Bill Borucki is the principal investigator for NASA's planet-finding Kepler spacecraft. At a recent meeting of the American Astronomical Society, Borucki explained how long it will be before Kepler can tell us whether habitable, Earth-like planets are common or rare:

“I really think by three years from now we will have that answer. The mission is designed to do it in that period of time.”

That's for Earth-like planets orbiting sun-like stars. For habitable planets orbiting brighter, hotter stars, the search will take longer. Kepler has reserve fuel if NASA extends the mission.

“Each year we give you more information. We find more Earths, we find more in the habitable zone. Because if you found one in the habitable zone, would that mean it had life? It might not. Earth might be very special. But if you find 10 of them, or a hundred, or a thousand, almost certainly some of them have life. There are so many opportunities for life to develop, it will have taken advantage of some of those. So each year we will be able to give you a more full answer, a more certain answer. Certainly in three years, we will give you the first good answers. And after that they will be even better.”

—John Matson

[The above text is a transcript of this podcast.]