The Outer Planets
There is a very great distance between the inner and outer planets. This region is called the Asteroid Belt and large chunks of rock swarm around and around in it.
Except for Pluto, the outer planets are alike in a lot of ways.
They are much bigger than the inner planets.
They are made mostly of hydrogen and helium.
The hydrogen and helium are in the form of gas in the planets' atmospheres. The outer planets have a lot of atmosphere.
The outermost part of these planets may be slushy. Below the surface, hydrogen and helium are in a liquid form. At the center is a rocky core.
Each of these huge planets has its own system of moons.
Rings around Saturn have been seen for many years. They are probably made mostly of water, ice, and other icy particles. Recently, rings have been discovered around two more giant planets — Jupiter and Uranus. Scientists expect to find rings around Neptune, too.
Jupiter, the first of the outer planets, is the largest of all the planets. Jupiter is so far away from Earth that it was hard to learn much about it with only telescopes.
In 1977, "Voyagers 1" and "2" — United States space probes — were sent off into space from Earth. They sped through space all the way to Jupiter and sent back a lot of new information.
Jupiter's outer atmosphere is very cold. But inside the planet, it gets hotter and hotter closer to the center.
Scientists do not think life exists on Jupiter.
Jupiter spins fast on its axis. But this giant planet takes a long time to go around the sun. One Jupiter year is as long as 12 Earth years.
Astronomers expect the new information about Jupiter's moons to help explain how our solar system began, how it changed, and how Earth came to be the way it is.
Fast-moving clouds whirl around Jupiter. They form a pattern of wide bands called ones and narrow bands called belts.
In one of the zones is a tremendous oval called the Great Red Spot. It is three times as big as Earth. Most scientists are not sure what the Great Red Spot really is. Some scientists think it may be a giant hurricane.
Jupiter has rings. There is also a magnetic field around Jupiter 10,000 times more powerful than Earth's. And Jupiter, the super planet, has super lightning bolts!
For hundreds of years, people could see four of Jupiter's moons through telescopes. Now we know Jupiter has at least 17 moons. Jupiter's mass is much greater than all the other planets together. Remember, greater mass means greater gravity. That's probably why Jupiter has so many satellites which keep revolving around this planet with its tremendous gravitational pull. They orbit Jupiter as Jupiter orbits the sun.
Beautiful Saturn, with its bright rings, is the second largest planet in our solar system. Saturn is almost 10 times bigger than Earth.
Saturn is much, much farther from the sun than Earth. It is very cold out there! And it takes almost 30 of our years for Saturn to go once around the sun.
Like the other very big planets, Saturn is mostly hydrogen and helium.
From far away, Saturn looks yellowish. But close-up photographs taken from the two "Voyager" space probes show it has bands of different colors — pale yellow, golden brown, and reddish brown.
Saturn has many moons. Twenty-two have been discovered already. The moon named Titan, one of the largest in the solar system, has a lot of atmosphere.
Maybe Titan's atmosphere, like that around Venus, has kept heat from escaping into space. Maybe Titan's atmosphere and surface have stayed warm over billions of years. And maybe here we may find some form of life.
All the planets described so far have been known for a long, long time. Uranus was discovered only about 200 years ago by a scientist looking through a telescope.
Uranus is the third of the four big outer planets. It is a small giant — fuzzy and blue — only four times bigger than Earth. It has at least 15 moons.
The orbit of Uranus is very, very far from the sun. The planet is tipped over so that it rotates like a top spinning almost on its side. Sunlight and darkness at the North Pole of Uranus each lasts 42 Earth years.
Life on Uranus is not likely.
Uranus also has rings, but they are narrow, dark, and hard to see. They were discovered while astronomers watched Uranus come close to a star. The star began to flicker before and after Uranus passed in front of it. Astronomers realized the flickering was due to rings that partly blocked the star's light for a few moments. When "Voyager 2" passed Uranus in 1986, it showed a close-up of the planet's rings.
When scientists studied Uranus, they found that its orbit was slightly different from what they thought it would be. They thought the difference might be due to the gravitational pull of another planet. So they looked for another planet. They discovered two — Neptune and Pluto.
Of the four giant planets, Neptune is farthest away from the sun. It is a very cold planet.
Neptune is a little smaller than Uranus.
It takes a little less time than Earth to rotate once. But it takes 165 Earth years for Neptune to make one orbit around the sun.
Neptune has three known satellites.
Scientists believe that two rings circle Neptune.
Pluto is a puzzle. It is farthest away from the sun* but it is not a giant planet. Some scientists think this planet once may have been a satellite of Neptune. If that is so, Pluto may have been pulled away by the gravity of still another planet — a planet not yet discovered.
Other scientists believe that Pluto and its very close moon, Charon, are really a double planet sharing the same atmosphere. This would be the only double planet in our solar system.
- Because of the way Pluto orbits, there is a time when Neptune is farther from the sun than Pluto. It happens every 248 years. In 1999, Pluto is again the farthest planet from the sun.
Adapted from the Scholastic "A Book About Planets and Stars" by Betty Polisar Reigot.