What Are Gas Giants?
Gas giants are planets that are almost entirely composed of gas rather than being composed of rock and other solid substances. At their center, gas giants have a core composed of rock, ice or such liquid compounds as molten heavy metals. However, unlike terrestrial planets, gas giants do not have a clearly defined difference between this core and their atmospheres. The atmosphere of a gas giant simply becomes denser as it nears the core.
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Gas Giant Composition
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Gas giants can be composed of different types of gases. For example, Jupiter and Saturn are primarily composed of hydrogen and helium. Uranus and Neptune are primarily composed of water, large fractions of oxygen, carbon, nitrogen, sulfur and ammonia in addition to hydrogen and helium gas. Due to their distance from the sun, Uranus and Neptune are also known as Ice Giants because many of the gases and liquids they are made of are frozen into layers of unusual ices. Spectroscopy is a method that is used to determine the composition of gas giants.
Gas Giants in Our Solar System
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We have four gas giants in our solar system: Jupiter, Saturn, Uranus and Neptune. In our solar system, the gas giants have many common features including being among the planets furthest from our sun, being extremely large in size and in mass, and rotating very quickly on their axes. An interesting fact about three of the gas giants in our solar system, Jupiter, Saturn and Neptune, is that they emit more energy than they receive from the sun. This means that these planets must have some internal source of energy. The gas giants in our solar system are sometimes called "Jovians" because Saturn, Uranus and Neptune are similar to Jupiter and "Jove" is the Latin variation for Jupiter.
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Discovery of the Gas Giants in Our Solar System
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Jupiter and Saturn were discovered in ancient times. Uranus was discovered in 1781 by William Herschel. Neptune was discovered in 1846 by Urbain Le Verrier, John Couch Adams and Johann Galle.
Gas Giants Outside Our Solar System
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Gas giants have also been discovered orbiting other stars. The most common forms of gas giant that have been located outside our solar system are "Hot Jupiters." These are gas giants that orbit their stars very closely and, as a result, experience extremely high temperatures. Hot Jupiters are relatively easy to discover because of their large sizes. Some gas giants that have been discovered outside our solar system are 51 Pegasi B--Bellerophon, 47 Ursae Majoris, 55 Cancri, 70 Virginis--Goldilocks and Tau Bootis.
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References
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