An exoplanet cloud is suspended condensed material in an exoplanet atmosphere. Condensation makes particles when local composition and temperature permit; mixing can keep them aloft while atmospheric condensate rainout removes them by settling. The resulting distribution depends on transport as well as on local chemical equilibrium.
Particles absorb and scatter radiation, changing the reflected brightness, thermal spectrum and exoplanet transmission spectrum. An opaque exoplanet cloud deck hides deeper molecular absorption, while a thinner cloud can leave wavelength-dependent structure. Particle size, composition and coverage therefore matter as much as the statement that a cloud is present. A weak gas band can indicate obscuration rather than low molecular abundance, making clouds a central uncertainty in atmospheric retrieval.
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