A brown dwarf is an object that's too big to be a planet - despite the lack of a good definition of what a planet actually is - and too small to be a star (most astronomers think of a star as an object that can have sustained fusion of hydrogen into helium in its core). They were theoretical until the first one was spotted in the 90s, and now we know of hundreds.

Brown dwarfs don't generate any heat through fusion, so once formed, they just basically sit there, cooling off. Once they're a few billion years old they're pretty hard to spot, since they don't give off much light. They're brightest in the infrared, and that's how most are discovered. The hotter they are the easier they are to find, and most have temperatures around 1000°C, much hotter than your oven (but far cooler than, say, the Sun).

WISE, NASA's Wide-field Infrared Survey Explorer, was a brown dwarf-finding machine, since it was built to look at just the wavelengths brown dwarfs emit. It's found quite a few, and most were easy to spot - due to the way the images were put together, brown dwarfs appear green in the pictures (though really the colors of brown dwarfs in visible light vary wildly). You can see one circled in the image above - it's called WISE 1828+2650 and what's amazing about it is that its temperature is a mere 25° C! That's right: it's basically room temperature. Now, it's actually a giant ball of gas the size of Jupiter but with probably dozens of times the mass, so you'd be crushed to death if you could stand on its surface, which you couldn't, because it doesn't really have a surface. But you'd be at a comfy temperature while it happened.

I studied brown dwarfs for a while when I was working on Hubble; at the time we didn't know much about them but I found them fascinating. And now I can say, without fear of being contradicted, that they are literally cool.

Image credit: NASA/JPL-Caltech/UCLA

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