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The study of atmosphere formation (or atmospheric formation) of planets (including Earth's) is of interest in astronomy to explain their observed atmospheres, and develop principles that explain their variety. The gases are assumed to come from accretion of gas from the protoplanetary disk, and from outgassing of the planet's's solid body, such as the gas in Earth's atmosphere expelled by volcanos. Gas can be removed or changed by chemical reactions within the atmosphere itself (e.g., oxidation, reactions of constituents with opposing oxidation states), by escape of gas through hydrodynamic escape or impacts, or by energy supplied by electromagnetic radiation (photoevaporation). As the Earth cooled from its initial very hot state, it is assumed the atmosphere went through a rock vapor phase and a steam phase.