The gas giant planets, also known as the Jovian Planets, comprise the outer planets in the solar system: Jupiter, Saturn, Uranus and Neptune. These planets are composed of gas; however, only Jupiter and Saturn are composed mostly of hydrogen and helium, while Uranus and Neptune are sometimes classified as ice giants due to their different composition, which consists mostly of methane, ammonia and water. The gas giants do not have a solid surface, but the gases comprising these planets become thinner with increased distance from their solid rocky cores. Even though the giant planets are larger and more massive than the Earth they´re less dense.
Gas giant planets lack a solid surface; however, they´re thought to have a solid metallic or rocky core due to the high pressures and temperatures in their cores. This accounts for the density of these planets, which increases with increasing depth to their interior. Hydrogen and helium constitute most of the atmospheres of Jupiter and Saturn, while Uranus and Neptune are composed primarily of methane, ammonia and water, with an outer envelope of hydrogen and helium. All the gas giant planets share a characteristic that differentiates then from the inner rocky planets. They all have rings and numerous moons orbiting them.
Jupiter
The largest planet in the solar system is 300 times more massive than the Earth. It´s composed of approximately 90 percent hydrogen (H), 9 percent helium (He) and 1 percent traces of other elements. These gases become liquid with increasing pressure and temperature. Liquid hydrogen at its core behaves like a molten metal with free electrons conducting electricity, creating Jupiter´s magnetosphere. Its rocky core is believed to be of a diameter the size of the Earth. Jupiter possesses more than 60 moons, including the Galilean moons: Ganymede, Calisto, Io and Europa. Jupiter can be easily observed during a cloudless, moonless night with a small telescope, including the bands across its atmosphere and the Great Red Spot, which is a swirling hurricane of about twice the size of the Earth.
Saturn
Saturn is the second largest planet in the solar system and the sixth planet from the Sun. The atmosphere of Saturn is similar to Jupiter´s, with about 92.4 percent hydrogen and 7.4 percent helium, with traces of ammonia and methane. Its internal, metallic hydrogen layer is thinner than Jupiter´s, while its core might be larger. Saturn´s internal pressure and temperature are less extreme than in Jupiter. The rapid rate of rotation and the conductive metallic hydrogen in Saturn´s core are thought to generate its magnetic field. Saturn´s ring system consists of nine rings composed mainly of ice, dust and rocks. 62 known moons orbit the planet. Saturn can be easily observed with the naked eye during moonless, clear nights, but a four-inch aperture telescope will enable you to observe Saturn´s rings and moons.
Uranus
This planet is believed to have a rocky core similar in size to Jupiter´s, surrounded by a layer of water clouds and ammonia. Spectroscopic studies have revealed that Uranus contains about 82 percent hydrogen and 14 percent helium in its atmosphere, with about 2 percent methane and 1 percent ammonia, along with traces of hydrocarbons. Methane absorbs red light and reflects blue, which is what gives Uranus a blue-green color. Uranus has a ring system, a magnetosphere and various moons, like the other gas giants. Its axis of rotation is tilted 98º sideways, resulting in extreme seasonal conditions at the poles, with 42 years of continuous sunlight, followed by another 42 years of continuous darkness at the other pole.
Neptune
This planet is the eighth farthest planet from the Sun. Neptune is the most dense gas giant, with about 17 times the mass of the Earth. Neptune was discovered on September 23, 1846, due to variations in the orbit of Uranus, which led astronomers to the prediction of another planet in its vicinity. Neptune´s outer envelope is composed primarily of hydrogen and helium, while methane and ammonia constitute most of its interior. There are traces of hydrocarbons, as well. Neptune´s core is composed principally of ices and rock. The small quantities of methane give Neptune its bluish appearance. Neptune possesses one of the coldest temperatures, -218 ºC (55 K), in the solar system.
While the gas giant planets are the largest and most massive planets in the solar system, their outer hydrogen and helium layers are less dense than those of the terrestrial planets. The bands that are characteristic of gas giant planets are known as zones and belts and correspond to low pressure and high pressure regions. It´s thought that the central part of the gas giant planets is made of liquid metal or rock, thus increasing their density with increased depth. According to Universe Today, Jupiter is considered a model among the gas giants; thus astronomers refer to these planets as the Jovian planets.
