Aerosols In Exoplanet Atmospheres

Condensation temperatures of various cloud species as a function of atmospheric pressure, assuming solar metallicity, compared to temperature-pressure (TP) profiles of several objects. Condensation of a given species can occur when the planet TP profile becomes lower than its condensation temperature profile. TP profiles for Jupiter and Uranus are taken from Moses and Poppe (2017) while those of HR 8799b and HD 209458b are generated by a thermal structure model (Saumon & Marley, 2008) assuming appropriate planetary parameters. The condensation curve for CH4 is computed by combining the CH4 saturation vapor pressure (Lodders & Fegley, 1998) with its mixing ratio in a solar metallicity gas (Lodders, 2010), assuming that all carbon is in the form of CH4. The condensation curves for NH3, NH4SH, and H2O are taken from Lodders and Fegley (2002); that of H2S is from Visscher et al. (2006); those of KCl, ZnS, Na2S, MnS, and Cr are from Morley et al. (2012); those of MgSiO3, Mg2SiO4, and Fe are from Visscher et al. (2010); that of TiO2 is from Helling et al. (2001); and that of Al2O3 is from Wakeford, Visscher, et al. (2017). The CH4/CO and NH3/CO transition curves are from Lodders and Fegley (2002).

Observations of exoplanet atmospheres have shown that aerosols, like in the Solar System, are common across a variety of temperatures and planet types. The formation and distribution of these aerosols are inextricably intertwined with the composition and thermal structure of the atmosphere.

At the same time, these aerosols also interfere with our probes of atmospheric composition and thermal structure, and thus a better understanding of aerosols lead to a better understanding of exoplanet atmospheres as a whole.

Here we review the current state of knowledge of exoplanet aerosols as determined from observations, modeling, and laboratory experiments. Measurements of the transmission spectra, dayside emission, and phase curves of transiting exoplanets, as well as the emission spectrum and light curves of directly imaged exoplanets and brown dwarfs have shown that aerosols are distributed inhomogeneously in exoplanet atmospheres, with aerosol distributions varying significantly with planet equilibrium temperature and gravity.

Parameterized and microphysical models predict that these aerosols are likely composed of oxidized minerals like silicates for the hottest exoplanets, while at lower temperatures the dominant aerosols may be composed of alkali salts and sulfides. Particles originating from photochemical processes are also likely at low temperatures, though their formation process is highly complex, as revealed by laboratory work. In the years to come, new ground- and space-based observatories will have the capability to assess the composition of exoplanet aerosols, while new modeling and laboratory efforts will improve upon our picture of aerosol formation and dynamics.

Peter Gao, Hannah R. Wakeford, Sarah E. Moran, Vivien Parmentier

Comments: Invited review for JGR-Planets's Exoplanets: The Nexus of Astronomy and Geoscience special section. Accepted for publication. 75 pages, 12 figures, 1 table, 1 helluva year
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2102.03480 [astro-ph.EP] (or arXiv:2102.03480v1 [astro-ph.EP] for this version)
Submission history
From: Peter Gao
[v1] Sat, 6 Feb 2021 02:34:08 UTC (6,307 KB)

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