Mercury is the planet closest to the sun. It was named after the Roman messenger god Mercury because it moves quickly around the sun. While no single person discovered Mercury, it has been known since the 7th century BC. Mercury orbits the sun every 87.9 Earth days and rotates once on its axis every 55.64 Earth days, making a day on Mercury over twice as long as a year. It is a hot planet with large temperature variations and many impact craters, and cannot support life on its surface.