Gliese 86 / HR 637 AB
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© Torben Krogh & Mogens Winther,
(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)
Gliese 86 A is an orange-red
dwarf star, similar to Epsilon
Eridani at left center of meteor.
(See a Digitized Sky Survey
image of Gliese 86 from
the Nearby Stars Database.)
Gliese 86 is located about 35.6 light-years from Sol. It lies in the southwestern corner (2:10:25.9-50:49:25.4 ICRS 2000.0) of Constellation Eridanus -- slightly northwest of Phi Eridani and northeast of Chi Eridani, and northeast of Achernar (Alpha Eridani). Many astronomers now refer to this star by its designation in the famous Gliese Catalogue of Nearby Stars (CNS, now ARICNS database) of Wilhelm Gliese (1915-93), who was a longtime astronomer at the Astronomiches Rechen-Institut at Heidelberg (even when it was located at Berlin).
As a relatively bright star in Earth's night sky, Gliese 86 is catalogued as Harvard Revised (HR) 637, a numbering system derived from the 1908 Revised Harvard Photometry catalogue of stars visible to many Humans with the naked eye. The HR system has been preserved through its successor, the Yale Bright Star Catalogue -- updated and expanded through the hard work of E. Dorrit Hoffleit and others. HR 637 is also listed as HD 13445 in the Henry Draper (1837-82) Catalogue with extension (HDE), a massive photographic stellar spectrum survey carried out by Annie Jump Cannon (1863-1941) and Edward Charles Pickering (1846-1919) from 1911 to 1915 under the sponsorship of a memorial fund created by Henry's wife, Anna Mary Palmer. (More discussion on star names and catalogue numbers is available from Alan MacRobert at Sky and Telescope and from Professor James B. Kaler's Star Names.)
On November 24, 1998, astronomers announced the discovery of a substellar companion to this star (see ESO press release, initial summary data, and exoplanets.org, with more details below). In 2001, astronomers announced that that they had imaged a brown dwarf in an outer orbit (more below). Beginning in 2005, some astronomers with additional data began arguing that the outer companion is more likely to be a white dwarf depending on the age of the star (more below). (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)
Gliese 86 is an orange-red main-sequence dwarf star of spectral and luminosity type K0-1 V. The star may have 77 percent of Sol's mass (Desidera and Barbieri, 2007, pp. 10, 11, and 16; and Observatoire de Genève), 86 percent of its diameter (Johnson and Wright, 1983, page 649), and 35 percent of its luminosity. The star appears to be around 63 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Desidera and Barbieri, 2007, pp. 10, 11, and 16; and Observatoire de Genève; and Morell et al, 1994). It may be around 10 billion years old (Saffe et al, 2005). Gliese 86 A has a very close substellar companion "b" orbiting the star at around 0.11 AUs brown dwarf "c" around 20 AUs away. Useful catalogue numbers and designations for the star include: HR 637, Gl 86, Hip 10138, HD 13445, CD-51 532, CP(D)-51 282, SAO 232658, LHS 13, LTT 1130, LFT 186, and LPM 103.
The orbit of an Earth-like planet with surface water would be centered within 0.594 AU -- between the orbital distances of Venus and Earth in the Solar System -- and take around 188 days (more than half a year) to complete. However, the inner orbit of a giant planetary companion "b" recently discovered around Gliese 86 would probably disturb the orbit of such an Earth-type planet. Astronomers would find it very difficult to detect an Earth-sized planet around this star using present methods.
Whatmough -- larger image
(Artwork from Extrasolar Visions, used with permission)
Gliese 86 "b" is an inner orbit superplanet or brown dwarf,
similar to 55 Cancri "A1" as imagined by Whatmough.
