Solar analog
Encyclopedia
Solar-type, solar analog, and solar twin stars are those stars that are particularly similar to the Sun
. The classification is a hierarchy with solar twin being most like the Sun followed by solar analog and then solar-type. Observations of these stars are important for understanding better the properties of the Sun in relation to other stars and the habitability of planets.
Similarity to the Sun allows for checking derived quantities — like temperature, which is derived from the color index — against the Sun, the only star whose temperature is confidently known. For stars which aren't similar to the Sun, this cross-checking can't be done.
stars with a B-V color between 0.48 and 0.80, the Sun having a B-V color of 0.65. Alternatively, a definition based on spectral type can be used, such as F8 V through K2 V, which would correspond to B-V color of 0.50 to 1.00. This definition fits approximately 10% of stars, so a list of solar-type stars would be quite extensive.
Solar-type stars show highly correlated behavior between their rotation rates and their chromospheric activity (e.g. Calcium H & K line emission) and coronal activity (e.g. X-ray emission). As solar-type stars spin-down during their main sequence lifetimes due to magnetic braking, these correlations allow rough ages to be derived. Mamajek & Hillenbrand (2008) have estimated the ages for the 108 solar-type (F8V-K2V) main sequence stars within 16 parsecs of the Sun based on their chromospheric activity (as measured via Ca H & K emission lines).
The following table shows a sample of solar-type stars within 50 light years that nearly satisfy the criteria for solar analogs, based on current measurements.
Solar analogs not meeting the stricter solar twin criteria include, within 50 light years and in order of increasing distance
:
The following are the known stars that come closest to satisfying the criteria for a solar twin. (The Sun is listed for comparison.)
Some other stars are sometimes mentioned as promising solar twin candidates, particularly: Beta Canum Venaticorum (see references for Turnbull & Tarter), 37 Geminorum (see references for Turnbull & Tarter) and 16 Cygni B (Porto de Mello et al. 2000). However, all three have temperatures and/or luminosities that are too high for true solar twins. Furthermore, Beta Canum Venaticorum and 37 Geminorum have too low metallicities for solar twins. Finally, 16 Cygni B is part of a (very wide) binary system and is very old for a solar twin (at least 7 to 8 Gyr). Beta Canum Venaticorum is mentioned above as a nearby solar analog.
The requirement that the star remain on the main sequence for at least 3 Ga sets an upper limit of approximately 1.5 solar masses, corresponding to a hottest spectral type of F5 V. Such stars can reach an absolute magnitude of 2.5, or 8.55 times as bright as the Sun, at the end of the main sequence.
Non-variability is ideally defined as variability of less than 1%, but 3% is the practical limit due to limits in available data. Variation in irradiance in a star's habitable zone due to a companion star with an eccentric orbit is also a concern.
Terrestrial planets in multiple star
systems, those containing three or more stars, are not likely to have stable orbits in the long term. Stable orbits in binary systems take one of two forms: S-Type (satellite or circumstellar) orbits around one of the stars, and P-Type (planetary or circumbinary) orbits around the entire binary pair. Eccentric Jupiter
s may also disrupt the orbits of planets in habitable zones.
Metallicity of at least 40% solar ([Fe/H] = -0.4) is required for the formation of an Earth-like terrestrial planet. High metallicity strongly correlates to the formation of hot Jupiter
s, but these are not absolute bars to life, as some gas giant
s end up orbiting within the habitable zone themselves, and could potentially host Earth-like moons.
One example of such a star is HD 70642
.
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. The classification is a hierarchy with solar twin being most like the Sun followed by solar analog and then solar-type. Observations of these stars are important for understanding better the properties of the Sun in relation to other stars and the habitability of planets.
By similarity to the Sun
Defining the three categories by their similarity to the Sun reflects the evolution of astronomical observational techniques. Originally, solar-type was the closest that similarity to the Sun could be defined. Later, more precise measurement techniques and improved observatories allowed for greater precision of key details like temperature, enabling the creation of a solar analog category for stars that were particularly similar to the Sun. Later still, continued improvements in precision allowed for the creation of a solar twin category for near-perfect matches.Similarity to the Sun allows for checking derived quantities — like temperature, which is derived from the color index — against the Sun, the only star whose temperature is confidently known. For stars which aren't similar to the Sun, this cross-checking can't be done.
