Diameter: about 940 km (about 580 miles)
Average distance from Sun: 2.8 astronomical units (AU), about 414 million km (257 million miles)
Orbital period (year): 4.6 Earth years (1680.5 days)
Rotation period (day): 9 Earth hours
Atmosphere: May have an extremely thin atmosphere, but it hasn't been detected
The Sky Police were hunting for a planet between Mars and Jupiter.
Bode's Law said there should be a planet between Mars and Jupiter. [To find out more about Bode's Law, click here.] In Germany, a group of astronomers formed a society to search for this planet. They were nicknamed the "Celestial Police" because they wanted order in the Solar System.
Piazzi didn't get an invitation to join the Celestial Police, but he found a planet.
Italian astronomer Giuseppe Piazzi was on the Celestial Police's list to get an invitation to join them. He didn't get an invitation, but on New Year's Day 1801 Piazzi discovered Ceres, and everyone assumed that it was the missing planet.
The new planet is called Hera. No, it's not. Oh, whatever.
Many years before the discovery of Ceres, someone in Germany suggested a name for the "missing planet". It was Hera, after the wife of Zeus in Greek mythology. So Hera is what people in Germany were calling Piazzi's discovery and he wasn't pleased. He wrote, "If the Germans think they have the right to name somebody else's discoveries they can call [it] anything they like." However Piazzi was calling it Ceres Ferdinandea. Ceres was a Roman goddess of agriculture and the patron goddess of Sicily, and Ferdinand was the king of Sicily. Piazzi was the director of the Palermo Observatory in Sicily, but his tribute to the king didn't catch on elsewhere.
Ceres is a planet . . . um, no . . . an asteroid.
Astronomers accepted Ceres as a planet. When Pallas was discovered the following year, they were both listed as planets. William Herschel (discoverer of Uranus) was the first to suggest that they were, in fact, a new kind of object. He called them asteroids (little stars), because they looked like stars through a telescope. The next two that were discovered, Juno and Vesta, were also called planets. Fifty years later, these four were listed both as planets and as asteroids. By the end of the 19th century, they were all asteroids.
Ceres is an asteroid . . . um. . . make that a dwarf planet.
In 2006 the International Astronomical Union (IAU) defined a planet. Famously, Pluto wasn't a planet any more, but became a dwarf planet. Ceres also qualified as a dwarf planet. However it's still an asteroid too - its full name is "1 Ceres", because it was the first asteroid discovered.
Ceres has layers like a planet does.
Planets have layers with heavier materials in the center and lighter ones farther out. For example, Earth has a dense core, a mantle surrounding the core and a thin crust on the outside. Astronomers think that Ceres has a rocky core, a mantle made of water ice and a thin crust.
Ceres has almost a third of the mass of the whole Asteroid Belt, but it's still tiny.
Ceres is by far the biggest object in the Asteroid Belt, even though it's only as big across as the US state of Texas. If you could peel off the crust of Ceres and flatten it out, it would almost cover Argentina, or the USA east of the Mississippi. Yet it has so little mass that someone weighing 100 pounds (45 kg) on Earth would weigh less than two pounds (a kilogram) on Ceres.
The days are short, but the years are long.
Since the Asteroid Belt is beyond Mars, Ceres has a long trip around the Sun. Click here for a diagram. The days are only about nine hours long, but years are nearly four and a half Earth years long.
It gets quite warm considering how far away from the Sun it is.
The temperature on Ceres can get up to -38 C (-36 F). True, that isn't very warm, but it's higher than the annual mean temperature of the interior of Antarctica, which is -57 C (-70 F). However the mean annual temperature on Ceres is around -100 degrees C (-150 F), but Ceres is nearly three times farther away from the Sun than Earth is.
Ceres might have more water than Earth does.
There is evidence of water, but we're still guessing about how much. This is one of the questions NASA's Dawn mission may answer. It was launched in 2007 and arrives at Ceres in 2015, after leaving Vesta.
(1) Parker, J. et al., "Largest Asteroid May Be 'Mini Planet' with Water Ice," http://hubblesite.org/newscenter/archive/releases/2005/27/
(2) Serio, G.F., Manara, A., Sicoli, P., "Giuseppe Piazzi and the Discovery of Ceres," http://www.lpi.usra.edu/books/AsteroidsIII/pdf/3027.pdf
(3)"Dawn's Targets -- Vesta and Ceres" (accessed 06.06.11) http://www.nasa.gov/mission_pages/dawn/ceresvesta/index.html