terrestrial planet, the earth or a planet that resembles the earth in its physical characteristics. The terrestrial planets in the solar system are the earth, Mercury, Venus, and Mars. These planets are approximately the same size, with the earth the largest. They are considerably denser than the Jovian planets, ranging from a specific gravity of 4 for Mars to 5.5 for the earth. Because they spin less rapidly than the Jovian planets, the terrestrial planets are less flattened at their poles.