PALS: William Misener (Carnegie EPL), Sub-Neptunes from top to bottom: How interior-atmosphere coupling affects small exoplanets

Sub-Neptunes from top to bottom: How interior-atmosphere coupling affects small exoplanets

William Misener
Carnegie Postdoctoral Fellow
Earth & Planets Laboratory

Abstract: Sub-Neptunes and super-Earths are the most abundant class of planet yet discovered, but their formation, evolution, and even bulk properties remain mysterious. Many processes, from initial accretion to atmosphere-interior interactions to atmospheric escape, are thought to play a major role in shaping the planets we observe today. A key result thus far is that treating these processes together, that is, considering sub-Neptunes as complete, coupled systems from their upper atmospheres to deep interiors, is vital for understanding their evolution, structure, and composition. In this talk, I will present my recent work on how considering feedbacks between sub-Neptune interiors and atmospheres changes our understanding of these ubiquitous planets. First, I will show that allowing silicates to evaporate from an underlying magma ocean into a hydrogen-rich atmosphere leads to mean molecular weight gradients that inhibit convection, implying that sub-Neptune envelope structure may be strikingly different than is typically assumed. These silicates also chemically react with the background hydrogen, producing abundant water and reduced magma species potentially observable with JWST. Second, I will use hydrodynamic models to show how core-powered mass loss, an escape mechanism thought to sculpt the exoplanet population, is highly sensitive to upper atmospheric composition. I find that altering the relative opacities to stellar and emitted light, as is expected if the gas composition changes, can change the expected atmospheric escape rate by orders of magnitude. These results underscore the need to model small exoplanets holistically, accounting for interactions between their interiors and atmospheres. 

Host: Hayley Beltz