Aerosol composition of hot giant exoplanets dominated by silicates and hydrocarbon hazes

Aerosols are common in the atmospheres of exoplanets across a wide swathof temperatures, masses and ages1-3. These aerosols stronglyimpact observations of transmitted, reflected and emitted light fromexoplanets, obfuscating our understanding of exoplanet thermal structureand composition4-6. Knowing the dominant aerosol compositionwould facilitate interpretations of exoplanet observations andtheoretical understanding of their atmospheres. A variety ofcompositions have been proposed, including metal oxides and sulfides,iron, chromium, sulfur and hydrocarbons7-11. However, therelative contributions of these species to exoplanet aerosol opacity isunknown. Here we show that the aerosol composition of giant exoplanetsobserved in transmission is dominated by silicates and hydrocarbons. Byconstraining an aerosol microphysics model with trends in giantexoplanet transmission spectra, we find that silicates dominate aerosolopacity above planetary equilibrium temperatures of 950 K due to lownucleation energy barriers and high elemental abundances, whilehydrocarbon aerosols dominate below 950 K due to an increase in methaneabundance. Our results are robust to variations in planet gravity andatmospheric metallicity within the range of most giant transitingexoplanets. We predict that spectral signatures of condensed silicatesin the mid-infrared are most prominent for hot (>1,600 K),low-gravity (