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A gas giant is a type of planet not composed of rock or solid matter. This means that neither probes nor astronauts could land on these planets because there is no solid surface. They would just start sinking. At the center, the gas giants have a small rocky core. This core is not very well understood by scientists who guess that it is at high temperatures (as high as 20,000 K) and extreme pressures. This means that the core is most likely liquid metals rather than a solid ball, as the name infers. Many scientists believe that the gases that comprise the planet may become gradually denser closer to the core. James Blish, who was a science fiction writer from the mid-1900’s, came up with the term gas giants.
Gas giants also fall into different categories. So-called “traditional” gas giants include Jupiter. Jupiter is such a classic example of a gas giant that gas giants are often referred to as Jovian planets despite the fact that some gas giants – ice giants for instance – are very different from Jupiter. Traditional gas giants have high levels of hydrogen and helium. Another category of gas giants is ice giants. Ice giants are also composed of small amounts of hydrogen and helium; however, they have high levels of what are called “ices.” These ices include methane, water, and ammonia.
There are four gas giants in our Solar System: Jupiter, Saturn, Uranus, and Neptune. As noted, Jupiter is a traditional gas giant. Saturn is also a traditional gas giant. However, Uranus and Neptune are ice giants. Their blue color is due to the large amounts of methane in the planets. All of the gas giants in our Solar System have planetary ring system, and they are all much larger than the terrestrial planets in the Solar System. For example, you could fit over 1200 Earths inside of the largest gas giant, Jupiter. All of the gas giants in our Solar System also have many moons. Again, Jupiter holds the record with 63 moons discovered so far.
Astronomers have also discovered many extra-solar gas giants. Gas giants are apparently very common around different stars as well. Astronomers have been able to discover that many extra-solar gas giants are closer to their stars, thus the term “Hot Jupiters” has been applied to them. Scientists continue to carry out studies of gas giants both in our Solar System as well as on those in other systems.
Astronomy Cast has episodes on all of the gas giants in our Solar System including Saturn.