Uranus dan Neptun mempunyai komposisi dalaman yang jelas berbeza, dengan kebanyakan dalamannya adalah dianggarkan terdiri daripada sebuah campuran (atau bahagian-bahagian berlapis) batu, air, metana, dan ammonia. Kedua-duanya mempunyai medan magnetik yang sangat cenderung kepada paksi putaran mereka.
The term was coined by the science fiction writer [[James Blish]]. Arguably it is a misnomer, since all of these planets are primarily liquid and not gaseous. In fact, for Neptune and Uranus, the gaseous atmospheres are quite thin compared to the planetary radii -- only extending perhaps one percent of the way to the center. However, at least for Jupiter and Saturn, the name is defensible because their compositions are dominated by hydrogen and helium, which are gases in the outer solar system when not under pressure.
Planetary scientists often use ' rock', 'gas', and ' ice' as shorthands for classes of elements and compounds commonly found as planetary constituents, irrespective of what [[ phase of matter| phase]] they appear in. In the outer solar system, hydrogen and helium are " gases"; water, methane, and ammonia are " ices"; and silicates are rock. When deep planetary interiors are considered, it may not be far off to say that, by "ice" astronomers mean oxygen and carbon, by " rock" they mean silicon, and by "gas" they mean hydrogen and helium.
With this terminology in mind, some astronomers are starting to refer to Uranus and Neptune as "ice giants", to indicate the apparent predominance of the "ices" (in liquid form) in their interior composition.
==Extrasolar gas giants==