For questions about atmospheres. An atmosphere is a gaseous shell around a body in space, typically a planet, but it can also be a star or a dwarf planet. Atmospheric pressure and composition varies widely between planets. The atmosphere of Earth consists mostly of nitrogen and oxygen, whereas the Atmosphere of Venus is composed mostly of carbon dioxide. Stellar atmospheres, which include the Corona and the Solar Wind, are quite different from planetary atmospheres.

The Wikipedia article is a good read.

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