Ceres has been classified as a dwarf planet that might also be classified as an asteroid. While there are three known dwarf planets, there are only two that have been classified further as plutoids. Ceres remains in the dwarf planet category because of its position in the asteroid belt between Mars and Jupiter. Its name is derived from the Roman goddess Ceres. Discovered on January 1, 1801, by Giuseppe Piazzi, Ceres has a diameter of about 950 kilometers and is by far the largest and most massive known body in the asteroid belt, as it contains approximately a third of the belt's total mass. The classification of Ceres has changed several times. Even though it was classified as a planet when it was first discovered, because it resembled similar bodies in the asteroid belt it was reclassified as an asteroid for over 150 years, and now retains its name as a dwarf planet. As the first such body to be discovered, its name was prefixed by the number 1, under the modern system of asteroid numbering. After the discovery of the trans-Neptunian object 2003 UB313 (Eris), a proposition was made by the International Astronomical Union to reinstate Ceres to the status of planet along with Pluto's moon, Charon, and Eris.