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

It would help if you actually named and linked to these conflicting sources. Guessing is not part of the BA forum way.

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The formation of Uranus and Neptune has been a nagging problem for a while. Computer models show they cannot form where they are because via accretion because it would take too long at that distance. So if they formed via core accretion they probably formed in the same region as Jupiter and Saturn and were ejected outwards by the bigger guys. Its pretty clear that gas giants can move inwards or out via interaction with other planets or the disk. I think the latest idea is Jupiter moved in by 2%, Saturn out by 10%, Uranus out by 15%-20%, and Neptune out by 30%.

The other theory for making gas giants is Alan Boss's disk instability you mentioned, in that case its been proposed that Uranus and Neptune were bigger but lost most of their envelopes due to a catastrophic close encounter with an O or B star in the birth cloud.

We probably need a bigger sampling of extrasolar systems to figure out which way ice giants form.

Sorry about that. Here are a couple of links. I don't have time to hunt for any more sites today.

http://www.seds.org/nineplanets/nineplanets/uranus.html
http://library.thinkquest.org/C00592...anInterior.htm

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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

From an article re extrasolar planets:
"...In the case of the nearby star 51 Pegasi...The picture that has emerged is of a planet being driven in towards its star during the late stages of planet formation by the gravitational force between the planet and the remnant of the swirling disc of dust out of which the planets formed. But, if this could happen to a gas-giant planet such as Jupiter, why could it not happen to an ice-giant planet like Uranus or Neptune?..."
11 February 2004
http://news.independent.co.uk/world/...p?story=489934