Binary brown dwarfs

Week of December 7, 1998

A tidbit of news on the brown dwarf front came in this week.

Regular readers may remember that brown dwarfs are objects that are intermediate in mass between a planet and a star. In other words, they are more massive than a planet, but less massive than the lightest known 'normal' stars. A star like the Sun generates heat by fusing hydrogen into helium in its core, and to do that it needs enough gravity to crush the atoms of hydrogen into each other. If the star doesn't have the mass, it won't be able to fuse atoms, and it cannot make any energy that way.

A brown dwarf by definition lacks the mass to sustain fusion in the core. The big question is, how do brown dwarfs form? Do they form like stars, condensing out of clouds of gas and dust, or do they form like planets, slowly gathering mass from a disk of material around a 'real' star forming nearby? We are pretty sure that planets like the Earth and Jupiter, and even the supergiant planets recently found orbiting other stars, don't form out of the gas cloud itself but actually are created later, after the cloud collapses into a disk. The star forms in the center, and the planets farther out.

Most brown dwarfs are found orbiting other stars like planets would, but that is what astronomers call a 'selection effect': they are easiest to find near other stars, so that's where we find them. More recently, brown dwarfs are being discovered on their own, and not near other stars (some people call them 'free floating'). But we still don't know if those free-floaters formed on their own like stars, or formed like planets and were somehow ejected from their solar system.

However, that may have changed. We now know of at least two systems of binary brown dwarfs, that is, a pair of brown dwarfs orbiting each other. Binary stars are very common; as a matter of fact most stars are in binary systems! The Sun is actually an exception to this. We know therefore that making binary stars from a gas cloud is easy, and is actually the way stars prefer to form. From our own solar system (granted, only one example, but it's all we've got) we know that forming binary planets is not so easy, and from the physics of solar systems, we believe that ejecting them both together is even less likely. So this is even better evidence that brown dwarfs form like stars and not planets.

Brown dwarfs may not turn out to be the 'missing link' between planets and stars the way some astronomers thought a few years ago, but they are providing clues on just how stars and planets do form. NASA (and the public they serve) is very gung-ho about finding planets around other stars, and findings like the binary brown dwarfs are helping us understand just how other solar systems are created.