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 2014-2015 , 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 _ { \earth } ) has mass 3.72 \pm 0.42 ~ { } M _ { \earth } and density 6.46 \pm 0.73 ~ { } 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 \pm 0.11 of the planet mass .
For Kepler-10c ( R _ { p } = 2.35 ~ { } R _ { \earth } ) we measure mass 13.98 \pm 1.79 ~ { } M _ { \earth } and density 5.94 \pm 0.76 ~ { } g cm ^ { -3 } , significantly lower than the mass computed in .
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 3 \sigma 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 1-7 ~ { } M _ { \earth } .