Pluto is smaller than Ceres.
Eris ( minor-planet designation 136199 Eris ) is the most massive and second-largest dwarf planet known in the Solar System . Under the IAU definition approved on August 24 , 2006 , Eris is a `` dwarf planet '' , along with objects such as Pluto , Ceres , Haumea and Makemake , thereby reducing the number of known planets in the Solar System to eight , the same as before Pluto 's discovery in 1930 . Pluto . Pluto is the largest and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun . It is the largest known trans-Neptunian object by volume but is less massive than Eris .