82 G. Eridani

82 G. Eridani

A star chart of the Eridanus constellation showing the position of 82 G. Eridani (circled)
Observation data Epoch J2000      Equinox J2000
Constellation	Eridanus
Right ascension	03h 19m 55.65093s
Declination	−43° 04′ 11.2175″
Apparent magnitude (V)	4.254
Characteristics
Spectral type	G8 V
U−B color index	+0.21
B−V color index	+0.71
Astrometry
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
Details
Mass	0.70 M☉
Radius	0.92 R☉
Luminosity	0.74 L☉
Surface gravity (log g)	4.40 cgs
Temperature	5,401 K
Metallicity [Fe/H]	–0.40 dex
Rotational velocity (v sin i)	4.0 km/s
Age	6.1–12.7 Gyr
Other designations
e Eri, e Eridani, 82 G. Eridani, 82 G. Eri, HD 20794, CD-43°1028, GCTP 703, GJ 139, LHS 19, LTT 1583, HR 1008, SAO 216263, FK5 119, and HIP 15510.
Database references
SIMBAD	The star
	planet b
	planet c
	planet d
Exoplanet Archive	data
ARICNS	data
Extrasolar Planets Encyclopaedia	data

82 G. Eridani (HD 20794, HR 1008, e Eridani) is a star about 20 light years away from Earth in the constellation Eridanus. It is a main-sequence star with a stellar classification of G8.

Observation

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.

Properties

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.

The 82 G. Eridani planetary system

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (days)	Eccentricity	Inclination	Radius
Hot dust	≲0.1 AU				—	—
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–~30 AU				—	—

Habitability

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

• 82 G. Eridani in fiction
• Map analysis of the 1961 Zeta Reticuli Incident (a purported alien abduction)