Forming super-Mercuries: Role of stellar abundances

Abstract

Rocky exoplanets with bulk iron mass fraction of more than 60%, known as super-Mercuries, appear to be preferentially hosted by stars with higher iron mass fraction than that of the Sun. It is unclear whether these iron-rich planets can form in the disc or whether giant impacts are necessary for their formation. Here, we investigate the formation of super-Mercuries in their natal protoplanetary discs by taking into account their host stars’ abundances (Fe, Mg, Si, and S). We employed a disc evolution model which includes the growth, drift, evaporation, and recondensation of pebbles to compute the pebble iron mass fraction. The recondensation of outwardly drifting iron vapour near the iron evaporation front is the key mechanism that facilitates an increase in the pebble iron mass fraction. We also simulated the growth of planetary seeds around the iron evaporation front using a planet formation model which includes pebble accretion and planet migration and we computed the final composition of the planets. Our simulations were able to reproduce the observed iron compositions of the super-Mercuries, provided that all the iron in the disc are locked in pure Fe grains and that the disc viscosity is low (α ~ 10−4). The combined effects of slow orbital migration of planets and long retention time of iron vapour in low-viscosity discs makes it easier to form iron-rich planets. Furthermore, we find that decreasing the stellar Mg/Si ratio results in an increase in the iron mass fraction of the planet due to a reduction in the abundance of Mg2SiO4, which has a very similar condensation temperature as iron, in the disc. Our results imply that super-Mercuries are more likely to form around stars with low Mg/Si (≲ 1), in agreement with observational data.