Between the planes of the inner and outer Solar System, there are some notable differences. The planets that resides clsoer to the Sun are terrestrial (i.e. rocky) in nature, meaning that they are composed of silicate minerals and metals. Beyond the Asteroid Belt, however, the planets are predominantly composed of gases, and are much larger than their terrestrial cousins.
This is why astronomers use the term “gas giant” to refer to planets of the outer Solar System, as well as extra-solar planets that conform to that definition.
A gas giant describes a planet that is not composed of mostly rock and other solid substances. Gas giants are almost entirely formed of various gases. These planets are not completely gas though. At the center, is what astronomers call a rocky center. This term is somewhat misleading though because the rocky center is actually liquid compounds, including molten heavy metals. The term was created by James Blish, a science fiction writer from the mid-1900’s. Gas giants are also called Jovian planets after Jupiter, the prototype of gas giants in our Solar System. 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 because they are composed almost entirely of gas. In addition to being large, these planets rotate extremely quickly. 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. Astronomers have been studying these planets using powerful telescopes, but they have not been able to find out much information about them so far. Some astronomers are actually searching for life on these planets. They have discovered some extra-solar planets in the habitable zones of other solar systems, and they believe that life could exist on these extra-solar planets or at least the moons of these planets.
Because the gas giants are farther away from Earth than the terrestrial planets, astronomers have not been able to study the gas giants extensively up close. Hopefully, that will change as NASA sends more spacecraft out to explore the outer planets.
If you are looking for more information on gas giants, take a look at NASA’s planets and ThinkQuest’s habitable moons around extra-solar gas giants.
Astronomy Cast has episodes on all of our Solar System’s gas giants, so start with Jupiter.
- NASA – Solar System Exploration: Planets
- NASA – Gas Giant Interiors
- Space Facts – Gas Giants
- Wikipedia – Gas Giants
- The Planets – Gas Giants