Infrared observations have uncovered a cool brown dwarf that’s only about 7 light-years away. The object is one of the closest stellar systems to the Sun and the coolest brown dwarf yet discovered.
Astronomy is a science pursued at a distance. Most of the light we see from distant stars and galaxies takes thousands to millions of years to reach us. That makes our solar neighborhood a valuable place for detailed observations: the closest companions to the Sun are benchmarks, because they are the easiest stars to study in detail.
While the census of the solar neighborhood has tallied more stellar citizens over time, most of the newly discovered neighbors have been relatively distant, usually at least 30 to 60 light-years away. But recently, an astronomer from Penn State discovered a solar neighbor about 7 light-years away, and it’s a "cool" result in more ways than one!
Using data from NASA's Wide-field Infrared Survey Explorer (WISE) and Spitzer Space Telescope, Kevin Luhman recently discovered an object known as WISE J085510.83−071442.5. The object is special for quite a few reasons. First, it is right next door, in astronomical terms. At 7.2 light-years away (6.5 to 8 is the error range), this is likely the fourth closest stellar system ever detected, farther only than the Alpha Centauri triple system (4.2 light-years), Barnard’s star (5.9 light-years) and the brown dwarf binary WISE J104915.57−531906.1 (6.6 light-years). (It displaces Wolf 359, which lies 7.8 light-years away).
Second, it’s moving fast. Using infrared images obtained by WISE and Spitzer, Luhman noticed the object was traveling extremely quickly across the sky in between images. Part of this motion is from parallax, the apparent back-and-forth change in position with respect to background objects. This motion is caused by Earth orbiting around the Sun: the closer a star is to the Sun, the larger its apparent shift in position as we look at it from different sides of our orbit. It’s the same effect you see if you hold your finger up at arm's length and blink your eyes one at a time: you will notice your finger appears to move back and forth as you blink each eye. If you move your finger closer to your face, that effect increases.
WISE J0855−0714’s parallax allowed Luhman to infer that WISE J0855−0714 was close to the Sun. But the object’s parallax is small compared with its proper motion, which is its apparent motion across the sky from point A to point B over time. The object is traversing 8.1 arcseconds per year, the third largest proper motion of any object outside the solar system (second only to Barnard’s star and Kapteyn’s star). In comparison, most of the brightest stars have a proper motion of a few tenths of an arcsecond per year or less — for example, Rigel only moves 0.004 arcsecond per year.
The other thing that makes WISE J0855−0714 "cool" is that it really is cold! Using images of the object taken in different filters, Luhman estimated its temperature to be about 250 kelvin, or about 10 degrees below zero in Fahrenheit. This makes WISE J0855−0714 not only the coldest neighbor to the Sun but also the coldest brown dwarf ever discovered.
This combination of close, fast, and cold makes WISE J0855−0714 unique among all of the solar neighborhood members. As Luhman states in a press release, “It is very exciting to discover a new neighbor of our solar system that is so close. In addition, its extreme temperature should tell us a lot about the atmospheres of planets, which often have similarly cold temperatures.”
The discovery of WISE J0855−0714 points out just how important large-scale surveys of the sky, such as WISE, really are. This cold brown dwarf was discovered relatively close to the plane of our Milky Way, which astronomers often avoid because "crowding" can occur — that is, there are so many stars along the galactic plane that it can be tough to tell one from the other, especially when they are moving. (Luhman actually had to use multiple filters to separate WISE J0855−0714 from the signals of two stationary background objects in order to study it.) But as Luhman has shown, this may be a fertile hunting ground for finding more close companions to the Sun.
Reference: K. Luhman. "Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun." Astrophysical Journal Letters, May 10, 2014.