Group B The Gas Giant Planets

**A gas giant (sometimes also known as a jovian planet after the planet Jupiter, or giant planet) is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune. Many extrasolar gas giants have been identified orbiting other stars.

Planets above 10 Earth masses are termed giant planets.[1] Lower-mass gassy planets are sometimes called "gas dwarfs".[2]

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 gas giant planets and the lowest mass stars. The 13 Jupiter mass (MJ) cutoff is a rule of thumb rather than something of precise physical significance. Larger objects will burn most of their deuterium and smaller ones will burn only a little, and the 13 MJ value is somewhere in between. The amount of deuterium burnt also depends not only on mass but on the composition of the planet, especially on the amount of helium and deuterium present.[3] The Extrasolar Planets Encyclopaedia includes objects up to 25 Jupiter masses, and the Exoplanet Data Explorer up to 24 Jupiter masses.**

**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.**

**The four solar system gas giants share a number of features. All have atmospheres that are mostly hydrogen and helium and that blend into the liquid interior at pressures greater than the critical pressure, so that there is no clear boundary between atmosphere and body. They have very hot interiors, ranging from about 5,000 kelvins (K) for Neptune to over 20,000 K for Jupiter. This great heat means that, beneath their atmospheres, the planets are most likely entirely liquid. Thus, when discussions refer to a "rocky core", one should not picture a ball of solid rock, or even, at 20,000 K, liquid rock. Rather, what is meant is a region in which the concentration of heavier elements such as iron and silicon is greater than that in the rest of the planet.

All four planets rotate relatively rapidly, which causes wind patterns to break up into east-west bands or stripes. These bands are prominent on Jupiter, muted on Saturn and Neptune, and barely detectable on Uranus. Uranus has an extreme tilt unlike the other gas giants that causes extreme seasons.

Finally, all four are accompanied by elaborate systems of rings and moons. Saturn's rings are the most spectacular, and were the only ones known before the 1970s. As of 2006, Jupiter is known to have the most moons, with sixty-three.**

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