I saw this in an article concerning Kepler and the SETI program:
What Kepler is telling us about habitable planets that we didn’t know beforehand is info that bears directly on the search for life," says Geoff Marcy, an astronomer at UC Berkeley and a co-investigator on the Kepler project. So far, he says, Kepler has discovered about 3000 planets, and about three-quarters of those are between the size of Earth and two to three times that of Earth. "What are these planets? We have some very good clues," he says. "We have been able to measure the density of 15 or so. They are nearly one gram per cubic centimeter. That’s the density of water." And water is the first and most important indicator of the possibility of life.
"When you look up at the night sky at the stars your naked eye can see, what you know is that a vast majority of those stars have these remarkable planets," Marcy says. "There must be large amounts of water . . . and water is the key solvent of biochemical reactions. I think what Kepler has discovered is it’s very likely these water worlds are a predominant type of planet in the universe and can certainly serve as the petri dishes for life as we know it."
Seth Shostak, senior astronomer at the SETI Institute, says Kepler’s data has helped SETI scientists sharpen their methods in the search for life. Without Kepler, they’d still be looking at the entire sky. With Kepler, they have a host of new exoplanets to study—and they know a lot more about the makeup of alien solar systems.
"The trump card in all this is the result that planets are very commonplace. Before 1995, we didn’t know that," Shostak says. "Essentially all stars have planets. The number of planets around stars is in the order of a trillion. That’s very good news for SETI."