Forming planets around stars with non-solar elemental composition

Abstract

Stars in the solar neighbourhood have refractory element ratios slightly different from the Sun. It is unclear how much the condensation of solids and thus the composition of planets forming around these stars is affected. We aim to understand the impact of changing the ratios of refractory elements Mg, Si, and Fe within the range observed in solar type stars within 150~pc on the composition of planets forming around them. We use the GGchem code to simulate the condensation of solids in protoplanetary disks with a Minimum Mass Solar Nebula around main sequence G-type stars in the Solar neighbourhood. We extract the stellar elemental composition from the Hypatia database. We find that a lower Mg/Si ratio shifts the condensation sequence from forsterite (Mg$_2$SiO$_4$) and SiO to enstatite (MgSiO$_3$) and quartz (SiO$_2$); a lower Fe/S ratio leads to the formation of FeS and FeS$_2$ and little or no Fe-bearing silicates. Ratios of refractory elements translate directly from the gas phase to the condensed phase for $T\,<\,1000$~K. However, ratios with respect to volatile elements (e.g.\ oxygen and sulphur) in the condensates -- the building blocks of planets -- differ from the original stellar composition. Our study shows that the composition of planets crucially depends on the abundances of the stellar system under investigation. Our results can have important implications for planet interiors, which depend strongly on the degree of oxidation and the sulphur abundance.