82 G. Eridani
A star chart of the Eridanus constellation showing the position of 82 G. Eridani (circled)
Epoch J2000 Equinox J2000
|Right ascension||03h 19m 55.65093s|
|Declination||−43° 04′ 11.2175″|
|Apparent magnitude (V)||4.254|
|Spectral type||G8 V|
|U−B color index||+0.21|
|B−V color index||+0.71|
|Radial velocity (Rv)||+87.2 km/s|
|Proper motion (μ)||RA: 3038.34 mas/yr
Dec.: 726.58 mas/yr
|Parallax (π)||165.47 ± 0.19 mas|
|Distance||19.71 ± 0.02 ly
(6.043 ± 0.007 pc)
|Absolute magnitude (MV)||5.34|
|Surface gravity (log g)||4.40|
|Metallicity [Fe/H]||–0.40 dex|
|Rotational velocity (v sin i)||4.0 km/s|
In the southern-sky catalog Uranometria Argentina, 82 G. Eridani (sometimes abbreviated to "82 Eridani") is the 82nd star listed in the constellation Eridanus. The Argentina catalog, compiled by the 19th-century astronomer Benjamin Gould, is a southern celestial hemisphere analog of the more famous Flamsteed catalog, and uses a similar numbering scheme. 82 G. Eridani, like other stars near the Sun, has held on to its Gould designation, even while other more distant stars have not.
This star is slightly smaller and less massive than the Sun, making it marginally dimmer than the Sun in terms of luminosity; about 20% more luminous than Tau Ceti or Alpha Centauri B. The projected equatorial rotation rate (v sin i) is 4.0 km/s, compared to 2 km/s for the Sun.
82 G. Eridani is a high-velocity star—it is moving quickly compared to the average—and hence is probably a member of Population II, generally older stars whose motions take them well outside the plane of the Milky Way. Like many other Population II stars, 82 G. Eridani is somewhat metal-deficient (though much less deficient than many), and is older than the Sun. It has a relatively high orbital eccentricity of 0.40 about the galaxy, ranging between 4.6 and 10.8 kiloparsecs from the core. Estimates of the age of this star ranged from 6 to 12 billion years.
This star is located in a region of low-density interstellar matter (ISM), so it is believed to have a large astropause that subtends an angle of 6″ across the sky. Relative to the Sun, this star is moving at a space velocity of 101 km/s, with the bow shock advancing at more than Mach 3 through the ISM.
Planetary system 
On August 17, 2011, European astronomers announced the discovery of three planets orbiting 82 G. Eridani. The mass range of these planets classifies them as super-Earths; objects with only a few times the Earth's mass. These planets were discovered by precise measurements of the radial velocity of the star, with the planets revealing their presence by their gravitational displacement of the star during each orbit. None of the planets display a significant orbital eccentricity. However, their orbital periods are all 90 days or less, indicating that they are orbiting close to the host star. The equilibrium temperature for the most distant planet, based on an assumed Bond albedo of 0.3, would be about 388 K (115 °C); significantly above the boiling point of water.
The number of planets in the system is slightly uncertain. At the time of planet c's detection, it exerted the lowest gravitational perturbation. There was also a similarity noted between its orbital period and the rotational period of the star. For these reasons the discovery team were somewhat more cautious regarding the verity of its candidate planet status than for the other two. Continued observation of the star will be required to determine the exact nature of the planetary system.
An infrared excess was discovered around the star by the Infrared Space Observatory at 60 μm, but was not later confirmed by the Spitzer Space Telescope, in 2006. However, in 2012, a dust disk was found around the star, by the Herschel Space Observatory. While not well-constrained, if assumed to have a similar composition to 61 Virginis' dust disk, it has a semi-major axis of 19 AU.
(in order from star)
|b||≥2.7 ± 0.3 M⊕||0.1207 ± 0.002||18.315 ± 0.008||0 (fixed)||—||—|
|c||≥2.4 ± 0.4 M⊕||0.2036 ± 0.0034||40.114 ± 0.053||0 (fixed)||—||—|
|d||≥4.8 ± 0.6 M⊕||0.3499 ± 0.0059||90.309 ± 0.184||0 (fixed)||—||—|
|Dust disk||~19 AU||—||—|
In his book Habitable Planets for Man, Stephen Dole gave 82 G. Eridani his highest estimate for habitability: 5.7%. Four other stars had this figure: Alpha Centauri B, 70 Ophiuchi A, Eta Cassiopeiae A, and Delta Pavonis. 82 G. Eridani (GJ 139) was also picked as a Tier 1 target star for NASA's proposed Space Interferometry Mission (SIM) mission to search for terrestrial-sized or larger planets.
