solar analog

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.{{citation needed|date=July 2022}}

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.

These stars are broadly similar to the Sun. They are main-sequence 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 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.{{Cite web|url=http://www.atlasoftheuniverse.com/startype.html|title=The Classification of Stars|website=www.atlasoftheuniverse.com}}

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){{Cite journal|url=https://ntrs.nasa.gov/search.jsp?R=19950048251|title=Chromospheric variations in main-sequence stars|first1=S. L.|last1=Baliunas|first2=R. A.|last2=Donahue|first3=J. H.|last3=Soon|first4=J. H.|last4=Horne|first5=J.|last5=Frazer|first6=L.|last6=Woodard-Eklund|first7=M.|last7=Bradford|first8=L. M.|last8=Rao|first9=O. C.|last9=Wilson|first10=Q.|last10=Zhang|date=January 1, 1995|journal=Astrophysical Journal, Part 1|volume=438|issue=1|via=ntrs.nasa.gov}} 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).{{citation needed|date=July 2022}}

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):

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 of 50–200% (± 0.3 dex) of that of the Sun, meaning the star's protoplanetary disk 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 (The Sun is listed for comparison.):

To date no solar twin that exactly matches the Sun has been found.{{Cite web|url=https://science.nasa.gov/science-research/planetary-science/08jan_sunclimate/|title=Solar Variability and Terrestrial Climate - NASA Science|website=science.nasa.gov}} However, there are some stars that come very close to being identical to 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 5,778 K surface temperature, be 4.6 billion years old, with the correct metallicity and a 0.1% solar luminosity variation. Stars with an age of 4.6 billion years are at the most stable state. Proper metallicity, radius, chemical composition, rotation, magnetic activity, and size are also very important to low luminosity variation.{{Cite journal|url=https://www.aanda.org/articles/aa/abs/2016/03/aa27472-15/aa27472-15.html|title=Solar twins in the ELODIE archive|first1=D.|last1=Mahdi|first2=C.|last2=Soubiran|first3=S.|last3=Blanco-Cuaresma|first4=L.|last4=Chemin|date=March 1, 2016|journal=Astronomy & Astrophysics|volume=587|pages=A131|via=www.aanda.org|doi=10.1051/0004-6361/201527472|arxiv=1601.01599|bibcode=2016A&A...587A.131M }}{{Cite web|url=http://astro.unl.edu/classaction/animations/stellarprops/stellarlum.html|title=Stellar Luminosity Calculator|website=astro.unl.edu}}{{Cite book|url=http://www.nap.edu/catalog/13519|title=The Effects of Solar Variability on Earth's Climate: A Workshop Report|date=November 12, 2012|publisher=National Academies Press|doi=10.17226/13519 |isbn=978-0-309-26564-5 }}{{Cite web|url=https://scienceblogs.com/startswithabang/2013/06/05/most-of-earths-twins-arent-identical-or-even-close|title=Most of Earth's twins aren't identical, or even close! | ScienceBlogs|website=scienceblogs.com}}

File:Morgan-Keenan spectral classification.svg

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){{efn|A true solar twins as noted by the Lowell Observatory should have a temperature within ~10 K of the Sun. Space Telescope Science Institute, Lowell Observatory, noted in 1996 that temperature precision of ~10 K can be measured. A temperature of ~10 K reduces the solar twin list to near zero, so ±50 K is used for the chart.}} (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 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)

Other Sun parameters:{{Cite journal|url=https://pressbooks.online.ucf.edu/astronomybc/chapter/15-1-the-structure-and-composition-of-the-sun/|title=15.1 The Structure and Composition of the Sun|date=January 23, 2017|via=pressbooks.online.ucf.edu}}

• Sun rotates on its axis once in about 27 days or {{convert|1.997|km/s|mi/s}}
• Sun radius is {{convert|700,000|km|mi}}
• Sun chemical composition by mass: hydrogen (73.4%); helium (25%); carbon (0.2%); nitrogen (0.09%);oxygen (0.80%); neon (0.16%); magnesium (0.06%); silicon (0.09&); sulfur (0.05%); iron (0.003%).[http://aa.springer.de/papers/0363003/2300947.pdf Bjarne Rosenkilde Jørgensen, “The G Dwarf Problem: Analysis of a New Data Set,” Astronomy & Astrophysics 363, November 2000, 947]

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; however it has too low metallicities (−0.21) for solar twin. 16 Cygni 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.

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.{{efn|habstar or habitability, is currently defined as an area, such as a planet or a moon, where liquid water can exist for at least a short duration of time.{{Cite web|url=http://www.solstation.com/habitable.htm|title=Sol Company, solstation.com, Stars and Habitable Planets, 2012}}{{Cite web|url=https://www.britannica.com/science/habitable-zone|title=Habitable zone | Astrobiology, Exoplanets & Habitability | Britannica|website=www.britannica.com}}}}

• 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
• 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.{{Cite web|url=https://www.space.com/19141-alien-planets-two-stars-dangerous.html|title=Double-Star Systems Can Be Dangerous for Exoplanets|author1=Mike Wall|date=January 6, 2013|website=Space.com}} Tardigrade-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.{{Efn|The supergiant and following supernova & neutron star (due to >{{Solar mass|8}} mass) would likely destroy the life at the end of the B1V star's lifetime.}}

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 Jupiters 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 Jupiters, but these are not absolute bars to life, as some gas giants end up orbiting within the habitable zone themselves, and could potentially host Earth-like moons.

