The coldest brown dwarf ever discovered is colder than boiling water

Just 63 light-years away, there's a failed star known as a brown dwarf barely any bigger than Jupiter. It's temperature is way less than 100 degrees Celsius, which blurs the line even more between the smallest stars and biggest planets.

Brown dwarfs occupy an odd middle ground between planets and stars. They're not quite massive enough to sustain the hydrogen fusion required to become a star - that kicks in at roughly 75 to 80 Jupiter masses. But even the smallest brown dwarfs - and this new discovery certainly falls into that category - have some key features that distinguish them from large gas giants. They emit radiation in the X-ray and infrared spectra, the latter of which allowed NASA's Spitzer Space Telescope to detect this cold new brown dwarf. They also are very dense, as they are almost always about the size of Jupiter, meaning they pack many times that planet's mass into roughly the same volume.

In the past few weeks, we've actually discovered two incredibly cool brown dwarfs. The first, which has the incredibly catchy name of CFBDSIR J1458+1013B, was discovered by the Keck II Telescope in Hawaii. Located 75 light-years away, it's only about six to fifteen times the mass of Jupiter, and initial observations suggest its temperature is well below 100 degrees Celsius (212 degrees Fahrenheit), also known as the boiling point of water. If it's not quite at room temperature, then it's certainly close to it - indeed, this brown dwarf's temperature is probably lower than that of some saunas - which is absolutely amazing for an object that might have become a star.

But Spitzer's find is even more incredible. The melodiously named WD 0806-661B appears to have a temperature of just 30 degrees Celsius - or 86 degrees Fahrenheit - which is just unheard of. That said, there might be a non-brown-dwarf explanation for this object's existence. WD 0806-661B actually orbits a white dwarf, which is the dense, cool remnants of a star that was once much like our own, although in this case it was about twice our sun's mass.

So couldn't WD 0806-661B just be a planet? The thing is that this object orbits at a distance 2500 times that of the distance between Earth and the Sun. That's way too far out for planets to form, so that makes it far more likely that it's a failed star. But even then, there's some uncertainty, because models suggest that the surviving planets after a dying star expands into a supergiant and then collapses back into a white dwarf will drift out to far more distant orbits. It's possible that WD 0806-661B is just a planet, but its orbit is an incredibly long way to wander out to.

In any event, further examination of CFBDSIR J1458+1013B and WD 0806-661B should reveal their specific natures. If both hold up as brown dwarfs, then they're the first of the theorized "Y"-class brown dwarfs, a part of the family that is far more planet-like than star-like in their nature. Fascinatingly, these brown dwarfs are actually cool enough for water vapor to condense into clouds in their atmospheres, potentially making them way more Earth-like than any failed star has any right to be.

The Astrophysical Journal Letters via Discovery News.