Brown dwarf

An artist’s conception shows WISE J085510.83-071442.5, the coldest known brown dwarf.

Image: Penn State University/NASA/JPL-Caltech

Written by Sarah Kellett

Our Sun has a neighbour, and it’s as cold as the North Pole. An invisible brown dwarf has been found a mere 7.2 light years away, by space telescopes searching in the infrared.

A cosmic friend

On a dark night, the sky sparkles with stars beyond counting, but even the darkest night cannot reveal every object above. It took two space telescopes to spot the star-like object called WISE J085510.83-071442.5, which is thought to be a brown dwarf.

At only 7.2 light years away, it takes the title of the fourth-closest system to the Sun. Our closest star, Proxima Centauri, is just 4.2 light years away by comparison.

Our newly-found neighbour is very good at hiding. Brown dwarfs do not have enough mass to fuse hydrogen into helium, so they don’t produce lots of visible light energy like larger stars do. This new object is invisible to us. But it does produce another kind of light – infrared. We can’t see it, but infrared sensors can. The hidden brown dwarf was revealed by two NASA space telescopes, the Wide-field Infrared Survey Explorer (WISE) and the Spitzer Space Telescope.

Cool star

There are plenty of reasons to be excited about our new neighbour. It is the coldest brown dwarf ever found, with a temperature between –48 and –13 degrees Celsius. That’s similar to the North Pole! It is also particularly tiny, estimated to be three to ten times the mass of Jupiter.

Sun's neighbours

This diagram illustrates the locations of the star systems closest to the Sun.

Image credit: Penn State University

At that size, it could be a gas giant planet that has lost its star system. Kevin Luhman, who discovered the object, believes it is more likely to be a brown dwarf because they are known to be fairly common. Kevin also found the third-closest system to the Sun, a pair of warmer brown dwarfs.

“It is remarkable that even after many decades of studying the sky, we still do not have a complete inventory of the Sun’s nearest neighbours,” said Michael Werner, from NASA’s Jet Propulsion Laboratory, in the press release. “This exciting new result demonstrates the power of exploring the Universe using new tools, such as the infrared eyes of WISE and Spitzer.”

After thousands of years of looking with eyes, and hundreds with telescopes, it seems space still has plenty of surprises.

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