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A giant planet is any massive planet. They are usually primarily composed of low-boiling-point materials (gas or ices), rather than rock or other solid matter, but massive solid planets can 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, after Jupiter. They are also sometimes known as gas giants. However, many astronomers apply the latter term only to Jupiter and Saturn, classifying Uranus and Neptune, which have different compositions, as ice giants.〔 〕〔See for example: 〕 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. Objects large enough to start deuterium fusion (above 13 Jupiter masses in the case of solar composition) are called brown dwarfs, which occupy the mass range between that of large giant planets and the lowest-mass stars. ==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.〔(Rocky core solubility in Jupiter and giant exoplanets ), Hugh F. Wilson, Burkhard Militzer, 2011〕 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". Among extrasolar planets, hot Jupiters and hot Neptunes are giant planets that orbit very close to their stars and thus have a very high surface temperature. Hot Jupiters were, until the advent of space-borne telescopes, the most common form of extrasolar planet known, due to the relative ease of detecting them with ground-based instruments. Giant planets are commonly said to lack solid surfaces, but it is more accurate to say that they lack surfaces altogether since the gases that constitute them simply become thinner and thinner with increasing distance from the planets' centers, eventually becoming indistinguishable from the interplanetary medium. Therefore landing on a giant planet may or may not be possible, depending on the size and composition of its core. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「giant planet」の詳細全文を読む スポンサード リンク
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