Ceres was the first object discovered in the asteroid belt and is named for the Roman goddess of corn and harvests. Italian astronomer Father Giuseppe Piazzi spotted the object in 1801. As more such objects were found in the same region, they became known as asteroids or minor planets. Ceres was initially classified as a planet and later classified as an asteroid. In recognition of its planet-like qualities, Ceres was designated a dwarf planet in 2006 along with Pluto and Eris.
NASA's Hubble Space Telescope observed that Ceres' rotation demonstrates that it has a nearly round body. Ceres' diameter at its equator is wider than at its poles. Ceres is approximately 590 miles (950 kilometers) across. And even though Ceres comprises 25 percent of the asteroid belt's total mass, tiny Pluto is still 14 times more massive.
But Ceres has more in common with Earth and Mars than its rocky neighbors. There are signs that Ceres contains large amounts of water ice beneath its surface. Scientists using the Herschel Space Observatory found evidence for water vapor on Ceres. The vapor may be produced by cryovolcanoes or by ice near the surface sublimating (transforming from solid to gas). This proves that Ceres has a icy surface and an atmosphere as well. Astronomers estimate that if Ceres were composed of 25 percent water, it may have more water than all the fresh water on Earth. Ceres' water, unlike Earth's, would be in the form of water ice and located in the mantle, which wraps around the dwarf planet's solid core.
Observations by NASA's Hubble Space Telescope show that Ceres shares characteristics of the rocky, terrestrial planets of our inner solar system. Computer models show that nearly round objects such as Ceres have a differentiated interior, with denser material at the core and lighter minerals near the surface. All the terrestrial planets -- including Earth -- have differentiated interiors. This sets Ceres apart from its asteroid neighbors.