Planetar. Substar. Failed star. Sub-stellar object. Astronomers have pinned each of these monikers on brown dwarfs, a category that has always perplexed scientists because it raises questions about what it means to be a star or a planet. And if that wasn’t enough, now they’ve discovered the coldest brown dwarf yet, blurring the line between planet and star even further.
It’s name is CFBDSIR J1458+1013B, and may be cooler than the boiling point of water (at the pressure of Earth’s atmosphere). This strange body is about 75 light-years from us, where it orbits its binary partner, another brown dwarf. Using the infrared capabilities of the 10-meter Keck II Telescope on Mauna Kea, University of Hawaii researcher Michael Liu and his team estimated the brown dwarf’s temperature, and have a ballpark range for its mass: between 6 and 15 times the mass of Jupiter.
It’s special because it may be a class Y dwarf (temperature less than 225 degrees Celsius (440 F)), a type of object whose existence astronomers had predicted but never actually found. Before this candidate arose, the coolest known brown dwarf was in the T spectral class; while there have been a few Y-class candidates in the past, scientists have a better grasp on the temperature of this one: 97 degrees Celsius, plus or minus 40C.
Another cool (ahem) thing about this particular brown dwarf is its mass. An object less than 13 Jupiter masses is too light to fuse atoms of deuterium, a heavy isotope of hydrogen; objects above 13 Jupiter masses can fuse deuterium. The uncertainty over CFBDSIR’s mass—estimated as between 6 and 15 Jupiter masses—could put it on either side of the line. And to top it off, it may be so cool that its gases could form clouds, a very planet-like thing to do.
So much still remains to be known about this particular brown dwarf and brown dwarfs in general, but one thing is set, at least for now: It’s the coolest one we’ve ever seen, and it may help us sort out this vague and messy mystery about the smudgy line between stars and planets.
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