Coldest 'Missing Link' Brown Dwarf Discovered

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Brown dwarfs are strange celestial objects. They’re not stars, they’re not planets, yet they exhibit characteristics of both. And now astronomers using NASA’s Wide-field Infrared Survey Explorer (WISE) have discovered the coolest of a cool class of brown dwarfs, potentially providing some answers as to what makes a planet and what makes a star.

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It seems simple enough; you get a huge cloud of gas, let it collapse under its mutual gravity for millions of years, and if there’s enough mass, perhaps a star will be born. But say if there isn’t quite enough mass contained in the nebula to spark the sustained nuclear fusion in the core? Well, it’s possible that a brown dwarf will be born.

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Although there might be some nuclear fusion in its core for a short period of time, a brown dwarf is commonly referred to as a “failed star.” Its atmosphere will have more similarities with the gaseous atmosphere of Jupiter than that of the hot plasma of the sun. However, because a brown dwarf’s atmosphere is constantly driven by convective currents by its hot core, the chemicals in its atmosphere do not settle and do not differentiate by height — a very star-like quality.

Despite the “failed star” designation, astronomers still classify brown dwarfs by their spectral type (a scale of letters assigned to the luminosity of stars), which relates to their temperature. At the lowest, coolest end of the scale, radiating in infrared wavelengths, are the oddball brown dwarfs.

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Until recently, the coolest brown dwarfs confirmed to exist are at the lowest end of the scale, with a spectral class of “T.” However, there is a theoretical class “Y” that is even cooler than the T-class brown dwarfs, thought to be the “missing link” that bridges the largest planets from the smallest stars — predicted to have a temperature less than 225 degrees Celsius (440 F).

But Y-class brown dwarfs are theoretical no more.

Where brown dwarf science gets really interesting is that because they are naturally very dim, conventional astronomy has a hard job spotting them. However, sophisticated infrared space telescopes are the best brown dwarf hunters out there, and WISE is the best there is.

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Astronomers using WISE have spotted six Y-class brown dwarfs close to our sun — within a distance of 40 light-years. The space telescope has spotted 100 brown dwarfs previously unknown to astronomy.

“WISE scanned the entire sky for these and other objects, and was able to spot their feeble light with its highly sensitive infrared vision,” said Jon Morse, Astrophysics Division director at NASA Headquarters, in a NASA release. “They are 5,000 times brighter at the longer infrared wavelengths WISE observed from space than those observable from the ground.”

But the best news is that WISE has found a record-breaking Y-class dwarf. It’s called WISE 1828+2650, and it has a positively comfortable surface temperature of 25 degrees Celsius (80F) — that’s room temperature.

“The brown dwarfs we were turning up before this discovery were more like the temperature of your oven,” said Davy Kirkpatrick, a WISE science team member at the Infrared Processing and Analysis Center at Caltech in Pasadena, Calif. “With the discovery of Y dwarfs, we’ve moved out of the kitchen and into the cooler parts of the house.”

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One of the six Y-class dwarfs, WISE 1541-2250, is very close to home — only 9 light-years from Earth, making it the seventh-closest stellar object to us (the closest star, Proxima Centauri, is four light-years from Earth). The red dwarf star Ross 154 looks as if it’s been relegated to the position of “eighth closest star.”

Now that WISE has identified the locations of these ultra-cool brown dwarfs, followup studies by NASA’s Spitzer Space Telescope and the Hubble Space Telescope will be carried out. Closer analysis of these Y-class dwarfs will surely turn up a few surprises and ultimately help us understand this mysterious class of stellar object.

Images (from top): An artist impression of a Y-class brown dwarf (NASA/JPL-Caltech); A comparison of the brown dwarf classes (NASA/JPL-Caltech); WISE observation of the coolest brown dwarf ever detected (circled) (NASA/JPL-Caltech/UCLA).

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