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

By P. Gao, D. Thorngren, Elspeth K. H. Lee, Jon Fortney, C. Morley, H. Wakeford, D. Powell, Kevin B. Stevenson, Xi Zhang. 2020

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Aerosols are common in the atmospheres of exoplanets across a wide swath of temperatures, masses and ages 1 – 3 . These aerosols strongly impact observations of transmitted, reflected and emitted light from exoplanets, obfuscating our understanding of exoplanet thermal structure and composition 4 – 6 . 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 sulfides, iron, chromium, sulfur and hydrocarbons 7 – 11 . 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 (>1,600 K), low-gravity (<10 m s −2 ) objects. A detailed microphysical model shows that there are two distinct regimes in the aerosol composition of exoplanetary atmospheres: silicates dominate at atmospheric temperatures above 950 K, whereas hydrocarbons prevail for temperatures below 950 K.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/1fc7883261050b66b3fe67519e00541d7183f99d

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High Tide or Riptide on the Cosmic Shoreline? A Water-rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ 486b from JWST Observations (2023, background): “Photochemical hazes could dampen the presence of any spectral features with a haze layer at this altitude and create a flat line spectrum (Gao et al. 2020; Pidhorodetska et al. 2021; Caballero et al. 2022); however, given the Bayesian evidence of the Gaussian absorption tests discussed above, the…”