82 Eridani

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NASA

82 Eridani is a yellow-orange star
like our Sun, Sol. (See a 2MASS
Survey image of 82 Eridani from
the NASA Star and Exoplanet
Database.)

System Summary

82 Eridani is located only about 19.8 light-years (ly) away in the southern part (03:19:55.65-43:04:11.22, ICRS 2000.0) of Constellation Eridanus, the River -- southeast of Acamar (Theta1 Eridani). Apparently, past spectroscopic analysis suggested that the star may have a dim companion in a close orbit, but no study presenting empirical data for evaluating that indication was cited (see "Remark: SB" at the Astronomiches Rechen-Institut at Heidelberg's ARICNS page for 82 Eridani). On August 17, 2011, astronomers associated with the High Accuracy Radial velocity Planet Searcher (HARPS) instrument at the European Southern Observatory (ESO) uploaded a pre-print that revealed the discovery of three hot super-Earths with at least 2.4 to 4.8 Earth-masses outside the potentially habitable zone orbit around this star (Pepe et al, 2011, Table 4 -- more below).

JPL, CalTech, NASA

Larger illustration

Astronomers have identified 82 Eridani
as a prime target for the Terrestrial Planet
Finder (TPF), and the Space Interferometry
Mission (SIM), now both indefinitely postponed.

Due to 82 Eridani's relative proximity and similarity of spectral type to Sol, the star has been an object of intense interest among astronomers. 82 Eridani became one of the top 100 target stars for NASA's proposed Terrestrial Planet Finder (TPF), but the project has been postponed indefinitely. It was also selected as a "Tier 1" target star for NASA's optical Space Interferometry Mission (SIM) to detect a planet as small as three Earth-masses within two AUs of its host star (and so some summary system information and images of 82 Eridani may still be available from the SIM Teams), but the SIM project manager announced on November 8, 2010 that the mission was indefinitely postponed due to withdrawal of NASA funding.

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Orbital
Distance
(a=AUs) Orbital
Period
(P=years) Orbital
Eccentricity
(e) Orbital
Inclination
(i=degrees)
Mass
(Earths)
Diameter
(Earths)
Density
(Earths) Surface
Gravity
(Earths)
Metallicity
(Solar)
82 Eridani 0.0 ... ... ... 230-280,000 81-98 ... ... 0.49-0.65
Planet b 0.12 0.05 0.? 90? =>2.7 >1 ... ... ...
Planet c 0.20 0.11 0.? 90? =>2.4 >1 ... ... ...
Planet d 0.25 0.25 0.? 90? =>4.8 >1 ... ... ...
Inner H.Z. Edge? 0.555 0.49 0.0 90? ... ... ... ... ...
Outer H.Z. Edge? 0.91 1.40 0.0 90? ... ... ... ... ...

	Orbital Distance (a=AUs)	Orbital Period (P=years)	Orbital Eccentricity (e)	Orbital Inclination (i=degrees)	Mass (Earths)	Diameter (Earths)	Density (Earths)	Surface Gravity (Earths)	Metallicity (Solar)
82 Eridani	0.0	...	...	...	230-280,000	81-98	...	...	0.49-0.65
Planet b	0.12	0.05	0.?	90?	=>2.7	>1	...	...	...
Planet c	0.20	0.11	0.?	90?	=>2.4	>1	...	...	...
Planet d	0.25	0.25	0.?	90?	=>4.8	>1	...	...	...
Inner H.Z. Edge?	0.555	0.49	0.0	90?	...	...	...	...	...
Outer H.Z. Edge?	0.91	1.40	0.0	90?	...	...	...	...	...

82 Eridani

This main sequence, yellow-orange dwarf of spectral and probable luminosity type G8 V, but classed as yellow as G5 (Pepe et al, 2011, Table 3; Perrin et al, 1977, page 782; Haisch and Basri, 1985, pages 180-181; and Olin Jeuck Eggen, 1956, page 463; and 1955, No. 703 in page 402), may have 70 to 97 percent of Sol's mass (Pepe et al, 2011, Table 3); SIM; and RECONS), 92 percent of its diameter (Johnson and Wright, 1983, page 653), around 61 percent of its visual luminosity and 65.6 +/- 0.3 percent of its bolometric luminosity (Pepe et al, 2011, Table 3). The star appears to be less enriched in elements heavier than hydrogen ("metals") because it appears to have around 39 to 65 percent of Sol's abundance of iron (Pepe et al, 2011, Table 3; NASA Star and Exoplanet Database; and Cayrel de Strobel et al, 1991, page 280).

Based on chromospheric activity and rotational period alone, 82 Eridani could be around 6.1 and 6.6 billion years old (Mamajek et al, 2008), but new calculations place its age around 5.76 +/- 0.66 billion years old with a rotational period of 33.19 +/- 3.61 days (Pepe et al, 2011, Table 3). The star, however, shows a radial velocity of about 52 miles per second (mps) -- about 84 km per second or kps -- in recession, but the true space velocity is about 80 mps (about 129 kps). Indeed, 82 Eridani's low metallicity and high space velocity, and its substantial galactic orbital eccentricity (e= 0.40) suggest that it may be more than 10 billion years old (see Table 1 for HD 20794 in Olin Jeuck Eggen, 1998 (1919-1998); and John B. Hearnshaw, 1973), although it does appear to have a slight overabundance of metals for its age. Since the star's orbit lies primarily within the galactic plane (as does Sol's own orbit), it is thought to be an old disk star.

