<P> The defining differences between a very low - mass brown dwarf and a gas giant (estimated at about 13 Jupiter masses) are debated . One school of thought is based on formation; the other, on the physics of the interior . Part of the debate concerns whether "brown dwarfs" must, by definition, have experienced nuclear fusion at some point in their history . </P> <P> The term gas giant was coined in 1952 by the science fiction writer James Blish and was originally used to refer to all giant planets . It is, arguably, something of a misnomer because throughout most of the volume of all giant planets, the pressure is so high that matter is not in gaseous form . Other than solids in the core and the upper layers of the atmosphere, all matter is above the critical point, where there is no distinction between liquids and gases . The term has nevertheless 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". Because Uranus and Neptune are primarily composed of, in this terminology, ices, not gas, they are increasingly referred to as ice giants and separated from the gas giants . </P> <P> Gas giants can, theoretically, be divided into five distinct classes according to their modeled physical atmospheric properties, and hence their appearance: ammonia clouds (I), water clouds (II), cloudless (III), alkali - metal clouds (IV), and silicate clouds (V). Jupiter and Saturn are both class I. Hot Jupiters are class IV or V . </P> <P> A cold hydrogen - rich gas giant more massive than Jupiter but less than about 500 M (1.6 M) will only be slightly larger in volume than Jupiter . For masses above 500 M, gravity will cause the planet to shrink (see degenerate matter). </P>

How do we know jupiter is a gas planet