Habitability of yellow dwarf systems

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Artistic interpretation of Kepler-452b, a potentially habitable exoplanet belonging to a yellow dwarf. Kepler-452b artist concept.jpg
Artistic interpretation of Kepler-452b, a potentially habitable exoplanet belonging to a yellow dwarf.

Habitability of G V stars of G V stars systems defines the suitability for life of exoplanets belonging to yellow dwarf stars. These systems are the object of study among the scientific community because they are considered the most suitable for harboring living organisms, together with those belonging to K-type stars. [1]

Contents

Yellow dwarfs comprise the G-type stars of the main sequence, with masses between 0.9 and 1.1 M☉ and surface temperatures between 5000 and 6000 K, like the Sun. [2] [3] They are the third most common in the Milky Way Galaxy and the only ones in which the habitable zone coincides completely with the ultraviolet habitable zone. [2] [4]

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 red and orange dwarfs. [5] Therefore, planets located in this zone of G-type stars are safe from the intense stellar emissions that occur after their formation and are not as affected by the gravitational influence of their star as those belonging to smaller stellar bodies. [6] [7] Thus, all planets in the habitable zone of such stars exceed the tidal locking limit and their rotation is therefore not synchronized with their orbit. [7]

The Earth, orbiting a yellow dwarf, represents the only known example of planetary habitability. For this reason, the main goal in the field of exoplanetology is to find an Earth analog planet that meets its main characteristics, such as size, average temperature and location around a star similar to the Sun. [8] [9] However, technological limitations make it difficult to find these objects due to the infrequency of their transits, a consequence of the distance that separates them from their stars or semi-major axis. [10]

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

Habitable zone of the stars Kepler-186 (red dwarf), Kepler-452 and the Sun (both yellow dwarfs) Kepler-452b System.jpg
Habitable zone of the stars Kepler-186 (red dwarf), Kepler-452 and the Sun (both yellow dwarfs)

The habitable zone around yellow dwarfs varies according to their size and luminosity, although the inner boundary is usually at 0.84 AU and the outer one at 1.67 in a G2V class dwarf like the Sun. [19] In a G5V class dwarf -smaller- of 0.95 R☉ the habitable zone would correspond to the region located between 0.8 and 1.58 AU with respect to the star, while in a G0V type — larger — it would be located at a distance of between 1 and 2 AU from the stellar body. [20] In orbits smaller than the inner boundary of the habitable zone, a process of water evaporation, hydrogen separation by photolysis and loss of hydrogen to space by hydrodynamic escape would be triggered. [21] Beyond the outer limit of the habitable zone, temperatures would be low enough to allow CO2 condensation, which would lead to an increase in albedo and a feedback reduction of the greenhouse effect until a permanent global glaciation would occur. [22]

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. [4] This area is determined by an inner limit beyond which exposure to ultraviolet radiation would be too high for DNA and by an outer limit that provides the minimum levels for living things to carry out their biogenic processes. [25] In the solar system, this region is located between 0.71 and 1.9 AU with respect to the Sun, compared to the 0.84-1.67 AU that mark the extremes of the habitable zone. [4] [19]

Life potential

Given the length of the main sequence in G-type stars, [26] the levels of ultraviolet radiation in their habitable zone, [4] the semi-major axis of the inner boundary of this region [19] and the distance to their tidal locking limit, [27] among other factors, yellow dwarfs are considered to be the most hospitable to life next to K-type stars. [1]

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 type F or higher, the only stars that might offer a bearable scenario for life would be those of type K and G. [1] Solar analogs used to be considered as the most likely candidates to host a solar-like planetary system, and as the best positioned to support carbon-based life forms and liquid water oceans. [30] Subsequent studies, such as "Superhabitable Worlds" [31] by René Heller and John Armstrong, establish that orange dwarfs may be more suitable for life than G-type dwarfs, and host hypothetical superhabitable planets. [4] [32]

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 Kepler space telescope, which uses the transit method to find planets around other systems. [33] [34] This procedure analyzes the brightness of stars to detect dips that indicate the passage of a planetary object in front of them from the perspective of the observatory. [35] It is the method that has been most successful in exoplanetary research, together with the radial velocity method, [36] which consists of analyzing the vibrations caused in the stars by the gravitational effects of the planets orbiting them. [37] The use of these procedures with the limitations of current telescopes makes it difficult to find objects with orbits similar to the Earth's orbits or higher, which generates a bias in favor of planets with a short semi-major axis. [28] As a consequence, most of the exoplanets detected are either excessively hot [37] or belong to low-mass stars, whose habitable zone is close to them and any object orbiting in this region will have a significantly shorter year than the Earth. [10]

