Pinning down the mass of Kepler-10c: the importance of sampling and model comparison

Initial radial velocity (RV) characterization of the enigmatic planet Kepler-10c suggested a mass of ∼17 M_⊕, which was remarkably high for a planet with radius 2.32 R_⊕; further observations and subsequent analysis hinted at a (possibly much) lower mass, but masses derived using RVs from two different spectrographs (HARPS-N and HIRES) were incompatible at a 3σ level. We demonstrate here how such mass discrepancies may readily arise from suboptimal sampling and/or neglecting to model even a single coherent signal (stellar, planetary or otherwise) that may be present in RVs. We then present a plausible resolution of the mass discrepancy, and ultimately characterize Kepler-10c as having mass 7.37_{-1.19}^{+1.32} M_⊕, and mean density 3.14^{+0.63}_{-0.55} g cm^-3.