General Astronomy/The Giant Planets

The giant planets are so called for their size. They are also called the gas planets because they consist mainly of hydrogen, or the Jovian planets for Jupiter. These planets each have a large system of moons and rings of varying size made of small particles, moonlets and dust.

There are four such planets in the Solar System: Jupiter, Saturn, Uranus and Neptune. They are all made mainly of fluids, like supercritical fluids, liquid and to a lesser extent gas. These envelope solid cores of rock and ice. All the planets have relatively fast rotation; the slowest rotator, Uranus, has a rotation rate of 17 hours and 14 minutes. This and the radiated heat generated from the crushing pressure within these planets give these planets wild wind speeds, and all of these planets experience storms at certain times.

The Gas Giants

The gas giants are the planets composed of hydrogen and helium. They are much larger than the ice giants and have liquid mantles. They are composed mainly of hydrogen and helium, with small amounts of methane, ammonia, water vapor and other compounds. Their atmospheres are gaseous, but they gradually transition into a liquid mantle. They have solid cores and layers of metallic hydrogen.

Jupiter

This image of Jupiter was taken on 21 April 2014 by Hubble's Wide Field Camera

Jupiter is the innermost 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) in equatorial diameter. Jupiter has much metallic hydrogen. When 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. Under all of this is thought to be a somewhat solid, but diffuse core of rock and ice.

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 possesses seventy-nine known satellites, four of which - Callisto, Europa, Ganymede and Io - are round were first discovered by Galileo in 1610 and are commonly referred to as the Galilean Moons. It also has very faint and very small rings made of dust knocked from its moons.

Saturn

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 big and rich system of (mostly) icy rings of small to tiny pieces of dust and ice.. It has eight round moons: Titan, Rhea, Dione, Mimas, Enceladus, Thethys and Iapetus, the largest being Titan, which is unique among moons for having a significant atmosphere and bodies of liquid on its surface. Saturn has more than a hundred smaller moons. It is composed mainly of hydrogen (96% by volume) and (4% by volume) helium, with small amounts of methane, ammonia, water vapor and other compounds. Saturn is differentiated similarly to Jupiter, though it has much less metallic hydrogen and its core is more rigid than Jupiter's. It has the second highest wind speeds of these planets. It has an axial tilt of 26.73 degrees, and it has the most eccentric orbit of the giant planets. Saturn's bands are fainter than those of Jupiter's due to it being further from the Sun. It has a hexagonal cloud pattern at its North Pole, which may be a jet stream. This "hexagon" contains its northern vortex. Periodically, so-called Great White Spots occur. They are large enough to be seen from Earth and can span a whole latitude.

The Ice Giants

The ice giants are so called because they are mostly made of "ices", substances with a freezing point above ~100 K or ~-175 degrees Celsius. They have thick atmospheres above supercritical mantles; that means their mantles are made of a state of matter compressible like a gas, but dense like a liquid; dense enough for the molecules to interact. This state of matter fills its container, like a gas. Under these are liquid cores, which may produce their magnetic fields.

Uranus

Uranus' mass is 8.686 x 10²⁵ kg (~14.5x the mass of Earth ). Its equatorial diameter is 51,800 kilometers (32,190 miles). It also has a dark faint ring system somewhat comparable to Saturn's. Its rotational axis is tilted 98 degrees, so it appears at times as if it is rolling on its orbit. This produces extreme seasons, which plunge almost one half of Uranus is darkness and another in permanent daylight. This is also why its polar regions are lighter in colour than the rest of the planet. Uranus's has 29 moons, 5 of which are round. Its magnetic field is tilted 60 degrees to its rotational axis, and off center. It is the coldest planet in the Solar System, being 6 degrees Celsius colder than Neptune, which is much further fromt he Sun. Uranus is somewhat inert compared to the other giant planets, likely due to its low thermal flux. It is the only giant planet to receive more radiation from the Sun than it gives off.

Neptune

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 round 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, highly inclined and faint rings. It has the fastest wind speeds of any planet, and they are notable for being faster than the speed of sound on Earth. Its strong gravity influences trans-Neptunian Objects, and many of them are locked in an orbital resonance with it. For example, when Neptune orbits thrice, Pluto orbits twice.