A giant planet is any planet much larger than Earth. They are usually mostly composed of low-boiling-point materials (gases or ices), rather than rock or other solid matter. Massive solid planets also exist. There are four giant planets in the Solar System: Jupiter, Saturn, Uranus and Neptune. Many extrasolar giant planets have been identified orbiting other stars.
Giant planets are also sometimes called jovian planets, the adjective for the name Jupiter. They are sometimes known as gas giants. Many astronomers use the latter term only for Jupiter and Saturn. They classify Uranus and Neptune, which have different compositions, as ice giants. Both names are potentially misleading: all of the giant planets consist primarily of fluids above their critical points, where distinct gas and liquid phases do not exist. The principal components are hydrogen and helium in the case of Jupiter and Saturn, and water, ammonia and methane in the case of Uranus and Neptune.
The defining differences between a very low-mass brown dwarf and a gas giant are not clear. One school of thought is based on formation; the other, on the physics of the interior. Part of the debate is whether "brown dwarfs" must, by definition, have had nuclear fusion at some point in their history.
Terminology
The term gas giant was coined in 1952 by the science fiction writer James Blish. It was originally used to refer to all giant planets. Arguably it is something of a misnomer, because for most of thire volume the pressure is so high that matter is not in gaseous form. Other than the upper layers of the atmosphere, all matter is likely beyond the critical point, where there is no distinction between liquids and gases. Fluid planet would be a more accurate term. Jupiter also has metallic hydrogen near its center, but much of its volume is hydrogen, helium, and traces of other gases above their critical points. The observable atmospheres of all these planets (at less than unit optical depth) are quite thin compared to their 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 can 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 found on planets, whatever the actual phase of matter. 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 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 have started referring to Uranus and Neptune as ice giants to indicate the predominance of the ices (in fluid form) in their interior composition.
The alternative term jovian planet refers to the Roman god Jupiter, and meant all these planets were similar to Jupiter.
Objects large enough to start deuterium fusion (above 13 Jupiter masses for solar composition) are called brown dwarfs, and these occupy the mass range between that of large giant planets and the lowest-mass stars. The 13-Jupiter-mass cutoff is a rule of thumb, not something of precise physical significance. Larger objects will burn most of their deuterium and smaller ones will burn only a little, and the Jupiter mass value is somewhere in between. The amount of deuterium burnt depends not only on the mass but also on the composition of the planet, especially on the amount of helium and deuterium present. The Extrasolar Planets Encyclopaedia includes objects up to 60 Jupiter masses, and the Exoplanet Data Explorer up to 24 Jupiter masses.
Description
A giant planet is a massive planet and has a thick atmosphere of hydrogen and helium. They may have a dense molten core of rocky elements, or the core may have completely dissolved and dispersed throughout the planet if the planet is hot enough. In "traditional" giant planets such as Jupiter and Saturn (the gas giants) hydrogen and helium constitute most of the mass of the planet, whereas they only make up an outer envelope on Uranus and Neptune, which are instead mostly composed of water, ammonia, and methane and therefore increasingly referred to as "ice giants".