Reason one: Atmosphere Earth has a breathable atmosphere. Oxygen is the gas that is required for the life of most creatures. This is present in Earth's atmosphere, along with nitrogen, and in water, along with hydrogen. Oxygen is constantly put into the atmosphere by plants and trees. Fortunately, Earth's atmosphere does not contain much carbon dioxide, a poisonous gas which makes up most of the atmosphere of planets like Venus and Mars. In densely-populated areas, carbon dioxide is more present. This is because cars and factories produce it. This causes the air to be polluted. The Earth's atmosphere is kept on the planet by its pull of gravity. Mars and Mercury are too small to keep very thin, containing gases which have not managed to escape into space yet. Earth's atmosphere is thick enough to prevent poisonous rays of radiation from getting through it (this is what has happened on Mars).