Gas Giant
A gas giant is a giant planet composed mainly of hydrogen and helium. There are two gas giants in the Solar System, Jupiter and Saturn. The term "gas giant" was originally synonymous with "giant planet". However, starting in the 1970s and continuing into the 1980s, it became increasingly common to classify Uranus and Neptune separately as ice giants, a distinct class of giant planets composed mainly of heavier volatile substances (referred to as "ices").
Jupiter and Saturn consist mostly of hydrogen and helium, with heavier elements making up between 3 and 13 percent of their mass. They are thought to have a deep atmosphere of molecular hydrogen, with lower layers being more compressed, surrounding a layer of liquid metallic hydrogen, and with a molten rocky core inside. The outermost portions of their hydrogen atmospheres contain many layers of visible clouds that are mostly composed of water and ammonia. The layer of metallic hydrogen located in the mid-interior would make up the bulk of the mass of every gas giant, and it is referred to as "metallic" because under the immense weight of the atmosphere, the very high pressure turns hydrogen into an electrical conductor. The gas giants' cores are thought to consist of heavier elements at such high temperatures (20,000 K [19,700 °C; 35,500 °F]) and pressures that their properties are not yet completely understood. The placement of the solar system's gas giants can be explained by the grand tack hypothesis.
The line of delineation between very low-mass brown dwarf stars (which can have masses as low as 13 times that of Jupiter) and gas giants is debated. While one definition depends on the manner of formation of the object, the other relies only 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.
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