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NASA’s Kepler Mission has identified more than 750 possible extrasolar planets after monitoring more than 156,000 stars. But it’s keeping the best data back.

The Kepler space observatory looks for the data signatures of planets by measuring tiny decreases in the brightness of stars when planets cross in front ofthem. The size of the planet can be derived from the change in the star’s brightness.

NASA is examining 43 days of science data on the stars, looking for subtle brightness changes to determine if orbiting planets are responsible, and has released much of it to the scientific community.

But data on some of the stars is being kept back, while the team uses ground-based telescopes and the Hubble Space Telescope and Spitzer Space Telescope to perform follow-up observations on a specific set of 400 objects of interest.

The rest of the data will be released to the scientific community next February, after the Kepler team was granted an extension to the period during which it can keep it under wraps

Kepler will carry on working until at least November 2012, searching for planets – especially Earthlike ones.

Since transits of planets in the habitable zone of solar-like stars occur about once a year and require three transits for verification, it is expected to take at least three years to locate and verify an Earth-size planet.