Astronomers have in the past 20 years located several hundred planets orbiting distant stars, and they have only scratched the surface. In a small patch of stars—less than 1 percent of the sky—in the Northern Hemisphere, NASA's Kepler mission has already found more than 100 planets, along with strong hints of thousands more. Stars across the sky ought to be similarly laden with planets. A recent study indicated that each star hosts, on average, 1.6 planets. Exoplanets, as these strange worlds are called, are as plentiful as weeds—they crop up wherever they can. Whether any of them harbors life remains to be seen, but the odds of finding such a world are getting better.

More data in an interactive graphic at