An ice dwarf is a planetary body that is larger than the nucleus of a normal comet and much icier than any asteroid. They are a few hundred kilometers across, and they are found mostly in the Oort Cloud and the Kuiper Belt in very large numbers. "Thousands of these planetoids lie beyond the nine planets," says Alan Stern of the Southwest Research Institute. He thinks that there may be trillions of ice dwarfs that we have never seen. A small explosion on Halley's Comet suggests that it could have collided with one of these icy planetoids.^{[citation needed]} Stern says that the three outer planets show signs of collisions with ice dwarfs, 1000 to 2000 kilometers in diameter. Uranus was tipped off its axis by an ice dwarf, and Triton, the moon of Neptune, hit an ice dwarf as well. Nucleus usually refers to the center of something, but can mean: In science: Atomic nucleus, the collection of protons and neutrons in the center of an atom that carries the bulk of the atoms mass and positive charge Cell nucleus, the membrane-bound subcellular organelle found in eukaryotes, visible... Comet Hale-Bopp For other uses, see Comet (disambiguation). ... An asteroid is a predominantly rocky body that orbits around its star. ... This image is an artists rendering of the Oort cloud and the Kuiper Belt. ... Artists rendering of the Kuiper Belt and hypothetical more distant Oort cloud. ... This page may meet Wikipedias criteria for speedy deletion. ... Southwest Research Institute (SwRI) is an independent, nonprofit applied research and development organization. ... Comet Halley as taken with the Halley Multicolor Camera on the ESA Giotto mission. ... Adjective Uranian Atmospheric characteristics Atmospheric pressure 120 kPa (at the cloud level) Hydrogen 83% Helium 15% Methane 1. ... Adjective Neptunian Atmospheric characteristics Surface pressure ≫100 MPa Hydrogen - H2 80% ±3. ...

External links

• Encyclopedia of Astrobiology, Astronomy and Spaceflight (David Darling)
• Astronomers struggle to define 'planet'