Brown dwarfs straddle the divide between planets and stars—they are celestial objects too small to burn hydrogen in fusion reactions, as stars do, but they are large enough to sustain other kinds of fusion. At least a few even harbor orbiting planets. The International Astronomical Union sets the planet–brown dwarf boundary at 13 times the mass of Jupiter. But that mass limit is an imperfect definition—what of brown dwarf–size bodies that orbit stars, behaving themselves like supersized planets?

Firming up the line between planets and brown dwarfs will become more pressing as the coolest, dimmest brown dwarfs soon reveal themselves to the new generation of astronomical instruments. NASA's WISE satellite, for instance, is highly adept at spotting cool, dim objects. Since its launch in December 2009, WISE has already discovered seven comets and 30 near-Earth asteroids, and its operators expect that WISE will spot many brown dwarfs as well during its sky-surveying mission.

In the meantime, an international team led by Philip Lucas of the University of Hertfordshire in England claims discovery of what could be the nearest and coolest brown dwarf yet. Lucas and his colleagues reported their finding in a paper posted to the preprint repository on April 2. They used data from a survey of the sky taken at the U.K. Infrared Telescope in Hawaii to uncover UGPS 0722-05, a brown dwarf with a temperature of only about 125 to 225 degrees Celsius. The coolest previously known brown dwarfs are in the range of 225 to 275 degrees C.

Based on preliminary distance measurements, the object appears to be floating free in interstellar space some 9.5 light-years away, closer than the binary brown dwarfs of the Epsilon Indi system 11.8 light-years distant. If it were a normal star it would rank among the dozen nearest stellar companions to the sun. (The closest star, Proxima Centauri, is 4.2 light-years from the sun.)

Astrophysicist Adam Burgasser of the University of California, San Diego, was not a member of Lucas's team but has already been involved in follow-up observations of the object. "I think it's a very interesting find," Burgasser says, "although I would caution it is still unclear how close the source is, as there are only a few astrometric measurements made thus far." He notes also that temperatures can be difficult to pin down for the coolest varieties of brown dwarfs, and that the research has not yet been peer-reviewed. "In any case, the group's discovery has gotten several of us very excited," Burgasser says, "and we do hope it pans out to be as close as they claim."

Artist's impression of a cool brown dwarf: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)