Brown Dwarfs: The Coolest Stars or the Hottest Planets?

Brown dwarfs mix features of stars and planets, but they are their own unique entities. This artist’s illustration shows the brown dwarf 2MASSJ22282889-431026, which NASA’s Spitzer and Hubble space telescopes simultaneously observed. Credit: NASA/JPL-Caltech
It sounds like the start of a really bad riddle: „I’m bigger than a planet, but I’m not a star. And I’m smaller than a star, but I’m not a planet. Who am I?“

By Paul Sutter |

But instead of a cheesy brain-teaser, this is actually an entry point into exploring just how astronomers classify the exotic objects of our universe. It’s also an opportunity to understand some of the weirdest citizens of the Milky Way.

The answer to the riddle, by the way, is „I’m a brown dwarf.“ [Brown Dwarfs: Strange Failed Stars of the Universe Explained (Infographic)]

What’s in the name?

Despite the name, brown dwarfs are not very brown. These objects, with masses ranging from 12 times that of Jupiter up to half the mass of the sun, emit light on their own … just usually not very much. The largest and youngest ones are quite hot, giving off a steady glow of warm light. From a distance, those stars would look indistinguishable from their stellar cousins, the red dwarfs. The smallest and oldest ones, by contrast, are barely visible, emitting radiation firmly in the infrared part of the spectrum. You wouldn’t even be able to pick them out without the help of night-vision goggles.

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