Pluto and Charon. Artwork of the surface of the planet Pluto with its moon, Charon, rising on the horizon. Pluto's thin atmosphere of methane can be seen along the horizon and the Sun is seen at top center. Pluto is the smallest, coldest planet in the solar system, and is sometimes the outermost. Pluto is, on average, 39 times further from the Sun than Earth, and its elliptical orbit takes 247.7 years to complete. It is 2284 kilometers (km) across and is largely made of rocky material. Charon has a diameter of 1192 km and orbits Pluto at a distance of 12000 km. Pluto may have originally been a moon of Neptune. Pluto & Charon were discovered in 1930 and 1978 respectively.