82 Eridani was once thought to be unusually bright for its spectral type. The star was the brightest dwarf in a 1964 catalogue of high velocity stars with space motions exceeding 100 kps (62 mps) collected by Olin Jeuck Eggen (1919-1998). In the past, it was sometimes classified as a giant (1982 Bright Star Catalogue, 4th Edition; and Slee et al, 1989) or a subgiant star -- evolving off the main sequence as it begins to fuse the increasing amounts of helium "ash" mixed with hydrogen at its core. Moreover, the star has been called e Eridani, presumably because it was suspected of variability. Some useful catalogue numbers for this star are: e Eri, 82 Eri, HR 1008, Gl 139, Hip 15510, HD 20794, CD-43 1028, CP(D)-43 354, SAO 216263, FK5 119, LPM 147, LHS 19, LTT 1583, LFT 277, and GC 4000.

Habitable Zone

Calculations performed using one type of planetary climate model by the NASA Star and Exoplanet Database indicate that the inner edge of the 82 Eridani's habitable zone may be located around 0.555 AU from the star, with the outer edge around 1.109 AUs. Similarly, the distance from this star where a planet like Earth could have liquid water on its surface was been estimated to be around 0.784, or 0.80 AU according to calculations by another team of astronomers (SIM) -- midway between the orbital distances of Venus and Earth in the Solar System. At that distance from the star, such a planet would have an orbital period of about 265 days, or close to three fourths of an Earth year.

Planetary System

On August 17, 2011, astronomers associated with the High Accuracy Radial velocity Planet Searcher (HARPS) instrument at the European Southern Observatory (ESO) uploaded a pre-print that revealed the discovery of three hot super-Earths. The three possibly rocky planets are estimated to have at least 2.4 to 2.7 Earth-masses outside the potentially habitable zone orbit around this star, and all three have average orbital distances that would be well within the orbital distance of Mercury in the Solar System (Pepe et al, 2011).

Trent Schindler, NSF

Larger and jumbo illustrations
(more images and videos).

All three super-Earth candidates around
82 Eridani may be hot and rocky planets
outside of the star's habitable zone, like
Gl 876 d as imagined by Schindler (more).

The innermost super-Earth planetary candidate "b" has a minimum of 2.7 +/- 0.3 Earth-masses with an average orbital distance (semi-major axis) of a=0.1207 +/- 0.0020 AU (Pepe et al, 2011, Table 4). Assuming that the new estimate of 82 Eridani at 0.70 Earth-masses is correct, then planet b's orbit may take over 18 days to complete.

The middle super-Earth planetary candidate "c" has a minimum of 2.4 +/- 4 Earth-masses with an average orbital distance (semi-major axis) of a=0.2036 +/- 0.0034 AU (Pepe et al, 2011, Table 4). Assuming that the new estimate of 82 Eridani at 0.70 Earth-masses is correct, then planet b's orbit may take over 40 days to complete.

The outermost super-Earth planetary candidate "d" has a minimum of 4.8 +/- 0.6 Earth-masses with an average orbital distance (semi-major axis) of a=0.3499 +/- 0.0059 AU (Pepe et al, 2011, Table 4). Assuming that the new estimate of 82 Eridani at 0.70 Earth-masses is correct, then planet b's orbit may take over 90 days to complete.

82 Eridani B?

As no data have been available from up-to-date astronomical databases on this suspected spectroscopic companion, the original observation was probably spurious.

Hunt for Substellar Companions

No large substellar companion has been found thus far (Murdoch et al, 1993). Astronomers had been hoping to use NASA's proposed Terrestrial Planet Finder (TPF) and the ESA's Darwin planned groups of observatories to search for a rocky inner planet in the so-called "habitable zone" (HZ) around 82 Eridani. As originally proposed, the TPF would include two complementary observatory groups, a visible-light coronagraph and a "formation-flying" infrared interferometer, while Darwin would have included a flotilla of three mid-infrared telescopes and a fourth communications hub. Both projects, however, has been indefinitely postponed.

Closest Neighbors

The following star systems are located within 10 ly of 82 Eridani.

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4.4

Star System	Spectra & Luminosity	Distance (light-years)
(LP 944-20)	brown dwarf
LHS 1565	M5.5 V	7.9
L 302-89	DA8 /VII	8.1
Kapteyn's Star	M0-1.5-3 VI	8.9

Other Information

Up-to-date technical summaries on this star can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS, the NASA Star and Exoplanet Database, the Research Consortium on Nearby Stars (RECONS) list of the 100 Nearest Star Systems, and the SIMBAD Astronomical Database. Additional information may be available at Roger Wilcox's Internet Stellar Database.

Eridanus, the river, wends its way from the Hunter's foot of Orion then southwest to the southern circumpolar zone to enclose a larger area of sky than any other constellation. For more information on stars and other objects in this Constellation and an illustration, go to Christine Kronberg's Eridanus. For another illustration, see David Haworth's Eridanus.

For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.