UCLA

Planets with radii between 1 and 4 Earth radii and orbital periods shorter than Mercury’s are the most common class of planet discovered to date. Detailed measurements of the masses and radii of these planets indicate that a ‘radius valley’ separates the smaller super-Earths, with densities consistent with an Earth-like composition, from the sub-Neptunes, larger planets with lower densities. A leading theory consistent with astronomical observations posits that these planets all formed with hydrogen atmospheres that greatly increase their observed radii. Over time, the super-Earths lost these atmospheres via a hydrodynamic wind, heated by the star’s bolometric luminosity and sustained by the heat released from the silicate-rich interior, while sub-Neptunes retained them. My dissertation work reveals that the nature of the connection between the interior silicates and atmospheric hydrogen has broad implications for the atmospheric composition and evolution of these planets. I find that the thermal coupling of the interior and atmosphere, which sustains atmospheric escape, can also lead to its end, as the interiors cool more efficiently as the atmosphere is stripped. I demonstrate that chemical equilibrium in the deep envelopes of sub-Neptunes, which implies high silicate vapor concentrations that decline with altitude, leads to structural changes in the atmosphere. Convection is inhibited by the strong molecular weight gradients, creating a highly super-adiabatic region that increases the atmospheric mass inferred around observed planets. I show that reactions between the silicates and hydrogen produce novel reduced silicon species and abundant endogenic water vapor in sub-Neptunes. Finally, I apply a hydrodynamic radiative transfer model to core-powered mass loss for the first time, demonstrating that the inclusion of multi-band opacities fundamentally alters predicted mass loss rates. Together, this work opens a new window into the atmospheres and interiors of the most abundant planets known in the Galaxy.