Solar Twins
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Solar-type star, solar analogs (also analogues), and solar twins are stars that are particularly similar to 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 ...
. The
stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction grati ...
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—such as temperature, which is derived from the color index—against the Sun, the only star whose temperature is confidently known. For stars that are not similar to the Sun, this cross-checking cannot be done.


Solar-type

These stars are broadly similar to the Sun. They are
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 Hert ...
stars with a
B−V color In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. The lower the color index, the more blue (or hotter) the object is. Conversely, the large ...
between 0.48 and 0.80, the Sun having a B−V color of 0.65. Alternatively, a definition based on
spectral type In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction grati ...
can be used, such as F8V through K2V, 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) Because 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 52 light-years (16 parsecs) of the Sun based on their chromospheric activity (as measured via Ca, H, and 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 (B−V color between 0.48 and 0.80), based on current measurements (the Sun is listed for comparison):


Solar analog

These stars are photometrically similar to the Sun, having the following qualities: * Temperature within 500 K from that of the Sun (5278 to 6278 K) *
Metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...
of 50–200% (± 0.3 dex) of that of the Sun, meaning the star's
protoplanetary disk A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, be ...
would have had similar amounts of dust from which planets could form * No close companion (orbital period of ten days or less), because 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 Distance is a numerical or occasionally qualitative measurement of how far apart objects or points are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two counties over"). ...
(The Sun is listed for comparison.):


Solar twin

To date no solar twin that exactly matches the Sun has been found. However, there are some stars that come very close to being identical to that of the Sun, and are such considered solar twins by members of the astronomical community. An exact solar twin would be a G2V star with a 5,778K surface temperature, be 4.6 billion years old, with the correct metallicity and a 0.1%
solar luminosity The solar luminosity (), is a unit of radiant flux (power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxies and other celestial objects in terms of the output of the Sun. One nominal s ...
variation. Stars with an age of 4.6 billion years are at the most stable state. Proper metallicity and size are also very important to low luminosity variation.University of Nebraska-Lincoln astronomy education group, Stellar Luminosity Calculator
/ref>National Center for Atmospheric Research, The Effects of Solar Variability on Earth's Climate, 2012 Report
/ref> The stars below are more similar to the Sun and having the following qualities: * Temperature within 50 K from that of the Sun (5728 to 5828 K) (within 10 K of sun (5768–5788 K)). * Metallicity of 89–112% (± 0.05 dex) of that of the Sun, meaning the star's
proplyd A proplyd, short for ionized protoplanetary disk, is an externally illuminated photoevaporating protoplanetary disk around a young star. Nearly 180 proplyds have been discovered in the Orion Nebula. Images of proplyds in other star-forming regio ...
would have had almost exactly the same amount of dust for planetary formation * No stellar companion, because the Sun itself is a solitary star * An age within 1 billion years from that of the Sun (3.6 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. Highlighted boxes are out of range for a solar twin. The star may have been noted as solar twin in the past, but are more of a solar analog. Some other stars are sometimes mentioned as solar-twin candidates such as:
Beta Canum Venaticorum Beta Canum Venaticorum (β Canum Venaticorum, abbreviated Beta CVn, β CVn), also named Chara , is a G-type main-sequence star in the northern constellation of Canes Venatici. At an apparent visual magnitude of 4.25, it is the second-brigh ...
; however it has too low metallicities (−0.21) for solar twin.
16 Cygni 16 Cygni or 16 Cyg is the Flamsteed designation of a triple star system approximately 69 light-years away from Earth in the constellation of Cygnus. It consists of two Sun-like yellow dwarf stars, 16 Cygni A and 16 Cygni B, together wit ...
B is sometimes noted as twin, but is part of a triple star system and is very old for a solar twin at 6.8 Ga. Two solar sibling candidates (similar age, metallicity, and kinematics) are ''Gaia'' DR2 1927143514955658880 and 1966383465746413568.


