The terrestrial planet zone is generally considered to be the part of the solar system containing the planets Mercury, Venus, Earth, and Mars. These four objects are called the terrestrial planets because they resemble one another (specifically, Earth) in their structure: a metallic core, surrounded by a rocky mantle and thin crust. There are three moons in the terrestrial zone as well: Earth’s moon, and the two moons of Mars: Phobos and Deimos.