Pluto as seen from its moon Charon, artwork. Pluto is the smallest of the nine planets in the solar system, with a diameter of 2300 kilometres. Charon's diameter is about half that. This compares to our own Moon, which has a diameter of about 3500 kilometres. Pluto has an erratic orbit; varying from a distance of 4.4 to 7.3 billion kilometres from the Sun and taking 248 years for one full circle. Scientists speculate that the surface of Charon is mainly water ice, whereas Pluto is covered in methane ice. But no spacecraft has yet confirmed this.