Ceres

Ceres is the largest main-belt asteroid. It is classified as a dwarf planet because of its size which is almost 2 times the 2nd largest asteroid, Vesta.

Surface features
Like many other asteroids, its surface contains rock and ice.

Discovery
Astronomers in the late 1700s thought there was a planet in between Mars and Jupiter because the gap was so large there must be something in the gap.

On 1 January 1801 (yes, exactly new years day), Giuseppe Piazzi discovered ceres. It was first classified as a planet. In around 1845 many more ceres-like (but much smaller than ceres) objects have been found, and astronomers classified all objects between mars and jupiter, including ceres, as asteroids.

In 2006, due to its large size, it was classified as a dwarf planet along with Pluto, Eris, Makemake, and Haumea (the original 5 dwarf planets).