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 ± 1.02 M ⊕ and R p = 1.71 − 0.057 + 0.069 R ⊕ , which leads to a density of ρ p = 9.4 − 1.5 + 1.6 g cm −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 an iron-free, MgSiO 3 mantle, and we determine that the range of the core mass fractions are consistent with the observed mass and radius. We use existing high-resolution spectra of the host star to derive the Fe/Mg/Si abundances ([Fe/H] = −0.03 ± 0.01, [Mg/H] = 0.04 ± 0.02, [Si/H] = 0.03 ± 0.06) to infer the composition of K2-106b. We find that K2-106b has a density and core mass fraction ( 44 − 15 + 12 % ) consistent with that of Earth (CMF ⊕ = 32%). Furthermore, 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.