The Solar System (太陽系) comprises the Sun and the various planetary objects that rotate around it. According to current scientific thinking, the Solar System contains eight planets. However, within the Sailor Moon canon, there are ten planets, including Pluto (classified as a planet up until August 2006) and Nemesis (the fictional home of the Black Moon Clan).
In the Sailor Moon series, each of the planets, and some of the larger asteroids, were protected by the Sailor Senshi who bore the Sailor Crystal of that planetary body. The only planet which was not shown to have an associated Sailor Crystal was Nemesis.