NASA’s WISE spacecraft spent a little over a year mapping stars in infrared light. Although NASA put it into hibernation in 2011, the agency turned it back on last year to continue its mission. Since then, it’s taken images of over 700 million celestial bodies and galaxies. After going over WISE’s data and comparing it to images from other telescopes, scientists have discovered a brown dwarf that is one of the closest objects to our Sun. But this brown dwarf is even more special than that, because it’s also the coldest one we’ve ever seen.
A brown dwarf is an object that sits somewhere in between a very big gas giant (like Jupiter) and a very small star. It doesn't contain sufficient mass to get hot enough to sustain fusion reactions in its core that allows it to emit light and heat. This results in brown dwarfs being cooler than most other celestial bodies, which makes them more difficult to spot and even invisible to visible light telescopes, since they don't emit visible light.
WISE, however, uses infrared light and scanned this part of the sky several times. These scans revealed a lot of movement from a particular object, which made it not only stand out, but also tells us that the object is relatively close. What we know about distance and movement is that the closer an object is, the faster we see it move. After careful study, astronomers estimated that this brown dwarf is just 7.2 light-years away from Earth, making it the fourth closest system to our Sun.
But what makes this brown dwarf truly extraordinary is its temperature, which scientists determined after examining images from two other telescopes. Comparing the images allowed astronomers to measure the temperature of the brown dwarf at somewhere between -4 and 9 degrees Fahrenheit. This is considerably colder than the coldest brown dwarf we’ve previously discovered, which was at a balmy room temperature.
So what does this really cold temperature tell us about the brown dwarf? Primarily, it’s a sign that the object is really, really old. Because a brown dwarf starts its life at thousands of degrees Fahrenheit right after it forms, a considerable amount of time has passed to make it so cool, at least a billion years or so. The brown dwarf also has a very low mass, which might indicate it was actually a rogue planet that got kicked out of its star system. Despite how difficult it is to see objects like these, astronomers believe that they're relatively common, suggesting that if we look hard enough, there might even be others out there, even closer to Earth.