Ultra-cold neighbor star discovered — or is it a planet?


Time-lapse image of of WISE 0855–0714’s movement in the sky. [Credit: NASA/JPL-Caltech/Penn State University]

Think all stars are hot? Think again. The coldest star ever discovered, a sub-brown dwarf called WISE J085510.83-071442.5, has been found to have a surface temperature somewhere in the range of -54 to 9 degrees F. It’s also one of the closest stars to our solar system at 7.2 light-years. 

That is, IF it is a star.

WISE J085510.83-071442.5 (the WISE stands for Wide-field Infrared Survey Explorer; the numbers stand for its celestial coordinates) is only 3-10 times the mass of Jupiter. We know of exoplanets — planets orbiting stars other than the Sun — with masses greater than this. So, with such a low mass, one might reasonably wonder why this object isn’t classified as a planet — and the distinction is not always clear to scientists, either. At such low masses, the distinction is how the body formed. In the case of our super-cold neighbor, it is believed to have formed by the collapse of a gas cloud rather than as a planet that formed in a star system and was later ejected. But scientists haven’t ruled out the latter scenario.

relative star sizes

Relative star sizes [Credit: NASA]

As the image above shows, there is a gradual progression from “normal” stars, like our Sun, to cooler, redder stars down to gas giant planets. Sometimes distinctions in science are a bit arbitrary — unlike, say, the distinction between a human and a volcano — but scientists make them in an attempt to better understand what they’re studying.