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 ...