Mercury is the closest planet to the Sun and is also the smallest of the eight planets in our solar syste. Mercury is the Roman name for Hermes the god of commerce, travel and thievery. Hermes is the messenger of the gods, known for his speed and swiftness. That is why the planet received this name - because it moves so quickly across the sky (Mercury takes only 88 days to orbit the Sun). Mercury has been known since at least the time of the Sumerians (3rd millennium BC).