Solar-type
These stars are broadly similar to the Sun. They are main sequenceMain sequence
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 Hertzsprung and Henry Norris Russell...
stars with a B-V color between 0.48 and 0.80, the Sun having a B-V color of 0.65. Alternatively, a definition based on spectral type can be used, such as F8 V through K2 V, which would correspond to B-V color of 0.50 to 1.00. This definition fits approximately 10% of stars, so a list of solar-type stars would be quite extensive.
Solar-type stars show highly correlated behavior between their rotation rates and their chromospheric activity (e.g. Calcium H & K line emission) and coronal activity (e.g. X-ray emission). As solar-type stars spin-down during their main sequence lifetimes due to magnetic braking, these correlations allow rough ages to be derived. Mamajek & Hillenbrand (2008) have estimated the ages for the 108 solar-type (F8V-K2V) main sequence stars within 16 parsecs of the Sun based on their chromospheric activity (as measured via Ca H & K emission lines).
The following table shows a sample of solar-type stars within 50 light years that nearly satisfy the criteria for solar analogs, based on current measurements.
| Identifier | Coordinates | Distance (ly) |
Stellar Class Stellar classification In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure... |
Temperature (K) |
Metallicity (dex) |
Notes | |
|---|---|---|---|---|---|---|---|
| Right ascension Right ascension Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:... |
Declination Declination In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and... |
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| Tau Ceti Tau Ceti Tau Ceti is a 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 12 light-years from the Solar System, it is a relatively close star. Tau Ceti is metal-deficient and so is thought to be less likely to... |
11.9 | G8V | 5,344 | –0.52 | |||
| 40 Eridani 40 Eridani 40 Eridani is a triple star system less than 16.5 light years away from Earth. It is in the constellation Eridanus. The primary star of the system, 40 Eridani A, is easily visible to the naked eye... A |
16.5 | K1V | 5,126 | –0.31 | |||
| 82 Eridani 82 Eridani 82 G. Eridani is a star about 20 light years away from Earth. It is in the constellation Eridanus. It is a main sequence star of spectral type G5.-Observation:... |
19.8 | G8V | 5,338 | –0.54 | |||
| Delta Pavonis Delta Pavonis Delta Pavonis is a star about 19.9 light years away from Earth. It is in the constellation Pavo.-Observations:It is a subgiant of spectral type G8 IV, meaning it is about to stop fusing hydrogen in its core and is starting the process of becoming a red giant. Because of that, Delta Pavonis is... |
19.9 | G8IV | 5,604 | +0.33 | |||
| HR 7722 | 28.8 | K0V | 5,166 | –0.04 | |||
| Gliese 86 Gliese 86 Gliese 86 is a K-type dwarf star approximately 35 light-years away in the constellation of Eridanus. It has been confirmed that a white dwarf orbits the primary star... A |
35.2 | K1V | 5,163 | −0.24 | |||
| 54 Piscium 54 Piscium 54 Piscium is an orange dwarf star approximately 36 light-years away in the constellation of Pisces. In 2002, an extrasolar planet was confirmed to be orbiting the star, and in 2006, a brown dwarf was also discovered orbiting it.... |
36.1 | K0V | 5,129 | +0.19 | |||
| V538 Aurigae V538 Aurigae V538 Aurigae is variable of BY Dra type. It has a spectral class of K1e and is located 39.9 light-years from Earth. The stars diameter is 0.84 times the sun and it has a metallicity 63% that of the Sun making it relatively metal poor.... |
39.9 | K1V | 3,500-5,000 | -0.20 | |||
| HD 14412 HD 14412 HD 14412 is a yellow dwarf star in the constellation Fornax.This star is located about 41 light years from the Sun. Based on its spectral classification, it is slightly less massive and luminous than the Sun. The temperature of the star's photosphere has been measured at 5,368 ± 24 °K.... |
41.3 | G5V | 5,432 | −0.46 | |||
| HR 4587 HR 4587 HR 4587 has been studied at length by S. Feltzing and G. Gonzalez in their study of the metallicity of stars. It has an as-yet-unconfirmed planet, HD 104304b, discovered in 2007, which is believed to have a mass of 17.2 Jupiters and an orbital period of 2752 days at an eccentricity of... |
42.1 | G8IV | ? | ? | |||
| HD 172051 HD 172051 HD 172051 is a main sequence dwarf star in theconstellation Sagittarius.It is located about 42 light years from the Sun and is considered a solar analog. It is similar in mass to the Sun, although it is cooler and has a lower luminosity... |
42.7 | G5V | 5,610 | −0.32 | |||
| 72 Herculis 72 Herculis 72 Herculis is a main sequence dwarf star in the constellation Hercules. The Flamsteed designation for this star comes from the publication Historia Coelestis Britannica by John Flamsteed. It is the 72nd star in Flamsteed's list of stars in Hercules.... |
46.9 | G0V | 5,662 | −0.37 | |||
| HD 196761 HD 196761 HD 196761 is a main sequence dwarf star in the constellation Capricornus and is located about 47 light years from the Solar System. It has a stellar classification of G8V and is about 88% of the radius of the Sun... |
46.9 | G8V | 5,415 | -0.31 | |||
| Nu² Lupi | 47.5 | G4V | 5,664 | −0.34 | |||
Solar analog
These stars are photometrically similar to the Sun, having the following qualities:- Temperature within 500 K Solar (roughly 5200 to 6300 K)
- MetallicityMetallicityIn astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
of 50—200% (± 0.3 dex) Solar, meaning the star's proplyd would have had similar amounts of dust from which planets could form - No close companion (orbital period of ten days or less), as such a companion stimulates stellar activity
Solar analogs not meeting the stricter solar twin criteria include, within 50 light years and in order of increasing distance
Cosmic distance ladder
The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth...
