Counting Planets 
The number of planets discovered in other star systems has topped 500, with the census growing every month. But when you consider that the galaxy contains several hundred billion stars, the number isn't all that impressive. It's not really enough to answer the basic questions about planetary systems -- how they form, how they evolve, what types of planets are most common, and how the systems are laid out.
A planet-hunting space telescope known as Kepler is helping beef up the census. It's watching about 150,000 stars to see if their light dims a little bit as planets cross in front of them as seen from Earth. The observations can help answer several questions, as mission scientist Natalie Batalha explains:
BATALHA: We are trying to answer a very specific question, which is the frequency of potentially habitable Earth-size planets, but that doesn't mean that we don't want to understand the whole zoology of planets....You want to be able to see the big picture, so you need lots of numbers. Kepler is going to yield hundreds of planet discoveries. We already have 706 stars that show compelling signals. Many of them will turn out to be false positives, but we're still talking hundreds that are going to be studied in the years to come. So now you're really changing the way you look at planet formation and evolution, and you can start to study all of these statistics in a broader sense and get a feel for that big picture. [:43]
We'll have more about Kepler's quest for the "big picture" tomorrow.
Script by Damond Benningfield, Copyright 2010
Production and distribution of this episode made possible in part by a grant from NASA