On November 24, 1998, a team of astronomers (including Michel Mayor, Didier Queloz, and Stephane Udry) announced the discovery of a substellar companion "b," whose latest minimum mass estimate has been calculated as 4.02 times that of Jupiter (exoplanets.org; Butler et al, 2002; initial summary data, and ESO press release) with a similar diameter. This object moves around Gliese 86 A at an average separation of 0.11 AUs, which would be well within the orbital distance of Mercury in the Solar System. Its highly circular orbit (e= 0.04) takes less than 15.8 days to complete (exoplanets.org; and Queloz et al, 2000; in ps). (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)
Astrometric analysis suggests that the object may actually have 15 times the mass of Jupiter with an inclination of 164.0° from Earth's line of sight (Han et al, 2001, in pdf). Thus, the object could be an extremely dim brown dwarf, substellar companion of Gliese 86 A. The authors consider their analysis to be preliminary, needing confirmation with additional astrometric as well as other observations.
Gliese 86 or "B" (or "c")
(left of star) may be
a white (or yet a
brown dwarf "c")
Past analysis suggested that Gliese 86 A had a spectroscopic companion at an orbital distance of over 11 AUs. Subsequent recent radial velocity analysis of Gliese 86 A suggests that it has a low mass stellar companion with an orbital period substantially exceeding 10 years (Butler et al, 2002; and Queloz et al, 2000; in ps). In 2001, astronomers announced that coronographic images revealed a faint object at a separation of over 16 AUs (1.5" at a HIPPARCOS distance estimate of 35.6 ly) with a mass below 70 Jupiters (or 0.07 Solar) which is too low to initiate core hydrogen fusion necessary for the object to be a star (Els et al, 2001). Beginning in 2005, astronomers with improved photometric data began arguing that the colors detected are compatible with a white dwarf of around 0.5 Solar-mass as well as a ~70 Jupiter-mass brown dwarf since "both types of objects fit the available, still limited astrometric data"; on the other hand, the hypothesized formation of the Gl 86 system with an inner Jupiter-class planet and an originally more massive companion -- a Solar-mass stellar progenitor that subsequently evolved into a white dwarf at around 18.4 AUs -- "represents a challenge to current models" given that the binary orbit may have widened by 40-50 percent with the mass-loss of the progenitor by the time it cast off its outer hydrogen and helium layers (Desidera and Barbieri, 2007, pp. 10, 11, and 16; (Lagrange et al, 2006; and (Mugrauer and Neuhäuser, 2005).
The following table includes all star systems known to be located within 10 light-years (ly), plus more bright stars within 10 to 20 ly, of Gliese 86.
|Star System||Spectra &|
|CC Eridani B||?||5.5|
|CC Eridani A||K6-7 Ve||5.9|
|L 227-140||DZ7 /VII||6.1|
|CD-53 570||K-M V||8.3|
|L 127-97||M0 V||9.1|
|CD-45 1184||M3.5 V||9.3|
|p Eridani ABab||K2-5 V |
|* plus bright stars *||. . .|
|Zeta1,2 Reticuli||G2.5-5 V |
|Zeta Tucanae 2?||F8-G0 V||13|
|Nu Phoenicis||F8 V||16|
|Zeta Doradus AB||F7-8 V |
|HR 209||G1-5 IV||17|
|82 Eridani AB||G5-8 V |
|HR 683||G5-8 V||18|
|Beta Hydri||G2 IV||18|
The late John Whatmough created illustrated web pages on this system in Extrasolar Visions.
Up-to-date technical summaries on these stars can be found at: Jean Schneiders's Extrasolar Planets Encyclopaedia; the Astronomiches Rechen-Institut at Heidelberg's ARICNS, and the Nearby Stars 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. Towards the western edge of Eridanus, is Gamma Eridani, which is also known as Zaurak. Epsilon Eridani is located northwest of Zaurak. For more information on stars and other objects in Constellation Eridanus and an illustration, go to Christine Kronberg's Eridanus. Another illustration is available at 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.
Note: Thanks to Andrew Tribick for notifying us of the updated status of the outer substellar companion to Gliese 86.
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