:
| Identifier | Coordinates | Distance (ly) |
Stellar Class Stellar classification In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure... |
Temperature (K) |
Metallicity (dex) |
Notes | |
|---|---|---|---|---|---|---|---|
| Right ascension Right ascension Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:... |
Declination Declination In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and... |
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| Alpha Centauri Alpha Centauri Alpha Centauri is the brightest star in the southern constellation of Centaurus... A |
4.37 | G2V | 5,847 | +0.24 | |||
| Alpha Centauri Alpha Centauri Alpha Centauri is the brightest star in the southern constellation of Centaurus... B |
4.37 | K1V | 5,316 | +0.25 | |||
| 70 Ophiuchi 70 Ophiuchi 70 Ophiuchi a binary star system located 16.6 light years away from the Earth. It is in the constellation Ophiuchus. At magnitude 4 it is a typical less bright star usually visible to the unaided eye away from city lights.-Binary star:... A |
16.6 | K0V | 5,314 | –0.02 | |||
| Sigma Draconis Sigma Draconis Sigma Draconis is a 4.7 magnitude star located at a distance of 18.8 light-years in the constellation Draco. Its traditional name is Alsafi.-Name:... |
18.8 | K0V | 5,297 | –0.20 | |||
| Eta Cassiopeiae Eta Cassiopeiae Eta Cassiopeiae is a star system 19.4 light years away from Earth, in the constellation Cassiopeia. Sometimes the traditional name Achird is used.... A |
19.4 | G0V | 5,941 | –0.17 | |||
| 107 Piscium | 24.4 | K1V | 5,242 | –0.04 | |||
| Beta Canum Venaticorum | 27.4 | G0V | 5,930 | −0.30 | |||
| 61 Virginis | 27.8 | G5V | 5,558 | –0.02 | |||
| Zeta Tucanae Zeta Tucanae Zeta Tucanae is a star in the constellation Tucana. It is a spectral class F9.5 main sequence star with an apparent magnitude of +4.23. Despite having a slightly lower mass, this star is more luminous than the Sun. Based upon parallax measurements by the Hipparcos spacecraft, it is approximately... |
28.0 | F9.5V | 5,956 | –0.14 | |||
| Chi¹ Orionis A | 28.3 | G0V | 5,902 | –0.16 | |||
| Beta Comae Berenices Beta Comae Berenices Beta Comae Berenices is a main sequence dwarf star in the constellation of Coma Berenices. It is located at a distance of about 30 light years. The Greek letter beta usually indicates that the star has the second highest visual magnitude in the constellation... |
29.8 | G0V | 5,970 | –0.06 | |||
| HR 4523 HR 4523 HR 4523 is a binary star system that is located in the northeastern part of the Centaurus constellation, at a distance of about from the Solar System. The larger member of the system is a G-type star that is smaller than the Sun but of similar mass. It has a common proper motion companion that... A |
30.1 | G5V | 5,629 | –0.29 | |||
| 61 Ursae Majoris 61 Ursae Majoris 61 Ursae Majoris is an orange-yellow main sequence dwarf star in the constellation Ursa Major. This star is somewhat smaller and fainter than the Sun, and can just barely be seen by the unaided eye .... |
31.1 | G8V | 5,483 | –0.12 | |||
| HR 4458 HR 4458 HR 4458 is a binary star system in the constellation Hydra. At a distance of 31 light years, they are the closest stars to the solar system within this constellation. The pair has an angular separation of 16.2″.... A |
31.1 | K0V | 5,629 | –0.29 | |||
| HR 511 HR 511 HR 511 is an orange dwarf of spectral type K0V in the constellation Cassiopeia. The star is relatively close, 32.5 light years from the Sun.... |
32.8 | K0V | 5,333 | +0.05 | |||
| Alpha Mensae Alpha Mensae Alpha Mensae is the brightest star in the constellation Mensa. At a magnitude of 5.09, it is the dimmest lucida in all the heavens. It is a main sequence dwarf star only slightly smaller and cooler in temperature than the Sun... |
33.1 | G5V | 5,594 | +0.10 | |||
| Zeta1 Reticuli Zeta Reticuli Zeta Reticuli is a binary star system located about away from Earth. It is located in the constellation Reticulum, and is visible to the unaided eye in very dark skies... |
39.5 | G3-5V | 5,733 | −0.22 | |||
