Solar analog

Solar-type star, solar analogs (also analogues), and solar twins are 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.^[1] 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.^[1]

Solar-type

The Sun (left) compared to the similar but slightly smaller and less active Tau Ceti (right).

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.^[1] 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)^[2] 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, 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, based on current measurements.

Sample of solar-type stars

Identifier	J2000 Coordinates[3]		Distance[3] (ly)	Stellar Class[3]	Temperature (K)	Metallicity (dex)	Notes
	Right ascension	Declination
Epsilon Eridani [4]	03^h 32^m 55.8^s	−09° 27′ 29.7″	10.5	K2V	5,153	−0.11	[5]
Tau Ceti [6]	01^h 44^m 04.1^s	−15° 56′ 15″	11.9	G8V	5,344	–0.52	[7]
40 Eridani [8] A	04^h 15^m 16.3^s	−07° 39′ 10″	16.5	K1V	5,126	–0.31	[7]
82 Eridani [9]	03^h 19^m 55.7^s	−43° 04′ 11.2″	19.8	G8V	5,338	–0.54	[10]
Delta Pavonis [11]	20^h 08^m 43.6^s	−66° 10′ 55″	19.9	G8IV	5,604	+0.33	[5]
HR 7722 [12]	20^h 15^m 17.4^s	−27° 01′ 59″	28.8	K2V	5,166	−0.04	[5]
Gliese 86 [13] A	02^h 10^m 25.9^s	−50° 49′ 25″	35.2	G9V	5,163	−0.24	[7]
54 Piscium [14]	00^h 39^m 21.8^s	+21° 15′ 02″	36.1	K0V	5,129	+0.19	[10]
V538 Aurigae [15]	05^h 41^m 20.3^s	+53° 28′ 51.8″	39.9	K1V	3,500–5,000	−0.20	[10]
HD 14412 [16]	02^h 18^m 58.5^s	−25° 56′ 45″	41.3	G5V	5,432	−0.46	[10]
HR 4587 [17]	12^h 00^m 44.3^s	−10° 26′ 45.7″	42.1	G8IV	5,538	0.18	[10]
HD 172051 [18]	18^h 38^m 53.4^s	−21° 03′ 07″	42.7	G5V	5,610	−0.32	[10]
72 Herculis [19]	17^h 20^m 39.6^s	+32° 28′ 04″	46.9	G0V	5,662	−0.37	[10]
HD 196761 [20]	20^h 40^m 11.8^s	−23° 46′ 26″	46.9	G8V	5,415	−0.31	[5]
Nu² Lupi [21]	15^h 21^m 48.1^s	−48° 19′ 03″	47.5	G4V	5,664	−0.34	[5]

Solar analog

These stars are photometrically similar to the Sun, having the following qualities:^[1]

• Temperature within 500 K from that of the Sun (roughly 5200 to 6300 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:

