Formation, structure and habitability of super-Earth and sub-Neptune exoplanets

Abstract

Insights into a distant exoplanet's interior are possible given a synergy between models and observations. Spectral observations of a star's radial velocity wobble induced by an orbiting planet's gravitational pull measure the planet mass. Photometric transit observations of a planet crossing the disk of its star measure the planet radius. This thesis interprets the measured masses and radii of super-Earth and sub-Neptune exoplanets, employing models to constrain the planets' bulk compositions, formation histories, and habitability. We develop a model for the internal structure of low-mass exoplanets consisting of up to four layers: an iron core, silicate mantle, ice layer, and gas layer. We quantify the span of plausible bulk compositions for low-mass transiting planets CoRoT-7b, GJ 436b, and HAT-P-11b, and describe how Bayesian analysis can be applied to rigorously account for observational, model, and inherent uncertainties. We present a detailed case study of GJ 1214b, the first exemplar of a new class of volatile-rich super-Earth exoplanets. At 6.5 Mo and 2.7 Ro, GJ 1214b must have a gas layer to account for its low mean density. We present three possible scenarios for the origin of the gas layer on GJ 1214b: direct accretion of H/He gas from the protoplanetary nebula, sublimation of ices, and outgassing of volatiles from a rocky interior. We next explore the low-density extreme of the mass-radius relations for volatilerich super-Earth exoplanets. Using models of planet formation, structure, and survival, we constrain the minimum plausible planet mass for a measured planet radius and equilibrium temperature. We explore both core-nucleated accretion and outgassing as two separate formation pathways for Neptune-size planets with voluminous atmospheres of light gases. Finally, we present a practical method to assess whether a hydrogen-rich sub-Neptune planet with measured mass and radius could potentially harbor a liquid water ocean. Using a one-dimensional radiative-convective model of energy transport through water-saturated hydrogen-rich envelopes, we constrain the combinations of planet properties (mass, radius, equilibrium temperature, intrinsic luminosity) that are conducive to liquid water oceans. The pace of low-mass exoplanet discoveries is poised to accelerate, and this thesis will contribute to constraining the interior properties of newfound planets.