The Jovian Worlds of the Solar System

the jovian worlds of jupiter, saturn, uranus, and neptune
These are the "Jovian" worlds of our solar system. They are the gas and ice giants Jupiter, Saturn, Uranus, and Neptune. NASA/Lunar and Planetary Institute

Looking at our own solar system can give you a good sense of the kinds of planets that orbit around many other stars. There are rocky worlds, ice worlds, and giant planets that can be made up of gas, ice, and a mixture of the two. Planetary scientists often refer to these last ones as "Jovian worlds" or "gas giants". "Jovian" comes from the god Jove, who became Jupiter, and in Roman mythology, ruled all the other planets.

At one time, scientists just assumed that all gas giants were like Jupiter, which is where the name "jovian" originates. In reality, the giant planets of this solar system can be remarkably different from each other in certain ways. It also turns out that other stars sport their own type of "jovians". 

Meet the Solar System's Jovians

The Jovians in our solar system are Jupiter, Saturn, Uranus, and Neptune. They are made largely of hydrogen in the form of gas in their upper layers and liquid metallic hydrogen in their interiors. They have small rocky, icy cores. Beyond those similarities, however, they can be divided into two further classes: the gas giants and the ice giants. Jupiter and Saturn at the "typical" gas giants, while Uranus and Neptune have more ice in their compositions, particularly in their atmospheric layers. So, they are the ice giants.

A closer look at Jupiter shows a world made mostly of hydrogen, but with a quarter of its mass being helium. If you could descend to Jupiter's core, you'd pass through its atmosphere, which is a turbulent mass of ammonia clouds and possibly some water clouds floating in a hydrogen layer. Below the atmosphere is a layer of liquid metallic hydrogen that has droplets of helium passing through it. That layer surrounds a dense, probably rocky core. Some theories suggest that the core could be very densely squeezed, making it almost like a diamond.

Saturn has roughly the same layered structure as Jupiter, with a mostly hydrogen atmosphere, ammonia clouds, and a bit of helium. Below that lies a layer of metallic hydrogen, and a rocky core at the center.

Out at chilly, shrouded Uranus and distant Neptune, solar system temperatures drop drastically. That means a lot more ice exists out there. That's reflected in the makeup of Uranus, which has a gaseous hydrogen, helium, and methane clouds under a high thin haze. Beneath that atmosphere lies a mix of water, ammonia, and methane ices. And buried underneath it all is a rocky core.

The same structural layout is true for Neptune. The upper atmosphere is largely hydrogen, with traces of helium and methane. The next layer down has water, ammonia, and methane ices, and like the other giants, there's a small rocky core at the heart.

Are They Typical?

Are all jovian worlds like this throughout the galaxy? It's a good question. In this era of exoplanet discovery, led by ground-based and space-based observatories, astronomers have found a good many giant worlds orbiting other stars. They go by various names: superJupiters, hot Jupiters, super-Neptunes, and gas giants. (That's in addition to the water worlds, super-Earths, and Earth-type smaller worlds that have been detected.)

What do we know about distant Jovians? Astronomers can determine their orbits and how close they lie to their stars. They can also measure the temperatures of distant worlds, which is how we get "Hot Jupiters". Those are Jovians that formed close to their stars or migrated inwards after being born elsewhere in their systems. Some of them can be quite hot, more than 2400 K (3860 F, 2126 C). These also happen to be the most commonly found exoplanets, likely because they're easier to spot than smaller, dimmer, cooler worlds.

Their structures remain largely unknown, but astronomers can make some good deductions based on their temperatures and where these worlds exist in relation to their stars. If they're farther out, they're likely to be much cooler, and that might mean that ice giants could be "out there". Better instruments will soon be able to give scientists a way to measure the atmospheres of these worlds quite accurately. That data would tell whether a planet had a largely hydrogen atmosphere, for exmaple. It seems likely they would, since the physical laws governing gases in atmospheres are the same everywhere. Whether or not those worlds have rings and moons as our outer solar system planets do is also something that scientists are looking to determine.

Jovian Worlds Exploration Helps Our Understanding

Our own studies of the gas giants in the solar system by the Pioneer missions, the Voyager 1 and Voyager 2 missions, and the Cassini spacecraft, as well as by such orbiting missions as Hubble Space Telescope, can help scientists make very educated deductions about worlds around other stars. Eventually, what they learn about those planets and how they formed will be very helpful in the understanding of our own solar system and others that astronomers will find as the search for exoplanets continues.