Identifier	J2000 Coordinates[3]		Distance[3] (ly)	Stellar Class[3]	Temperature (K)	Metallicity (dex)	Notes
	Right ascension	Declination
Alpha Centauri [22] A	14^h 39^m 36.5^s	−60° 50′ 02″	4.37	G2V	5,847	+0.24	[23]
Alpha Centauri [24] B	14^h 39^m 35.0^s	−60° 50′ 14″	4.37	K1V	5,316	+0.25	[23]
70 Ophiuchi [25] A	18^h 05^m 27.3^s	+02° 30′ 00″	16.6	K0V	5,314	−0.02	[26]
Sigma Draconis [27]	19^h 32^m 21.6^s	+69° 39′ 40″	18.8	K0V	5,297	−0.20	[28]
Eta Cassiopeiae [29] A	00^h 49^m 06.3^s	+57° 48′ 55″	19.4	G0V	5,941	−0.17	[30]
107 Piscium [31]	01^h 42^m 29.8^s	+20° 16′ 07″	24.4	K1V	5,242	−0.04	[10][32]
Beta Canum Venaticorum [33]	12^h 33^m 44.5^s	+41° 21′ 27″	27.4	G0V	5,930	−0.30	[10]
61 Virginis [34]	13^h 18^m 24.3^s	−18° 18′ 40″	27.8	G5V	5,558	−0.02	[5]
Zeta Tucanae [35]	00^h 20^m 04.3^s	–64° 52′ 29″	28.0	F9.5V	5,956	−0.14	[7]
Chi¹ Orionis [36] A	05^h 54^m 23.0^s	+20° 16′ 34″	28.3	G0V	5,902	−0.16	[10]
Beta Comae Berenices [37]	13^h 11^m 52.4^s	+27° 52′ 41″	29.8	G0V	5,970	−0.06	[10]
HR 4523 [38] A	11^h 46^m 31.1^s	–40° 30′ 01″	30.1	G5V	5,629	−0.29	[5]
61 Ursae Majoris [39]	11^h 41^m 03.0^s	+34° 12′ 06″	31.1	G8V	5,483	−0.12	[10]
HR 4458 [40] A	11^h 34^m 29.5^s	–32° 49′ 53″	31.1	K0V	5,629	−0.29	[5]
HR 511 [41]	01^h 47^m 44.8^s	+63° 51′ 09″	32.8	K0V	5,333	+0.05	[10]
Alpha Mensae [42]	06^h 10^m 14.5^s	–74° 45′ 11″	33.1	G5V	5,594	+0.10	[7]
Zeta^1 Reticuli [43]	03^h 17^m 46.2^s	−62° 34′ 31″	39.5	G3–5V	5,733	−0.22	[7]
Zeta^2 Reticuli [44]	03^h 18^m 12.8^s	−62° 30′ 23″	39.5	G2V	5,843	−0.23	[7]
55 Cancri [45]	08^h 52^m 35.81^s	+28° 19′ 51″	40.3	G8V	5,235	+0.25	[30]
HD 69830 [46]	08^h 18^m 23.9^s	−12° 37′ 56″	40.6	K0V	5,410	−0.03	[7]
HD 10307 [47]	01^h 41^m 47.1^s	+42° 36′ 48″	41.2	G1.5V	5,848	−0.05	[10]
HD 147513 [48]	16^h 24^m 01.3^s	−39° 11′ 35″	42.0	G1V	5,858	+0.03	[5]
58 Eridani [49]	04^h 47^m 36.3^s	−16° 56′ 04″	43.3	G3V	5,868	+0.02	[7]
Upsilon Andromedae A [50]	01^h 36^m 47.8^s	+41° 24′ 20″	44.0	F8V	6,212	+0.13	[7]
HD 211415 [51] A	22^h 18^m 15.6^s	–53° 37′ 37″	44.4	G1–3V	5,890	−0.17	[7]
47 Ursae Majoris [52]	10^h 59^m 28.0^s	+40° 25′ 49″	45.9	G1V	5,954	+0.06	[7]
Alpha Fornacis [53] A	03^h 12^m 04.3^s	−28° 59′ 21″	46.0	F6V	6,275	−0.19	[7]
Psi Serpentis [54] A	15^h 44^m 01.8^s	+02° 30′ 55″	47.9	G5V	5,636	−0.03	[10]
HD 84117 [55]	09^h 42^m 14.4^s	–23° 54′ 56″	48.5	F8V	6,167	−0.03	[7]
HD 4391 [56]	00^h 45^m 45.6^s	–47° 33′ 07″	48.6	G3V	5,878	−0.03	[7]
20 Leonis Minoris [57]	10^h 01^m 00.7^s	+31° 55′ 25″	49.1	G3V	5,741	+0.20	[10]
Nu Phoenicis [58]	01^h 15^m 11.1^s	–45° 31′ 54″	49.3	F8V	6,140	+0.18	[7]
51 Pegasi [59]	22^h 57^m 28.0^s	+20° 46′ 08″	50.9	G2.5IVa	5,804	+0.20	[7]

Solar twin

These stars are more similar to the Sun still, having the following qualities:^[1]

