Inferring the Compositions and Interior Structures of Small Planets

Although there are now over 10,000 confirmed and candidate extrasolar planets, accurate masses and radii have been determined for around 200 planets of less than 10 Earth-masses. This number will grow exponentially over the next decade as extreme precision radial velocity spectrographs see first light. The densities of small planets hint at a diverse range of compositions for terrestrial worlds from super-Mercuries to super-Ganymedes. Uncertainty in density is as low as 3% in the Trappist-1 system. To determine composition from observed parameters, the community uses a variety of interior structure models. Underlying these models are multiple computational techniques, numerous experimental measurements and theoretical estimates of the equations of state for planet-building materials, and differing treatments of temperature. I present MAGRATHEA, an open-source interior structure solver which can be customized to user-defined planet models for reproducible characterizations. Our code features adaptable phase diagrams for the core, mantle, hydrosphere, and atmosphere and transparent storage for equations of states. I demonstrate how I use MAGRATHEA in concert with high-pressure physicists to improve our interior models and with observational data to characterize planets. While previous works have looked at how models affect mass-radius relationships, I carry through uncertainties to the characterization of small planets with known density. I show how observed, model, and experimental uncertainties affect inferences of water mass fraction in the Trappist-1 system. The Earth’s composition and interior structure can soon be understood in the greater context of exoplanet diversity