Astronomers may have found the coldest star in the universe, a brown dwarf 75 light years from Earth.
Using NASA's Spitzer Space Telescope, Kevin Luhman and John Bochanski of Penn State University and Adam Burgasser of the University of California, San Diego, discovered what looks like a faint companion to a white dwarf star called WD 0806-661. The companion orbits at a distance of 2,500 astronomical units, or about 374 billion kilometers, far enough that light takes a full 11 days to get between them.
The scientists looked at the age of the white dwarf, and came up with a figure of about 1.5 billion years. They then estimated the mass of the companion, and used the data from Spitzer, which sees in the infrared part of the spectrum. From that, they got a temperature of about 300 degrees Kelvin, or 27 degrees C. That's about the average daytime temperature of Washington, D.C. in June. If confirmed it would be the coldest brown dwarf ever discovered. An average star has a surface temperature measured in the thousands of degrees. The Sun's surface temperature is 5,500 degrees C.
Brown dwarfs are bodies that are more massive than planets, but not massive enough that the pressure inside them can fuse hydrogen. That fusion is what makes all stars shine, including the Sun. Generally the upper limit for the mass of such an object is 75 to 80 times the mass of Jupiter.
There is some question as to whether it would count as a brown dwarf, though. It could be a planet that got pushed out to where it is when the star that would become the white dwarf went nova. Stars like the Sun, when they age, eventually blow off their outer layers and become white dwarfs.
If that happened in this case, the blast from the parent star could have pushed any planets out to wider orbits. If that isn't the case it probably wouldn't be a planet, because the gas and dust that surrounds young stars doesn't tend to form massive objects that far away from the primary.
If it turns out to be a brown dwarf, then it would fall into a class that has only been a theoretical one so far, and it would be cool enough that water vapor in its atmosphere could form clouds and water.