Why are there no restaurants on the moon? Great food. No atmosphere. The atmosphere is the layer of gases that surrounds a planet or other massive body. Although our atmosphere extends for tens of miles, almost ninety-nine percent of it is within eighteen miles of the earth's surface. Our own atmosphere is mostly nitrogen, with about twenty-percent being the oxygen we breathe. But, a planet doesn't start out having an atmosphere like ours. Much of the carbon dioxide that envelops earth, for example, was originally embedded inside rocks, and later released by heat from the sun. About a billion years ago, simple green plants started generating oxygen, which soon became a major part of our atmosphere as well.