Ice dwarf

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".cite web
title=ice dwarf
publisher=Encyclopedia of Astrobiology, Astronomy and Spaceflight
author=David Darling
url=http://www.daviddarling.info/encyclopedia/I/icedwarf.html
accessdate=2008-06-22] 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, is probably a captured ice dwarf from the Kuiper Belt.cite journal
author = Craig B Agnor, Douglas P Hamilton
title = Neptune's capture of its moon Triton in a binary–planet gravitational encounter
journal = Nature
year = 2006
date = May 2006
volume = 441
issue = 7090
pages = 192–194
doi = 10.1038/nature04792
url = http://www.nature.com/nature/journal/v441/n7090/abs/nature04792.html
accessdate = 2006-05-10]

Plutoids

Ice dwarfs that have achieved hydrostatic equilibrium are plutoids. There are four recognized plutoids:
# Eris (2400 km)
# Pluto (2306 km)
# Haumea (~1,960×1,518×996 km)
# Makemake (1300–1900 km)

Note also
# Triton (2707 km), the largest moon of Neptuneas well as other large icy moons of the gas giants.

References

External links

* [http://www.daviddarling.info/encyclopedia/I/icedwarf.html Encyclopedia of Astrobiology, Astronomy and Spaceflight (David Darling)]
* [http://www.physorg.com/news74703928.html Astronomers struggle to define 'planet']