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Published March 20, 2019 | Published + Accepted Version
Journal Article Open

Aggregate Hazes in Exoplanet Atmospheres


Photochemical hazes have frequently been used to interpret exoplanet transmission spectra that show an upward slope toward 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 the transmission spectra of warm exoplanets. We find that the wavelength dependence of the optical depth of aggregate particle hazes is flatter than for spheres because aggregates grow to larger radii. Consequently, while spherical haze opacity displays a scattering slope toward shorter wavelengths, aggregate haze opacity can be gray in the optical and near-infrared, 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, although 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 that is limited by delivery of methane to the upper atmosphere.

Additional Information

© 2019 The American Astronomical Society. Received 2018 October 1; revised 2019 February 1; accepted 2019 February 13; published 2019 March 22. We thank P. Rannou for helpful comments regarding aggregate scattering. D.A. acknowledges the UC Berkeley URAP program. P.G. acknowledges funding support from the 51 Pegasi b Fellowship in Planetary Astronomy from the Heising-Simons Foundation. This work was performed in part under contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Sagan Fellowship Program, which is executed by the NASA Exoplanet Science Institute.

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Accepted Version - 1902.05231.pdf

Published - Adams_2019_ApJ_874_61.pdf


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August 19, 2023
August 19, 2023