Earth

The Earth is the astronomical body where humans and all other known life is born. It may be divided into three parts: the atmosphere, the surface (which holds life), and the interior, each one being subdivided on its own (see below). Different views exist regarding Earth's age, shape, origin, and exact definition, with some being connected to religion and some not. According to mainstream science, the Earth is a planet with the shape of an oblate spheroid that revolves around the Sun, is approximately 4.54 billion years old, and was formed out of debris in the solar protoplanetary disk. Other views include, for example that of some traditional Christians, according to which the Earth is approximately 6,000 years old and was created by God, with some few maintaining that it is flat (and thus not a planet) and that the Sun and the Moon revolve above it.

Subdivision
There are several ways to divide Earth. Roughly speaking, one can say it is composed of three great parts:


 * The atmosphere, itself composed of several layers, namely exosphere, thermosphere, mesosphere, stratosphere, and troposphere (others, such as the ionosphere, are sometimes included).
 * The surface, which holds all known life and contains 70 percent of ocean and 30 percent of landmasses (continents and islands).
 * The interior, composed of the litosphere (also called crust), the mantle (upper mantle and mantle), and the nucleus (outer and inner).