One example of such a star is {{CelestialRefAll|HD 70642}}, a G5V, at temperature of 5533 K, but is much younger than the Sun, at 1.9 billion years old.{{cite news| url=http://news.bbc.co.uk/1/hi/sci/tech/3041220.stm | work=BBC News | title=Solar System 'twin' found | date=2003-07-03}}

Another such example would be HIP 11915, which has a planetary system containing a Jupiter-like planet orbiting at a similar distance that the planet Jupiter does in the Solar System.{{cite web|title=Jupiter Twin Discovered Around Solar Twin |url=http://www.eso.org/public/news/eso1529/|website=eso.org/|access-date=16 July 2015}} 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.{{cite web|url=https://science.nasa.gov/science-news/science-at-nasa/2013/08jan_sunclimate/|title=Solar Variability and Terrestrial Climate – NASA Science|access-date=8 January 2013}}

• Catalog of Nearby Habitable Systems (HabCat)
• G-dwarf problem
• List of nearest bright stars
• List of nearest stars and brown dwarfs
• Main sequence
• G-type main-sequence star
• Habitability of G-type main-sequence stars
• Planetary habitability
• Space colonization

{{notelist}}

{{Reflist|colwidth=30em|refs=

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{{cite journal | author=Boyajian, Tabetha S. | title=Angular Diameters of the G Subdwarf µ Cassiopeiae A and the K Dwarfs s Draconis and HR 511 from Interferometric Measurements with the CHARA Array | journal=The Astrophysical Journal | volume=683 | issue=1 | pages=424–432 |date=August 2008 | doi=10.1086/589554 | bibcode=2008ApJ...683..424B | display-authors=1 | last2=McAlister | first2=Harold A. | last3=Baines | first3=Ellyn K. | last4=Gies | first4=Douglas R. | last5=Henry | first5=Todd | last6=Jao | first6=Wei-Chun | last7=o’Brien | first7=David | last8=Raghavan | first8=Deepak | last9=Touhami | first9=Yamina | last10=Ten Brummelaar |arxiv = 0804.2719 | first10=Theo A. | last11=Farrington | first11=Chris | last12=Goldfinger | first12=P. J. | last13=Sturmann | first13=Laszlo | last14=Sturmann | first14=Judit | last15=Turner | first15=Nils H. | last16=Ridgway | first16=Stephen | s2cid=8886682 }}

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{{cite journal |author1=A. Önehag |author2=A. Korn |author3=B. Gustafsson |author4=E. Stempels |author5=D. A. VandenBerg | title=M67-1194, an unusually Sun-like solar twin in M67 | year=2011 | doi=10.1051/0004-6361/201015138 | journal=Astronomy and Astrophysics | volume=528 | pages=A85 | arxiv=1009.4579 | bibcode=2011A&A...528A..85O |s2cid=119116626 }}

{{cite journal | title=A photometric and spectroscopic survey of solar twin stars within 50 parsecs of the Sun; I. Atmospheric parameters and color similarity to the Sun |author1=Porto de Mello, G. F. |author2=da Silva, R. |author3=da Silva, L. |author4=de Nader, R. V. |name-list-style=amp | journal=Astronomy and Astrophysics | volume=563 | page=A52 | date=March 2014 | doi=10.1051/0004-6361/201322277 | bibcode= 2014A&A...563A..52P|arxiv = 1312.7571 |s2cid=119111150 }}

{{cite journal |arxiv=1601.05054|doi=10.1051/0004-6361/201527478|title=Correlation between lithium abundances and ages of solar twin stars|journal=Astronomy & Astrophysics|volume=587|pages=A100|year=2016|last1=Carlos|first1=Marília|last2=Nissen|first2=Poul E.|last3=Meléndez|first3=Jorge|bibcode=2016A&A...587A.100C|s2cid=119268561}}

{{cite journal | title=Solar twins in the ELODIE archive |last1=Mahdi | first1=D. | last2=Soubiran | first2=C.| last3=Blanco-Cuaresma | first3=S. | last4=Chemin | first4=L. | journal=Astronomy & Astrophysics | display-authors=1 | volume=587 | id=A131 | pages=9 | date=March 2016 | doi=10.1051/0004-6361/201527472 | bibcode=2016A&A...587A.131M|arxiv=1601.01599 |s2cid=119205608 }}

{{cite journal | title=The AMBRE project: searching for the closest solar siblings |last1=Adibekyan | first1=V. | last2=de Laverny | first2=P.| last3=Recio-Blanco | first3=A. | last4=Sousa | first4=S. | last5=Sousa | first5=S. | last6=Delgado-Mena | first6=E. | last7=Kordopatis | first7=G. | last8=Ferreira | first8=A. | last9=Santos | first9=N. | last10=Hakobyan | first10=A. | last11=Tsantaki | first11=M. | journal=Astronomy & Astrophysics | display-authors=1 | volume=619 | id=A130 | pages=19 | date=November 2018 | doi= 10.1051/0004-6361/201834285 | bibcode=2016A&A...587A.131M|arxiv=1601.01599 |s2cid=119205608 }}

{{citation

| title=Solar twins in the ELODIE archive

| last1=Mahdi | first1=D. | last2=Soubiran | first2=C.

| last3=Blanco-Cuaresma | first3=S. | last4=Chemin | first4=L.

| journal=Astronomy & Astrophysics | display-authors=1

| volume=587 | id=A131 | pages=9 | date=March 2016

| doi=10.1051/0004-6361/201527472 | bibcode=2016A&A...587A.131M

| arxiv=1601.01599 | s2cid=119205608 | postscript=. }}

}}

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