From Wikipedia. Pluto is a dwarf planet in the solar system and is the prototype of a yet-to-be-named family of trans-Neptunian objects.
From its discovery in 1930 until 2006, Pluto was considered the ninth planet of the solar system by the International Astronomical Union (IAU). After recent discoveries of other trans-Neptunian objects, most notably 2003 UB313, which is larger than Pluto, the IAU redefined the term 'planet'; this resulted in Pluto being reclassified as a dwarf planet.
