Fifth largest planet of the solar system and the only one known to support life. Its mean distance from the sun is 150 million km. The change of seasons is caused by the tilt of the earth's equator to the plane of the orbit. The earth is surrounded by an envelope of gases, mostly oxygen and nitrogen, called the atmosphere. Gravitational forces have molded the earth into a spherical shape that bulges slightly at the equator. Studies indicate that the earth consists of concentric layers that differ in size, chemistry, and density. The outer shell, or crust, consists of the continents and the ocean basins. The crust is broken into vast plates that slide around on a plastic zone, or asthenosphere, within the middle shell, or mantle. At the center of the earth is an outer core, believed to be liquid, and an inner, solid core. The earth is estimated to be 4.5-5 billion years old, and its origin is a controversial subject. The earth has one natural satellite, the moon.