Chemical Fingerprints of Formation in Rocky Super-Earths' Data

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 ±0.9, while the planets’ distribution peaks at lower values and has a longer tail, 1.15 (+1.4/-0.8). It is easier to see the discrepancy in cmf space, where primordial stellar composition is 0.32 ±0.13, while rocky planets’ follow a broader distribution 0.24 (+0.33/-0.18). (2) We introduce uncompressed density (ρ₀) 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 (ρ₀ ~ 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.