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What's an Exoplanet?
An artist's depiction of an extrasolar gas giant planet with two moons in a young star system still filled with gas and dust.
Credit: Luke137 | Dreamstime

An exoplanet, or extrasolar planet, is a planet located outside our solar system, orbiting around a star other than our Sun. They range from gaseous giants 60 times the mass of Jupiter that whip around their stars in frenzying orbits to rocky "super-Earths," far more massive than our planet.

Astronomers theorized that planets orbit other stars for some time, but it wasn't until as recently as the 1990s that researchers first conclusively detected exoplanets. Now, astronomers have found 464 of these worlds to date. They range from about 20 light years to 1,000 light years away from us.

Finding exoplanets can tell us more about the universe and gives us tantalizing clues for finding planets which could have other forms of life . While none discovered so far seem to have conditions that would be habitable to life forms, astronomers speculate that potentially billions of exoplanets probably exist just in our own Milky Way galaxy, so the chances of finding Earth-like planets is possible.

But detecting exoplanets is tricky because planets reflect so little light, and are washed out in the glare of their star. Astronomers have detected a few directly, but usually rely on indirect methods such as observing the movement of a planet between the Earth and its star (which dims the starlight that reaches us by a minuscule amount), or by measuring slight gravitational shifts of a star as caused by its orbiting planets.

Many of the exoplanets discovered so far are Jupiter-like large and with hot, gassy atmospheres, with short orbits around their star (with "years" as short as a few days), because their size and nearness to their stars make them easier to detect. NASA's proposed Terrestrial Planet Finder program plans to further search for exoplanets that are in "habitable zones"with temperatures and atmospheres hospitable to humans.

The most recent discovery of exoplanets, which astronomers announced in June 2010, includes 6 diverse planets ranging from "shrunken Saturns to bloated Jupiters." One of the exoplanets, CoRoT-15b, is a brown dwarfa kind of cross between a very dense planet and a star. Last month, researchers also observed a "superstorm" on an exoplanet for the first time: a carbon monoxide wind on the Jupiter-like exoplanet HD209458b that streamed as fast as 6,200 mph (10,000 kph).How Big is Jupiter?

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