Saturn, Uranus, and Neptune: The Other Gas Giants


By Robert Hazen, Ph.D.George Mason University

There is clear evidence that the planets in our solar system are unique worlds, with unique characteristics. There are four inner terrestrial planets, and then four larger, Jovian planets, or gas giant planets farther out. The biggest of the Jovian planets is Jupiter, beyond which lie Saturn, Uranus, and Neptune.

An illustration of Saturn and its rings.
Saturn’s rings are made up of smaller ringlets. (Image:

Saturn and Its Rings

Saturn is the second-largest planet in the solar system. It has breathtaking rings, which you can see with a good pair of 10x binoculars. It’s primarily hydrogen, with a little bit of helium. It has severe weather patterns and a ten-hour rotation period, which streams out the atmosphere into bands and bulges the planet.

Saturn is about ten times farther out from the Sun than Earth. It takes almost 30 years for Saturn to complete an orbit around the Sun, and so it’s fairly easy to keep track of Saturn from year to year; it doesn’t move much in the night sky.

Saturn’s ring complex is very magnificent. It’s actually a complex of many, many little ringlets, which appear, in detail, like the grooves of a record. 

The dynamics of this fine structure are fascinating. Here you have a structure composed of thousands of millions of countless particles, tiny particles; detailed photographs from Pioneer and Voyager 2 show that these rings are also made of water ice, little bits of rock that are coated with ice. 

They’re herded into line by a series of tiny satellites, perhaps a kilometer in diameter, that keep all the smaller particles in line. Astonishingly, that ring is extremely thin; the thickness of Saturn’s rings may be no more than a few tens of meters, even though they’re thousands of kilometers across.

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

Titan: A Fascinating Moon

Given the nature of Saturn’s rings, it’s rather difficult to exactly tell what to call the moon. Astronomers have identified about 20 objects greater than 15 kilometers in diameter, and maybe that’s a reasonable definition. While most are irregular asteroid-like bodies, the largest moon, Titan, is a uniquely fascinating world.

The name Titan is worth remembering because when you think of Titan, you think of the largest moon, and the most unique moon, in the solar system. So when you think of Saturn, think of Titan. Titan is larger than Mercury, although it’s only half as massive because it’s about half ice and half rock, and Mercury is a solid, rocky, terrestrial planet. 

Titan has a substantial nitrogen atmosphere. Its surface pressure is about 1.5 times Earth’s atmosphere, so that’s a pretty significant atmosphere on Titan. Voyager 1 photographed Titan and found that an orange smog obscures the moon’s surface and that orange smog is made of carbon-based compounds. So here we have organic compounds and an atmosphere that’s similar—at least in pressure—to Earth’s; a fascinating world to think about.

Nevertheless, the surface of Titan is quite cold; it’s frigid, and so it may be covered with lakes of liquid nitrogen, and methane, and ethane. Someday, travelers in the solar system might stop by Titan, mining or collecting fuels for their voyages in deeper space.

Learn more about Edwin Hubble’s contribution to the discovery of galaxies.

An illustration of Uranus with its rings.
Uranus was discovered in 1781 by the celebrated English astronomer William Herschel. (Image: quasar171/Shutterstock)

Uranus: Gas Giant That’s on Its Side

Uranus was discovered in 1781 by the celebrated English astronomer William Herschel, which won him a pension from the Hanoverian king, George III. This generous annuity allowed him to devote the rest of his life to astronomy.

Uranus is about 19 times the Earth-Sun distance and has an orbital period of 84 years. The planet is four times the diameter of Earth, and it’s 100 times more massive than the Earth. Voyager 2 flew by the planet and revealed that a Uranian day is about 17 hours. 

The rotation axis of Uranus is extraordinary. It’s on its side, so the planet is orbiting with one pole pointing toward the Sun and one pole pointing away from the Sun, the only planet to do this. 

Of course, like the other gas giant planets, it’s composed primarily of hydrogen and helium. Uranus has five major moons, all of which appear to be frozen and inactive, as well as a complex system of rings.

Learn more about the life cycle of stars.

Predicting a Discovery Before It Even Happened: Neptune

An illustration of Neptune
Neptune’s existence and its approximate location were predicted by the perturbations in Uranus’s orbit before it was discovered physically. (Image: 19 STUDIO/Shutterstock)

Neptune is the eighth planet in our solar system. The discovery of Neptune was a triumph of Newtonian models because its existence and its approximate location were predicted by the perturbations in Uranus’s orbit. It was done independently by astronomers in England and France and then was discovered by French astronomers in 1846. 

When you think of Neptune, think of the words “gas giant” and “blue.” Neptune is 30 times the Earth-Sun distance, with an orbital period of 165 years. It takes four light hours to travel from the Sun to this distant planet.

Most of our knowledge comes from Voyager 2, the NASA fly-by in 1989, which photographed the beautiful blue planet. It has a frigid, hydrogen-rich atmosphere laced with methane; it is 60 degrees above absolute zero at the surface. 

The great, dynamic weather patterns are driven by a 16-hour rotation period, so it’s not unlike Saturn in that regard. Neptune also has several moons and a ring complex. Its largest moon, Triton, is an important world; it has its own methane-rich atmosphere.

Common Questions about Saturn, Uranus, and Neptune

Q: How do Saturn’s rings appear?

Saturn’s ring complex is a complex of many, many little ringlets, which appear, in detail, like the grooves of a record.

Q: Who discovered Uranus?

The celebrated English astronomer William Herschel was the one who discovered Uranus in 1781.

Q: How is the rotation axis of Uranus unusual when compared to the other planets in the solar system?

Uranus rotates on its side, with one of its poles facing the Sun and the other facing the opposite. In this regard, Uranus is unique compared to the other planets of the solar system.

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