A Reanalysis of the Composition of K2-106b: an Ultra-short Period Super-Mercury Candidate

Abstract

We present a reanalysis of the K2-106 transiting planetary system, with a focus on the composition of K2-106b, an ultra-short period, super-Mercury candidate. We globally model existing photometric and radial velocity data and derive a planetary mass and radius for K2-106b of $M_p=8.53\pm 1.02M_{\oplus }$ and $R_p=1.71_{-0.057}^{+0.069}{R}_{\oplus }$, which leads to a density of $\ddot{I}{}_p=9.4_{-1.5}^{+1.6}$ $\mathrm{g}\mathrm{c}{\mathrm{m}}^{-3}$, a significantly lower value than previously reported in the literature. We use planet interior models that assume a two-layer planet comprised of a liquid, pure Fe core and iron-free, $\mathrm{MgSi}{\mathrm{O}}_3$ mantle, and we determine the range of core mass fractions that are consistent with the observed mass and radius. We use existing high-resolution spectra of the host star to derive Fe/Mg/Si abundances ([Fe/H]$=-0.03\pm 0.01$, [Mg/H]$=0.04\pm 0.02$, [Si/H]$=0.03\pm 0.06$) to infer the composition of K2-106b. We find that although K2-106b has a high density and core mass fraction ($44_{-15}^{+12}\%$) compared to the Earth ($33\%$), its composition is consistent with what is expected assuming that it reflects the relative refractory abundances of its host star. K2-106b is therefore unlikely to be a super-Mercury, as has been suggested in previous literature.

19 pages, 8 figures, submitted to AJ