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The ice giants Uranus and Neptune are often being distinguished from Saturn and Jupiter who consist mostly of hydrogen and helium, while the ice giants have more of heavier elements than hydrogen and helium. However, is there any clear distinction or is it just that gas giants who consist of more than 50% of heavier components than hydrogen and helium are considered ice giants, or something like that?

Saturn has a little bit more ices than Jupiter but still less than two per cent. Uranus and Neptune have ices in the deeper atmosphere while their upper atmosphere is hydrogen and helium too. Do planets inbetween exist that would be hard to classify whether they are ice giants or Jupiter-/Saturn-like gas giants?

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The distinction between gas giants and ice giants is a distinction that works nicely in our solar system. (And don't forget the distinction between the giant planets and the terrestrial planets, which also works nicely in our solar system.) I strongly doubt that these distinctions are universal.

Just before the end of the 20th century, astronomers and physicists thought they had developed a nice and simple explanation of how the solar system formed. Then astronomers started discovering exoplanets. Lots and lots of exoplanet systems. Some of those systems utterly defied that nice and simple explanation. In addition to the oddball systems, the exoplanets appear to lie on a spectrum rather than a nice clearcut classification into terrestrial / gas giant / ice giant.

There are many issues with that nice and simple explanation from the previous century, even in our solar system. The Nice Model, the Grand Tack Model, and the Grand Attack Model are attempts at addressing those many issues. All of these models are somewhat specific to our solar system, but are also somewhat generic. The formation of a star system appears to involve more than a bit of chaos.

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    $\begingroup$ Is there some introductory text on the Grand Attack Model you would suggest? $\endgroup$
    – B--rian
    Jun 16 at 19:59
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    $\begingroup$ @B--rian Edited in. I've become less of a fan of wikipedia than I used to be, but I left your edits on the Nice Model and the Grand Tack Model in place. $\endgroup$ Jun 16 at 22:47

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