Is K2-106b truly a super-Mercury?

Super-Mercuries are a class of exoplanets with radii less than ~1.5 Re, high bulk densities and relatively large core-mass fractions (CMFs). The study of super-Mercuries will shed light on the composition of low-mass, terrestrial exoplanets as well as on the mechanisms that lead to the formation of iron-rich planets. However, only a few exoplanets have been confirmed as super-Mercuries, in part because of the challenges of obtaining the precise stellar and planetary parameters required to confirm them. We perform a reanalysis of the K2-106 system, which contains an ultra-short period, super-Mercury candidate with a density from the literature of 13.1 (+5.4 -3.6) g/cc, approximately twice the density of Earth. We globally model extant photometry and radial velocity of the system and derive a planetary mass and radius that leads to a considerably lower density than previously reported. We derive the host star’s Fe, Mg and Si abundances and combine them with planet interior models to infer the CMF and interior composition of K2-106b. Using a statistical framework, we compared the planet’s CMF as expected from the planet’s density and the CMF as expected from the host star. Our analysis suggests that, although K2-106b has a high density and CMF, it is statistically unlikely to be a super-Mercury.