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NASA (Gliese 229 is a red dwarf star with a brown dwarf companion)
On March 4, 2014, a team of astronomers announced that analysis of new and older radial-velocity data from nearby red dwarf stars revealed a planet with a minimum of 32 (max 49) Earth-masses at an average orbital distance of 0.97 AU from host star Gl 229, with an orbital period around 471 days (UH news release; and Tuomi et al, 2014).
Gliese 229 is located only about 18.8 light-years away in the east central part (06:10:34.62-21:51:52.72, ICRS 2000.0) of Constellation Lepus, the Hare -- east of Delta Leporis. The high proper motion of this star may have been noticed first by Willem Jacob Luyten (1899-1994), who found the proper motions of over 520,000 stars despite the loss of sight in one eye since 1925 by building an automated photographic plate scanner and measuring machine. However, 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). The star has a cool methane brown dwarf companion that was discovered in October 1994 by Caltech and Johns Hopkins astronomers using the 60-inch reflecting telescope on Mt. Palomar (Nakajima et al, 1995), which was confirmed in November 1995 with the Hubble Space Telescope (NASA press release).
|Center of Mass||0.0||...||...||...||...||...||...||...||...|
|Inner H.Z. Edge?||0.231||0.148||0.0||?||...||...||...||...||...|
|Outer H.Z. Edge?||0.450||0.403||0.0||?||...||...||...||...||...|
|Gliese 229 b||35.9||340?||?||?||0.025-0.065||0.009-0.0011||...||...||...|
This cool, main sequence red dwarf (M1-2 Ve) may have only 56 to 58 percent of Sol's mass (Ansgar Reiners, 2007, from Jenkins et al, 2009; and RECONS), less than 53 percent of its diameter, and 1.6 percent of its luminosity and 5.5 percent of its theoretical bolometric luminosity, correcting for infrared output (NASA Star and Exoplanet Database, derived using exponential formula from Kenneth R. Lang, 1980). The star may have only around 81 percent of Sol's abundance of elements heavier than hydrogen -- "metallicity" (Ansgar Reiners, 2007, from Jenkins et al, 2009). Gliese 229 may be as much as 3 billion years old. The star exhibits a long-term periodicity in its chromospheric activity cycle of around four years (Buccino et al, 2011). It is a flare star with the New Suspected Variable designation NSV 2863. Some alternative names and useful star catalogue numbers are: Gl 229, Hip 29295, HD 42581, BD-21 1377, CP(D)-21 1138, SAO 171334, LHS 1827, LTT 2475, LPM 230, LFT 459, L 668-21, V 471, and 2MASS J06103462-2151521.
NASA -- larger image
Gliese 229 is a dim red dwarf star, like Gliese
623 A (M2.5V) and B (M5.8Ve) at lower right.
(A 2MASS Survey image of Gliese 229 from the NASA
Star and Exoplanet Database may become available.)
According to one type of model calculations performed for the NASA Star and Exoplanet Database, the inner edge of Barnard's habitable zone should be located at around 0.231 AU from the star, and the outer edge would lie at around 0.450 AUs. In order to be warmed sufficiently have liquid water at the surface, an Earth-type rocky planet would have to be located very close to such a cool and dim red dwarf star like Gliese 229, at around 0.340 AUs. At such close distances, such a planet can easily become tidally locked -- with one side in perpetual day -- and race around the star in less than 97 days (or less than 14 weeks).
This brown dwarf has about 25 to 65 times Jupiter's mass (Allard et al, 1996), 0.9 to 1.1 times its diameter (Allard et al, 1996; and Marley et al, 1996), and a surface temperature of 1,000 to 1,200 °C. Like Jupiter, Gliese 229 b it has an abundance of methane on its surface. Under one proposed classification scheme, it has been given a spectral type of T6.5 (see Dr. Adam J. Burgasser's T Dwarfs page). The object is currently separated from Gliese 229 by about 39 AUs -- the orbital distance of Pluto in the Solar System.
© John Whatmough (Artwork from Extrasolar Visions, used with permission)
The brown dwarf companion to Gliese 229 -- with its own dark satellite,
as imagined by Whatmough
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.
