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The term photoevaporation in astronomy is the dispersal of a gas by the energy of XEUV radiation. (Thus it is not "evaporation" in the sense of a liquid changing to a gas.) The term is used for such dispersal from molecular clouds, protoplanetary disks, and the atmosphere of planets (atmospheric escape). Due to this process, extra-solar planets close enough to stars emitting significant ultraviolet radiation are expected to have lost their atmospheres.