The telescope, named Kepler, rode into a starry night sky aboard an unmanned Delta rocket that blasted off at 10:49pm EST (0349 GMT) from Cape Canaveral Air Force Station in Florida.
"So far, although we've discovered more than 300 planets (beyond the solar system), we haven't discovered any Earths," said Nasa's associate administrator for space science Ed Weiler.
Kepler, named for the 17th century astronomer Johannes Kepler, is designed to do just that.
Once in position trailing Earth around the sun, Kepler will turn its gaze onto a patch of sky between the constellations Cygnus and Lyra that is filled with more than 4 million stars. Scientists plan to scrutinize Kepler's observations of more than 100,000 targets in hopes of catching tiny blinks of light caused by passing planets.
"Trying to detect Jupiter-size planets crossing in front of their stars is like trying to measure the effect of a mosquito flying by a car's headlight," said Jim Fanson, Kepler project manager. "Finding Earth-sized planets is like trying to detect a very tiny flea."
The measurements will not only be difficult to make, they will be time-consuming.
A planet the size of Earth that is about as far from its parent star as Earth is, will pass by Kepler's view just once a year. Scientists say they'll need to catch three transits to verify existence of an Earth-sized world.
Nasa hopes to follow up the $591-million (Dh2.2 billion) Kepler mission with a new generation of powerful telescopes capable of directly imaging Earth-sized planets and analyzing their atmospheres for gases indicative of life.
"I think we would be absolutely astonished if Kepler didn't find any Earth-like planets," said astronomer Alan Boss with the Carnegie Institution of Washington. "I think we're going to find that the number of Earths is quite large."
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