Plutoid

A plutoid or ice dwarf is a trans-Neptunian dwarf planet, i.e. a body orbiting beyond Neptune that is massive enough to be rounded in shape. The term plutoid was adopted by the International Astronomical Union (IAU) working group Committee on Small Bodies Nomenclature, but was rejected by the IAU working group Planetary System Nomenclature. The term plutoid is not widely used by astronomers, though ice dwarf is not uncommon.

There are thought to be thousands of plutoids in the Solar System, although only four have been formally designated as such by the IAU.

The IAU developed this category of astronomical objects as a consequence of its 2006 resolution defining the word "planet". The IAU's formal definition of "plutoid", announced 11 June 2008, is:

In light of the difficulty of remotely ascertaining hydrostasis, the IAU only formally confers "dwarf planet" (and by extension, "plutoid") status to those bodies whose minimum estimated size is substantially greater than what is generally thought necessary to guarantee hydrostatic equilibrium. As of 2009^[update], Pluto, Eris, Haumea, and Makemake are the only objects officially recognized as plutoids, while upwards of seventy more bodies that currently lack formal recognition are thought likely to meet the definition, and can expect formal recognition at some time in the future.

Alan Stern of the Southwest Research Institute believes the outer planets show signs of collisions with plutoids 1,000 to 2,000 kilometers in diameter: Uranus could have been tipped off its axis by a plutoid, and Triton, the largest moon of Neptune, is probably a captured plutoid from the Kuiper belt.

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