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" Of the more than 230 planets that have been discovered outside of our solar system since 1992, only one even remotely resembles Earth. Gliese 581c, which was spotted this past April, is 20.5 light-years distant, and appears to have liquid water and an average temperature between 32 F and 104 F. NASA, however, 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. Until now, planet chasers -- such as the European astronomers who found 581c -- have worked from the ground. But the optical distortion caused by the atmos\u00ADphere 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&amp;#039;s a larger field of view than all the telescopes on Earth, combined. The spacecraft will look for planets by measuring each star&amp;#039;s brightness. A passing world dims a star ever so slightly -- the size and length of the discoloration tell NASA how big the planet is, and how far it is from its star. William Borucki, a former Apollo engineer and Kepler&amp;#039;s principal investigator, is looking for planets from one-half to 10 times the Earth&amp;#039;s mass, with orbits within a star&amp;#039;s habitable zone. That&amp;#039;s what Borucki calls &amp;quot;the Goldilocks zone, where the star is just far enough away. The planet&amp;#039;s water doesn&amp;#039;t boil, and it doesn&amp;#039;t freeze, either.&amp;quot; Kepler is a single-minded spacecraft -- any planet that falls outside of Borucki&amp;#039;s parameters, whether it&amp;#039;s too large or its orbit is too wide, won&amp;#039;t register at all. And while Kepler would never have spotted 581c, because it orbits a red dwarf, NASA hopes to find close to fifty companion Earths by 2012. The Kepler spacecraft will seek Earth-like worlds (Planet 1) as they orbit a star. Planets that are too large (Planet 2) or that never cross Kepler&amp;#039;s viewpoint (Planet 3) won&amp;#039;t register.  "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/space/telescopes/a1716/4217638/>