Chemical Fingerprints of Formation in Rocky Super-Earths' Data

TL;DR

This study analyzes the composition of rocky super-Earths, revealing their wider compositional diversity compared to stars, and introduces new metrics to understand their formation and iron enrichment limits.

Contribution

It provides new insights into planetary composition diversity, introduces uncompressed density as a comparison metric, and highlights the need for improved formation theories for iron-depleted planets.

Findings

Rocky planets have a wider composition range than their host stars.

Maximum iron enrichment in planets is indicated by specific density and core-mass fraction values.

Highly irradiated planets show diverse compositions, suggesting complex formation and evolution processes.

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.