Gas Giants: Definition & Explanation

Instructor: Richard Cardenas

Richard Cardenas has taught Physics for 15 years. He has a Ph.D. in Physics with a focus on Biological Physics.

In this lesson you will learn about the group of planets composed mainly of gas, called the gas giants. You will read about theories regarding their formation, common properties of the group of gas giants and information specific to each gas giant planet.

Definition

The solar system is composed of two groups of planets. One group is made up of planets that are rocky. These planets are called the terrestrial planets. The second group is the group made up of gassy planets. The planets in this group are collectively called the gas giants. The term gas giant refers to a planet that is not made up of rock or metal or anything solid. A gas giant is predominantly made up of a variety of gases. The following planets belong to the group of gas giants: Jupiter, Saturn, Uranus, and Neptune. The terrestrial planets inhabit the region closest to the sun, while the gas giants are further away from the sun. The figure below shows the gas giants.

The gas giants: Jupiter, Saturn, Uranus, and Neptune
Gas Giants

Origin and Composition of the Gas Giants

Currently, there are two theories about how the gas giants formed. Both theories rely on the fact that the solar system formed from a large cloud of gas, rock and ice. The first theory claims that the gas giants formed as a result of slowly growing ice and rock cores, followed by a relatively quick accretion of gas around these cores. The rock, ice, and gas all came from the disk of gas surrounding the new born sun (also called the protosun). The second theory suggests that the gas, rock, and ice was spinning forming a set of spiral arms around the protosun. These rotating spiral arms continued to clump up and increase in mass and density until the gas giants were formed.

So what are these gas giants made of? The cores of all of the gas giants are made up of essentially the same things: rock, metal, and hydrogen compounds. Jupiter and Saturn are very similar since their cores are made up of layers of the following: metallic hydrogen, liquid hydrogen, and gaseous hydrogen. Meanwhile, the cores of Uranus and Neptune are made up of rock and metal, followed by layers of water, methane, ammonia, then gaseous hydrogen. The figure below shows a superimposed image of the cores of the gas giants and their compositions.

The Composition of the cores of the Gas Giants
Cores

As you can see from this figure, Jupiter and Saturn are really gas giants while Uranus and Neptune are sometimes called ice giants since they are mostly composed of ice.

Properties of Each Gas Giant

Jupiter

Jupiter is the largest of the gas giants. It is composed predominantly of hydrogen and helium, but it also contains traces of methane, water, ammonia, and rock. The core of Jupiter is very hot, with temperatures of about 20,000 degrees Centigrade. Jupiter has about 63 known satellites, the most famous of which are the Galilean moons: Io, Europa, Ganymede, and Callisto. The rest of the satellites are small and still unnamed. Jupiter also has three rings. The most prominent feature of Jupiter is the red spot. The red spot is a great high pressure storm, much like a hurricane on Earth but much larger. This storm has been raging for approximately 400 years.

Saturn

Saturn is the second largest planet. It has about the same composition as Jupiter. Saturn's core is also hot, but it's a bit cooler than Jupiter's, at 12,000 degrees Centigrade. Saturn has 53 known satellites. It also has 14 rings. But why are Saturn's rings more prominent than the rings of Jupiter or any other ringed planet? The reason lies in the composition of the rings of Saturn. Astronomers once thought that the rings of Saturn were solid flat sheets, but images from the Voyager spacecraft revealed that the rings were composed of separate pieces of ice and rock. These pieces are highly reflective, like millions of tiny little moons reflecting light from the sun. As a result, the 14 rings of Saturn are very prominent and bright. We therefore typically think of Saturn when we say ringed planet, even though the other gas giants have rings.

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