What is the conventional explanation for the formation of the "ice giants" Uranus and Neptune? I understand that formation of giants like Jupiter and Saturn is the result of a large core (10+ Earth masses) accreting an extensive hydrogen envelope. However, the ice giants do not have large enough cores to accomplish this, and also have much different compositions (primarily superheated ices, especially water ice), as has been discussed in the thread about Neptune's interior. The only other theory I've heard is the "disk instability" model, but that that theory is still only a minority view.

Also, I've heard two conflicting models for the internal structure of Uranus, specifically its core. Many sources state that Uranus' interior structure is nearly identical to Neptune's, complete with a solid core about the mass of Earth. However, some sources state that Uranus doesn't have a core at all. Bill Arnett's Nine Planets website states that "It appears that Uranus does not have a rocky core like Jupiter and Saturn but rather that its material is more or less uniformly distributed." The entry on Uranus in the World Almanac likewise makes the same point. So, which of these models is true?

Thanks.

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"The universe we observe has precisely the properties we should expect if there is, at bottom, no design, no purpose, no evil and no good, nothing but blind pitiless indifference." - Richard Dawkins