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On August 25, 2014, a team of astronomers (including Jacqueline Faherty) revealed the first detection of water-ice clouds outside the Solar System around this brown dwarf. The astronomers used the 6.5-meter Magellan Baade telescope at the Las Campanas Observatory in Chile to acquire 151 near-infrared images over three nights in May 2014 to detect the clouds. In a forthcoming article, the team reports that the observed colors match models (developed by others) of a brown dwarf with clouds of water ice and clouds of sodium sulfide. The brown brown dwarf appears to be about half obscured by these clouds -- a "partly cloudy" atmosphere (Ken Croswell, Science Daily News, August, 2014).
On April 25, 2014, astronomer Kevin L. Luhman announced the discovery of an extremely dim brown dwarf around 7.2 +0.8/-0.7 light-years (2.20+0.24/-0.20 pc) from our Sun, Sol, using infrared images collected by NASA's Wide-field Infrared Survey Explorer (WISE) . Designated WISE J085510.83-071442.5 (but can be shortened to WISE 0855-0714), it can be found in the northern part (8:66:10.83-07:14:42.5) of Constellation Hydra, the Water Serpent, in the southern hemisphere -- northwest of Alphard (Alpha Hydrae) and southeast of open cluster M48. Currently, the substellar object is the fourth closest to the Solar System (NASA JPL news release; NASA science news; and Kevin L. Luhman, 2014).
After noticing the fast proper motion of WISE J0855-0714 across infrared images in March of 2013 (the third highest observed outside the Solar System, behind Bernard's and Kapteyn's stars), Luhman analyzed additional images taken with Spitzer infrared space telescope and the Gemini South telescope. Spitzer's infrared observations helped to determine its chilly temperature of between minus 54 and 9 degrees Fahrenheit (minus 48 to minus 13 degrees Celsius). As a result, the object is the coldest brown dwarf discovered as of April 25, 2014, as the previous coldest dwarfs were no colder than room temperature. Also the reddest as well as coldest brown dwarf known, it should probably be classified as a "Y" dwarf (Kevin L. Luhman, 2014).
If WISE J0855-0714 is between one and 10 billion years old like most nearby stars, then it may have around 3 to 10 times Jupiter's mass. At that mass range, it is one of the least massive brown dwarfs known as of April 25, 2014. While it could be a gas giant that was ejected by gravitational perturbations from its star system, it probably developed as a solitary brown dwarf rather than a planet around a star.
Larger illustration: Sol; M,L,T dwarfs;
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 (more; and Kevin L. Luhman, 2013).
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 young 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.
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.
Astronomer Ben R. Oppenheimer, who helped to discover the other 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 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.
Hydra is the largest constellation of the southern hemisphere. Latin for "water snake," its name refers to the constellation's shape, which resembles a twisting snake. The constellation was created by the Greek astronomer Ptolemy in the 2nd century. In addition, the Chinese refer the constellation as "Liu," or Willow, while the Japanese call it "Nuriko." For more information and illustrations of Constellation Vela, go to Christine Kronberg's Hydra. For another illustration, see David Haworth's Hydra.
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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