By potential habitability

Another way of defining solar twin is as a "habstar"—a star with qualities believed to be particularly hospitable to a life-hosting planet. Qualities considered include variability, mass, age, metallicity, and close companions. * At least 0.5–1 billion years old * On the main sequence * Non-variable * Capable of harboring terrestrial planets * Support a dynamically stable
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 ...
* 0–1 non-wide stellar companion stars. The requirement that the star remain on the main sequence for at least 0.5–1 Ga sets an upper limit of approximately 2.2–3.4 solar masses, corresponding to a hottest spectral type of A0- B7V. Such stars can be 100x as bright as the Sun.
Tardigrade Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbär ...
-like life (due to the UV flux) could potentially survive on planets orbiting stars as hot as B1V, with a mass of 10 M☉, and a temperature of 25,000 K, a main-sequence lifetime of about 20 million years. 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 A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a ''star cluster'' or ''galaxy'', although, broadly speaking, ...
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 An eccentric Jupiter is a Jovian planet that orbits its star in an eccentric orbit. Note: this study treats eccentric Jupiters as giant planets having an orbital eccentricity of 0.1 or greater. Eccentric Jupiters may probably disqualify a planetar ...
s may also disrupt the orbits of planets in habitable zones. Metallicity of at least 40% solar ( e/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 Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (). The close proximity to their stars and high surface-atmosphere temp ...
s, but these are not absolute bars to life, as some
gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "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 , a G5V, at temperature of 5533 K, but is much younger than the Sun, at 1.9 billion years old. Another such example would be
HIP 11915 HIP 11915 is a G-type main-sequence star located about 190 light-years from Earth in the constellation Cetus. It is best known for its characteristics, which are very similar to those of the Sun, including the mass, radius, temperature, ...
, which has a planetary system containing a Jupiter-like planet orbiting at a similar distance that the planet
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
does in the Solar System. To strengthen the similarities, the star is class G5V, has a temperature of 5750 K, has a Sun-like mass and radius, and is only 500 million years younger than the Sun. As such, the habitable zone would extend in the same area as the zone in the Solar System, around 1 AU. This would allow an Earth-like planet to exist around 1 AU.


See also

*
Catalog of Nearby Habitable Systems The Catalog of Nearby Habitable Systems (HabCat) 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 SET ...
(HabCat) *
List of nearest bright stars This list of nearest bright stars is a table of stars found within 15 parsecs (48.9 light-years) of the nearest star, the Sun, that have an absolute magnitude of +8.5 or brighter, which is approximately comparable to a listing of stars more lumi ...
*
List of nearest stars and brown dwarfs This list covers all known stars, brown dwarfs, and sub-brown dwarfs within of the Sun. So far, 131 such objects have been found, of which only 22 are bright enough to be visible without a telescope. The visible light needs to reach or exce ...
*
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 ...
**
G-type main-sequence star A G-type main-sequence star (Spectral type: G-V), also often, and imprecisely called a yellow dwarf, or G star, is a main-sequence star (luminosity class V) of spectral type G. Such a star has about 0.9 to 1.1 solar masses and an effective temp ...
*
Planetary habitability 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 ...
*
Space colonization Space colonization (also called space settlement or extraterrestrial colonization) is the use of outer space or celestial bodies other than Earth for permanent habitation or as extraterrestrial territory. The inhabitation and territori ...


Footnotes


References


Further reading

* * * * * {{cite journal , first1=Jose Dias , last1=do Nascimento Jr. , first2=Matthieu Sebastien , last2=Castro , first3=Jorge , last3=Meléndez , first4=Michaël , last4=Bazot , first5=Sylvie , last5=Théado , first6=Gustavo Frederico , last6=Porto de Mello , first7=José Renan , last7=De Medeiros , title=Age and mass of solar twins constrained by lithium abundance , journal=Astronomy and Astrophysics , year=2009 , volume=501 , issue=1 , pages=687–694 , doi=10.1051/0004-6361/200911935 , bibcode=2009A&A...501..687D , arxiv = 0904.3580 , s2cid=9565600