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OTHER DWARF PLANETS: Eris Eris, previously known as 2003 UB313 Pluto Pluto

Images of Ceres taken by the Hubble Space Telescope over a period of 2 hr 20 min, the time it takes the asteroid to complete one quarter of a rotation. The bright spot that appears in each image is probably an impact crater. It is brighter than its surroundings, yet still very dark – reflecting only a small portion of the sunlight that shines on it. Credit: NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell University), and L. McFadden (University of Maryland, College Park)
Hubble image of Ceres taken in 2007
Hubble image of Ceres taken in 2007
A dwarf planet according to a new classification scheme adopted by astronomers on August 24, 2006. Before this elevation in status, which may yet be contested, Ceres was considered to be merely the largest asteroid in the Solar System (though exceeded in size by some Kuiper Belt objects). It was also the first asteroid to be discovered, by Giuseppe Piazzi on Jan. 1, 1801.

Ceres contains about one third of the mass of all the asteroids in the main asteroid belt put together. With a surface composition probably similar to that of a carbonaceous chondrite, Ceres is so dark that if placed at the same distance as Vesta (the third largest asteroid, which has only 55% of Ceres' diameter but an albedo of 0.35) it would appear the fainter of the two; even so, Ceres reaches magnitude 7.4 at opposition, second only to Vesta.

Discovery and recovery

For a while after its discovery, Ceres was lost. The observations made by Piazzi before his new-found object entered the daytime sky were too few to allow an orbit to be calculated that was accurate enough to predict where the object would reappear when it moved back into the night sky. There matters might have stood were it not for the fact that Ceres lay at the heliocentric distance predicted by the Titius-Bode law and therefore seemed as if it might be the missing planet that Johann Bode argued must exist between Mars and Jupiter.

The discovery of Uranus in 1781 at a distance that closely fit the one predicted by the Titius-Bode law had already bolstered confidence that the law was valid. So convinced were some astronomers that Ceres was Bode's missing planet that, during an astronomical conference in 1796, they agreed to undertake a systematic search. This led Carl Gauss to develop, in 1801, a method for computing the orbit of an asteroid from only a few observations – a technique that hasn't been significantly improved since. Using Gauss's predictions, the German astronomer Franz von Zach recovered Bode's mystery world on Jan. 1, 1802. Piazzi named it Ceres after the Roman goddess of grain and patron goddess of Sicily, thereby starting a tradition that continues to the this day: asteroids are named by their discoverers (in contrast to comets, which are named for their discoverers).

Observation and theory

infrared map of Ceres
Ceres reveals a textured surface in this 3D color model produced from infrared data. Blue corresponds to dark patches in the infrared, yellow to bright. The blackout at the edges is due to a lack of data at the poles.
Observations by the Hubble Space Telescope at visible wavelengths in 2003–2003 and by the Keck Observatory at near-infrared wavelengths in 2006 have shown that Ceres has characteristics of the rocky, terrestrial planets like Earth. Its shape is roughly round but noticeably oblate (i.e., with a bulge at the equator). This shape suggests that its interior is differentiated, with a rocky inner core, a mantle containing as much as 25% pristine water ice as old as the Solar System, and a thin, dusty outer crust. Gravitational perturbations from Jupiter, billions of years ago, prevented Ceres from accreting more material to become a larger world.

comparison of asteroids

If the estimated high water content of the mantle, based on the oblateness, is correct, Ceres may contain more fresh water than the Earth. Some of this water may have migrated to the surface through natural channels, having collected minerals along with it, and, be contributing to the mottled pattern seen in the Keck infrared images. There are some indications that the surface of Ceres is relatively warm and that it may have a very thin atmosphere. The maximum temperature with the Sun overhead was estimated from measurements to be 235K (about -38C) on May 5, 1991. Taking into account also the heliocentric distance at the time, gives an estimated maximum of ~239K (-34°C) at perihelion.

diameter 975 × 909 km
density 2.08 g/cm³
class G
albedo 0.09
rotation period 9.08 hours
semimajor axis 2.767 AU
perihelion 2.544 AU
aphelion 2.987 AU
period 4.599 years
eccentricity 0.078
inclination 10.587°

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