Habitability of yellow dwarf systems
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Habitability of yellow dwarf systems defines the suitability for life of
Yellow dwarfs comprise the
Since the habitable zone is farther away in more massive and luminous stars, the separation between the main star and the inner edge of this region is greater in yellow dwarfs than in
The
Characteristics
Yellow dwarf stars correspond to the G-class stars of the main sequence, with a mass between 0.9 and 1.1 M☉,[2] and surface temperatures between 5000 and 6000 K.[3] Since the Sun itself is a yellow dwarf, of type G2V,[11] these types of stars are also known as solar analogs.[12][13] They rank third among the most common main sequence stars, after red and orange dwarfs, with a representativeness of 10% of the total Milky Way.[2] They remain in the main sequence for approximately 10 billion years. After the Sun, the closest G-type star to the Earth is Alpha Centauri A, 4.4 light-years away and belonging to a multiple star system.[2][14]
All stars go through a phase of intense activity after their formation due to their rotation, which is much faster at the beginning of their lives.[6] The duration of this period varies according to the mass of the object: the least massive stars can remain in this state for up to 3 billion years, compared to 500 million for G-type stars.[15][16] Studies by the team of Edward Guinan, an astrophysicist at Villanova University, reveal that the Sun rotated ten times faster in its early days. Since the rotation speed of a star affects its magnetic field, the Sun's X-ray and UV emissions were hundreds of times more intense than they are today.[6]
The extension of this phase in red dwarfs, as well as the probable tidal locking[17] of their potentially habitable planets with respect to them, could wipe out the magnetic field of these planets, resulting in the loss of almost all their atmosphere and water to space by interaction with the stellar wind.[6] In contrast, the semi-major axis of planetary objects belonging to the habitable zone of G-type stars is wide enough to allow planetary rotation.[7][18] In addition, the duration of the period of intense stellar activity is too short to eliminate a significant part of the atmosphere on planets with masses similar to or greater than that of the Earth, which have a gravity and magnetosphere capable of counteracting the effects of stellar winds.[16]
Habitable area
The
The size of the habitable zone is directly proportional to the mass and luminosity of its star, so the larger the star, the larger the habitable zone and the farther from its surface.[5] Red dwarfs, the smallest of the main sequence, have a very small habitable zone close to them, which subjects any potentially habitable planets in the system to the effects of their star, including probable tidal locking.[23] Even in a small yellow dwarf like Tau Ceti, of type G8.5V, the locking limit is at 0.4237 AU versus the 0.522 AU that marks the inner boundary of the habitable zone, so any planetary object orbiting a G-class star in this region will far exceed the locking limit, and will have day-night cycles like Earth.[24]
In yellow dwarfs, this region coincides entirely with the ultraviolet habitability zone.
Life potential
Given the length of the main sequence in G-type stars,
One goal in exoplanetary research is to find an object that has the main characteristics of our planet, such as radius, mass, temperature, atmospheric composition and belonging to a star similar to the Sun.[9][28] In theory, these Earth analogs should have comparable habitability conditions that would allow the proliferation of extraterrestrial life.[9][29]
Based on the serious problems for planetary habitability presented by red dwarf systems and stellar bodies of
However, yellow dwarfs still represent the only stellar type for which there is evidence of their suitability for life. Moreover, while in other types of stars the habitable zone does not coincide entirely with the ultraviolet habitable zone, in G-class stars the habitable zone lies entirely within the limits of the latter.[4] Finally, yellow dwarfs have a much shorter initial phase of intense stellar activity than K-type stars, which allows planets belonging to solar analogs to preserve their primordial atmospheres more easily and to maintain them for much of the main sequence.[16]
Discoveries
Most of the exoplanets discovered have been detected by the
Planetary bodies belonging to the habitable zone of yellow dwarfs, such as Kepler-22b, Kepler-452b or Earth, take hundreds of days to complete an orbit around their star.[38] The higher luminosity of these stars, the scarcity of transits and the semi-major axis of their planets located in the habitable zone reduce the probabilities of detecting this class of objects and considerably increase the number of false positives, as in the cases of KOI-5123.01 and KOI-5927.01.[39][40] The ground-based and orbital observatories projected for the next ten years may increase the discoveries of Earth analogs in yellow dwarf systems.[41][42][43][44]
Kepler-452b
Kepler-452b lies 1400
The mass of its star is slightly higher than that of the Sun, 1.04 M☉, so despite the fact that it completes an orbit around it every 385 days versus 365 terrestrial days, it is warmer than the Earth. If it has similar albedo and atmospheric composition, the average surface temperature will be around 29 °C.[48]
According to Jon Jenkins of NASA's Ames Research Center, it is not known whether Kepler-452b is a terrestrial planet, an ocean world or a mini-Neptune.[45] If it is an Earth-like telluric object, it is likely to have a higher concentration of clouds, intense volcanic activity, and is about to suffer an uncontrolled greenhouse effect similar to that of Venus due to the constant increase in the luminosity of its star, after having remained throughout the main sequence in its habitable zone.[49] Doug Caldwell, a SETI Institute scientist and member of the Kepler mission, estimates that Kepler-452b may be undergoing the same process that the Earth will undergo in a billion years.[50]
Tau Ceti e
Tau Ceti e orbits a G8.5V-type star in the constellation Cetus, 12 light-years from Earth.[48] It has a radius of 1.59 R⊕ and a mass of 4.29 M⊕, so like Kepler-452b it lies at the separation boundary between terrestrial and gaseous planets. With an orbital period of only 168 days, its temperature assuming an Earth-like atmospheric composition and albedo would be about 50 °C.[48]
The planet is located just at the inner edge of the habitable zone and receives about 60% more light than Earth. Its size may also imply a higher concentration of gases in its atmosphere, making it a
Kepler-22b
Kepler-22b is at a distance of 600 light-years, in the Cygnus constellation.[48] It completes one orbit around its G5V-type star every 290 days.[53] Its radius is 2.35 R⊕ and its estimated mass, for an Earth-like density, would be 20.36 M⊕. If the planet's atmosphere and albedo were similar to Earth's, its surface temperature would be around 22 °C.[54]
It was the first exoplanet found by the Kepler telescope belonging to the habitability zone of its star.[55] Because of its size, considering the limit established by Courtney Dressing's team, its probability to be a mini-Neptune is very high.[47][48]
See also
- Astrobiology
- Circumstellar habitable zone
- Earth analog
- Superhabitable planet
- Habitability of natural satellites
- Habitability of red dwarf systems
- Habitability of K-type main-sequence star systems
- Habitability of F-type main-sequence star systems
- List of potentially habitable exoplanets
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