Extreme compositional diversity among rocky exoplanets

To date, more than 5000 exoplanets have been discovered, of which roughly 1800 are thought to be rocky on the basis of mass and radius measurements. However, the resulting densities are degenerate with respect to the compositions of these planets, necessitating other constraints. Of these, the most readily available is the composition of the host star (e.g. Adibekyan et al. 2021). On this basis, the compositions of rocky exoplanetary mantles are expected to be similar to those found in our own Solar System (e.g. Putirka and Rarick, 2019; Spargaaren et al. 2022). Here, we evaluate this conclusion by examining stellar compositions in the system Fe-O-Mg-Si-Ca-Al for F- and G-type stars in the Hypatia and GALAH databases. In reducing the multidimensionality of the dataset, we apply a Principal Component Analysis (PCA) to identify the prevailing chemical trends among the stellar population. We find that the first principal component describes the positive correlation among major element abundances, though O abundance increases less markedly with Fe than those of metals. Therefore, increasing metallicity (as defined by Fe/H) results in an increase in the metal/oxygen ratio of the star. Consequently, the core mass fraction of rocky planets around such stars cannot be treated as a free parameter. Instead, we predict compositions of hypothetical Earth-like planets (i.e., assuming planet/star chemical fractionation equivalent to Earth/Sun) and show that planets around low metallicity stars can be coreless, while those orbiting high metallicity stars should have Fe-free mantles and abundant Si in their cores.