Group V The Gas Giant Planets
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A gas giant is a large planet that is not primarily composed of rock or other solid matter. 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 gases hydrogen and helium, with traces of water, methane, ammonia, and other hydrogen compounds. 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. There are four gas giants in our solar system: Jupiter, Saturn, Uranus, and Neptune. The gas giants in our Solar Systems have a number of similar characteristics. All of our Solar System’s gas giants are outer planets, which means they are the furthest planets from the Sun. Compared to terrestrial planets, gas giants are extremely large and massive. For example, Jupiter has a mass 318 times the mass of Earth, which is a terrestrial planet. Despite their size, gas giants are low-density planets. In addition to being large, these planets rotate extremely quickly, rotation periods ranging from about 10 to 17 hours, as contrasted with the Earth's 24-hour rotation. Jupiter rotates so quickly that it has actually flattened at its poles. The gas giants are extremely cold planets, although that is mostly due to the fact that they are very far from the Sun. Gas giants also have dozens of satellites and ring systems. Saturn is famous for its beautiful rings, which can be seen with the unaided eye from Earth.

Astronomers have also discovered gas giants around stars in other solar systems. In fact, these are the only extra-solar planets that scientists have been able to discover as of yet. These extra-solar gas giants seem similar to Jupiter and the other gas giants in our own Solar System.
In our solar system the four giant planets look radically different from each other. Jupiter and Saturn are predominantly tan-colored, while Uranus and Neptune are blue. Note that many images online have enhanced or exaggerated colors; the true colors of all of the planets are muted and subtle. Saturn has a uniquely prominent ring system. Uranus and Neptune are so far away that they barely show as disks in the largest telescopes. Little was known about Uranus and Neptune until the Voyager 2 space probe visited them in the 1980s.

The gases that formed the Sun and its surroundings were mostly hydrogen and helium, but helium is inert and doesn’t form chemical compounds. So the ices that formed in the outer Solar System are a variety of frozen compounds of hydrogen. One is very familiar: frozen water (H2O), or ordinary ice. Others are less familiar: for example, frozen methane (CH4), and ammonia (NH3). The "H" in the formula of each of these icy compounds is the symbol for the universe's most common element, hydrogen. Though hydrogen compounds dominate, there are still more ices in the outer Solar System. One is frozen carbon dioxide (CO2), which is also a component of the polar ice caps of Mars. On Earth, we know it as "dry ice." Instead of forming rocky worlds like the terrestrial planets, the outer Solar System formed worlds of rock plus ice, in roughly equal mixtures.

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