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)
We report the discovery of one newly confirmed planet (P = 66.06 days, R P = 2.68 ± 0.17 R ?) and mass determinations of two previously validated Kepler planets, Kepler-289 b (P = 34.55 days, R P = 2.15 ± 0.10 R ?) and Kepler-289-c (P = 125.85 days, R P = 11.59 ± 0.10 R ?), 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 (~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 (~1 Gyr as determined by isochrones and gyrochronology), Sun-like star with M * = 1.08 ± 0.02 M ?, R * = 1.00 ± 0.02 R ?, and T eff = 5990 ± 38 K. The middle planet's large TTV amplitude (~5 hr) 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 ± 6.8 M ?, 4.0 ± 0.9M ?, and M = 132 ± 17 M ?, which we designate PH3 b, c, and d, respectively. Furthermore, the middle planet, PH3 c, has a relatively low density, ? = 1.2 ± 0.3 g cm-3 for a planet of its mass, requiring a substantial H/He atmosphere of 2.1 0.8-0.3% by mass, and joins a growing population of low-mass, low-density planets.