| Zeta2 Reticuli Zeta Reticuli Zeta Reticuli is a binary star system located about away from Earth. It is located in the constellation Reticulum, and is visible to the unaided eye in very dark skies... |
39.5 | G2V | 5,843 | −0.23 | |||
| 55 Cancri 55 Cancri 55 Cancri , also cataloged Rho1 Cancri or abbreviated 55 Cnc, is a binary star approximately 41 light-years away from Earth in the constellation of Cancer... |
40.3 | G8V | 5,235 | +0.25 | |||
| HD 69830 HD 69830 HD 69830 is an orange dwarf star approximately 41 light-years away in the constellation of Puppis. It has the Gould designation 285 G. Puppis, though this is infrequently used. In 2005, the Spitzer Space Telescope discovered a debris disk orbiting the star. The disk contains substantially more... |
40.6 | K0V | 5,410 | −0.03 | |||
| HD 10307 HD 10307 HD 10307 is a star similar to the sun in mass, temperature and metal content, situated about 41 light-years from Earth in the constellation Andromeda... |
41.2 | G1.5V | 5,848 | −0.05 | |||
| HD 147513 HD 147513 HD 147513 is the 5th-magnitude star in the constellation of Scorpius. It was first catalogued by Italian astronomer Piazzi in his star catalogue as "XVI 55".... |
42.0 | G1V | 5,858 | +0.03 | |||
| 58 Eridani 58 Eridani 58 Eridani is a main sequence star in the constellation Eridanus. It is considered a solar analogue, which means it has similar physical properties to the Sun... |
43.3 | G3V | 5,868 | +0.02 | |||
| Upsilon Andromedae Upsilon Andromedae Upsilon Andromedae is a binary star located approximately 44 light-years away from Earth in the constellation Andromeda. The primary star is a yellow-white dwarf star that is somewhat younger than the Sun... A |
44.0 | F8V | 6,212 | +0.13 | |||
| HD 211415 HD 211415 HD 211415 is a binary star system in the constellation Grus. It has a relatively high proper motion and is located about 44 light years from the Sun.... A |
44.4 | G1-3V | 5,890 | −0.17 | |||
| 47 Ursae Majoris 47 Ursae Majoris 47 Ursae Majoris is a solar analog, yellow dwarf star approximately 46 light-years away from Earth in the constellation of Ursa Major. , it has been confirmed that three Jupiter-like extrasolar planets orbit the star... |
45.9 | G1V | 5,954 | +0.06 | |||
| Alpha Fornacis Alpha Fornacis Alpha Fornacis is the brightest star in the constellation Fornax, its only star brighter than magnitude 4.0. It has the proper names Dalim and Fornacis... A |
46.0 | F8IV | 6,275 | −0.19 | |||
| Psi Serpentis Psi Serpentis Psi Serpentis is a star system in the constellation Serpens. It is approximately 47.9 light years from Earth.The primary component, Psi Serpentis A, is a yellow G-type main sequence dwarf with an apparent magnitude of +5.86... A |
47.9 | G5V | 5,636 | −0.03 | |||
| HD 84117 HD 84117 HD 84117 is a F-type main sequence star in the constellation of Hydra. It has an apparent visual magnitude of approximately 4.94.... |
48.5 | F8V | 6,167 | –0.03 | |||
| HD 4391 HD 4391 HD 4391 is a star in the constellation Phoenix that is located at a distance of 48.7 light years from the Sun. It has a stellar classification of G3V, which is a... |
48.6 | G3V | 5,878 | –0.03 | |||
| 20 Leonis Minoris 20 Leonis Minoris 20 Leonis Minoris is a binary star system in the constellation Leo Minor. It has a relatively high proper motion. The companion is an old, active red dwarf star that has a relatively high metallicity. The two stars are currently separated by 14.5 arc seconds.... |
49.1 | G3 V | 5,741 | +0.20 | |||
| Nu Phoenicis Nu Phoenicis Nu Phoenicis is a main sequence dwarf star in the constellation Phoenix. It is similar to the Sun, although somewhat more massive and luminous... |
49.3 | F8V | 6,140 | +0.18 | |||
| 51 Pegasi 51 Pegasi 51 Pegasi is a Sun-like star located 15.6 parsecs from Earth in the constellation Pegasus... |
50.9 | G2.5IVa | 5,804 | +0.20 | |||
Solar twin
These stars are more similar to the Sun still, having the following qualities:- Temperature within 50 K Solar (roughly 5720 to 5830 K)
