The rest of the Jovian planets include Saturn, Uranus, and Neptune. Like Jupiter, these planets are gas giants. The structure and Saturn is fundamentally identical to the structure of Jupiter, although the liquid metallic hydrogen region is not as large in radius as that same region on Jupiter. Therefore the magnetic field of Saturn, although strong, is not as large as the field on Jupiter.

Saturn is certainly the most famous of these planets. This is because it is visible from the Earth and its ring system is prominent even through a pair of binoculars. The ring system of Saturn was first discovered by Galileo. However the orientation of Saturn when Galileo saw it was such that the rings looked more like ears on the disk of the planet. The rings were not seen as rings until Saturn was viewed by Christian Huygens. The view below shows the rings of Saturn edge on, much like the view seen by Galileo. However, this image was taken with the Hubble Space Telescope.

What are the rings and where did they come from? When first discovered the rings were thought to be a solid disk surrounding the planet. But observations of the rings soon showed that they obeyed Kepler's laws. That is to say, the inner rings orbit Saturn in a shorter period than the outer rings. If the rings were solid, the inner and outer rings would orbit in the same time. Therefore the rings must be made of individual particles.

When Voyager flew by Saturn some interesting facts about the rings were discovered. The extent of the rings would show them to be larger than originally thought. In fact the ring system stretches all the way to the planet's surface. In addition, a small ring was found outside the main ring. This thin ring seems to hold its shape by virtue of two small satellites referred to as shepherding satellites. The gravitational pull of these two tiny moons allows the ring to keep its structure. You can barely see this ring (called the F ring) in the photo above. In addition, the main rings seem to be composed of ringlets braided together. This complex structure was quite different than the haphazard distribution of material expected.

The origin of the rings is not well understood. The most likely origin is that the rings formed when a moon or moons of Saturn came too close to the planet. The tidal forces from Saturn would have torn the moon into tiny fragments. These fragments are the current ring system.

In addition to the rings, Saturn has at least 33 moons. The most interesting of these moons is Titan. Although slightly smaller than Ganymede, Titan is larger than Mercury or Pluto. The characteristic of Titan which stands out however, is that it is the first moon to show an atmosphere. In fact the atmosphere of Titan is more dense than the atmosphere of Earth. This atmosphere is comprised mostly of nitrogen with a small percentage of methane. It is believed possible that this methane has formed organic materials which may be swimming in an ocean on Titan.

Uranus and Neptune are also gas giants, though smaller than either Saturn or Jupiter. Their structure is similar but not identical. They also have a rocky core and a liquid mantle, but the liquid mantle in both of these cases is water and not liquid metallic hydrogen. Surrounding this water mantle is an atmosphere of hydrogen and helium. Both planets tend to have a bluish color from methane in the upper atmosphere. The atmosphere of Uranus shows no distinctive markings but the atmosphere of Neptune shows some stripes and two storms similar to the great red spot of Jupiter. The storms were first discovered by Voyager but the Hubble space telescope has indicated that one of the storms has disappeared.

Uranus has one characteristic which sets it off from all the other planets. The axis of Uranus is tilted almost into its orbital plane. To compare this to Earth, instead of a small polar region roughly 23 degrees from the poles, the majority of Uranus would be polar regions. And these polar regions would have 42 years of dark and 42 years of light.

This orientation turned out to be fortuitous. In 1977, while watching Uranus occult a star, the ring system of Uranus was discovered. Occultation occurs when a planet moves in front of a star. Astronomers time how long the planet is in front of the star in order to determine the radius of the planet. Because of the orientation of Uranus, the rings caused the stars light to blink on and off nine times before the planet blocked the star and nine times after the planet blocked the star. Since the pattern of these nine occultations were mirror images we knew the culprits must be rings and not moons. When Voyager flew by Uranus it verified the existence of these rings. In addition it discovered several moons orbiting Uranus.

In a similar fashion Voyager discovered moons around Neptune. It also discovered that Neptune had a ring system which was unknown to exist until Voyager passed by Neptune. But possibly the most interesting discovery was that one of the moons of Neptune, Triton, had an atmosphere. This would be only the second moon to be found with an atmosphere. Unlike the atmosphere of Titan, the atmosphere of Triton is very thin.

This page was last updated on 08/31/04.