In-depth characterization of the Kepler-10 three-planet system with HARPS-N RVs and Kepler TTVs

The old G3V star Kepler-10 is known to host two transiting planets, the ultra-short-period super-Earth Kepler-10b (P=0.837 d; R_p=1.47 R_⊕) and the long-period sub-Neptune Kepler-10c (P=45.294 d; R_p=2.35 R_⊕), and a non-transiting planet causing variations in the Kepler-10c transit times. Measurements of the mass of Kepler-10c in the literature have shown disagreement, depending on the radial-velocity dataset and/or the modeling technique used. Here we report on the analysis of almost 300 high-precision radial velocities gathered with the HARPS-N spectrograph at the Telescopio Nazionale Galileo over ∼11~years, and extracted with the YARARA-v2 tool correcting for possible systematics and/or low-level activity variations at the spectrum level. To model these radial velocities, we used three different noise models and various numerical techniques, which all converged to the solution: M_p,b=3.24±0.32 M_⊕ (10σ) and ρ_p,b=5.54±0.64 gcm⁻³ for planet b; M_p,c=11.29±1.24 M_⊕ (9σ) and ρ_p,c=4.75±0.53 gcm⁻³ for planet c; and M_p,dsini=12.00±2.15 M_⊕ (6σ) and P=151.06±0.48 d for the non-transiting planet Kepler-10d. This solution is further supported by the analysis of the Kepler-10c transit timing variations and their simultaneous modeling with the HARPS-N radial velocities. While Kepler-10b is consistent with a rocky composition and a small or no iron core, Kepler-10c may be a water world that formed beyond the water snowline and subsequently migrated inward.