- Metallicity of 89—112% (± 0.05 dexDexDex may refer to:* DEX is a graph database.* In astronomy, a contraction of decimal exponent, for use with values on a log10 scale* Acronym for Dexterity in role playing games...
) Solar, meaning the star's proplyd would have had almost exactly the same amount of dust for planetary formation - No stellar companion, because the Sun itself is solitary
- An age within 1 billion years Solar (roughly 3.5 to 5.6 Ga)
The following are the known stars that come closest to satisfying the criteria for a solar twin. (The Sun is listed for comparison.)
| Identifier | Coordinates | Distance (ly) |
Stellar Class Stellar classification In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure... |
Temperature (K) |
Metallicity (dex) |
Age (Gyr) |
Notes | |
|---|---|---|---|---|---|---|---|---|
| Right ascension Right ascension Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:... |
Declination Declination In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and... |
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| Sun Sun The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields... |
— | — | 0.00 | G2V | 5,778 | +0.00 | 4.6 | |
| 18 Scorpii 18 Scorpii 18 Scorpii is a star located some 45.3 light years from Earth at the northern edge of the Scorpius constellation.18 Scorpii has many physical properties in common with the Sun. Cayrel de Strobel included it in her review of the stars most similar to the Sun, and Porto de Mello & da Silva ... |
45.1 | G2Va | 5,835 | +0.04 | 4.2 | |||
| HD 44594 HD 44594 HD 44594 is a star in the constellationPuppis that is located about 84 light years fromSun. It has a stellar classification of G3V,making it a G-type main sequence star. In the... |
84 | G3V | 5,840 | +0.15 | 4.1 | |||
| HD 195034 HD 195034 HD 195034 is a G-type star in the constellation of Vulpecula. It has an apparent visual magnitude of approximately 7.088. It has similar mass, metallicity and temperature to the Sun and was called a solar twin in a 2009 study, although its lithium abundance is three to four times that of the... |
92 | G5 | 5,760 | -0.04 | 5.1 | |||
| HD 138573 HD 138573 HD 138573 is a G-type subgiant star in the constellation of Serpens. It has an apparent visual magnitude of approximately 7.206. Although it is a solar-type star, similar to the Sun, a 2005 analysis rejected this star as a solar twin owing to its lower mass, lower metallicity, and older age.... |
101 | G5IV-V | 5,710 | –0.03 | 7.8 | |||
| HD 142093 HD 142093 HD 142093 is a G-type main sequence star in the constellation of Serpens. It has an apparent visual magnitude of approximately 7.33. Although it is a solar-type star, similar to the Sun, a 2005 analysis rejected this star as a solar twin owing to its lower mass and metallicity. It was also... |
103 | G2V | 5,841 | –0.15 | 5.0 | |||
| HD 98618 HD 98618 HD 98618 is a star 126 light years from Earth that is almost identical in most respects to the sun; it has therefore been proposed as a candidate solar twin. However, like the solar twin 18 Scorpii, HD 98618 has a lithium abundance significantly higher than that of the Sun... |
126 | G5V | 5,851 | +0.03 | 4.7 | |||
| HD 143436 | 141 | G0 | 5,768 | +0.00 | 3.8 | |||
| HD 129357 HD 129357 HD 129357 is a star in the constellation Boötes that is located about 154 light years from the Sun. The measured properties of this star are very similar to those of the Sun, making it a candidate solar twin. However, it has a lower abundance of lithium than the Sun and appears over 3 billion years... |
154 | G2V | 5,749 | –0.02 | 8.2 | |||
| HD 133600 HD 133600 HD 133600 is a G-type star in the constellation of Virgo. It has an apparent visual magnitude of approximately 8.219. It is very similar to the Sun and has been called a solar twin.... |
171 | G0 | 5,808 | +0.02 | 6.3 | |||
| HD 101364 HIP 56948 HIP 56948 is a solar twin star of type G5V, which as of March 2011, is the most sun-like star yet discovered in terms of size, mass, temperature, chemical makeup and, arguably, age... |
217 | G5V | 5,783 | +0.01 | 5.4 | |||
