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
NASA's NStars Database.)
Gliese 86 is located about 35.6 light-years from Sol. It lies in the 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 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 222 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.)
The star is too dim and faraway to be visible with the naked eye (but has been included in the on-line ChView "Bright Star" map). 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). (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 79 percent of Sol's mass (Observatoire de Genève), 86 percent of its diameter (Johnson and Wright, 1983, page 685), and 40 percent of its luminosity. The star appears to be less than 58 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Morrell et al, 1994). It may be around billion years old. Gliese 86 A has a spectroscopic stellar companion (Star B) over 11 AUs away, and a very close substellar companion "b" orbiting Star A at around 0.11 AUs. 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 0.608 AU -- between the orbital distances of Venus and Earth in the Solar System -- and take around 195 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.
© Tomislav Stimac
Artwork from Planets
at Digital Eye
(used with permission).
A view of Gliese 86 and its
substellar companion "b" from
the surface of a satellite, as
imagined by Stimac (more).
On July 6, 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) 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, 2000). 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 is an orange-red main-sequence dwarf star of spectral and luminosity type K0-1 V. The star may have 79 percent of Sol's mass (Observatoire de Genève), 86 percent of its diameter (Johnson and Wright, 1983, page 685), and 75 percent of its luminosity. spec.dou.: sepAB>11AU;lowMet; The star appears to be half (50 percent) as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (ESO Press Release, 200?). It may be around billion years old. 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 0.608 AU -- between the orbital distances of Venus and Earth in the Solar System -- and take around 195 days (more than half a year) to complete. However, the eccentric 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.
Brown Dwarfs or Planets?
When brown dwarfs were just a theoretical concern, astronomers differentiated those hypothetical objects from planets by how they were formed. If a substellar object was formed the way a star does, from a collapsing cloud of interstellar gas and dust, then it would be called a brown dwarf. If it was formed by gradually accumulating gas and dust inside a star's circumstellar disk, however, it was called a planet. Once the first brown dwarf candidates were actually found, however, astronomers realized that it was actually quite difficult to definitely rule on the validity of competing hypotheses about how a substellar object was actually formed without having been there. This problem is particularly difficult to resolve in the case of stellar companions, objects that orbit a star -- or two.
University of California at Berkeley astronomer Ben R. Oppenheimer, who helped to discover a nearby brown dwarf, Gliese 229 b, is part of a growing group that would like to define a brown dwarf as an substellar object with the mass of 13 to 80 (or so) Jupiters. While these objects cannot fuse "ordinary" hydrogen (a single proton nucleus) like stars, they have enough mass to briefly fuse deuterium (hydrogen with a proton-neutron nucleus). Therefore, stellar companions with less than 13 Jupiter masses would be defined as planets.
Other prominent astronomers, such as San Francisco State University astronomer Geoffrey W. Marcy who also has helped to discover many extrasolar planets, note that there may in fact be many different physical processes that lead to the formation of planets. Similarly, there may also be many different processes that lead to the creation of brown dwarfs, and some of these may also lead to planets. Hence, more observational data may be needed before astronomers can determine how to make justifiable distinctions in the classification of such substellar objects.
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|
John Whatmough has developed 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 NASA's NStar 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.
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