Revised Masses and Densities of the Planets around Kepler-10

March 2016 • 2016ApJ...819...83W

Authors • Weiss, Lauren M. • Rogers, Leslie A. • Isaacson, Howard T. • Agol, Eric • Marcy, Geoffrey W. • Rowe, Jason F. • Kipping, David • Fulton, Benjamin J. • Lissauer, Jack J. • Howard, Andrew W. • Fabrycky, Daniel

Abstract • Determining which small exoplanets have stony-iron compositions is necessary for quantifying the occurrence of such planets and for understanding the physics of planet formation. Kepler-10 hosts the stony-iron world Kepler-10b, and also contains what has been reported to be the largest solid silicate-ice planet, Kepler-10c. Using 220 radial velocities (RVs), including 72 precise RVs from Keck-HIRES of which 20 are new from 2014 to 2015, and 17 quarters of Kepler photometry, we obtain the most complete picture of the Kepler-10 system to date. We find that Kepler-10b ({R}_{{p}}=1.47 {R}_\oplus) has mass 3.72\quad +/- \quad 0.42\quad {M}_\oplus and density 6.46\quad +/- \quad 0.73\quad {{g}} {{cm}}^-3. Modeling the interior of Kepler-10b as an iron core overlaid with a silicate mantle, we find that the iron core constitutes 0.17 ± 0.11 of the planet mass. For Kepler-10c ({R}_{{p}}=2.35 {R}_\oplus) we measure mass 13.98\quad +/- \quad 1.79\quad {M}_\oplus and density 5.94\quad +/- \quad 0.76\quad {{g}} {{cm}}^-3, significantly lower than the mass computed in Dumusque et al. (17.2+/- 1.9 {M}_\oplus). Our mass measurement of Kepler-10c rules out a pure stony-iron composition. Internal compositional modeling reveals that at least 10% of the radius of Kepler-10c is a volatile envelope composed of hydrogen-helium (0.2% of the mass, 16% of the radius) or super-ionic water (28% of the mass, 29% of the radius). However, we note that analysis of only HIRES data yields a higher mass for planet b and a lower mass for planet c than does analysis of the HARPS-N data alone, with the mass estimates for Kepler-10 c being formally inconsistent at the 3σ level. Moreover, dividing the data for each instrument into two parts also leads to somewhat inconsistent measurements for the mass of planet c derived from each observatory. Together, this suggests that time-correlated noise is present and that the uncertainties in the masses of the planets (especially planet c) likely exceed our formal estimates. Transit timing variations (TTVs) of Kepler-10c indicate the likely presence of a third planet in the system, KOI-72.X. The TTVs and RVs are consistent with KOI-72.X having an orbital period of 24, 71, or 101 days, and a mass from 1 to 7 {M}_\oplus.

W.M.O. Keck Observatory.

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Benjamin Fulton

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