Mercury Mercury is the eighth largest planet in our solar system, equaling roughly one-third of Earth’s size. It is also the planet closest to the sun, and is often visible with binoculars, or even the naked eye. It’s difficult to see in the twilight sky. One of Mercury’s most distinguishable features is its rugged surface, which is similar to the surface of the Earth’s moon. In Roman mythology, Mercury is the god of commerce and thievery. He is considered the Roman counterpart of the Greek god Hermes, the messenger of the Olympian gods. The planet probably received this name because it moves so quickly across the sky.