Ceres is the largest object in the asteroid belt that lies between the orbits of Mars and Jupiter, slightly closer to Mars’s orbit. With a diameter of 945 km (587 mi), Ceres is both the largest of the asteroids and the only unambiguous dwarf planet inside Neptune’s orbit. It is the 33rd-largest known body in the Solar System.
Ceres is composed of rock and ice, and contains approximately one-third of the mass of the entire asteroid belt. Ceres is the only object in the asteroid belt known to be rounded by its own gravity, although detailed analysis was required to exclude Vesta. From Earth, the apparent magnitude of Ceres ranges from 6.7 to 9.3, peaking once at opposition every 15 to 16 months, which is its synodic period. Thus even at its brightest, it is too dim to be seen by the naked eye, except under extremely dark skies.
Ceres was the first asteroid to be discovered (by Giuseppe Piazzi at Palermo Astronomical Observatory on 1 January 1801). It was originally considered a planet, but was reclassified as an asteroid in the 1850s after many other objects in similar orbits were discovered.