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 light-years from Earth, in the Cygnus constellation. [45] Its radius of about 1.6 R⊕ [46] places it right on the boundary separating telluric planets from mini-Neptunes established by the team of Courtney Dressing, a researcher at the Harvard-Smithsonian Center for Astrophysics (CfA). [47] If the planet's density is similar to Earth's, its mass will be about 5 M⊕ and its gravity twice as great. [46] A G2V-type yellow dwarf like the Sun belongs to Kepler-452, with an estimated age of 6 billion years (6 Ga) versus the solar system's 4.5 Ga. [46]

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 super-Venus [51] type object. Otherwise, it could be the first thermoplanet discovered. [52] [48]

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

Related Research Articles

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An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not then recognized as such. The first confirmation of the detection occurred in 1992. A different planet, first detected in 1988, was confirmed in 2003. As of 1 May 2024, there are 5,662 confirmed exoplanets in 4,169 planetary systems, with 896 systems having more than one planet. The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and to give more insight into their traits, such as their composition, environmental conditions, and potential for life.

<span class="mw-page-title-main">Habitable zone</span> Orbits where planets may have liquid surface water

In astronomy and astrobiology, the habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure. The bounds of the HZ are based on Earth's position in the Solar System and the amount of radiant energy it receives from the Sun. Due to the importance of liquid water to Earth's biosphere, the nature of the HZ and the objects within it may be instrumental in determining the scope and distribution of planets capable of supporting Earth-like extraterrestrial life and intelligence.

<span class="mw-page-title-main">K-type main-sequence star</span> Stellar classification

A K-type main-sequence star, also referred to as a K-type dwarf, or orange dwarf, is a main-sequence (hydrogen-burning) star of spectral type K and luminosity class V. These stars are intermediate in size between red M-type main-sequence stars and yellow/white G-type main-sequence stars. They have masses between 0.6 and 0.9 times the mass of the Sun and surface temperatures between 3,900 and 5,300 K. These stars are of particular interest in the search for extraterrestrial life due to their stability and long lifespan. Many of these stars have not left the main sequence as their low masses mean they stay on the main sequence for up to 70 billion years, a length of time much larger than the time the universe has existed. Well-known examples include Alpha Centauri B and Epsilon Indi.

<span class="mw-page-title-main">Planetary habitability</span> Known extent to which a planet is suitable for life

Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and maintain environments hospitable to life. Life may be generated directly on a planet or satellite endogenously or be transferred to it from another body, through a hypothetical process known as panspermia. Environments do not need to contain life to be considered habitable nor are accepted habitable zones (HZ) the only areas in which life might arise.

<span class="mw-page-title-main">Habitability of natural satellites</span> Measure of the potential of natural satellites to have environments hospitable to life

The habitability of natural satellites is the potential of moons to provide habitats for life, though it is not an indicator that they harbor it. Natural satellites are expected to outnumber planets by a large margin and the study of their habitability is therefore important to astrobiology and the search for extraterrestrial life. There are, nevertheless, significant environmental variables specific to moons.

<span class="mw-page-title-main">Discoveries of exoplanets</span> Detecting planets located outside the Solar System

An exoplanet is a planet located outside the Solar System. The first evidence of an exoplanet was noted as early as 1917, but was not recognized as such until 2016; no planet discovery has yet come from that evidence. What turned out to be the first detection of an exoplanet was published among a list of possible candidates in 1988, though not confirmed until 2003. The first confirmed detection came in 1992, with the discovery of terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. As of 1 May 2024, there are 5,662 confirmed exoplanets in 4,169 planetary systems, with 896 systems having more than one planet. This is a list of the most notable discoveries.

<span class="mw-page-title-main">Habitability of K-type main-sequence star systems</span> Overview of the habitability of K-type main-sequence star systems

K-type main-sequence stars, also known as orange dwarfs, may be candidates for supporting extraterrestrial life. These stars are known as "Goldilocks stars" as they emit enough radiation in the non-UV ray spectrum to provide a temperature that allows liquid water to exist on the surface of a planet; they also remain stable in the main sequence longer than the Sun by burning their hydrogen slower, allowing more time for life to form on a planet around a K-type main-sequence star. The planet's habitable zone, ranging from 0.1–0.4 to 0.3–1.3 astronomical units (AU), depending on the size of the star, is often far enough from the star so as not to be tidally locked to the star, and to have a sufficiently low solar flare activity not to be lethal to life. In comparison, red dwarf stars have too much solar activity and quickly tidally lock the planets in their habitable zones, making them less suitable for life. The odds of complex life arising may be better on planets around K-type main-sequence stars than around Sun-like stars, given the suitable temperature and extra time available for it to evolve. Some planets around K-type main-sequence stars are potential candidates for extraterrestrial life.