• Temperature within 50 K from that of the Sun (roughly 5720 to 5830 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 solitary
• An age within 1 billion years from that of the Sun (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	J2000 Coordinates[3]		Distance[3] (ly)	Stellar Class[3]	Temperature (K)	Metallicity (dex)	Age (Gyr)	Notes
	Right ascension	Declination
Sun	—	—	0.00	G2V	5,778	+0.00	4.6	[60]
18 Scorpii [61]	16^h 15^m 37.3^s	–08° 22′ 06″	45.1	G2Va	5,790	−0.03	5.0	[62][63]
HD 9986 [64]	01^h 37^m 40.9^s	+12° 04′ 42″	84	G2V	5,785	+0.09	3.4	[63]
HD 150248 [65]	16^h 41^m 49.8^s	–45° 22′ 07″	88	G2	5,750	−0.04	6.2	[63]
HD 164595 [66]	18^h 00^m 38.9^s	+29° 34′ 19″	91	G2	5,810	−0.04	4.5	[63]
HD 195034 [67]	20^h 28^m 11.8^s	+22° 07′ 44″	92	G5	5,760	−0.04	2.9	[68]
HD 117939 [69]	13^h 34^m 32.6^s	–38° 54′ 26″	98	G3	5,730	−0.10	6.1	[63]
HD 138573 [70]	15^h 32^m 43.7^s	+10° 58′ 06″	101	G5IV-V	5,760	+0.00	5.6	[71][63]
HD 71334 [72]	08^h 25^m 49.5^s	−29° 55′ 50″	124	G2	5,770	−0.06	5.1	[63]
HD 98649 [73]	11^h 20^m 51.769^s	–23° 13′ 02″	137	G3	5,770	−0.02	4.7	[63]
HD 134664 [74]	15^h 12^m 10.4^s	–30° 53′ 11″	140	G3	5,810	+0.13	2.6	[63]
HD 143436 [75]	16^h 00^m 18.8^s	+00° 08′ 13″	141	G0	5,768	+0.00	3.8	[71]
HD 129357 [76]	14^h 41^m 22.4^s	+29° 03′ 32″	154	G2V	5,749	−0.02	8.2	[71]
HD 118598 [77]	13^h 38^m 13.7^s	−23° 41′ 20″	160	G2	5,800	+0.02	4.3	[63]
HD 133600 [78]	15^h 05^m 13.2^s	+06° 17′ 24″	171	G0	5,808	+0.02	6.3	[62]
HD 115382 [79]	13^h 16^m 48.4^s	+12° 24′ 56″	176	G1	5,780	−0.08	6.1	[63]
HIP 11915 [80]	02^h 33^m 49.02^s	−19° 36′ 42.5″	190	G5V	5,760	-0.059	4.0
HD 101364 [81]	11^h 40^m 28.5^s	+69° 00′ 31″	208	G5V	5,795	+0.02	3.5	[62][82]
BD +15 3364 [83]	18^h 07^m 18.6^s	+15° 56′ 46″	209	G2	5,785	0.07	3.8	[63]
HD 197027 [84]	20^h 41^m 54.6^s	–27° 12′ 57″	250	G3V	5,723	−0.013	8.2	[85]
YBP 1194 [86]	08^h 51^m 00.8^s	+11° 48′ 53″	2934	G5V	5,780	+0.023	4.2	[87]

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.^[88]

• At least 3 billion years old
• On the main sequence
• Non-variable
• Capable of harboring terrestrial planets
• Support a dynamically stable habitable zone

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.^[88]

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.^[88]

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.^[88]

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.^[88]

One example of such a star is HD 70642 ^[89].^[90]

See also

• Catalog of Nearby Habitable Systems (HabCat)
• List of nearest bright stars
• List of nearest stars
• Planetary habitability
• Space colonization

