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

Abstract

Initial RV characterisation of the enigmatic planet Kepler-10c suggested a mass of $\sim 17$ M${}_{\oplus }$, which was remarkably high for a planet with radius $2.32$ R${}_{\oplus }$; 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 $3\sigma$-level. We demonstrate here how such mass discrepancies may readily arise from sub-optimal 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 characterise Kepler-10c as having mass $7.37_{-1.19}^{+1.32}$ M${}_{\oplus }$, and mean density $3.14_{-0.55}^{+0.63}$ g cm${}^{-3}$.

7 pages, 4 figures. Accepted for publication in MNRAS Letters

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