<span class="mw-page-title-main">Kepler-62f</span> Super-Earth orbiting Kepler-62

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<span class="mw-page-title-main">Habitability of red dwarf systems</span> Possible factors for life around red dwarf stars

The theorized habitability of red dwarf systems is determined by a large number of factors. Modern evidence indicates that planets in red dwarf systems are unlikely to be habitable, due to their low stellar flux, high probability of tidal locking and thus likely lack of magnetospheres and atmospheres, small circumstellar habitable zones and the high stellar variation experienced by planets of red dwarf stars. However, the sheer numbers and longevity of red dwarfs could provide ample opportunity to realize any small possibility of habitability.

<span class="mw-page-title-main">Kepler-186</span> Star in the constellation Cygnus

Kepler-186 is a main-sequence M1-type dwarf star, located 177.5 parsecs away in the constellation of Cygnus. The star is slightly cooler than the sun, with roughly half its metallicity. It is known to have five planets, including the first Earth-sized world discovered in the habitable zone: Kepler-186f. The star hosts four other planets discovered so far, though they all orbit interior to the habitable zone.

<span class="mw-page-title-main">Kepler-438b</span> Super-Earth orbiting Kepler-438

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<span class="mw-page-title-main">Kepler-442b</span> Super-Earth orbiting Kepler-442

Kepler-442b is a confirmed near-Earth-sized exoplanet, likely rocky, orbiting within the habitable zone of the K-type main-sequence star Kepler-442, about 1,206 light-years (370 pc) from Earth in the constellation of Lyra.

<span class="mw-page-title-main">Kepler-452b</span> Super-Earth exoplanet orbiting Kepler-452

Kepler-452b is a super-Earth exoplanet orbiting within the inner edge of the habitable zone of the sun-like star Kepler-452 and is the only planet in the system discovered by Kepler. It is located about 1,400 light-years (430 pc) from Earth in the constellation of Cygnus.

<span class="mw-page-title-main">Kepler-452</span> G-type main-sequence star in the constellation Cygnus

Kepler-452 is a G-type main-sequence star located about 1,810 light-years away from Earth in the Cygnus constellation. Although similar in temperature to the Sun, it is 20% brighter, 3.7% more massive and 11% larger. Alongside this, the star is approximately six billion years old and possesses a high metallicity.

<span class="mw-page-title-main">Superhabitable world</span> Hypothetical type of planet or moon that may be better-suited for life than Earth

A superhabitable world is a hypothetical type of planet or moon that is better suited than Earth for the emergence and evolution of life. The concept was introduced in a 2014 paper by René Heller and John Armstrong, in which they criticized the language used in the search for habitable exoplanets and proposed clarifications. The authors argued that knowing whether a world is located within the star's habitable zone is insufficient to determine its habitability, that the principle of mediocrity cannot adequately explain why Earth should represent the archetypal habitable world, and that the prevailing model of characterization was geocentric or anthropocentric in nature. Instead, they proposed a biocentric approach that prioritized astrophysical characteristics affecting the abundance and variety of life on a world's surface.

<span class="mw-page-title-main">Kepler-1229b</span> Super-Earth orbiting Kepler-1229

Kepler-1229b is a confirmed super-Earth exoplanet, likely rocky, orbiting within the habitable zone of the red dwarf Kepler-1229, located about 870 light years from Earth in the constellation of Cygnus. It was discovered in 2016 by the Kepler space telescope. The exoplanet was found by using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured.

TOI-700 is a red dwarf 101.4 light-years away from Earth located in the Dorado constellation that hosts TOI-700 d, the first Earth-sized exoplanet in the habitable zone discovered by the Transiting Exoplanet Survey Satellite (TESS).

<span class="mw-page-title-main">Kepler-1649c</span> Earth-size exoplanet orbiting Kepler-1649

Kepler-1649c is an Earth-sized exoplanet, likely rocky, orbiting within the habitable zone of the red dwarf star Kepler-1649, the outermost planet of the planetary system discovered by Kepler’s space telescope. It is located about 301 light-years (92 pc) away from Earth, in the constellation of Cygnus.

<span class="mw-page-title-main">Habitability of F-type main-sequence star systems</span> Overview of the habitability of F-type main-sequence star systems

The habitability of F-type main-sequence starsystems is disputed due to the shorter lifetimes and higher levels of UV radiation. Indeed, F0 stars are considered by many scientists as the hottest and most massive stars capable of supporting habitable planets. A planet orbiting an F-type star at the Earth boundary within the HZ would receive 2.5 to 7.1 times the UV that Earth gets from the sun.

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Bibliography

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