General Astronomy/The Jovian Planets
So called Jovian planets are named after Jupiter, the largest planet in the Solar System. They are also called the gas planets because they consist mainly of hydrogen, or the giant planets because of their size. These planets usually have complicated system of many moons and often even rings of ice and/or dust.
There are four Jovian planets in the Solar System: Jupiter, Saturn, Uranus and Neptune.
The Solar System Jovians
Jupiter is the innermost planet of the gas giants. It has a mass of 1.9 x 1027 kg (approx. 318x the mass of Earth) and is 142,800 kilometers (88,736 miles) across the equator. Jupiter possesses 63 known satellites, four of which - Callisto, Europa, Ganymede and Io - were observed long ago by Galileo in 1610. It is composed mainly of hydrogen and helium, with small amounts of methane, ammonia, water vapor and other compounds. At great depths within Jupiter, the pressure is so high that the hydrogen atoms are broken up and the electrons are freed so that the resulting atoms consist of bare protons. This produces a state in which the hydrogen becomes metallic. Fluid motions in this metallic conducting layer produce Jupiter's powerful magnetic field.
Jupiter has a very dynamic weather system. Fluid motions are organized by the planet's rapid rotation into a series of dark belts and bright zones around the planet. The visible cloud layers are made of frozen ammonia, while deeper cloud layers are thought to contain ammonium hydrosulfide and water. Jupiter's "Great Red Spot" is a rotating storm system that has been observed for hundreds of years.
Jupiter also has a very lean dusty ring that is low opacity and difficult to observe.
Jupiter has a very powerful magnetosphere and is so enormous, that if it could be observed from the Earth would appear as large as the Moon.
Saturn's mass is 5.688 x 1026 kg (approx. 95x the mass of Earth) and its equatorial diameter is 119,300 kilometers (74,130 miles). Saturn is the only planet less dense than water (about 30 percent less). It has a beautiful, big and rich system of (mostly) icy rings. It has one large moon called Titan and many smaller moons.
Uranus' mass is 8.686 x 1025 kg (approx. 14.5x the mass of Earth ). Its equatorial diameter is 51,800 kilometers (32,190 miles). It also has a ring family, five big moons and many smaller moons. Its rotational axis lies near its orbital plane, so it appears at times as if it is rolling on its orbit.
Neptune is the outermost planet of the gas giants. Its mass is 1,024 x 1026 kg (approx. 17x the mass of Earth). It has an equatorial diameter of 49,500 kilometers (30,760 miles). It has one large moon, Triton (the coolest known object in our Solar system - around -235° C) as well as various smaller moons including Nereid. Neptune has four known, very dark rings.