Habitability of yellow dwarf systems defines the suitability for life of

exoplanets 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 recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

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 In biology, an organism () is any life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multicellular o ...

, together with those belonging to

K-type stars K-type may refer to: *AEC K-type, a bus chassis *K-type star, a stellar spectral classification *K-type filter, a type of electronic filter *K-type asteroid K-type asteroids are relatively uncommon asteroids with a moderately reddish spectrum shor ...

.Perryman, 2011, p. 285 Yellow dwarfs comprise the

G-type stars Gaea Gaea is one of the Elder Gods of Earth. Gaia Gaia, also known as the Guardian of the Universal Amalgamator, is a fictional superhero, depicted as possibly being a mutant or extraterrestrial. Created by Larry Hama, she first appeared in ...

of the

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

, with masses between 0.9 and 1.1

M☉ The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass of ...

and surface temperatures between 5000 and 6000 K, like the Sun.''Stellar classification''. '' British Encyclopedia''. Retrieved october 8th, 2015. They are the third most common in the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

Galaxy and the only ones in which the

habitable zone In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kas ...

coincides completely with the ultraviolet habitable zone. 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 Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondar ...

and orange dwarfs.Niels Bohr Institute. (2015
Planets in the habitable zone around most stars, calculate researchers
''Astrobiology Magazine'' 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.Astrobio. (2009
Stars Choose the Life Around Them
''Astrobiology Magazine''Ulmschneider, 2006 p. 61 Thus, all planets in the habitable zone of such stars exceed the

tidal locking Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked b ...

limit and their

rotation Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...

is therefore not synchronized with their

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

. The

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

, orbiting a yellow dwarf, represents the only known example of planetary habitability. For this reason, the main goal in the field of

exoplanetology This page describes exoplanet orbital and physical parameters. Orbital parameters Most known extrasolar planet candidates have been discovered using indirect methods and therefore only some of their physical and orbital parameters can be determi ...

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.Heller and Armstrong, 2014 p. 50 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 In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the long ...

Characteristics

Yellow

dwarf stars The Dwarf Stars Award is an annual award presented by the Science Fiction Poetry Association to the author of the best horror, fantasy, or science fiction poem of ten lines or fewer published in the previous year. The award was established in 20 ...

correspond to the G-class stars of the

, with a mass between 0.9 and 1.1

, and surface temperatures between 5000 and 6000 K. Since the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

itself is a yellow dwarf, of type G2V, these types of stars are also known as

solar analogs Solar may refer to: Astronomy * Of or relating to the Sun ** Solar telescope, a special purpose telescope used to observe the Sun ** A device that utilizes solar energy (e.g. " solar panels") ** Solar calendar, a calendar whose dates indica ...

. They rank third among the most common main sequence stars, after

and

orange Orange most often refers to: *Orange (fruit), the fruit of the tree species '' Citrus'' × ''sinensis'' ** Orange blossom, its fragrant flower *Orange (colour), from the color of an orange, occurs between red and yellow in the visible spectrum * ...

dwarfs, with a representativeness of 10% of the total

. 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 Alpha Centauri (Latinisation of names, Latinized from α Centauri and often abbreviated Alpha Cen or α Cen) is a Star System, triple star system in the constellation of Centaurus (constellation), Centaurus. It consists of 3 stars: ...

, 4.4

light-years A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

away and belonging to a multiple star system. All stars go through a phase of intense activity after their formation due to their

, which is much faster at the beginning of their lives. 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. Studies by the team of Edward Guinan, an astrophysicist at

Villanova University Villanova University is a Private university, private Catholic church, Roman Catholic research university in Villanova, Pennsylvania. It was founded by the Order of Saint Augustine, Augustinians in 1842 and named after Thomas of Villanova, Sa ...

, reveal that the Sun rotated ten times faster in its early days. Since the rotation speed of a star affects its

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

, the Sun's

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

and UV emissions were hundreds of times more intense than they are today. The extension of this phase in red dwarfs, as well as the probable

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 An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

and

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

to space by interaction with the

stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric. D ...

. In contrast, the

of planetary objects belonging to the habitable zone of G-type stars is wide enough to allow planetary rotation. 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 In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

and

magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...

capable of counteracting the effects of stellar winds.

Habitable area

The

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.Kasting, Whitmire, and Reynolds, 1993, pp.114-115 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. In orbits smaller than the inner boundary of the habitable zone, a process of water evaporation, hydrogen separation by

photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...

and loss of hydrogen to space by

hydrodynamic escape Hydrodynamic escape refers to a thermal atmospheric escape mechanism that can lead to the escape of heavier atoms of a planetary atmosphere through numerous collisions with lighter atoms. Description Hydrodynamic escape occurs if there is a str ...

would be triggered. Beyond the outer limit of the habitable zone, temperatures would be low enough to allow CO₂

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...

, which would lead to an increase in

albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...

and a feedback reduction of the

greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...

until a permanent global glaciation would occur. 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. 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

. Even in a small yellow dwarf like

Tau Ceti Tau Ceti, Latinized from τ Ceti, is a single star in the constellation Cetus that is spectrally similar to the Sun, although it has only about 78% of the Sun's mass. At a distance of just under from the Solar System, it is a relativ ...

