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Gas planets are a category of planets without any solid metals or rock. Gas planets are also referred to as gas giants, giant planets, and Jovian planets. Since gas planets do not have a solid surface, you would not be able to walk on them. Gas planets are said to have a rocky center; however, that term is somewhat misleading. Scientists believe that due to the high temperatures and extreme pressures in the center of gas planets their rocky centers are actually liquid metal or rock. Thus, the density of the gas planets simply increases as you go deeper to the center.
There are four gas planets in our Solar System: Jupiter, Saturn, Uranus, and Neptune. The other four planets are known as terrestrial planets. The gas giants in our Solar System are also called the outer planets because they are the four planets in the Solar System furthest from the Sun.
Although the gas planets are lumped into one category, there are also subcategories among them. Astronomers differentiate between the “classic” or “traditional” gas giants and the “ice” giants. The traditional gas giants are like Jupiter and Saturn. Jupiter is considered the model for traditional gas giants. In fact, gas giants are sometimes also called Jovian planets after Jupiter. They are comprised of mostly hydrogen and helium. While ice giants do have some hydrogen and helium, they are mostly made up of “ices” such as methane, ammonia, and water. Methane is what gives Uranus and Neptune their blue color. (Uranus and Neptune are the two ice giants in our Solar System.)
Despite their differences, the gas planets in our Solar System do have quite a few similarities. One of these similarities is that they all are much larger – have a much bigger volume – than the terrestrial planets. Additionally, all of Solar System’s gas planets have planetary ring systems and many moons. Because the gas giants are composed of gas, they are less dense than the terrestrial planets. Their large size is what makes them more massive than the inner (terrestrial) planets.
Gas planets do not just exist in our Solar System. Astronomers have discovered many gas giants orbiting other stars. Hot Jupiters are a type of gas planets that astronomers have found. Very similar to Jupiter in composition, the Hot Jupiters are closer to their suns, so they have much higher temperatures. So far, astronomers have found more Hot Jupiters than any other type of extra-solar planet.
Astronomy Cast has episodes on each gas planet including Neptune.