The Neptune-sized Circumbinary Planet Kepler-38b

We discuss the discovery and characterization of the circumbinary planet Kepler-38b. The stellar binary is single-lined, with a period of 18.8 days, and consists of a moderately evolved main-sequence star (M_A = 0.949 ± 0.059 M _⊙ and R_A = 1.757 ± 0.034 R _⊙) paired with a low-mass star (M_B = 0.249 ± 0.010 M _⊙ and R_B = 0.2724 ± 0.0053 R _⊙) in a mildly eccentric (e = 0.103) orbit. A total of eight transits due to a circumbinary planet crossing the primary star were identified in the Kepler light curve (using Kepler Quarters 1-11), from which a planetary period of 105.595 ± 0.053 days can be established. A photometric dynamical model fit to the radial velocity curve and Kepler light curve yields a planetary radius of 4.35 ± 0.11 R _⊕, or equivalently 1.12 ± 0.03 R _Nep. Since the planet is not sufficiently massive to observably alter the orbit of the binary from Keplerian motion, we can only place an upper limit on the mass of the planet of 122 M _⊕ (7.11 M _Nep or equivalently 0.384 M _Jup) at 95% confidence. This upper limit should decrease as more Kepler data become available.

Based on observations obtained with the Hobby-Eberly Telescope, which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen.