A gas giantis agiant planetcomposed mainly ofhydrogenandhelium.JupiterandSaturnare the gas giants of theSolar System. The term “gas giant” was originally synonymous with “giant planet”, but in the 1990s it became known thatUranusandNeptuneare really a distinct class of giant planet, being composed mainly of heavier volatile substances (which are referred to as ‘ices’). For this reason, Uranus and Neptune are now often classified in the separate category ofice giants.
Jupiter and Saturn consist mostly of hydrogen and helium, with heavier elements making up between 3 and 13 percent of the mass.They are thought to consist of an outer layer ofmolecular hydrogensurrounding a layer of liquidmetallic hydrogen, with probably a molten rocky core. The outermost portion of their hydrogen atmosphere is characterized by many layers of visible clouds that are mostly composed of water and ammonia. The layer of metallic hydrogen makes up the bulk of each planet, and is referred to as “metallic” because the very large pressure turns hydrogen into an electrical conductor. The gas giants’ cores are thought to consist of heavier elements at such high temperatures (20,000K) and pressures that their properties are poorly understood.
The defining differences between avery low-mass brown dwarfand a gas giant (~13MJ) 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 experiencednuclear fusionat some point in their history.
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