The Terrestrial Planets

Earth and the three other inner planets—Mercury, Venus, and Mars—are more similar to each other than they are to the four outer planets. The four inner planets are small and dense and have rocky surfaces. The inner planets are often called the terrestrial planets, from the Latin word terra, which means “Earth.”

The inner planets (a) are smaller than the outer planets (b).  Pluto is too small to be considered a planet.

On Earth the crust (lithosphere) is thinner and the mantle is thickest.