Compared to the other planets in our Solar System, Earth is astonishingly active. Its surface is continually recycled and its atmosphere sustains a stunning variety of life.
Earth is the largest of the four rocky, or terrestrial, planets. While radically different from. one another, Mercury, Venus, Earth, and Mars are easily distinguished from the gas-giant planets— Jupiter, Saturn, Uranus, and Neptune—by their size, density, and chemical composition.
Terrestrial planets are composed mainly of dense, rocky materials, such as basalt, and metals, such as iron. Gas-giant planets are more like the Sun in composition: they have enormous atmospheres, mainly of hydrogen and helium, wrapped around rocky centers. The terrestrial planets have far greater densities than the gas planets, but are low in mass—Earth has only one-fifteenth the mass of the smallest gas giant, Uranus.