Small but mighty
Dwarf Planets
A dwarf planet is round and orbits the Sun, but unlike the eight planets it has not cleared other objects from its orbital path. That definition, set by the International Astronomical Union in 2006, is why Pluto was reclassified. These are three of the best-studied dwarf planets.
The famous dwarf planetPlutoPluto is the best-known dwarf planet, orbiting in the Kuiper Belt beyond Neptune. Discovered in 1930, it was called the ninth planet until 2006, when the International Astronomical Union defined "planet" and reclassified Pluto because it has not cleared its orbital neighborhood of other objects.5.9 billion km (39.5 AU average) • 2,377 km (0.19x Earth)The asteroid belt dwarfCeresCeres is the largest object in the asteroid belt between Mars and Jupiter, and the only dwarf planet in the inner solar system. Discovered in 1801, it was first called a planet, then an asteroid, and finally a dwarf planet in 2006.413 million km (2.8 AU) • 940 kmThe planet-definition triggerErisEris is one of the largest known dwarf planets, orbiting far beyond Pluto in the scattered disc region of the outer solar system. Its discovery in 2005 directly triggered the 2006 debate that led to the official definition of "planet" and Pluto's reclassification — Eris is about the same size as Pluto but more massive, so astronomers had to decide whether both were planets or neither.10.1 billion km (68 AU average) • 2,326 km