  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.
