Aerosol Formation on Eccentric Exoplanets: The Interplay between Disequilibrium Chemistry and Microphysics

Giant exoplanets with large orbital eccentricities are intriguing targets for upcoming atmospheric characterization. Such planets may have a formation and migration history that is distinct from some or all planets on circular orbits (e.g., Dawson & Johnson 2018). Eccentric giant planets also provide diverse opportunities to study atmospheric physics and chemistry, as eccentric orbits yield both hot and cold environments in the same planet. These planets are natural laboratories to understand the relative timescales of chemical reactions, atmospheric mixing, aerosol formation and loss, and time-variable stellar forcing (e.g., Visscher 2012, Lewis et al. 2017). Recent statistical studies also suggest that eccentric planets might have clearer atmospheres (Dymont et al. 2022), which facilitates atmospheric characterization. Despite the crucial importance of eccentric exoplanets, there have been no comprehensive studies on the chemical and microphysical processes of aerosol formation on these worlds, which greatly impact the observable atmospheric spectra.

Here, we provide the first microphysical model of aerosol formation on eccentric giant planets that hierarchically combines radiative transfer, disequilibrium chemistry, and aerosol microphysical models. We present several intriguing results of our radiative-chemical-microphysical model. These include the drastic orbital variation of the CH4 abundance at highly eccentric planets, a long-lived photochemical haze throughout the entire orbit, vertical oscillation of the level of haze formation caused by orbit-varying UV irradiation, and large differences in vertical CH4 and haze profiles on highly eccentric planets depending on the strength of eddy diffusion.

We compute synthetic transmission and emission spectra and find that the observable spectra of highly eccentric planets strongly depends on the eddy diffusion strength. Thus, atmospheric observations of eccentric planets enable us not only to probe the atmospheric compositions but also to test the current understanding of atmospheric circulation. Our modeling framework provides a powerful and unique way to thoroughly understand the upcoming observations of eccentric giant planets delivered by the James Webb Space Telescope.