Appearance and Habitability

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Earth almost touches the inner edge of the habitable zone of the solar system-the area where temperatures allow Earth-like planets have liquid water surface. From this perspective, the Earth is only marginally habitable. That led us to the question: What could be more hospitable environments where terrestrial planets?

René Heller.[83]

The appearance of a superhabitable planet should be, in general, very similar to Earth.[5] The main differences, in compliance with the profile seen previously, would be derived from its mass. Its denser atmosphere probably prevent the formation of ice sheets as a result of lower thermal difference between different regions of the planet.[50] Also it has a higher concentration of clouds and abundant rainfall.[n. 5]

Probably the vegetation is very different due to the increased air density, precipitation and temperature; and the different starlight. For the type of light emitted from the K-type stars, plants may take colors like yellow, orange or red compared to the predominant green on Earth.[85][1] The vegetation would cover more regions than vegetation here on Earth, making this clearly visible from space.[5]

In general, the climate of a superhabitable planet would be warmer, moist, homogeneous and stable land, allowing life to be extended across the surface without presenting large population differences. Characteristics of the most inhospitable areas on Earth compared to the tropical regions.[41] The conditions of these planets may be bearable to humans even unprotected without a space suit, provided that the atmosphere does not contain excessive toxic gases, but require some adaptation to the increased gravitational attraction could naturally develop an increase in muscles, and in bone density, etc.[n. 6][86][87]

Abundance

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Set and subsets of terrestrial worlds.[9]

The number of planets superhabitables can far exceed that of the Earth analogs:[9] less massive stars in the main sequence are more abundant than the larger and brighter stars, so there are more oranges dwarfs than solar analogues.[88] It is estimated that, approximately, 9% of stars in the Milky Way are K-type stars.[89]

Another point favoring the predominance of superhabitables in regard to Earth analogs is that, unlike the latter, most of the requirements of a superhabitable world can occur spontaneously and jointly simply by having a higher mass.[90] A planetary body close to 2M🜨 will be better at plate tectonics and will have a larger surface area similar to Earth's surface.[31] Similarly, it is likely that its oceans are shallower by the effect of gravity on the planet's crust, its gravitational field more intense and that has a denser atmosphere -of this last point, it follows that its temperature possibly It will be higher and more homogeneous than in a less massive.[14]

By contrast, Earth-mass planets may have a wide range of very different to that of Earth analog states. For example, to have a less active tectonics[n. 7] and a lower air density, the probability of developing a permanent global ice is much greater.[50] Another negative effect of lower atmospheric density is represented in the form of thermal oscillation, which can lead to a high variability in the global climate and exposure to catastrophic events like the one above. In addition, by having a weaker magnetosphere, they may lose their hydrogen levels by hydrodynamic escape more easily and become a desert planet.[50] Any of these examples could prevent the appearance of organisms on the planet's surface.[91]

Considering one's life as a habitability factor which modifies its surroundings, optimizing their conditions-, since the superhabitables planets are more suitable for life than Earth-like as Heller and Armstrong, it must also occur more easily in places that meet most of its key features.[8] Assuming number of planets potentially superhabitable are identical to planets with potential to be similar to the Earth would have a higher percentage of some kind of life on its surface, which could alter their conditions and become true superhabitable planets.[9]

In any case, the multitude of scenarios that can turn an Earth-mass planet located in the habitable zone of a solar analogue in an inhospitable place, far from the image of a Earth's twin are less likely on a planet that meets the basic features of a superhabitable world, so that the latter should be more common.[9]

See also

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Notas

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  1. ^ The habitable zone (HZ) is a region present around each star where any terrestrial body that had an atmospheric pressure and a suitable combination of gases could maintain liquid water on its surface. If the orbit of a planet crosses the inner ends of the ZH, it could unleash a greenhouse effect on Venus similar to the wild. If you move its external frontiers, the CO2 would condense into clouds and fall in solid or liquid, and possibly fall on its surface, further cooling the planet and unleashing a nasty Ice Age.[10]
  2. ^ The main components of the Earth are iron, silica and magnesium, among others.[21]
  3. ^ Las siglas «HZD» o «Habitable Zone Distance» marcan la posición de un planeta respecto al centro de la zona de habitabilidad del sistema (valor 0). Un HZD negativo significa que la órbita de un planeta es más pequeña —y próxima a su estrella— que el centro de la zona habitable, mientras que uno positivo supone una mayor lejanía respecto a la estrella. Los valores 1 y -1 marcan el límite de la zona de habitabilidad.[49] Un planeta superhabitable debería tener un HZD más próximo a 0 —centro de la zona verde intenso— que la Tierra.[18]
  4. ^ Experts have proposed the possibility that a similar process takes place in Kepler-186f, one of the Earth-like exoplanets found to date, but with an average surface temperature significantly lower, which places it in the limit the hypopsychroplanet.[81]
  5. ^ Kepler-62e, discovered in 2013, it is a super-Earth with the potential to harbor life belonging to a star type K2V. Computer models suggest that more massive than Earth and with significant amounts of water on its surface terrestrial planets tend to have more cloud concentration Earth.[84]
  6. ^ In the conference that announced the discovery of Kepler-62e and Kepler-62f, experts discussed this possibility, although both planetary bodies are too massive to fall into the category of "superhabitables" and most likely to result in atmospheric density dependence of a team that would allow breathing normally.[5][27]
  7. ^ If the activity of tectonic plates is relatively low, it is likely that fewer volcanoes that can increase levels of CO2 if necessary.

Referencias

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  4. ^ Perryman 2011, p. 286
  5. ^ a b c d e f Heller & Armstrong 2014, p. 54-59
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  9. ^ a b c d e Heller & Armstrong 2014, p. 61
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  13. ^ a b c d e Heller & Armstrong 2014, p. 55
  14. ^ a b Heller & Armstrong 2014, p. 55-58
  15. ^ Heller & Armstrong 2014, p. 54-56
  16. ^ a b c d e f g h i j Heller & Armstrong 2014, p. 57
  17. ^ a b Heller & Armstrong 2014, p. 56-57
  18. ^ a b c Heller & Armstrong 2014, p. 56
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Bibliografía

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Enlaces externos

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{{Exoplaneta}}

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