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Ceres is the smallest identified dwarf planet in the Solar System and the only one in the asteroid belt. It was discovered on 1 January 1801, by Giuseppe Piazzi, and for half a century was classified as the eighth planet. It is named after Ceres, a Roman goddess. With a diameter of about 950 km (590 miles), Ceres is the largest and most massive body in the asteroid belt and contains about a third of the belt's total mass. It is spheroidal and its surface probably consists of a mixture of water ice and various hydrated minerals like carbonates and clays. Ceres appears to be differentiated into a rocky core and icy mantle, with brines reaching the surface. From the Earth, the apparent magnitude of Ceres ranges from 6.7 to 9.3; at its brightest it is generally too dim to be seen with the naked eye. On 27 September 2007, NASA launched the space probe Dawn to explore the asteroid Vesta from 2011 to 2012, and Ceres from 2015 to 2018. (This article is part of a featured topic: Solar System.)