Aggregate Hazes in Exoplanet Atmospheres

TL;DR

This study explores how aggregate photochemical hazes, common in the solar system, affect exoplanet transmission spectra, revealing they produce flatter, more featureless spectra than spherical hazes, and better match observations of GJ 1214b.

Contribution

It introduces an aerosol microphysics model for aggregate hazes and demonstrates their impact on transmission spectra, improving interpretation of exoplanet atmospheric data.

Findings

Aggregate hazes cause flatter optical and NIR spectra than spherical hazes.

Higher haze production rates increase haze opacity and spectral flatness.

Aggregate hazes better fit the observed spectra of GJ 1214b than spherical hazes.

Abstract

Photochemical hazes have been frequently used to interpret exoplanet transmission spectra that show an upward slope towards shorter wavelengths and weak molecular features. While previous studies have only considered spherical haze particles, photochemical hazes composed of hydrocarbon aggregate particles are common throughout the solar system. We use an aerosol microphysics model to investigate the effect of aggregate photochemical haze particles on transmission spectra of warm exoplanets. We find that the wavelength dependence of the optical depth of aggregate particle hazes is flatter than for spheres since aggregates grow to larger radii. As a result, while spherical haze opacity displays a scattering slope towards shorter wavelengths, aggregate haze opacity can be gray in the optical and NIR, similar to those assumed for condensate cloud decks. We further find that haze opacity increases with increasing production rate, decreasing eddy diffusivity, and increasing monomer size, though the magnitude of the latter effect is dependent on production rate and the atmospheric pressure levels probed. We generate synthetic exoplanet transmission spectra to investigate the effect of these hazes on spectral features. For high haze opacity cases, aggregate hazes lead to flat, nearly featureless spectra, while spherical hazes produce sloped spectra with clear spectral features at long wavelengths. Finally, we generate synthetic transmission spectra of GJ 1214b for aggregate and spherical hazes and compare them to space-based observations. We find that aggregate hazes can reproduce the data significantly better than spherical hazes, assuming a production rate limited by delivery of methane to the upper atmosphere.