References

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34. 61 Virginis at SIMBAD - Ids - Bibliography - Image.
35. Zeta Tucanae at SIMBAD - Ids - Bibliography - Image.
36. Chi1 Orionis at SIMBAD - Ids - Bibliography - Image.
37. Beta Comae Berenices at SIMBAD - Ids - Bibliography - Image.
38. HR 4523 at SIMBAD - Ids - Bibliography - Image.
39. 61 Ursae Majoris at SIMBAD - Ids - Bibliography - Image.
40. HR 4458 at SIMBAD - Ids - Bibliography - Image.
41. HR 511 at SIMBAD - Ids - Bibliography - Image.
42. Alpha Mensae at SIMBAD - Ids - Bibliography - Image.
43. Zeta1 Reticuli at SIMBAD - Ids - Bibliography - Image.
44. Zeta1 Reticuli at SIMBAD - Ids - Bibliography - Image.
45. 55 Cancri at SIMBAD - Ids - Bibliography - Image.
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47. HD 10307 at SIMBAD - Ids - Bibliography - Image.
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49. 58 Eridani at SIMBAD - Ids - Bibliography - Image.
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53. Alpha Fornacis at SIMBAD - Ids - Bibliography - Image.
54. Psi Serpentis at SIMBAD - Ids - Bibliography - Image.
55. HD 84117 at SIMBAD - Ids - Bibliography - Image.
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72. HD 71334 at SIMBAD - Ids - Bibliography - Image.
73. HD 98649 at SIMBAD - Ids - Bibliography - Image.
74. HD 134664 at SIMBAD - Ids - Bibliography - Image.
75. HD 143436 at SIMBAD - Ids - Bibliography - Image.
76. HD 129357 at SIMBAD - Ids - Bibliography - Image.
77. HD 118598 at SIMBAD - Ids - Bibliography - Image.
78. HD 133600 at SIMBAD - Ids - Bibliography - Image.
79. HD 115382 at SIMBAD - Ids - Bibliography - Image.
80. HIP 11915 at SIMBAD - Ids - Bibliography - Image.
81. HIP 56948 at SIMBAD - Ids - Bibliography - Image.
82. Vázquez, M.; Pallé, E.; Rodríguez, P. Montañés (2010). "Is Our Environment Special?". The Earth as a Distant Planet: A Rosetta Stone for the Search of Earth-Like Worlds. Astronomy and Astrophysics Library. Springer New York. pp. 391–418. doi:10.1007/978-1-4419-1684-6. ISBN 978-1-4419-1683-9. See table 9.1.
83. BD +15 3364 at SIMBAD - Ids - Bibliography - Image.
84. HIP 102152 at SIMBAD - Ids - Bibliography - Image.
85. Monroe, T. R.;; et al. (2013). "High Precision Abundances of the Old Solar Twin HIP 102152: Insights on Li Depletion from the Oldest Sun". The Astrophysical Journal Letters 774 (2): 22. arXiv:1308.5744. Bibcode:2013ApJL..774..L32. doi:10.1088/2041-8205/774/2/L32.
86. Cl* NGC 2682 YBP 1194 at SIMBAD - Ids - Bibliography - Image.
87. A. Önehag; A. Korn; B. Gustafsson; E. Stempels; D. A. VandenBerg (2011). "M67-1194, an unusually Sun-like solar twin in M67". Astronomy and Astrophysics 528: A85. arXiv:1009.4579. Bibcode:2011A&A...528A..85O. doi:10.1051/0004-6361/201015138.
88. Turnbull, M. C.; Tarter, J. C. (2002). "Target Selection for SETI. I. A Catalog of Nearby Habitable Stellar Systems". The Astrophysical Journal Supplement Series 145: 181. arXiv:astro-ph/0210675. Bibcode:2003ApJS..145..181T. doi:10.1086/345779.
89. HD 70642 at SIMBAD - Ids - Bibliography - Image.
90. "Solar System 'twin' found". BBC News. 2003-07-03.

Further reading

• G. W. Lockwood & B. A. Skiff; Skiff; Radick (1997). "The Photometric Variability of Sun-like Stars: Observations and Results, 1984—1995". The Astrophysical Journal 485 (2): 789–811. Bibcode:1997ApJ...485..789L. doi:10.1086/304453.
• G. Porto de Mello, R. da Silva, & L. da Silva; Da Silva; Da Silva (2000). "A Survey of Solar Twin Stars within 50 Parsecs of the Sun". Bioastronomy 99: A New Era in the Search for Life 213: 73. Bibcode:2000ASPC..213...73P.
• M. C. Turnbull & J. C. Tarter; Tarter (2003). "Target Selection for SETI. II. Tycho-2 Dwarfs, Old Open Clusters, and the Nearest 100 Stars". The Astrophysical Journal Supplement Series 149 (2): 423–436. Bibcode:2003ApJS..149..423T. doi:10.1086/379320.
• J. C. Hall & G. W. Lockwood; Lockwood (2004). "The Chromospheric Activity and Variability of Cycling and Flat Activity Solar-Analog Stars". The Astrophysical Journal 614 (2): 942–946. Bibcode:2004ApJ...614..942H. doi:10.1086/423926.
• J. D. Jr do Nascimento, M. Castro, J. Meléndez, M. Bazot, S. Théado, G. F. Porto de Mello; Castro; Meléndez; Bazot; Théado; Porto De Mello; De Medeiros (2009). "Age and mass of solar twins constrained by lithium abundance". Astronomy and Astrophysics 501 (1): 687–694. arXiv:0904.3580. Bibcode:2009A&A...501..687D. doi:10.1051/0004-6361/200911935.