An atmosphere (New Latin atmosphaera, created in the 17th century from Greek ἀτμός “vapor” and σφαῖρα “sphere”) is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere’s temperature is low. Some planets consist mainly of various gases, but only their outer layer is their atmosphere.