Before we start feeling too special about our home planet, scientists have a message: There are at least 17 billion planets the size of Earth in the Milky Way alone, Space.com reports. About 17% of our galaxy's stars have Earth-size exoplanets closely orbiting them—so 100 billion stars in the galaxy means 17 billion such planets. And half those stars have tightly-orbiting planets that are Earth-size or larger.
That's just the beginning: Further from the stars, there are probably planets that could actually support life. "These kind of rocky objects are everywhere," says an astronomer. Indeed, the Kepler Space Telescope, which finds planets by noting stars' dimming as planets pass, found 2,700 planetary candidates in 22 months of searching—and simulations suggest that 90% of the candidates the telescope finds are in fact planets. Overall, there are likely 100 billion alien planets in our galaxy. (Read more galaxy stories.)