Heart of Stone: The open-source mass-radius calculator, ExoPlex, phase diagrams and adiabats for Exoplanets across a wide range of compositions

Among the observables for an individual exoplanet, mass and radius provide our only direct constraints on interior composition. Inferences on exoplanet bulk composition are usually made using so-called "mass-radius relations". In their most common form, mass-radius models assume simple mixtures of pure bridgmanite (MgSiO₃) mantles, pure Fe cores, and volatile layers in the form of water, or H₂/He atmospheres. These simple models often only allow for the coarse characterization of an exoplanet as either being water-, rock- or iron-dominated, while providing little information on its specific mineralogy or core chemistry. Determination of mineralogy is important for constraining mantle convection, dynamo action, and surface outgassing rates of volatile specie, thus providing first order constraints on atmospheric models needed for interpretation of transmission spectroscopic data. More advanced mass-radius models utilizing Gibbs minimization techniques have emerged in recent years, allowing for self-consistent determinations of exoplanet bulk compositions. These new models have allowed us to more specifically characterize individual planetary systems such as TRAPPIST-1. To date though, there exists no open-source mass-radius code capable of these sophisticated phase-equilibria calculations that is available to the exoplanet community at-large.

Here we present the open-source ExoPlex mass-radius-composition calculator, as well as detailed radial grids of pressure, adiabatic temperature, density, and mantle mineralogy for modeled rocky exoplanets between 0.1 and ~2 Earth radii. We consider a wide range of elemental compositions in the Fe-Mg-Si-Ca-Al-O system. ExoPlex includes full phase diagrams for each individual composition across a range of pressures and temperatures between 1 bar - 2.8 TPa and 1400 - 7000 K. We include the effects of light-element core chemistry, mantle FeO content and a water phase diagram including high-pressure ices. Representing over 200 Gb of publicly available data, the ExoPlex mass-radius-composition calculator and associated grids not only provide the exoplanet community with the most powerful tool to characterize exoplanet interiors, these results will aid the geophysical community looking to better constrain exoplanet dynamics.