Ice dwarf is a term for a small icy planetary body (or asteroid or planetoid) that orbits beyond Neptune, that was coined as part of a conception of a threefold division of the Solar system into inner terrestrial planets, central gas giants, and outer ice dwarfs, of which Pluto was the principal member. This conception foreshadowed the demotion of Pluto to the status of a dwarf planet after the discovery of Eris.
'Ice dwarf' is not an IAU classification, and it overlaps with the official cubewano and plutino classifications. One attempted definition is that an ice dwarf "is larger than the nucleus of a normal comet and icier than a typical asteroid".[1] There are large numbers of such objects in the Oort Cloud and the Kuiper Belt. However, it is not clear that all ice dwarfs are actually icier than icy asteroids such as Ceres (now considered a dwarf planet). Nonetheless, Ceres is sometimes called a terrestrial dwarf to distinguish it from Pluto and Eris.
Alan Stern of the Southwest Research Institute believes there may be trillions of ice dwarfs beyond Neptune, and that the outer planets show signs of collisions with ice dwarfs, 1000 to 2000 kilometers in diameter: Uranus could have been tipped off its axis by an ice dwarf, and Triton, the largest moon of Neptune, may have hit an ice dwarf as well.[2]
