Solar eclipse on Pluto as viewed from its moon Charon, artwork. Charon's shadow can be seen projected onto the centre of Pluto. From within that shaded region the Sun would appear eclipsed. 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. 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.