What used to be a planet but is not now?
Since its discovery till 2006, Pluto was considered a planet.
International Astronomical Union decided on 24 August 2006 that Pluto is to be considered a dwarf planet instead.
Pluto, recently demoted from major Planet status, is now a "Dwarf Planet". However it's not even the biggest dwarf planet. "Eris" is at present the biggest of the dwarf planets that are known. "minor planets" used to mean asteroids only, but the term now includes the dwarf planets. So, the answer is ERIS.
Pluto used to be one of the nine planets (the ninth planet), but it was reclassified as a Dwarf Planet on August 24th 2006. Now there are only eight planets recognized. you know RoryHem is correct but hes forgetting Ceres. Ceres was considered a planet too at one point but is now classified as a dwarf planet. so if dwarf planets counted their would be 10 planets in the solar system.