Dusty clouds have been known to form in Brown Dwarf atmospheres since the early 2000s. Observations of Brown Dwarfs have found depletion of elements and a time varibility which have been attributed to cloud cover. The properties of clouds particles in exoplanet atmospheres are yet to be known with certainty due to the difficulty in gaining high resolution spectra of their atmospheres. However, this may change with the advent of space based instruments such as JWST and WFIRST.
Clouds are thought to form in the atmospheres of hot Jupiters and Brown Dwarfs differently to Earth clouds. Solid seed cloud particles form at the upper cooler regions of the atmosphere and grow larger and larger as they fall due to gravity. Chemical reactions on the dust particles surface continue to grow the grain to micrometer sizes. The grain can be composed of many different mixtures of mineral materials such as MgSiO3, SiO2, TiO2 and otheres depending on the local thermochemical conditions. The cloud particle eventually evaportates once the temperature gets too high.
Of all the current exoplanets the hot Jupiter HD 189733b is the most likely canditate to host dust clouds similar to the ones observed in Brown Dwarfs. Observations using the Hubble Space Telescope and Spitzer Infrared Space Telescope have shown that there is signifcant scattering of light off the atmosphere which thick cloud cover could explain. Astronomers also deduced that the planet would appear blue in colour due to this scattering effect.