Chemical Fingerprints of Formation in Rocky Super-Earths' Data

Abstract

The composition of rocky exoplanets in the context of stars' composition provides important constraints to formation theories. In this study, we select a sample of exoplanets with mass and radius measurements with an uncertainty <25% and obtain their interior structure. We calculate compositional markers, ratios of iron to magnesium and silicon, as well as core-mass fractions (cmf) that fit the planetary parameters, and compare them to the stars'. We find four key results that successful planet formation theories need to predict: (1) In a population sense, the composition of rocky planets spans a wider range than stars. The stars' Fe/Si distribution is close to a Gaussian distribution $1.63^{+0.91}_{-0.85}$, while the planets' distribution peaks at lower values and has a longer tail, $1.15^{+1.43}_{-0.76}$. It is easier to see the discrepancy in cmf space, where primordial stellar composition is $0.32^{+0.14}_{-0.12}$, while rocky planets' follow a broader distribution $0.24^{+0.33}_{-0.18}$. (2) We introduce uncompressed density ($\overline{\rho_0}$ at reference pressure/temperature) as a metric to compare compositions. With this, we find what seems to be the maximum iron enrichment that rocky planets attain during formation ($\overline{\rho_0}$ ~ 6 and cmf ~ 0.8). (3) Highly irradiated planets exhibit a large range of compositions. If these planets are the result of atmospheric evaporation, iron enrichment and perhaps depletion must happen before gas dispersal. And (4), we identify a group of highly-irradiated planets that, if rocky, would be 2-fold depleted in Fe/Si with respect to the stars. Without a reliable theory for forming iron-depleted planets, these are interesting targets for follow up.