Planet Hunters VII. Discovery of a New Low-Mass, Low-Density Planet (PH3 c) Orbiting Kepler-289 with Mass Measurements of Two Additional Planets (PH3 b and d)

Abstract

We report the discovery of one newly confirmed planet ($P=66.06$ days, $R_{\mathrm{P}}=2.68\pm 0.17R_{\oplus }$) and mass determinations of two previously validated Kepler planets, Kepler-289 b ($P=34.55$ days, $R_{\mathrm{P}}=2.15\pm 0.10R_{\oplus }$) and Kepler-289-c ($P=125.85$ days, $R_{\mathrm{P}}=11.59\pm 0.10R_{\oplus }$), through their transit timing variations (TTVs). We also exclude the possibility that these three planets reside in a $1:2:4$ Laplace resonance. The outer planet has very deep ($\sim 1.3$), high signal-to-noise transits, which puts extremely tight constraints on its host star's stellar properties via Kepler's Third Law. The star PH3 is a young ($\sim 1$ Gyr as determined by isochrones and gyrochronology), Sun-like star with $M_{\ast }=1.08\pm 0.02M_{\odot }$, $R_{\ast }=1.00\pm 0.02R_{\odot }$, and $T_{\mathrm{eff}}=5990\pm 38$ K. The middle planet's large TTV amplitude ($\sim 5$ hours) resulted either in non-detections or inaccurate detections in previous searches. A strong chopping signal, a shorter period sinusoid in the TTVs, allows us to break the mass-eccentricity degeneracy and uniquely determine the masses of the inner, middle, and outer planets to be $M=7.3\pm 6.8M_{\oplus }$, $4.0\pm 0.9M_{\oplus }$, and $M=132\pm 17M_{\oplus }$, which we designate PH3 b, c, and d, respectively. Furthermore, the middle planet, PH3 c, has a relatively low density, $\ddot{I}=1.2\pm 0.3$ g/cm${}^3$ for a planet of its mass, requiring a substantial H/He atmosphere of $2.1_{-0.3}^{+0.8}$ by mass, and joins a growing population of low-mass, low-density planets.

21 pages, 10 figures, 5 tables, accepted into ApJ