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2 stars and 2 planets?

An artist's depiction of the Kepler-47 system. Kepler-47c is the large planet on the left; Kepler 47-b appears as the small blue crescent to the right of the two stars.
[Image courtesy of NASA/JPL-Caltech/T. Pyle]

Astronomers have known that, unlike Earth, some planets orbit two stars instead of just one. But now, data from NASA's Kepler spacecraft has revealed a planetary system that consists of two planets orbiting around two stars. The discovery shows that a pair of stars, or a binary star system, isn't limited to just one planet.

Jerome Orosz and colleagues identified this system, known as Kepler-47, and say that it challenges the traditional model of planetary formation.

According to the researchers, Kepler-47's inner planet is 3.0 times the size of Earth and its outer planet is 4.6 times the size. The binary star system consists of a Sun-like star—about the same size and mass of our Sun—and a companion star that is roughly one-third its size. The inner planet orbits the two stars every 49.5 days while the outer planet takes 303.2 days to complete an orbit, they say.

The outer planet is probably a gas giant and not suitable for life, but Orosz and his colleagues say that it orbits the two stars in a "habitable zone," where liquid water could technically exist.