What are gas giants and how are they formed?
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.
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.
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.
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.
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.
Gas Giant Facts:
- Gas Giants have no known surface but have a rocky or metallic core.
- Jupiter is the largest of the Gas Giants in our solar system.
- The gas giants take longer to orbit the Sun because of their great distances. The farther away they are, the more time it takes to make one trip around the Sun.
- The densities of the gas giants are much less than the densities of the rocky, terrestrial worlds of the solar system.
- Gas giants have been found around more than a thousand stars by the Kepler mission. These large exoplanets are often referred to by such names as Hot Jupiters, SuperJupiters, and Giant Neptunes.
- Gas giants are not all gas. Beneath the heavy atmospheres of these Jupiter and Saturn are layers of molecular hydrogen and liquid metallic hydrogen.
- Uranus has an icy layer over its solid rock core, and covered with a gaseous atmosphere. Neptune has a water-ammonia ocean for a mantle overlying its rocky core.
- The metallic hydrogen layers in Jupiter and Saturn conduct electricity.
- The cores of the gas giants are crushed under tremendously high pressures and they are very hot (up to 20,000 K), while the cores of the ice giants Uranus and Neptune are at 5000K and 5,400K respectively.