A gas giant (sometimes also known as a Jovian planet after the planet Jupiter) is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our Solar System; Jupiter, Saturn, Uranus, and Neptune. Uranus and Neptune may be considered a separate subclass of giant planets, 'ice giants', or 'Uranian planets', as they are mostly composed of ice, rock, as well as gases of water, ammonia and methane, unlike the "traditional" gas giants Jupiter or Saturn. However, they share the same qualities of the lack of the solid surface; their differences stem from the fact that their proportion of hydrogen and helium is lower, due to their greater distance from the Sun.
Gas giants may have a rocky or metallic core—in fact, such a core is thought to be required for a gas giant to form—but the majority of its mass is in the form of the gaseous hydrogen and helium, with traces of water, methane, ammonia, and other hydrogen compounds. (Although familiar to us as gases on Earth, these constituents are expected to be compressed into liquids or solids deep in a gas giant's atmosphere.)
Unlike rocky planets, which have a clearly defined difference between atmosphere and surface, gas giants do not have a well-defined surface; their atmospheres simply become gradually denser toward the core, perhaps with liquid or liquid-like states in between. One cannot "land on" such planets in the traditional sense. Thus, terms such as diameter, surface area, volume, surface temperature and surface density may refer only to the outermost layer visible from space.
สมัครสมาชิก:
ส่งความคิดเห็น (Atom)
ไม่มีความคิดเห็น:
แสดงความคิดเห็น