Scaling the Earth: A Sensitivity Analysis of Terrestrial Exoplanetary Interior Models

TL;DR

This paper investigates how variations in terrestrial exoplanet interior composition and structure influence observable properties like mass and radius, using Earth as a model, and provides a new grid of models to assess exoplanet similarity to Earth.

Contribution

It introduces a comprehensive sensitivity analysis of exoplanet interior models, highlighting key factors affecting mass-radius relationships and providing a new grid for evaluating Earth-like exoplanets.

Findings

Core radius and light elements significantly impact mass at fixed radius.

Current models often omit critical interior factors like core composition.

Kepler-36b has only a ~20% chance of being Earth-like in structure.

Abstract

An exoplanet's structure and composition are first-order controls of the planet's habitability. We explore which aspects of bulk terrestrial planet composition and interior structure affect the chief observables of an exoplanet: its mass and radius. We apply these perturbations to the Earth, the planet we know best. Using the mineral physics toolkit BurnMan to self-consistently calculate mass-radius models, we find that core radius, presence of light elements in the core and an upper-mantle consisting of low-pressure silicates have the largest effect on the final calculated mass at a given radius, none of which are included in current mass-radius models. We expand these results provide a self-consistent grid of compositionally as well as structurally constrained terrestrial mass-radius models for quantifying the likelihood of exoplanets being "Earth-like." We further apply this grid to Kepler-36b, finding that it is only ~20% likely to be structurally similar to the Earth with Si/Fe = 0.9 compared to Earth's Si/Fe = 1 and Sun's Si/Fe = 1.19.