Steepening Optical Slopes in Exoplanet Transmission Spectra with Charged Hazes

Abstract

Observations of exoplanetary atmospheres have revealed the ubiquity of aerosols, yet their microphysics remain largely uncertain. Haze charging is a key microphysical process in aerosol-rich Solar System atmospheres, but it has been ignored in exoplanetary haze models. Here we develop a microphysical framework that includes haze charging for exoplanet atmospheres and investigate its effects on transmission spectra. Electrostatic repulsion suppresses coagulation, maintaining smaller particle radii and higher mixing ratios in the upper atmospheres. We find that particle charging steepens the optical spectral slopes when vertical mixing is weak. This behavior arises from the combination of reduced particle sizes and a steepened vertical gradient in haze opacity. Furthermore, we find distinct spectral responses to charging for different haze analogs, owing to the different contributions of scattering and absorption. For absorption-dominated materials such as soot, particle-size reduction has little impact on opacity profiles, leading to only minor charging-induced spectral changes. By contrast, scattering-dominated materials such as tholin and diamond show a stronger spectral signature of particle charging because of the strong dependence of scattering opacity on particle size. While previous studies have invoked haze formation under vigorous vertical mixing as a mechanism for producing super-Rayleigh optical slopes, we demonstrate that highly charged scattering haze can produce such slopes even under weak-to-moderate vertical mixing. Our results highlight haze charging as a previously neglected microphysical process that can affect particle growth and leave observable signatures in transmission spectra, providing a quantitative framework for assessing atmospheric electrical conditions.