On average, Mercury is the closest planet to Earth. Although Venus is the planet that comes closest to Earth at their respective closest approaches, Mercury is closer to Earth on average because it orbits the Sun more closely than Venus. Mercury is the closest planet to Earth not only in our solar system but also to every other planet in the solar system. The distance between Earth and Mercury is, on average, only 1.04 astronomical units (AU) . An AU is a unit of length equal to the average distance between Earth and the Sun.