Title: When Physical Chemistry Met Exoplanet Atmospheres…
Speaker: Jeehyun Yang (Chicago)
Abstract:
Exoplanet science lies at the intersection of observation, theory, and laboratory experiments. While JWST is rapidly expanding our view of exoplanet atmospheres, interpreting these data requires models grounded in physical chemistry. In this seminar, I show how first-principles physical chemistry can be used to link molecular-scale processes to planetary-scale observables. I first introduce a framework that couples automatic computer-generated reaction networks with one-dimensional photochemical kinetic-transport modeling. This approach enables self-consistent modeling across a wide range of planetary regimes, from hot Jupiters to temperate sub-Neptunes. Then, I will talk about how to apply this framework to study temperate sub-Neptunes. I demonstrate that the observable CO₂/CH₄ ratio can gauge the deep interior H₂O/H₂ ratio, providing a useful diagnostic for interpreting planning observations. Finally, I will explain how the role of deep-atmosphere thermochemistry can ‘kick start’ the haze formation process and how this naturally explains the parabolic temperature dependence in aerosol coverage described by Brande et al. (2024). Enabled by a computer-automated rate-based chemical network generator, we construct the most comprehensive carbon chemical network to date, consisting of 700 species and 8258 reactions, capable of describing the formation of polycyclic aromatic hydrocarbons (PAHs) with up to four rings (pyrene, C16H10). We focus on PAHs because these large molecules are well established as precursors to soot formation in combustion chemistry. In short, we find that the deep atmospheres of sub-Neptunes can behave analogously to combustion engines, producing soot precursors that are then transported upward into the atmospheric regions accessible to JWST observations.