Aerosol Composition of Hot Giant Exoplanets Dominated by Silicates and Hydrocarbon Hazes

TL;DR

This study reveals that the aerosol composition in hot giant exoplanets' atmospheres is mainly silicates above 950 K and hydrocarbons below, based on transmission spectra analysis, aiding interpretation of exoplanet observations.

Contribution

The paper identifies the dominant aerosol types in giant exoplanets and links their prevalence to temperature, providing a microphysics-based understanding of atmospheric aerosols.

Findings

Silicates dominate aerosol opacity above 950 K.

Hydrocarbons dominate below 950 K.

Spectral signatures of silicates are prominent in hot, low-gravity exoplanets.

Abstract

Aerosols are common in the atmospheres of exoplanets across a wide swath of temperatures, masses, and ages. These aerosols strongly impact observations of transmitted, reflected, and emitted light from exoplanets, obfuscating our understanding of exoplanet thermal structure and composition. Knowing the dominant aerosol composition would facilitate interpretations of exoplanet observations and theoretical understanding of their atmospheres. A variety of compositions have been proposed, including metal oxides and sulphides, iron, chromium, sulphur, and hydrocarbons. However, the relative contributions of these species to exoplanet aerosol opacity is unknown. Here we show that the aerosol composition of giant exoplanets observed in transmission is dominated by silicates and hydrocarbons. By constraining an aerosol microphysics model with trends in giant exoplanet transmission spectra, we find that silicates dominate aerosol opacity above planetary equilibrium temperatures of 950 K due to low nucleation energy barriers and high elemental abundances, while hydrocarbon aerosols dominate below 950 K due to an increase in methane abundance. Our results are robust to variations in planet gravity and atmospheric metallicity within the range of most giant transiting exoplanets. We predict that spectral signatures of condensed silicates in the mid-infrared are most prominent for hot (>1600 K), low-gravity (<10 m s$^{-2}$) objects.