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Larger composite image (where
the brown dwarf "b" is bluish,
here, in infrared but would
appear magenta in visible light)
Wolf 940 is a red dwarf star
with a very cool, methane
brown dwarf companion in
a wide orbit (more).
Wolf 940 is located about 39.9 light-years away in the north central part (21:46:40.5-0:10:25.4, ICRS 2000.0) of Constellation Aquarius, the Waterbearer -- southwest of Sadalmelik (Alpha Aquarii); east of Globular Cluster M2; northeast of Sadalsuud (Beta Aquarii); south of Enif (Epsilon Pegasi) and north of Deneb Algedi (Delta Capricorni). The relatively high proper motion of Wolf 940 was probably first discovered in the early 20th Century by Max (Maximilian Franz Joseph Cornelius) Wolf (1863-1932), a pioneer of astrophotography who discovered hundreds of variable stars and asteroids, and about 5,000 nebulae by analyzing photographic plates and developing the "dry plate" in 1880 and the "blink comparator" in 1900 with the Carl Zeiss optics company in Jena, Germany. On April 20, 2009, a team of astronomers announced that they had imaged a cool methane brown dwarf companion in a wide orbit around Wolf 940 (UKIRT/JAC press release; and Gemini press release -- more below). (See an animation of the possible orbits of Wolf 940, its potentially habitable zone, and its substellar companion, with a table of basic orbital and physical characteristics.)
Based on RECONS estimates for similar stars, this cool, main sequence red dwarf (M3.5-4 V) may have less than a fifth of Sol's mass, probably just under half of its diameter, and less than 0.11 percent of its visual luminosity. As it has not been designated as a flare or variable stars, Wolf 940 is probably more than a billion years old. The orbital distance from the star where an Earth-type planet would be comfortable with liquid water is around 0.033 AU, with a period around 17 days. At such a close distance to its host star, such a planet can easily become tidally locked -- with one side in perpetual day. Some alternative names and useful star catalogue numbers are: GJ 1263, LHS 3708, LTT 8692, LPM 797, LFT 1660, G 26-30, G 93-35, USNO 467, and 2MASS J21464040-0010233.
On April 20, 2009, a team of astronomers announced that they had imaged a cool methane brown dwarf companion "b" in a wide orbit around Wolf 940, using the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea in Hawaii as part of the UKIRT Infrared Deep Sky Survey (UKIRT/JAC press release; and Gemini press release). One of the coolest known brown dwarfs detected as of April 20, 2009, the object has about 20 to 30 times Jupiter's mass, roughly the same diameter, and a surface temperature of around 300 °C. Although Wolf 940 b glows brightly in infrared wavelengths, it does not emit much visible light because of its low surface temperature. Like Jupiter and the methane brown dwarf Gliese 229 b it has an abundance of methane on its surface and has been given a spectral type of T8.5. The object is currently separated from Wolf 940 by about 440 AUs (beyond the observed orbital distance of Edgeworth-Kuper Belt objects in the Solar System), and an orbital period of around 18,000 years. (See an animation of the possible orbits of Wolf 940, its potentially habitable zone, and its substellar companion, with a table of basic orbital and physical characteristics.)
JPL, CalTech, NASA
As a cool methane brown dwarf,
Wolf 940 b should appear magenta
in visible light, like the nearby
binary T-8 dwarfs 2MASS
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.)
Larger illustration: Sol; M,L,T dwarfs; & Jupiter.
In visible light at one billion years in age,
large brown dwarfs are reddish like the smallest
M-type stars, but cooler, dimmer T-dwarfs are
more magenta in hue. At least 13 brown dwarfs
may be located within 10 parsecs of Sol (more).
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. While methane is a atmospheric characteristic of giant gas planets like Jupiter, the first brown dwarf found to have a trace of methane was Gliese 229 b.
In Spring 1999, 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 provided 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. Astronomers then began speculating 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. By May 1, 2009, a list of T dwarfs compiled at DwarfArchives.org indicated that at least 155 methane brown dwarfs had been detected and confirmed over the past decade.
The following star systems are located within 10 light-years, plus more bright stars within 10 to 20 ly, of Wolf 940.
|Star System||Spectra &|
|LP 639-1||M5 V||4.2|
|BD+00 4810||K8-M0.5 V||7.0|
|LTT 16303 AB||M V||8.2|
|Hip 109119||A2 V||9.3|
|Wolf 1561 AB||M4.5 Ve |
M 5 Ve
|* plus bright stars *||. . .|
|Deneb Algedi 2||A5 IVm |
|Iota Pegasi AB||F5 V |
Aquarius was "Latinized" by the Romans from Ganymede in Greek mythology, who was "cup-bearer to the gods." For more information on stars and other objects in Constellation Aquarius and an illustration, go to Christine Kronberg's Aquarius. For another illustration, see David Haworth's Aquarius.
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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