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

TL;DR

This paper emphasizes the importance of proper sampling and comprehensive modeling in radial velocity data analysis, resolving previous mass discrepancies of Kepler-10c and accurately determining its mass and density.

Contribution

It demonstrates how sampling issues and neglecting signals can cause mass measurement discrepancies and provides a refined mass estimate for Kepler-10c.

Findings

Proper sampling and modeling resolve RV mass discrepancies.

Kepler-10c's mass is approximately 7.37 Earth masses.

The planet's mean density is about 3.14 g/cm³.

Abstract

Initial RV characterisation of the enigmatic planet Kepler-10c suggested a mass of $\sim17$ 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 $3.14^{+0.63}_{-0.55}$ g cm$^{-3}$.