NUMBER 10
Brown Dwarfs

Brown dwarfs are the misfits of the universe, something like bit actors in a Hollywood movie -- not just extras, but not genuine stars, either. They are too big to be called planets, yet too small to generate the thermonuclear fusion required to become real stars.

They exist in a world all their own. Sometimes, it's a cold, dark world -- many wander through space alone. Others make a home in the shadow of a great star, gravitationally tethered in what scientists call a binary system.

Either way, they are weird things. And, as researchers are only beginning to learn, there may be as many brown dwarfs as there are stars. But only a few have been discovered since the first was found in 1995.

"Determining how many brown dwarfs exist will tell us about where these objects come from and what is their contribution to the overall chemical and dynamical budget of the Milky Way," says Eduardo Martin of the University of Hawaii.

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More About Brown Dwarfs

Suspected since 1963 and confirmed to exist in 1995, brown dwarfs are enormous compared to the planets in our solar system. They can be up to 75 times as massive as Jupiter. Somewhere near or above that range, an object has enough mass to generate thermonuclear fusion -- converting hydrogen to helium -- and a star is born.

Being less energetic than real stars, brown dwarfs burn deuterium, which causes them to emit just enough light to be detected (unlike planets, which emit no visible light).

Yet despite the differences, some brown dwarfs appear very planet-like. They can have a diameter close to that of Jupiter. And like a planet, brown dwarfs often orbit stars, implying that they form out of the swirling disk of gas and dust left after the formation of the star, which is exactly how the planets in our solar system got their start.

But like real stars, brown dwarfs can also be born out of an otherwise unorganized cloud of gas and dust, when gravity forces a direct collapse of the cloud.

The nearest confirmed brown dwarf is 16 light-years away. (The nearest known star is Proxima Centauri, at 4.2 light-years away. A light-year is the distance light travels in one year, about 5.88 trillion miles, or 9.7 trillion kilometers.)

But recently, researchers discovered what might be a brown dwarf wandering alone through space just 13 light-years from Earth -- practically in our backyard. And there might be many more, some even closer, researchers say. But they would be cooler, fainter and even tougher to spot with existing telescopes.

Understanding more about these weird objects will allow researchers to develop firmer definitions for stars and planets.

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