Aerosols in Exoplanet Atmospheres

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. Plain Language Summary For nearly 2 decades we have had the opportunity to probe the atmospheres of planets orbiting other stars (exoplanets). These efforts have revealed the existence of clouds and hazes in these atmospheres, which prevent us from learning more about exoplanet atmospheres as a whole by blocking us from probing parts of the atmosphere below the cloud and haze layers. Here we summarize our current understanding of these structures. Using data from telescopes on the ground and in space, we have found that exoplanet clouds are patchy and are distributed mostly according to the temperature of the local atmosphere. Using computer simulations, we have surmised that these clouds are likely made of materials that make up rocks on Earth, as the exoplanets we have probed thus far orbit their stars closely, resulting in very high temperatures in their atmospheres. At lower temperatures, but still several hundred degrees above room temperature, hazes composed of organic material are possible. These hazes are likely formed from complex chemical reactions, which are the current focus of laboratory experiments. Future efforts in data collection, computer simulations, and lab work will lead to a better understanding of exoplanet clouds and hazes.

Automated summary

Observations of exoplanet atmospheres have shown that aerosols are common and their formation is closely related to the composition and thermal structure of the atmosphere. Aerosols are distributed unevenly in exoplanet atmospheres, with composition varying based on temperature and gravity. Modeling and laboratory experiments suggest that the composition of aerosols may include silicates, alkali salts, sulfides, and particles from photochemical processes, with future research aiming to improve our understanding of aerosols in exoplanet atmospheres.