The conclusions are based on the detection of two Saturn-size, gas giant planets—Kepler-34b and Kepler-35b—each orbiting its own pair of stars that are held close to each other by gravity. Until now only one circumbinary planet, Kepler-16b, had been documented.

All three discoveries were made using NASA's Earth-orbiting Kepler space telescope, which has detected more than 750 exoplanets—planets outside our solar system—by watching for dips in brightness of starlight as planets cross in front of their parent stars, as seen from Earth. (Related: "Fifty New Planets Found—Largest Haul Yet.")

The two new planets mean that Kepler-16b isn't a freak of nature, said William Welsh, lead author of the new study, published online today by the journal