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Physics LibreTexts

7.1 The Gas Giant Planets

Beyond the asteroid belt lies the outer Solar System. This region is dominated by four giant planets, which range in size from about four to ten times the diameter of Earth. Jupiter, Saturn, Uranus, and Neptune have massive gaseous atmospheres, so are often called gas giant planets.  Because Jupiter dominates these planets, they are also referred to as Jovian planets. The gas giants dominate the mass and the real estate of the Solar System beyond the Sun. Together they have 99.7% of the mass of the eight planets and their orbits occupy 99.8% of the area swept out by the orbits of the eight planets.

 


The gas giant planets in scale to each other. Click here for original source URL

 


Saturn, in natural colors. Click here for original source URL.


Jupiter, the largest planet in the solar system, as seen by the Hubble Space Telescope. Click here for original source URL.

The name of the monarch of the Roman gods is fitting for Jupiter, which is by far the biggest planet in our solar system. Jupiter contains 71% of the total mass of all the planets, nearly 2½ times as much as all other planets combined! Jupiter's diameter is a little over 10 times the Earth's diameter. Placed on the face of Jupiter, Earth would look like a dime on a dinner plate. Saturn dwarfs the Earth by almost as much, while Uranus and Neptune have diameters about four times the Earth's.
 


Neptune as seen by Voyager 2. Click here for original source URL.


Uranus in natural colors. Click here for original source URL.

The four giant planets look radically different from each other. Jupiter and Saturn are predominantly tan-colored, while Uranus and Neptune are blue. Note that many images online have enhanced or exaggerated colors; the true colors of all of the planets are muted and subtle. Saturn has a uniquely prominent ring system. Uranus and Neptune are so far away that they barely show as disks in the largest telescopes. Little was known about Uranus and Neptune until the Voyager 2 space probe visited them in the 1980s.

The Jovian planets have many characteristics, other than their size, that set them apart from the terrestrial planets. All four giant planets rotate faster than Earth, with rotation periods ranging from about 10 to 17 hours, as contrasted with the Earth's 24-hour rotation. Each gas giant is surrounded by a system of rings, along with a whole family of moons or satellites. There are more than a hundred satellites in the outer solar system. Some are the size of planets, while smaller satellites are only a kilometer or so across, and additional tiny “moonlets” are still being discovered. To summarize, the four giant planets together contain over 99% of the total planetary mass, and they play host to more than 90% of the known satellites in the Solar System.

The gas giant planets are far from the Sun, and therefore very cold. They are composed primarily of hydrogen (H) and helium (He), the two most common elements in the Sun and in the universe as a whole. Hydrogen and helium remain in their gaseous forms down to extremely low temperatures. However, according to the laws of chemistry, other compounds like carbon dioxide, which is gaseous in the warm inner Solar System, condense into frozen solids (ices) in the cold outer Solar System. This is the most important single thing to remember about the outer Solar System: gaseous or icy material is a major building block of the outer worlds, while rocky material is only a secondary component.
 


Frozen carbon dioxide (CO2), or dry ice, can be found in the polar ice caps of Mars and on gas giant planets in the outer solar system. Click here for original source URL.

What do we mean by "ices" in the composition of giant planets? The gases that formed the Sun and its planet-spawning surroundings were mostly hydrogen and helium, but helium is inert and doesn’t form chemical compounds. So the ices that formed in the outer Solar System are a variety of frozen compounds of hydrogen. One is very familiar: frozen water (H2O), or ordinary ice. Others are less familiar: for example, frozen methane (CH4), and ammonia (NH3). The "H" in the formula of each of these icy compounds is the symbol for the universe's most common element, hydrogen. Though hydrogen compounds dominate, there are still more ices in the outer Solar System. One is frozen carbon dioxide (CO2), which is also a component of the polar ice caps of Mars. On Earth, we know it as "dry ice." Instead of forming rocky worlds like the terrestrial planets, the outer Solar System formed worlds of rock plus ice, in roughly equal mixtures.