Terminology
The term gas giant was coined in 1952 by the science fiction writer James Blish. Arguably it is something of a misnomer, since throughout most of the volume of these planets all the components (other than solid materials in the core) are above the critical point and therefore there is no distinction between liquids and gases. Fluid planet would be a more accurate term. Jupiter is an exceptional case, having metallic hydrogen near the center, but much of its volume is hydrogen, helium and traces of other gases above their critical points. The observable atmospheres of any of these planets (at less than unit optical depth) are quite thin compared to the planetary radii, only extending perhaps one percent of the way to the center. Thus the observable portions are gaseous (in contrast to Mars and Earth, which have gaseous atmospheres through which the crust may be seen).
The rather misleading term has caught on because planetary scientists typically use "rock", "gas", and "ice" as shorthands for classes of elements and compounds commonly found as planetary constituents, irrespective of what phase the matter may appear in. In the outer solar system, hydrogen and helium are referred to as "gases"; water, methane, and ammonia as "ices"; and silicates and metals as "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.
The alternative term jovian planet refers to the Roman god Jupiter—the genitive form of which is Jovis, hence Jovian—and was intended to indicate that all of these planets were similar to Jupiter. However, the many ways in which Uranus and Neptune differ from Jupiter and Saturn have led some to use the term only for the planets similar to the latter two.
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.
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