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By Mark Hofstadter, David Atkinson, Kevin Baines, Shawn Brooks, Leigh Fletcher, A. James Friedson, Patrick Irwin, Sanjay Limaye, Robert Moeller, Neil Murphy, Glenn Orton, Christophe Sotin, Thomas Spilker and Daniel Wenkert

Abstract

In the 1990s it was realized that Uranus and Neptune represent a distinct class of planet, much different than the more familiar gas giants, Jupiter and Saturn. The gas giants are composed mostly of hydrogen (more than 90 % by mass). Their hydrogen envelopes are thought to extend all the way to their relatively small rock/ice cores, with molecular H2 beginning a transition to ionized, metallic hydrogen at mega-bar pressures (Guillot 2005; Lissauer and Stevenson 2007). While Uranus and Neptune also possess hydrogen envelopes, they are much smaller, accounting for less than 20 % of the planet’s masses and never making the transition to metallic hydrogen (Guillot 2005). The bulk composition of these planets is dominated by much heavier elements. Based on cosmic abundances, oxygen, carbon, nitrogen, and sulfur are the likely candidates. Since these species are thought to have been incorporated into proto-planets primarily as ices—either as solids themselves or as gas trapped in a water-ice clathrate (Hersant et al. 2004)—the term “ice giants ” has been adopted. Today, however, there is probably very little ice in Uranus and Neptune, a supercritical fluid being the preferred phase of H2O at depth. In 2004, the first of many ice giant candidates was reported around another star (Butler et al

Year: 2011
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