NASA’s Kepler planet-finder has started an extended mission that could last as long as four more years, after finding more than 2,300 candidate extra-solar planets in its 3.5-year primary mission by measuring the faint flickers of distant stars.

The candidate list, which must be confirmed by Earth-based observations, includes “hundreds” of planets that are roughly the same size as Earth. A prime objective of the extended mission will be to find a true Earth analog in terms of size, star type and orbit.

“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,” said William Borucki of Ames Research Center, the mission’s principal investigator.

“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.”

Kepler has been monitoring more than 150,000 stars since May 12, 2009, for the tiny dip in brightness that occurs when a planet passes in front of one of them during the course of its orbit.

It has found multi-planet systems, a planet in the habitable zone where water would remain liquid, and rocky planets like Earth. But it hasn’t found one exactly like Earth , although the mission team sees a trend in the data to confirming smaller planets with Earth-like orbital periods.

“The most wonderful discovery of the mission has not been individual planets, but the systems of two, three, even six planets crowded close to their stars and, like the planets orbiting about our Sun, moving in nearly the same plane,” says Jack Lissauer, a planetary scientist at Ames. “ … What are the neighborhoods of Earth-size exoplanets like? This is the question I most hope Kepler will answer in the years to come.”