Closest Brown Dwarf System Discovered

Two newly discovered brown dwarfs lie just 6.5 light-years away, making them the closest brown dwarfs known, the third-closest star system known, and best of all, a promising target for exoplanet studies.

A newly discovered pair of brown dwarfs 6.5 light-years away is breaking records. The brown dwarfs are the closest known, and together they make up the closest star system discovered since 1916. Moreover, WISE J104915.57-531906 (or WISE J1049 for short) displaces Wolf 359 as the third-closest star system to Earth after Alpha Centauri (4.4 light-years away) and Barnard’s star (6 light-years away). Kevin Luhman (Pennsylvania State University) announced the discovery in a letter to be published in April 10th’s Astrophysical Journal.

Technically, the system doesn’t contain stars at all — brown dwarfs are often referred to as “failed stars” because they contain too little mass to sustain the hydrogen fusion by which stars shine. While many undergo some fusion early on, all of them shine by radiating away their internal heat at infrared wavelengths instead of visible light.

The now-hibernating Wide-field Infrared Survey Explorer (WISE) surveyed almost the entire infrared sky from 2009 to 2011, uncovering more than 100 brown dwarfs. A previous WISE study found that hydrogen-fusing stars outnumber brown dwarfs by about 6:1, even in the solar neighborhood, where brown dwarfs were once thought to equal stars in number.

"Now that we're finally seeing the solar neighborhood with keener, infrared vision, the little guys aren't as prevalent as we once thought," comments Davy Kirkpatrick (Caltech).

Most previous surveys found brown dwarfs by their infrared colors, but Luhman took a different approach. He searched for infrared sources in WISE data with high proper motion — the sources appear to speed across the night sky. By measuring an object’s displacement relative to more distant, background stars, Luhman could determine its distance via parallax. This technique, based on simple geometry, allows you to calculate the distance to a finger held steady in front of your face by watching it move back and forth as you close first one eye and then the other.

In Luhman’s case, the baseline was the width of Earth’s orbit rather than the distance between his eyes. He caught a dim but rapidly moving object in the WISE surveys and calculated where he might find it in older images taken between 1978 and 1999 as part of past infrared surveys. The combined detections give a parallax of 0.5 arcseconds, or a distance of 6.5±0.5 light-years, putting the object just past Barnard’s Star.

Follow-up data from the Gemini South Telescope in Chile resolved the blurry infrared source into two objects, both brown dwarfs hovering near the transition between L-type and T-type. Separated by three times the Earth-Sun distance, one brown dwarf is about 1.5 times brighter than the other.

“This pair of brown dwarfs is so bright because of its close proximity to us that when I first started examining it, I thought that it was surely too bright to be a brown dwarf,” Luhman says. The pair has gone undetected until now because previous surveys tend to avoid the star-dense plane of the Milky Way.

"It's likely that proper motion surveys will continue to uncover a few more brown dwarfs in the galactic plane," Luhman adds, "as well as objects with unusual colors that would be rejected in a color-based search."

Brown Dwarfs as Exoplanets

Brown dwarfs are so like the gas giants they outweigh, they have piqued astronomers’ interest as possible exoplanet analogues. And its close proximity makes this brown dwarf pair a particularly tantalizing prospect.

“Giant planets around other stars are very difficult to study directly because the glare from their star gets in the way,” Luhman explains, “but brown dwarfs are often found free-floating by themselves in space, without any glare from a star, making them attractive substitutes for giant planets when studying cool atmospheres.”

A planet orbiting the brown dwarf would be easier to image directly, too, though such planets would likely not host life. Binary systems have a harder time making planets in stable orbits, but the possibility of planets orbiting either one or both brown dwarfs is far from impossible.

“A planet could orbit either brown dwarf in a tight orbit, so close that the other brown dwarf doesn't gravitationally perturb the planet's orbital path. Alternatively, a planet could orbit far from both brown dwarfs, so far from them that the pair acts almost as one star,” says Geoffrey Marcy (University of California, Berkeley).

The search for brown dwarfs is far from over — the project AllWISE will combine data from WISE’s all-sky surveys to search systematically for nearby fast-moving objects, such as WISE J1049, as well as faint objects from the distant universe. AllWISE data will be available to the public in late 2013.

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Shari Balouchi is a sophomore pursuing a Physics/Astronomy minor at Sewanee: The University of the South and writes an astronomy column for The Sewanee Purple a student-run newspaper on campus.