More than 230 planets that have been discovered outside of our solar system since 1992. Only one even remotely resembles Earth: Gliese 581c, spotted this past April. NASA thinks there are many more worlds like ours out there. So the space agency is launching its first-ever planet-hunting mission next year, sending a $500 million spacecraft called Kepler on a four-year quest to find Earth-like planets. If it works out as planned, by 2012, NASA will have identified over 1,000 more planets – and close to 50 new Earths.
Until now, planet chasers — such as the European astronomers who found 581c — have worked from the ground. But the optical distortion caused by the atmosphere makes it nearly impossible to find relatively small worlds like ours. (Gas giants are easier to spot.) Kepler will get a clearer view by orbiting the sun. It will also look at a broader swath of the sky, using photoreceptors to scan more than 100,000 sunlike stars in the Cygnus constellation; that's a larger field of view than all the telescopes on Earth, combined.
My article in the current Popular Mechanics has additional details. This NASA site has way, way more.
