Gas Giants: Facts About the Outer Planets
The planets of the solar system as depicted by a NASA computer illustration. Orbits and sizes are not shown to scale.
Credit: NASA

A gas giant is a large planet composed mostly of gases, such as hydrogen and helium, with a relatively small rocky core. The gas giants of our solar system are Jupiter, Saturn, Uranus and Neptune. These four large planets, also called jovian planets after Jupiter, reside in the outer part of the solar system past the orbits of Mars and the asteroid belt. Jupiter and Saturn are substantially larger than Uranus and Neptune, revealing that the pairs of planets have a somewhat different composition.

Although there are only four large planets in our own solar system, astronomers have discovered thousands outside of it, particularly using NASA's Kepler space telescope. These exoplanets (as they are called) are being examined to learn more about how our solar system came to be.

Jupiter is the largest planet in our solar system. It has a radius almost 11 times the size of Earth. It has 50 known moons and 17 waiting to be confirmed, according to NASA. The planet is mostly made of hydrogen and helium surrounding a dense core of rocks and ice, with most of its bulk likely made up of liquid metallic hydrogen, which creates a huge magnetic field. Jupiter is visible with the naked eye and was known by the ancients. Its atmosphere consists mostly of hydrogen, helium, ammonia, and methane. [Related: Planet Jupiter: Facts About Its Size, Moons and Red Spot]

Saturn is about nine times Earth's radius and is characterized by large rings; their formation circumstances are unknown. It has 53 known moons and nine more awaiting confirmation, according to NASA. Like Jupiter, it is mostly made up of hydrogen and helium that surround a dense core and was also tracked by ancient cultures. Its atmosphere is similar to Jupiter’s. [Related: Planet Saturn: Facts About Saturn’s Rings, Moons & Size]

Uranus has a radius about four times that of Earth's. It is the only planet tilted on its side, and it also rotates backward relative to every planet but Venus, implying a huge collision disrupted it long ago. The planet has 27 moons, and its atmosphere is made up of hydrogen, helium and methane, according to NASA. It was discovered by William Herschel in 1781. [Related: Planet Uranus: Facts About Its Name, Moons & Orbit]

Neptune also has a radius about four times that of Earth's. Like Uranus, its atmosphere is mostly made up of hydrogen, helium and methane. It has 13 confirmed moons and an additional one awaiting confirmation, according to NASA. It was discovered by several people in 1846. [Related: Planet Neptune: Facts About Its Orbit, Moons & Rings]

It is believed that the giants first formed as rocky and icy planets similar to terrestrial planets. However, the size of the cores allowed these planets (particularly Jupiter and Saturn) to grab hydrogen and helium out of the gas cloud from which the sun was condensing, before the sun formed and blew most of the gas away. 

Since Uranus and Neptune are smaller and have bigger orbits, it was harder for them to collect hydrogen and helium as efficiently as Jupiter and Saturn. This likely explains why they are smaller than those two planets. On a percentage basis, their atmospheres are more “polluted” with heavier elements such as methane and ammonia because they are so much smaller.

Scientists have discovered thousands of exoplanets. Many of these happen to be “hot Jupiters,” or massive gas giants that are extremely close to their parent stars. For this reason, scientists speculate that the planets may have moved back and forth in their orbits before settling into their current configuration. But how much they moved is still a subject of debate.

There are dozens of moons around the giant planets. Many formed at the same time as their parent planets, which is implied if the planets rotate in the same direction as the planet close to the equator (such as the huge Jovian moons Io, Europa, Ganymede and Callisto.) But there are exceptions. 

One moon of Neptune, Triton, orbits the planet opposite to the direction Neptune spins — implying that Triton was captured, perhaps by Neptune’s once larger atmosphere, as it passed by. And there are many tiny moons in the solar system that rotate far from the equator, of their planets, implying that they were also snagged by the immense gravitational pull.

Each of these planets also have complex atmospheres and in many cases, giant storms. The Great Red Spot on Jupiter, for example, has been known for 400 years and is shrinking for reasons that are poorly understood. According to Mark Marley, an astrophysicist at the NASA Ames Research Institute, we need more long-term monitoring programs of gas giant atmospheres to better understand the nature of these storms.

While the Cassini mission is active at Saturn right now, NASA is also preparing for the 2016 arrival of Juno — a Jupiter-bound spacecraft that will measure the planet's interior in detail for the first time by measuring the planet's gravity field. Astronomers are also looking for “seismic waves," similar to what propagates through the Earth during earthquakes. "It would be really great if we could detect those for the giant planets, measure these waves that travel through the inside of the planet. It would give us a picture of the cross section," Marley told Space.com.

In the past five years or so, scientists have found a multitude of “super-Earths” (planets between the size of Earth and Neptune) in other solar systems. Because this class of planets doesn't exist in our own solar system, there are many questions as to whether they are more like small giant planets or big terrestrial planets.

More exoplanet discoveries will be attempted in the coming years with the Transiting Exoplanet Survey Satellite to launch in 2017, the James Webb Space Telescope (set to launch in 2018) and the Wide-Field Infrared Survey Telescope (WFIRST), which could launch no earlier than 2024 if approved.

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