NASA’s Kepler Space Telescope, which was launched to find potentially habitable, Earth-sized planets, has successfully completed its three-and-a-half-year prime mission and embarks on an extended one that could last four years.

Launched on March 6, 2009, scientists have used Kepler data to identify more than 2,300 planet candidates and confirm more than 100 planets – finding the galaxy is teeming with planetary systems, that planets are prolific and hints that nature makes small planets efficiently.

So far, hundreds of Earth-size planet candidates have been found as well as candidates that orbit in the habitable zone, the region in a planetary system where liquid water might exist on the surface of a planet, NASA said in a statement.

None of the candidates is exactly like Earth. With the completion of the prime mission, Kepler has now collected enough data to begin finding true sun-Earth analogs – Earth-size planets with a one-year orbit around stars similar to the sun.

“The initial discoveries of the Kepler mission indicate that at least a third of the stars have planets and that the number of planets in our galaxy must number in the billions,” said William Borucki, Kepler principal investigator at NASA’s Ames Research Center in Moffett Field, California.

“The planets of greatest interest are other Earths and these could already be in the data awaiting analysis. Kepler’s most exciting results are yet to come!” Borucki said.

The Telescope searched for planet candidates orbiting distant suns or exoplanets, by continuously measuring the brightness of more than 150,000 stars.

When a planet candidate passes, or transits, in front of the star from the spacecraft’s vantage point, light from the star is blocked.

Different sized planets block different amounts of starlight. The amount of starlight blocked by a planet reveals its size relative to its star.

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