NASA's planet-hunting Kepler space telescope discovered more than 100 confirmed planets in its just-completed main mission and will now begin to hunt for Earth-like planets on an extended mission that could last to 2016, the space agency announced this week.

The Kepler space observatory was launched on March 7, 2009 atop a Delta II rocket and has spent the last three-and-a-half years scanning more than 150,000 stars for signs of planetary activity from an Earth-trailing heliocentric orbit.

Kepler measures the brightness of stars, looking for interruptions in their light that indicate a planet may be crossing the star's face. So far, the telescope has identified more than 2,300 possible planetary bodies, of which more than 100 have been confirmed to be planets orbiting their own suns.

"The initial discoveries of the Kepler mission indicate at least a third of the stars have planets and the number of planets in our galaxy must number in the billions. The planets of greatest interest are other Earths and these could already be in the data awaiting analysis," William Borucki, Kepler principal investigator at NASA's Ames Research Center in Moffett Field, Calif., said in a statement.

"Kepler's most exciting results are yet to come," he added.

Among Kepler's discoveries are Earth-sized planets and planets orbiting their stars at a similar distance from their suns regarded as the "habitable zone" based on our own planetary home's particular ingredients for life, such as the existence of liquid water.

NASA said no candidate planet discovered thus far by Kepler "is exactly like Earth," but noted that the spacecraft "has collected enough data to begin finding true sun-Earth analogsEarth-size planets with a one-year orbit around stars similar to the sun."

Kepler's first major discoveries were five exoplanets called "hot Jupiters," enormous bodies orbiting their stars very closely, the space agency said. Over the next months and years, the spacecraft beamed back data pointing to a wide variety of planet and planetary system types, including multi-planet systems, small rocky planets, densely packed solar systems, and even a planet that exists in a binary star system like the desert planet Tatooine from Star Wars.

Last December, the space telescope identified its first Earth-like planet located in the habitable zone, dubbed Kepler-22b, which paved the way for an extension of Kepler's mission for as much as four years beyond its primary task.

NASA extended the Kepler mission in April.

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NASA's Kepler Renews Hunt for Earth-Like Planets