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A circumbinary planet is an extra-solar planet with an orbit around the pair of individual stars of a binary star. The first such system was identified in 2003 and as of 2014, about 15 identifications have been confirmed. A stable orbit of such a planet must be beyond a certain critical radius or radius of stability, which is generally at least 2-4 times the semi-major axis of the stars' orbits. An example of a circumbinary planet is Kepler-16b.