Habitable Zones 
Many people aren't fond of rainy days. But if you're a scientist looking for life elsewhere in the universe, planets with rainy days are just what you want to find.
BOSS: My name is Alan Boss. I'm a member of the Science Working Group for the Kepler Project. The habitable zone is the distance from a star where an Earth-like planet with an Earth-like atmosphere should have temperatures amenable to having liquid water somewhere on its surface. Basically it's a planet where you might have a rainy day. [:18]
The Kepler project is a space telescope that's been looking for Earth-sized worlds orbiting Sun-like planets at just the right distance for those rainy days. Planets that are too close would be too hot, while planets that are too far would be too cold. And as Alan Boss notes, it's in that "just-right" zone between too hot and too cold that life as we know it is most likely to form and survive.
BOSS: At the very least, we want to be able to find something which looks more or less like our Earth because we know for sure that, at least in one case, life has evolved there. [:08]
Kepler's looking for such planets at about 150,000 stars. And another project scientist, Natalie Batalha, says the science team has high hopes of finding them.
BATALHA: The mission was designed so that if Earth-size habitable planets are common in our galaxy -- that is, every star has one -- Kepler should expect to detect about 50 of them, 50 to 100. If you find zero, that means, 'Wait a second, those types of planets must be rare. I expected 50 to 100, I got zero.' Then you can say something statistically significant about that result. [:22]
More about Kepler's numbers game tomorrow.
Script by Damond Benningfield, Copyright 2010
Production and distribution of this episode made possible in part by a grant from NASA