This is Ceres. Before being reclassified as a dwarf planet, Ceres was the first asteroid to be spotted—by Guiseppe Piazzi in 1801. He announced that he'd found a comet. Privately, he suspected it might be "something better"—perhaps a planet? But a year later William Herschel, discoverer of the planet Uranus, showed that Ceres was much smaller than Earth's moon. Herschel proposed the word asteroid, and ever since, asteroids have been giving astronomers headaches and reasons to argue with one another.

It turns out that even the definition of the word asteroid isn't precise. Many asteroids are irregularly shaped chunks of rock, too small to have been rounded into spheres by gravity. Other asteroids are almost pure high-grade nickel steel, having been refined (geologists call it differentiated) during their formation. And we now know that at least a few other asteroids are spent cores of former comets with no more volatile gases to give off.

That Piazzi found Ceres first is really no surprise; with a diameter of 900 kilometers (540 miles), it was one of the largest. But more than 200 asteroids with diameters larger than 100 kilometers (60 miles) have been discovered. As of 2006, Ceres is now classified as a dwarf planet not an asteroid.