Some other stars are sometimes mentioned as promising solar twin candidates, particularly: Beta Canum Venaticorum (see references for Turnbull & Tarter), 37 Geminorum (see references for Turnbull & Tarter) and 16 Cygni B (Porto de Mello et al. 2000). However, all three have temperatures and/or luminosities that are too high for true solar twins. Furthermore, Beta Canum Venaticorum and 37 Geminorum have too low metallicities for solar twins. Finally, 16 Cygni B is part of a (very wide) binary system and is very old for a solar twin (at least 7 to 8 Gyr). Beta Canum Venaticorum is mentioned above as a nearby solar analog.
By potential habitability
Another way of defining solar twin is as a "habstar" — a star with qualities believed to be particularly hospitable to an Earth-like planet. Qualities considered include variability, mass, age, metallicity, and close companions.- At least 3 billion years old
- On the main sequence
- Non-variable
- Capable of harboring terrestrial planets
- Support a dynamically stable habitable zoneHabitable zoneIn astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...
The requirement that the star remain on the main sequence for at least 3 Ga sets an upper limit of approximately 1.5 solar masses, corresponding to a hottest spectral type of F5 V. Such stars can reach an absolute magnitude of 2.5, or 8.55 times as bright as the Sun, at the end of the main sequence.
Non-variability is ideally defined as variability of less than 1%, but 3% is the practical limit due to limits in available data. Variation in irradiance in a star's habitable zone due to a companion star with an eccentric orbit is also a concern.
Terrestrial planets in multiple star
Multiple star
A multiple star consists of three or more stars which appear from the Earth to be close to one another in the sky. This may result from the stars being physically close and gravitationally bound to each other, in which case it is physical, or this closeness may be merely apparent, in which case...
systems, those containing three or more stars, are not likely to have stable orbits in the long term. Stable orbits in binary systems take one of two forms: S-Type (satellite or circumstellar) orbits around one of the stars, and P-Type (planetary or circumbinary) orbits around the entire binary pair. Eccentric Jupiter
Eccentric Jupiter
An eccentric Jupiter is a Jovian planet that orbits its star in an eccentric orbit. Eccentric Jupiters may disqualify a planetary system from having Earth-like planets in it because a massive gas giant with an eccentric orbit may remove all Earth mass planets from the habitable zone.To date, it...
s may also disrupt the orbits of planets in habitable zones.
Metallicity of at least 40% solar ([Fe/H] = -0.4) is required for the formation of an Earth-like terrestrial planet. High metallicity strongly correlates to the formation of hot Jupiter
Hot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...
s, but these are not absolute bars to life, as some gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
s end up orbiting within the habitable zone themselves, and could potentially host Earth-like moons.
One example of such a star is HD 70642
HD 70642
HD 70642 is a yellow dwarf star in the constellation of Puppis located 93.8 light years away. This star has about the same mass and radius as the Sun, is slightly cooler and less luminous, and is richer in abundance of iron relative to hydrogen....
.
See also
- HabCatHabCatThe Catalog of Nearby Habitable Systems is a catalogue of star systems which conceivably have habitable planets. The list was developed by scientists Jill Tarter and Margaret Turnbull under the auspices of Project Phoenix, a part of SETI....
- List of nearest bright stars
- Planetary habitabilityPlanetary habitabilityPlanetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...
- Space colonizationSpace colonizationSpace colonization is the concept of permanent human habitation outside of Earth. Although hypothetical at the present time, there are many proposals and speculations about the first space colony...