Earth is the third planet from the Sun and the fifth largest: The main characteristic that sets Earth apart from the other planets, is the presence of diverse life forms and large quantities of liquid water. Its distance form the Sun, in combination with its particular atmospheric make-up, produces a range of suitable surface temperatures for this to happen. Nitrogen dominates the atmosphere (78%). Life sustaining oxygen (21%) has been present only for the past 2 billion years, originating form bacterial processes during life's early developments. Water vapor (clouds), carbon dioxide, and other trace gases account for the remaining percent. The thin crust is composed of rocks, some of which are 3.9 billion years old.
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