Ceres: The Largest Asteroid and the Smallest Dwarf Planet
The dwarf planet Ceres, which was discovered by astronomer Giuseppe Piazzi in 1801, is the largest body in the asteroid belt located between Jupiter and Mars. It takes its name from the Roman goddess of fertility and agriculture.
Although Ceres is about 600 miles in diameter (with 4% of the Moon’s mass), because of its distant orbit (about 2.8 AU from the Sun) it is not normally visible to the naked eye unless the conditions are ideal. Unlike most of the asteroids, Ceres is spherical in shape, like the planets, with an estimated maximum surface temperature of -38º C. Ceres orbits the Sun once every 4.6 years.
Like the other asteroids, most scientists believe Ceres was formed from the collapse of an interstellar cloud of gas and dust through the force of gravity, similar to the planets, gradually evolving into a protoplanet. Astronomers postulate that Ceres did not accrete into a planet because of the strong gravitational pull of nearby Jupiter.
Based on observations from the Keck telescope and scientific modelling, scientists have determined that Ceres has a rocky core overlain with a thick mantle (up to 100 km deep) that likely contains frozen water. Sightings from the Hubble Space Telescope have detected a number of bright and dark features on its surface that look like craters.
When Ceres was first discovered, it was classified as a planet mainly because it was located in the “empty” space between Mars and Jupiter that should have been occupied by a planet, according to the Titus-Bode Law (which theory has since been abandoned). After the discovery of many more bodies in this region of space over the next fifty years, astronomers decided to reclassify them all as “asteroids”. Ceres (along with three other large asteroids, Vesta, Juno, and Pallas) lost its planetary status in the process. There are currently over 100,000 known asteroids in this belt.
In 2006, the International Astronomical Union (IAU) was forced to debate the definition of “planet” in light of the recent discovery of Eris, a body in the Kuiper belt that was larger than Pluto. They had to determine whether to classify Eris as a planet or to demote Pluto from that status. Their deliberations also addressed the classification of Ceres and the other large asteroids. After much discussion, the IAU settled on a new definition for “planet” and created a new “dwarf planet” classification. Pluto was demoted from a planet to a dwarf planet, while Ceres (along with Eris) was promoted to the status of dwarf planet. Dwarf planets are similar to planets in most aspects, except they are not large enough to clear the debris from their orbital paths.
More of Ceres’ secrets will be revealed after scientists get a closer look at it in 2015, when the unmanned space probe Dawn (launched by NASA in 2007) begins to orbit this dwarf planet.