Discovery of a Third Transiting Planet in the Kepler-47 Circumbinary System

Of the nine confirmed transiting circumbinary planet systems, only Kepler-47 is known to contain more than one planet. Kepler-47 b (the "inner planet") has an orbital period of 49.5 days and a radius of about 3 R-circle plus. Kepler-47 c (the "outer planet") has an orbital period of 303.2 days and a radius of about 4.7 R-circle plus. Here we report the discovery of a third planet, Kepler-47 d (the "middle planet"), which has an orbital period of 187.4 days and a radius of about 7 R-circle plus. The presence of the middle planet allows us to place much better constraints on the masses of all three planets, where the la ranges are less than 26 M-circle plus, between 7-43 M-circle plus, and between 2-5 M-circle plus for the inner, middle, and outer planets, respectively. The middle and outer planets have low bulk densities, with rho(middle) < 0 . 68 g cm(-3) and rho(outer) < 0.26 g cm(-3) at the 1 sigma level. The two outer planets are "tightly packed," assuming the nominal masses, meaning no other planet could stably orbit between them. All of the orbits have low eccentricities and are nearly coplanar, disfavoring violent scattering scenarios and suggesting gentle migration in the protoplanetary disk.