Because they have a number of traits in common with Earth.The inner planets—Mars, Venus, Mercury, and Earth—are frequently referred to as terrestrial planets. Because they have a number of traits in common with Earth. The Latin word "Terra," which means Earth, is where the word "terrestrial" originates. They are known as terrestrial planets for the following reasons:Rocky, solid surfaces.Comparable structure.Because of their rocky and metallic makeup, they have a higher density than the gas giants.Reduced or absent moons.Nearer the Sun.