Astronomers have recently announced the discovery of the coldest known brown dwarf, WISE-J0855, with a temperature of -20 degrees Celsius. This sounds really cold (and it is), but for comparison, the temperature of Jupiter's upper atmosphere is more like -145 degrees Celsius.

This new object is only about 5-10 times more massive than Jupiter, which means the pressure in its core isn't high enough to sustain nuclear fusion of hydrogen atoms. Straddling the boundary between the highest-mass planets, and the lowest-mass brown dwarfs, it's interesting to consider how WISE-J0855 formed. Was it formed more like a planet, in a disc around a bigger star, then ejected into space as a result of gravitational interactions with other bodies in the system? Or did it condense out of a collapsing cloud of dust and gas, in a manner more akin to how stars form?

Another interesting aspect of WISE-J0855 is that it's only 7 lightyears away from us, which is very nearby. The fact that it's only just been discovered suggests there could be many more out there that we haven't been capable of detecting until now.