The Composition and Mineralogy of Rocky Exoplanets: A Survey of >4,000 Stars from the Hypatia Catalog

Abstract

We present a survey of >4,000 star compositions from the Hypatia Catalog to examine whether rocky exoplanets (i.e., those with rocky surfaces, dominated by silicates) might be geologically similar to Earth, at least with respect to composition and mineralogy. To do so, we explore the variety of reported stellar compositions to then determine a possible range of exoplanetary mantle mineralogies. We find that exoplanetary mantles will likely be dominated by olivine and/or orthopyroxene, depending upon Fe partitioning during core formation. Some exoplanets may be magnesiowustite- or quartz-saturated, and we present a new classification scheme based on the weight $\%$ ratio (FeO+MgO)/SiO$_{2}$, to differentiate rock types. But wholly exotic mineralogies should be rare to absent. We find that half or more of the range of exoplanet mantle mineralogy is controlled by core formation, which we model using $\alpha_{Fe}$ = $Fe^{BSP}$/$Fe^{BP}$, where $Fe^{BSP}$ is Fe in a Bulk Silicate Planet (bulk planet, minus core), on a cation weight $\%$ basis (elemental weight proportions, absent anions) and $Fe^{BP}$ is the cation weight $\%$ of Fe for a Bulk Planet. In our solar system, $\alpha_{Fe}$ varies from 0 (Mercury) to about 0.54 (Mars). Remaining variations in exoplanet mantle mineralogy result from non-trivial variations in star compositions. Our major oxide analysis also verifies earlier, isotopic studies indicating that Earth is non-solar (non-chondritic). We also find that major oxide estimates for Earth's mantle appear to preclude a hidden component in the deep mantle that would allow for a bulk solar/chondritic Earth. If it did exist, such a hidden component must comprise at least 28% of the mass of the total mantle (to avoid negative concentrations of some oxides) and would not look anything like the sources of ocean island or mid-ocean ridge basalts.