According to National Geographic, the primary reason Pluto was demoted from planet to dwarf planet status is its size relative to its satellite. All the other planets with moons are much larger than their satellites, while Pluto is only about twice the size of its moon, Charon. Since Pluto does not "dominate its neighborhood," the International Astronomical Union demoted the planet in 2006.Continue Reading
The controversy over Pluto's status ultimately began as improved telescope and detection technology revealed a number of bodies of similar size and classification as the former ninth planet in the solar system. The Kuiper Belt, a field of icy objects at the perimeter of the solar system, may be full of Pluto-like objects, and astronomers began to discover bodies that were almost as large as Pluto.
In 2005, the discovery of Eris sealed Pluto's fate. Eris is an object in the Kuiper Belt that is actually larger than Pluto, making it a candidate for planetary status. Since astronomers realized that allowing Pluto to remain a planet could ultimately add dozens or even scores of new planets to the solar system's tally, they came up with a new classification for Pluto and other objects similar to the former planet.Learn more about Planets