Another thing to keep in mind is that the Earth also has a lot of gas! Our atmosphere is made up of gas, but the gas does not go flying off away from the Earth because it is held there by the gravity of the Earth. (Gravity is the attraction that all matter feels toward other matter.) In the case of Jupiter, the entire planet is made out of gas; there is no solid core or crust like the surface of the Earth. However, the gas itself has enough mass that gravity works to keep the whole thing together. --Lindsay