2014ApJ...795..167S

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, ρ = 1.2±0.3 g/cm3 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.