The planet-hunting space telescope, Kepler, released its first big batch of data today.

That should be exciting, but the team held back the good stuff until February 2011, wanting to analyze and follow up on the early observations themselves. Kepler is trying to find Earth-like planets that exist at just the right distance from their home stars to retain water in liquid form.

Of the 156,000 target stars in the telescope's field of vision, the 43 days of observations found 706 possible extrasolar planets from Earth size up to a bit bigger than Jupiter. Today, the NASA Ames Research Center crew, led by William Borucki, released data on the 306 targets they're least excited about. Their top 400 candidates to investigate as possible Earth twins will not be announced for eight more months.

"Many of the candidates are likely to be false positives and the brighter stars, and those with the small-size candidates ... are among the 400 withheld targets and are thus not among those considered here, biasing the results toward the dimmer stars and larger candidates," Borucki wrote in an article posted to arXiv.org.

The data release plan was approved earlier this year by a special NASA advisory board, but has recently touched off controversy over its fairness and wisdom.

Without all the data in hand, it's hard to answer the question that Kepler was built to answer: How common are planets like Earth? Though we now know hundreds of exoplanets, most of them are big, hot Jupiters around very bright stars that could not sustain any kind of life that we recognize. It's easy to detect the bigger planets that orbit close to their stars because their gravity makes the star "wobble" more noticeably and their size dims its light more. So, the data we've collected on extrasolar planets over the last two decades is muddied by observation bias.

Borucki's mission, which he pushed for over decades before finally getting funding, is like a stellar census that may reshape our notions about the prevalence of life and the habitability of the universe. By figuring out how many Earth-like planets exist, we will have a much better idea about some key variables in the Drake equation, which attempts to describe the likelihood of finding intelligent life.

The mission is complicated by the fact that it finds planets by monitoring stars that periodically dim when planets cross in front of them. For us to see that happening from our solar system, we have to be very precisely aligned with the other star and planet. And we may have to wait a very long time for a planet like Earth to orbit its star twice: Alien astronomers would have to wait two years to observe our planet transiting the sun twice.

"If we were to assume that every star had an Earth-like planet in an Earth-like orbit, the likelihood we could see it is half of a percent," said Charles Sobeck, a systems engineer at NASA Ames and the deputy project manager of the Kepler mission. "You not only have to have it lined up, the star has to be bright enough too."

Image: NASA.

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