By John Matson
When NASA launched the WISE satellite in 2009, astronomers hoped it would be able to spot loads of cool, dim objects known as brown dwarfs. Bigger than a planet, a brown dwarf is not quite a star, either—it is too small to sustain the nuclear fusion reactions that turn hydrogen to helium. But it may burn to some degree, using a heavy isotope of hydrogen called deuterium as fusion fuel.
Because brown dwarfs are so dim, it is entirely possible that some of them lie very close to the sun—as close as any known star—and have yet to be discovered. But more than three years after WISE (short for the Wide-Field Infrared Survey Explorer) launched, the map of the sun’s immediate vicinity has remained largely unchanged. Until now.
In a study to appear in the Astrophysical Journal Letters (pdf), Kevin Luhman, an associate professor of astronomy and astrophysics at Pennsylvania State University, announced that he has located a previously unknown denizen of the sun’s neighborhood. Using data from WISE, Luhman has identified a pair of brown dwarfs, bound into a binary system, just 6.5 light-years away. That is nearer to the sun than all but two known star systems, both of which were located more than 95 years ago: the Alpha Centauri triple star system (about 4.3 light-years away) and Barnard’s Star (six light-years).
“I think it is a spectacular find,” WISE principal investigator Edward Wright of the University of California, Los Angeles, wrote in an email, adding that the distance measurement appears robust. “So while this is the third-nearest star (nearly tied for second with Barnard’s star), these are definitely the two nearest known brown dwarfs.”
By extrapolating the orbit back in time for the binary brown dwarf system, known as WISE 1049-5319, Luhman was able to find archival images from other telescopes that registered the object as a moving speck of light as far back as 1978. And he gathered some new imagery of his own—at the Gemini South Telescope in Chile, Luhman caught a glimpse of the object that revealed the speck to be not one but two brown dwarfs locked in a tight orbital dance. Separated by about three times the distance between Earth and the sun, Luhman estimates that the two brown dwarfs circle each other every 25 years or so.
WISE 1049-5319 would make an excellent target for exoplanet hunters, Luhman notes. At such close proximity, any planets that might orbit the brown dwarfs would offer astronomers the rare opportunity to photograph exoplanets and study their properties directly, rather than simply inferring the presence of planets through their influence on the stars that host them. A word about the possibility of extraterrestrial life in such a planetary system: although it is theoretically possible for life to exist on a planet orbiting a brown dwarf, such a world would “suffer a number of critical habitability issues,” according to a 2012 study. Such issues include strong tidal effects from a brown dwarf on the planets near enough to feel its feeble heat and the gradual cooling of the brown dwarf as it ages.