© American Scientist
(Artwork by Linda Huff for Martin et al, 1997; used with permission)
Although brown dwarfs lack sufficient mass (at least 75-80 Jupiters) to
ignite core hydrogen fusion, the smallest true stars (red dwarfs) can
have such cool atmospheric temperatures (below 4,000° K) that it is
difficult to distinguish them from brown dwarfs. While Jupiter-class planets
may be much less massive than brown dwarfs, they are about the same
diameter and may contain many of the same atmospheric molecules.
University of California at Berkeley astronomer Ben R. Oppenheimer, who helped to discover 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. (More information on this debate over definitions is available at exoplanets.org.)
Cool Methane Brown Dwarfs
While brown dwarfs have too little mass to fuse "regular" hydrogen (which has a single proton nucleus), virtually all of the ones discovered until 1999 were too hot -- that is "young" -- to show evidence of methane which is destroyed by stellar temperatures. In fact, while methane is a atmospheric characteristic of giant gas planets like Jupiter, the only brown dwarf found to even have a trace of methane was Gliese 229 b.
In Spring 1999, however, two very dim and reddish brown dwarfs were found as solitary objects (one 30 light-years away in Ophiuchus and another also relatively nearby in Virgo). Analysis of their spectra indicated that both have atmospheres that are rich in methane. In addition, four similar objects that are too cool to be observed in visible light were found using near-infrared telescopes also to have the methane fingerprint of extremely cool (that is "old") brown dwarfs. These discoveries represent strong evidence that, although hard for astronomers to detect, faint brown dwarfs which have had billions of years to cool may represent a significant population of the universe. Some astronomers speculate that these objects may well be as numerous as the stars, reviving theories of stellar formation that suggest the existence of uncountably numerous brown dwarfs, rather than the relatively few easy-to-detect, bright ones found thus far.
Life Around a Flare Star
Many dim, red (M) dwarf stars exhibit unusually violent flare activity for their size and brightness. These flare stars are actually common because red dwarfs make up more than half of all stars in our galaxy. Although flares do occur on our Sun every so often, the amount of energy released in a solar flare is small compared to the total amount of energy Sol produces. However, a flare the size of a solar flare occurring on a red dwarf star (such as Gliese 229) that is more than ten thousand times dimmer than our Sun would emit about as much or more light as the red dwarf itself, doubling its brightness or more.
High resolution and jumbo images (Benz et al, 1998).
Gliese 229 is a flare star, like UV Ceti (Luyten 726-8 B)
shown flaring at left. UV Ceti is an extreme example
of a flare star that can boost its brightness by five times
in less than a minute, then fall somewhat slower back
down to normal luminosity within two or three minutes
before flaring suddenly again after several hours.
Flare stars erupt sporadically, with successive flares spaced anywhere from an hour to a few days apart. A flare only takes a a few minutes to reach peak brightness, and more than one flare can occur at a time. Moreover, in addition to bursts of light and radio waves, flares on dim red dwarfs may emit up to 10,000 times as many X-rays as a comparably-sized solar flare on our own Sun, and so flares would be lethal to Earth-type life on planets near the flare star. Hence, Earth-type life around flare stars may be unlikely because their planets must be located very close to red dwarfs to be warmed sufficiently by star light to have liquid water (less than 0.13 AU and a 24-day orbital period for Gliese 229), which makes flares even more dangerous due to their dimness. In any case, the light emitted by red dwarfs may be too red in color for Earth-type plant life to perform photosynthesis efficiently.
The following star systems are located within 10 ly of Gliese 229.
|Star System||Spectra &|
|BD-03 1123||M1.5 V||6.7|
|G 99-44||DZ9 /VII||6.8|
|Ross 614 AB||M4.5 Ve |
|LP 656-38||M3.5 V||7.2|
|LTT 17897||M4 V||7.9|
|L 674-15||M3.5 V||9.2|
|Kapteyn's Star||M0-1 VI||9.3|
|Keid 3||K1 Ve |
More information on Gliese 229 and its brown dwarf companion 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 Roger Wilcox's Internet Stellar Database.
Located in the Southern Hemisphere, Constellation Lepus (the Hare) can be found just south of Constellation Orion (the Hunter), a favorite prey of the Hunter and his hunting dogs. While believed to be ancient, the cultural origin of this constellation is unknown. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Lepus. For another illustration, see David Haworth's Lepus.
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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