WE THINK of stars, even the failed ones called brown dwarfs, as being lethally hot. That picture has been confounded by a newly discovered brown dwarf - at room temperature.
Like fully fledged stars, brown dwarfs form from collapsing gas clouds, but they are not massive enough to sustain nuclear reactions. Instead, they briefly shine red from the heat of formation, then fade. Still, the coolest known brown dwarfs are all hot enough to roast any spacefarers who venture too close.
Now Kevin Luhman of Pennsylvania State University in University Park and colleagues have used NASA's infrared Spitzer Space Telescope to detect the glow of what appears to be a brown dwarf at just 30 °C (Astrophysical Journal Letters, DOI: 10.1088/2041-8205/730/1/L9).
The object, which orbits a white dwarf star 63 light years from Earth, weighs seven times as much as Jupiter. At that mass, it would normally be considered a planet. But planets form from discs of gas and dust around stars, and the researchers say that the object, known as WD 0806-661 B, lies too far from its star - at 2500 times the Earth's distance from the sun - to be deemed a planet if it formed where it is.
The object is a lot hotter than Jupiter, which is at a frigid -149 °C, and much cooler than the next coolest brown dwarf, at 100 °C. This means that WD 0806-661 B will act as a "missing link" to reveal how temperature affects the atmosphere and spectral features of objects that are roughly the size of Jupiter.
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Black
Thu Mar 10 14:55:33 GMT 2011 by Eric Kvaalen
"Brown dwarf" is not a very good name. Most of them are red, and this one would be basically black (although it may reflect some light from its companion star).
The temperature referred to by the way is the temperature of the layer that emits the light that gets to us. The interior is hot, and higher layers are presumably cooler than room temperature.
Black
Sat Mar 12 22:21:12 GMT 2011 by Jan-Willem
They used to be called black dwarfs, but then that name got used for white dwarfs that cooled down (not that we've ever seen one of those, since the universe isn't old enough for any to have cooled down that much yet).
Incorrect Definition
Sat Mar 12 18:11:58 GMT 2011 by Karl
http://www.lostworldarts.com
The article states, "brown dwarfs... are not massive enough to sustain nuclear reactions. Instead, they briefly shine red from the heat of formation, then fade."
This is wrong. A brown dwarf is massive enough to sustain the fusion of deuterium. If it wasn't, it wouldn't a star. It is not massive enough to continue fusing hydrogen after the deuterium runs out - hence the other name, "failed star".
Also, the IAU has defined a planet as an object which:
1. is in orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3. has "cleared the neighborhood" around its orbit.
How such a body formed is clearly not part of the definition.
And if you try to work out how the author's claim that WD 0806-661 B "lies too far from its star... to be deemed a planet" fits in, the author (and the IAU) might be embarrassed to find that NO extrasolar planet seems to meet the definition of planet, because none of them orbit the sun.
Incorrect Definition
Sat Mar 12 22:09:00 GMT 2011 by Jan-Willem
"This is wrong. A brown dwarf is massive enough to sustain the fusion of deuterium."
That is still subject to discussion according to the IAU website. It's true for brown dwarfs over 13 Jupiter masses (and over 65 they also fuse lithium), but people haven't agreed that 13 is the border between large planets and brown dwarfs.
Incorrect Definition
Sun Mar 13 00:55:43 GMT 2011 by Karl
http://www.lostworldarts.com
Well I certainly hope they won't decide to use an arbitrary mass as the cutoff, rather than the ability to sustain fusion.
In the meantime, I'd like to nominate the word "planetoid" to describe any object which:
a) has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
b) has sufficient mass to initiate any sort of nuclear fusion.
That would include dwarf planets, large moons, the original planets of antiquity, and WD 0806-661 B. In other words - if it looks like a planet and acts like a planet, it is a planetoid.
Incorrect Definition
Sun Mar 13 01:00:39 GMT 2011 by Karl
Oops, I meant:
"b) has insufficient mass to initiate any sort of nuclear fusion."
Always Blow On The Pie
Sun Mar 13 22:11:20 GMT 2011 by Dave
Can anyone suggest how long this thing is likely to stay above freezing, or how long it may have been in the liquid water range so far??
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