The Solar System | Mercury | Venus | Earth | Mars | Ceres | Jupiter
Saturn | Chiron | Uranus | Neptune | Pluto
The Kuiper Belt and the Oort Cloud | Quaoar | Eris | Sedna

Until 2006, Ceres was classed as an asteroid. However, when the IAU started trying to define a planet and came up with the following definition:

A dwarf planet is a celestial body that

• (a) is in orbit around the Sun,
• (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
• (c) has not cleared the neighbourhood around its orbit, and
• (d) is not a satellite.

the definition fit Ceres almost exactly.

The Facts

• Mass: 9.47×10²⁰kg
• Equatorial Radius: 487km
• Mean Density: 2.08 g/cm³
• Length of Day: 9.07 hours
• Period of Revolution about sun: 1679.8 days
• Acceleration due to Gravity: 0.27m/s²
• Mean Orbital Velocity: 17.882km/s
• Mean Distance from the Sun: 2.77 AU

Early History

Pythagoras once said 'There is music in the spacing of the spheres.' This spacing was formalised in the Bode-Titius law which was known before the discovery of the asteroid belt. Kepler predicted a planet would be found between Mars and Jupiter. Johann Elert Bode said: 'I cannot believe that the founder of the universe left this space empty.' He also proposed that there should be a planet there. Indeed something was there - however, it was not a planet.

Giuseppi Piazzi discovered it in 1801 using a telescope in Palermo, Sicily, and when it began to go retrograde he wrote Bode telling him of his discovery. He named it Ferdinanea-Ceres. This was shortened to Ceres. The name came from the goddess of agriculture, grain and maternal love. She was also patron goddess of Sicily where Ceres was discovered. Soon, other objects were found at this distance from the sun. We now call this region the asteroid belt. Among its thousands of residents, only Ceres is large enough to meet the definition of a Dwarf Planet. It is far smaller than Mars, and a full-size model of it would stretch from Berlin to London. This is about a third the size of the largest dwarf planet and too small to hold an atmosphere.

Current evaluation

After looking at Hubble Telescope photos of the entire rotational cycle of Ceres taken during December 2003-January 2004, scientists postulated that it has a differentiated interior similar to terrestrial planets. This is because with its speed of rotation (once in nine hours) and density (about twice that of water) the equatorial bulge for a solid snowball would give an equatorial radius 39.7 km larger than the polar radius. Instead they find a difference of 32.6 km. This implies the existence of a core and mantle.

One source gives Ceres a core made of silica with a radius of 350km¹. Bill Nye said recently that 'if Pluto had the same orbit as Mercury it would boil away until there was nothing left.' If Earth were put in that orbit our world would lose its oceans but keep its Surface. Since the mantle of Ceres is mostly frozen water, if Ceres were in Mercury's orbit all that would remain would be the core, making it about three-quarters of its current size. Other scientists have called Ceres a proto-planet. Scientist Lucy McFadden even went so far as to say that had it not been for the gravitational perturbations of Jupiter, it would have continued accreting matter and become a full-fledged planet. This process was apparently aborted and the result is a dwarf planet.

The Dawn Mission

It is believed that Earth accreted material for about fifty million years, while Ceres stopped after ten. If so, then Ceres and the Asteroid Vesta² are examples of the early Solar System. With this in mind NASA sent a probe named Dawn in the summer of 2007 to check out the region. Using an ion engine, this craft would reach Vesta in 2010, spend a few months there, then break orbit and go to Ceres, establishing an orbit there in 2016. It would be the first spacecraft ever to orbit two different objects (excluding the Earth). It is expected that the photos and scans from this probe will give us a much better understanding of the Dwarf Planet Ceres and the Asteroid Vesta.