Using NASA's Kepler spacecraft, astronomers are beginning to find Earth-sized planets orbiting distant stars. Kepler detects planetary candidates using the transit method, watching for a planet to cross a star and create a mini-eclipse that dims the star slightly. A new analysis of Kepler data shows that about 17 percent of stars have an Earth-sized planet in orbit. Since the Milky Way has about 100 billion stars, there are at least 17 billion Earth-sized worlds out there.