American astronomer Clyde W. Tombaugh discovers Pluto, generally the most distant planet from the sun. The existence of an unknown ninth planet was first proposed by Percival Lowell, who theorized that it was responsible for the wobbles in Uranus' and Neptune's orbits. In 1930, using Powell's calculations as a guide, Tombaugh discovered the tiny planet. With a surface temperature below -200 Celsius, Pluto was appropriately given the Roman name for the god of the underworld. Nearly 4 million miles from the sun, it takes approximately 248 years to complete one orbit. It also has the most elliptical orbit of any planet and at its closest point to the sun passes inside the orbit of Neptune, the eighth planet. In 1978, Pluto's moon, Charon, was discovered.