Discovery
The idea that an undiscovered planet could exist between the orbits of Mars and Jupiter was first suggested by Johann Elert Bode in 1772.
His considerations were based on the Titius–Bode law, a now abandoned theory which had been first proposed by Johann Daniel Titius in 1766, observing that there was a regular pattern in the semi-major axes of the known planets marred only by the large gap between Mars and Jupiter. The pattern predicted that the missing planet ought to have a semi-major axis near 2.8 AU.
William Herschel's discovery of Uranus in 1781 near the predicted distance for the next body beyond Saturn increased faith in the law of Titius and Bode. In 1800, requests were sent to twenty-four experienced astronomers, asking that they combine their efforts and begin a methodical search for the expected planet.
One of the astronomers selected for the search was Giuseppe Piazzi at the Academy of Palermo, Sicily. Before receiving his invitation to join the group, Giuseppe Piazzi discovered Ceres on 1 January 1801. He was searching for "the 87th [star] of the Catalogue of the Zodiacal stars of Mr la Caille", but found that "it was preceded by another". Instead of a star, Piazzi had found a moving star-like object, which he first thought was a comet.
Naming
Piazzi originally suggested the name Cerere Ferdinandea for his discovery, after both the mythological figure Ceres (Roman goddess of agriculture) and King Ferdinand III of Sicily. "Ferdinandea" was not acceptable to other nations of the world and was thus dropped.
Stats
Diameter: 952 km
Semi-major axis: 2.766 AU
Orbital Period: 4.60 years
Rotation period: 9.07 hrs
Date discovered: 1801.1.1
Class: G
Type: Main-belt Asteroid
Status
The classification of Ceres has changed more than once. disagreement. Johann Elert Bode believed Ceres to be the "missing planet" he had proposed to exist between Mars and Jupiter, at a distance of 419 million km (2.8 AU) from the Sun. Ceres remained listed as a planet in astronomy books and tables (along with 2 Pallas, 3 Juno and 4 Vesta) for about half a century.
As other objects were discovered in the area it was realised that Ceres represented the first of a class of many similar bodies. In 1802 Sir William Herschel coined the term asteroid ("star-like") for such bodies, writing "they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes". As the first such body to be discovered, it was given the designation "1 Ceres" under the modern system of asteroid numbering.
The 2006 debate surrounding Pluto and what constitutes a 'planet' led to Ceres being considered for reclassification as a planet. A proposal before the International Astronomical Union for the definition of a planet would have defined a planet as "a celestial body that:
(a) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(b) is in orbit around a star, and is neither a star nor a satellite of a planet".
Had this resolution been adopted, it would have made Ceres the fifth planet in order from the Sun. It was not accepted, and in its place an alternate definition came into effect as of 24 August 2006, carrying the additional requirement that:
(c) a "planet" must have "cleared the neighborhood around its orbit".
By this definition, Ceres is not a planet because it does not dominate its orbit, sharing it as it does with the thousands of other asteroids in the asteroid belt and constituting only about a third of the total mass.
It is instead now classified as a dwarf planet.
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