Ceres

Scientists describe Ceres as an "embryonic planet." Gravitational perturbations from Jupiter billions of years ago prevented it from becoming a full-fledged planet. Ceres ended up among the leftover debris of planetary formation in the main asteroid belt between Mars and Jupiter.

Ceres is named for the Roman goddess of corn and harvests.

Ceres was the first object discovered in the asteroid belt. Sicilian astronomer Father Giuseppe Piazzi spotted the object in 1801. Piazzi was looking for suspected planets in the large gap between the orbits of Mars and Jupiter. 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 580 miles (930 kilometers) across, about the size of Texas. 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 it may contain large amounts of pure water ice beneath its surface. 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.

Astronomers suspect water ice may be buried under the dwarf planet's crust because the density of Ceres is less than that of Earth's crust, and because the surface bears evidence of water-bearing minerals. They 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 asteroid's solid core.