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An example of one of the methane (T-type) dwarf discoveries. At left is a 5x5 arcminute piece of sky as seen in the near-infrared by the Two-Micron All-Sky Survey (2MASS). In the center of the frame, marked with an arrow, is the object known as 2MASSW J1217-03. On the right for comparison purposes is the same piece of sky seen in visible light. In this visible-light view, 2MASSW J1217-03 is not seen because it is at least 4000 times fainter here than in the near-infrared view.
In the 2MASS image, the object appears blue because of absorption by methane. The discovery of methane in this object proves that it is a brown dwarf -- an object which will forever continue to fade because, unlike our Sun, it will never achieve thermonuclear fusion in its core. In addition to this object, three similar methane dwarfs have also been discovered by 2MASS.
Because the spectrum, or chemical fingerprint, is very similar to that of Jupiter, it means that this object is extremely cold. Before this year, only one such object was known outside our own solar system: a companion to the nearby star Gliese 229, announced in 1995. Discovery of the four 2MASS methane dwarfs along with two others being announced today by the Sloan Digital Sky Survey (SDSS) shows that these Jupiter-like objects can form by themselves in the absence of a parent star. In fact, preliminary results suggest that at least 90 of these isolated methane dwarfs may be discovered after 2MASS has surveyed the entire sky, this despite the fact that 2MASS can detect such objects out to only about 13 parsecs (about 40 light years).
Because of methane in their spectra, a new spectral type "T" has been proposed for these new discoveries. Spectral type "T" would be used for objects just cooler than "L" dwarfs, which show no methane. Spectral type "L" was announced just last year and follows "OBAFGKM" in the menagerie of letters that astronomers use to denote various classes of stars.
These results were announced to the press at the American Astronomical Society meeting in Chicago, IL, on May 31, 1999.
PHOTO CREDIT: (Left): 2MASS image courtesy of the University of Massachusetts and the Infrared Processing and Analysis Center. (Right): Digitized Sky Survey image produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. This visible-light image is shown for comparison purposes only and is based on photographic data obtained using the Oschin Schmidt Telescope located on Palomar Mountain and operated by the California Institute of Technology and Palomar Observatory.