NASA's Kepler mission has found the first multi-planet system orbiting a binary star, according to a study to be published Friday in the journal Science.

Located roughly 5,000 light-years away in the constellation Cygnus, the system, known as Kepler-47, contains a pair of stars whirling around each other every 7.5 days.

The primary star is about the same mass as the Sun, and its companion is an M-dwarf star one-third its size. The inner planet is three times the size of Earth and orbits the binary star every 49.5 days, making it the smallest known transiting circumbinary planet. The outer planet is 4.6 times the size of Earth with an orbit of 303.2 days, making it the longest-period transiting planet currently known.

The outer planet is the first planet found to orbit a binary star within the "habitable zone," where liquid water could exist and thus create a home for life. However, the planet's size (about the same as Uranus) means that it is an icy giant, and not an abode for life.

The discovery shows that planetary systems can form and survive even in the chaotic environment around a binary star and such planets can exist in the habitable zone of their stars.

Launched in March 2009, Kepler was designed to discover Earth- size planets orbiting other stars. Kepler searches for distant worlds by looking for "transits," when a planet passes in front of a star, briefly causing it to dim. The amount of dimming reveals the size of the planet compared to the size of the star.

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