Aerosol dynamics on hot exoplanets: the role of radiation pressure

Abstract

Aerosols appear to be ubiquitous in exoplanetary atmospheres. However because our understanding of the physical processes that govern aerosols is incomplete, their presence makes the measurement of atmospheric properties, such as molecular abundance ratios, difficult. We show that aerosol particles in highly-irradiated exoplanets experience an additional acceleration due to stellar radiation pressure. The strength of this radiative acceleration often exceeds the planet's gravity and can approach values of ~10-20x gravity's for low-density planets (typically sub-Saturns) hosting ~0.1--1 micron aerosols. Since these highly irradiated, low-density planets are often the best targets for atmospheric characterisation with current instrumentation, radiation pressure is likely an important process when considering aerosol dynamics. We find that radiation pressure accelerates hazes produced by photochemistry at high altitudes to faster terminal velocities, causing them to grow more slowly. Hence, the particles are smaller and have lower mass concentrations in the presence of radiation pressure. By simulating haze-like aerosols in a 2D equatorial band model, we show that radiation pressure steepens optical slopes in transmission spectra, resulting in less muted molecular features in the Near-IR and gives rise to a correlation between the strength of radiation pressure and the molecular feature amplitude. Furthermore, the interaction of zonal winds and radiation pressure impacts both the optical slopes and amplitudes on the individual morning and evening terminators.