, 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. In yellow dwarfs, this region coincides entirely with the ultraviolet habitability zone. This area is determined by an inner limit beyond which exposure to

ultraviolet radiation Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

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. In the

solar system The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...

, 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.

Life potential

Given the length of the main sequence in G-type stars, the levels of ultraviolet radiation in their

, the semi-major axis of the inner boundary of this region and the distance to their

limit, among other factors, yellow dwarfs are considered to be the most hospitable to life next to K-type stars. One goal in exoplanetary research is to find an object that has the main characteristics of our planet, such as

radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

, atmospheric composition and belonging to a star similar to the Sun. In theory, these Earth analogs should have comparable habitability conditions that would allow the proliferation of

extraterrestrial life Extraterrestrial life, colloquially referred to as alien life, is life that may occur outside Earth and which did not originate on Earth. No extraterrestrial life has yet been conclusively detected, although efforts are underway. Such life might ...

. 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. 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. Subsequent studies, such as ''"Superhabitable Worlds"'' 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. 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. 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

Discoveries

Most of the exoplanets discovered have been detected by the

Kepler space telescope The Kepler space telescope is a disused space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orb ...

, which uses the

transit Transit may refer to: Arts and entertainment Film * ''Transit'' (1979 film), a 1979 Israeli film * ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countries in the world * ''Transit'' (2006 film), a 2006 ...

method to find planets around other systems. 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. It is the method that has been most successful in exoplanetary research, together with the

radial velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the temporal rate of change, rate of change of the distance or Slant range, range between the two points. It is e ...

method, which consists of analyzing the vibrations caused in the stars by the gravitational effects of the planets orbiting them. 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

. As a consequence, most of the exoplanets detected are either excessively hot 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. Planetary bodies belonging to the habitable zone of yellow dwarfs, such as

Kepler-22b Kepler-22b (also known by its Kepler object of interest designation KOI-087.01) is an exoplanet orbiting within the habitable zone of the Sun-like star Kepler-22. It is located about from Earth in the constellation of Cygnus. It was discovere ...

Kepler-452b Kepler-452b (sometimes quoted to be an ''Earth 2.0'' or ''Earth's Cousin'' based on its characteristics; also known by its Kepler Object of Interest designation ''KOI-7016.01'') is a super-Earth exoplanet orbiting within the inner edge of the ha ...

, take hundreds of days to complete an orbit around their star. 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. The ground-based and orbital observatories projected for the next ten years may increase the discoveries of Earth analogs in yellow dwarf systems.

Kepler-452b

Kepler-452b lies 1400

from Earth, in the Cygnus constellation. Its

of about 1.6 R_⊕ 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). If the planet's density is similar to Earth's, its mass will be about 5 M_⊕ and its gravity twice as great. 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. 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

and atmospheric composition, the average surface temperature will be around 29

°C The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...

. According to Jon Jenkins of

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

Ames Research Center The Ames Research Center (ARC), also known as NASA Ames, is a major NASA research center at Moffett Federal Airfield in California's Silicon Valley. It was founded in 1939 as the second National Advisory Committee for Aeronautics (NACA) laborat ...

, it is not known whether Kepler-452b is a

terrestrial planet A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, Ve ...

, an

ocean world An ocean world, ocean planet, panthalassic planet, maritime world, water world or aquaplanet, is a type of planet that contains a substantial amount of water in form of oceans, either beneath the surface, as subsurface oceans, or on the surfa ...

or a

mini-Neptune A Mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet less massive than Neptune but resembling Neptune in that it has a thick hydrogen–helium atmosphere, probably with deep layers of ice, rock or liquid oceans (made ...

. 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

similar to that of

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...

due to the constant increase in the luminosity of its star, after having remained throughout the main sequence in its habitable zone. Doug Caldwell, a

SETI The search for extraterrestrial intelligence (SETI) is a collective term for scientific searches for intelligent extraterrestrial life, for example, monitoring electromagnetic radiation for signs of transmissions from civilizations on other p ...

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.

Tau Ceti e

Tau Ceti e orbits a G8.5V-type star in the constellation

Cetus Cetus () is a constellation, sometimes called 'the whale' in English. The Cetus (mythology), Cetus was a sea monster in Greek mythology which both Perseus and Heracles needed to slay. Cetus is in the region of the sky that contains other water- ...

, 12 light-years from Earth. 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 The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

of only 168 days, its temperature assuming an Earth-like atmospheric composition and albedo would be about 50 °C. 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 A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to ...

'' type object. Otherwise, it could be the first thermoplanet discovered.

Kepler-22b

Kepler-22b is at a distance of 600 light-years, in the Cygnus constellation. It completes one orbit around its G5V-type star every 290 days. 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. It was the first exoplanet found by the

Kepler telescope The Kepler space telescope is a disused space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orbit ...

belonging to the habitability zone of its star. Because of its size, considering the limit established by Courtney Dressing's team, its probability to be a

is very high.

References

Bibliography

* * * * Exoplanets NASA Planets Stars Planetary habitability {{DEFAULTSORT:Habitability in yellow dwarf systems