Pluto and Charon. Artwork of the planet Pluto and its moon Charon (upper right). Pluto (foreground) is the smallest of the nine planets orbiting the Sun (upper left). It has a diameter of some 2300 kilometres. Its mass is around one 450th that of the Earth, and it completes an orbit of the Sun every 248 years. Its orbit is highly elliptical, ranging from 30 to 50 times the distance between the Earth and the Sun. Its surface is covered in frozen water, methane and nitrogen, and is at a temperature of around -223 degrees Celsius. It was discovered in 1930 by Clyde Tombaugh. Charon was discovered in 1978. Its rotation is linked to Pluto's, so it appears fixed in the Plutonian sky.