See also 
- van Leeuwen, Floor (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752v1, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357 Note: see VizieR catalogue I/311.
- Holmberg, J.; Nordstrom, B.; Andersen, J. (July 2009), "The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics", Astronomy and Astrophysics 501 (3): 941−947, arXiv:0811.3982, Bibcode:2009A&A...501..941H, doi:10.1051/0004-6361/200811191
- Gray, R. O. et al. (July 2006), "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample", The Astronomical Journal 132 (1): 161–170, arXiv:astro-ph/0603770, Bibcode:2006AJ....132..161G, doi:10.1086/504637
- "LHS 19 -- High proper-motion Star", SIMBAD (Centre de Données astronomiques de Strasbourg), retrieved 2007-07-26
- Pepe, F. et al. (2011), The HARPS search for Earth-like planets in the habitable zone: I – Very low-mass planets around HD20794, HD85512 and HD192310, arXiv:1108.3447, Bibcode:2011yCat..35349058P, doi:10.1051/0004-6361/201117055
- Johnson, H. M.; Wright, C. D. (1983), "Predicted infrared brightness of stars within 25 parsecs of the sun", Astrophysical Journal Supplement Series 53: 643–711, Bibcode:1983ApJS...53..643J, doi:10.1086/190905 — See the table on p. 653.
- Porto de Mello, Gustavo; del Peloso, Eduardo F. (April 2006), "Astrobiologically Interesting Stars Within 10 Parsecs of the Sun", Astrobiology 6 (2): 308–331, arXiv:astro-ph/0511180, Bibcode:2006AsBio...6..308P, doi:10.1089/ast.2006.6.308, PMID 16689649
- Sousa, S. G. et al. (August 2007), "Spectroscopic parameters for 451 stars in the HARPS GTO planet search program. Stellar [Fe/H] and the frequency of exo-Neptunes", Astronomy and Astrophysics 487 (1): 373–381, arXiv:0805.4826, Bibcode:2008A&A...487..373S, doi:10.1051/0004-6361:200809698
- Schröder, C.; Reiners, A.; Schmitt, J. H. M. M. (January 2009), "Ca II HK emission in rapidly rotating stars. Evidence for an onset of the solar-type dynamo", Astronomy and Astrophysics 493 (3): 1099–1107, Bibcode:2009A&A...493.1099S, doi:10.1051/0004-6361:200810377
- Hillenbrand, Lynne A. (November 2008), "Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics", The Astrophysical Journal 687 (2): 1264–1293, arXiv:0807.1686, Bibcode:2008ApJ...687.1264M, doi:10.1086/591785
- Gould, Benjamin Apthorp (1879), Uranometria Argentina: brightness and position of every fixed star, down to the seventh magnitude, within one hundred degrees of the South Pole, Resultados, Universidad Nacional de Córdoba Observatorio Astronómico 1, Observatorio Nacional Argentino, pp. 159–160 Coordinates are for the 1875 equinox.
- Hearnshaw, J. B. (1973), "The iron abundance of 82 Eridani", Astronomy and Astrophysics 29: 165–170, Bibcode:1973A&A....29..165H
- Frisch, P. C. (1993), "G-star astropauses - A test for interstellar pressure", Astrophysical Journal 407 (1): 198–206, Bibcode:1993ApJ...407..198F, doi:10.1086/172505
- Decin, G. et al. (May 2000). "The Vega phenomenon around G dwarfs". Astronomy and Astrophysics 357: 533–542. Bibcode:2000A&A...357..533D.
- Wyatt, M. C. et al. (2012). "Herschel imaging of 61 Vir: implications for the prevalence of debris in low-mass planetary systems". Monthly Notices of the Royal Astronomical Society. arXiv:1206.2370. Bibcode:2012MNRAS.tmp.3237W. doi:10.1111/j.1365-2966.2012.21298.x.
- Schneider, Jean, "Star: HD 20794", Extrasolar Planets Encyclopaedia (Paris Observatory), retrieved 2011-12-05
- Dole, Stephen H. (1970), Habitable Planets for Man (2nd ed.), London: American Elsevier Pub. Co, ISBN 0-444-00092-5
- McCarthy, Chris (2005). "SIM Planet Search Tier 1 Target Stars". San Francisco State University. Archived from the original on 10 August 2007. Retrieved 2007-07-26.
- "82 Eridani". SolStation. Retrieved 2005-11-03.