outer planets

pluto

Pluto was once considered one of the outer planets, but when the definition of a planet was changed in 2006, Pluto became one of the dwarf planets. It is one of the largest and brightest objects that make up this group. Look for Pluto in the next lesson, in the discussion of dwarf planets.

jupiter

Jupiter, shown in Figure 25.19, is the largest planet in our solar system. Jupiter is named for the king of the gods in Roman mythology. Jupiter is truly a giant! The planet has 318 times the mass of Earth, and over 1,300 times Earths volume. So Jupiter is much less dense than Earth. Because Jupiter is so large, it reflects a lot of sunlight. When it is visible, it is the brightest object in the night sky besides the Moon and Venus. Jupiter is quite far from the Earth. The planet is more than five times as far from Earth as the Sun. It takes Jupiter about 12 Earth years to orbit once around the Sun.

textbook_image

a ball of gas and liquid

Since Jupiter is a gas giant, could a spacecraft land on its surface? The answer is no. There is no solid surface at all! Jupiter is made mostly of hydrogen, with some helium, and small amounts of other elements. The outer layers of the planet are gas. Deeper within the planet, the intense pressure condenses the gases into a liquid. Jupiter may have a small rocky core at its center.

a stormy atmosphere

Jupiters atmosphere is unlike any other in the solar system! The upper layer contains clouds of ammonia. The ammonia is different colored bands. These bands rotate around the planet. The ammonia also swirls around in tremendous storms. The Great Red Spot, shown in Figure 25.20, is Jupiters most noticeable feature. The spot is an enormous, oval-shaped storm. It is more than three times as wide as the entire Earth! Clouds in the storm rotate counterclockwise. They make one complete turn every six days or so. The Great Red Spot has been on Jupiter for at least 300 years. It may have been observed as early as 1664. It is possible that this storm is a permanent feature on Jupiter. No one knows for sure.

jupiters moons and rings

Jupiter has lots of moons. As of 2011, we have discovered over 60 natural satellites of Jupiter. Four are big enough and bright enough to be seen from Earth using a pair of binoculars. These four moons were first discovered by Galileo in 1610. They are called the Galilean moons. Figure 25.21 shows the four Galilean moons and their sizes relative to Jupiters Great Red Spot. These moons are named Io, Europa, Ganymede, and Callisto. The Galilean moons are larger than even the biggest dwarf planets, Pluto and Eris. Ganymede is the biggest moon in the solar system. It is even larger than the planet Mercury! Scientists think that Europa is a good place to look for extraterrestrial life. Europa is the smallest of the Galilean moons. The moons surface is a smooth layer of ice. Scientists think that the ice may sit on top of an ocean of liquid water. How could Europa have liquid water when it is so far from the Sun? Europa is heated by Jupiter. Jupiters tidal forces are so great that they stretch and squash its moon. This could produce enough heat for there to be liquid water. Numerous missions have been planned to explore Europa, including plans to drill through the ice and send a probe into the ocean. However, no such mission has yet been attempted. In 1979, two spacecrafts, Voyager 1 and Voyager 2, visited Jupiter and its moons. Photos from the Voyager missions

textbook_image

textbook_image

saturn

Saturn, shown in Figure 25.22, is famous for its beautiful rings. Saturn is the second largest planet in the solar system. Saturns mass is about 95 times Earths mass. The gas giant is 755 times Earths volume. Despite its large size, Saturn is the least dense planet in our solar system. Saturn is actually less dense than water. This means that if there were a bathtub big enough, Saturn would float! In Roman mythology, Saturn was the father of Jupiter. Saturn orbits the Sun once about every 30 Earth years. Saturns composition is similar to Jupiters. The planet is made mostly of hydrogen and helium. These elements are gases in the outer layers and liquids in the deeper layers. Saturn may also have a small solid core. Saturns upper atmosphere has clouds in bands of different colors. These clouds rotate rapidly around the planet. But Saturn has fewer storms than Jupiter. Thunder and lightning have been seen in the storms on Saturn (Figure 25.23).

textbook_image

a weird hexagon

There is a strange feature at Saturns north pole. The clouds form a hexagonal pattern, as shown in the infrared image in Figure 25.24. This hexagon was viewed by Voyager 1 in the 1980s. It was still there when the Cassini Orbiter visited in 2006. No one is sure why the clouds form this pattern.

saturns rings

Saturns rings were first observed by Galileo in 1610. He didnt know they were rings and thought that they were two large moons. One moon was on either side of the planet. In 1659, the Dutch astronomer Christiaan Huygens realized that they were rings circling Saturns equator. The rings appear tilted. This is because Saturn is tilted about 27 degrees to its side. The Voyager 1 spacecraft visited Saturn in 1980. Voyager 2 followed in 1981. These probes sent back detailed pictures of Saturn, its rings, and some of its moons. From the Voyager data, we learned that Saturns rings are made of particles of water and ice with a little bit of dust. There are several gaps in the rings. These gaps were cleared out by moons within the rings. Ring dust and gas are attracted to the moon by its gravity. This leaves a gap in the rings. Other gaps in the rings are caused by the competing forces of Saturn and its moons outside the rings.

textbook_image

textbook_image

saturns moons

As of 2011, over 60 moons have been identified around Saturn. Only seven of Saturns moons are round. All but one is smaller than Earths Moon. Some of the very small moons are found within the rings. All the particles in the rings are like little moons, because they orbit around Saturn. Someone must decide which ones are large enough to call moons. Saturns largest moon, Titan, is about one and a half times the size of Earths Moon. Titan is even larger than the planet Mercury. Figure 25.25 compares the size of Titan to Earth. Scientists are very interested in Titan. The moon has an atmosphere that is thought to be like Earths first atmosphere. This atmosphere was around before life developed on Earth. Like Jupiters moon, Europa, Titan may have a layer of liquid water under a layer of ice. Scientists now think that there are lakes on Titans surface. Dont take a dip, though. These lakes contain liquid methane and ethane instead of water! Methane and ethane are compounds found in natural gas.

textbook_image

uranus

Uranus, shown in Figure 25.26, is named for the Greek god of the sky, the father of Saturn. Astronomers pronounce the name YOOR-uh-nuhs. Uranus was not known to ancient observers. The planet was first discovered with a telescope by the astronomer William Herschel in 1781. Uranus is faint because it is very far away. Its distance from the Sun is 2.8 billion kilometers (1.8 billion miles). A photon from the Sun takes about 2 hours and 40 minutes to reach Uranus. Uranus orbits the Sun once about every 84 Earth years.

textbook_image

neptunes rings and moons

Like the other outer planets, Neptune has rings of ice and dust. These rings are much thinner and fainter than Saturns. Neptunes rings may be unstable. They may change or disappear in a relatively short time. Neptune has 13 known moons. Only Triton, shown in Figure 25.30, has enough mass to be round. Triton orbits in the direction opposite to Neptunes orbit. Scientists think Triton did not form around Neptune. The satellite was captured by Neptunes gravity as it passed by.

textbook_image

extremes of cold and wind

Like Uranus, Neptune is blue. The blue color is caused by gases in its atmosphere, including methane. Neptune is not a smooth looking ball like Uranus. The planet has a few darker and lighter spots. When Voyager 2 visited Neptune in 1986, there was a large dark-blue spot south of the equator. This spot was called the Great Dark Spot. When the Hubble Space Telescope photographed Neptune in 1994, the Great Dark Spot had disappeared. Another dark spot had appeared north of the equator. Astronomers believe that both of these spots represent gaps in the methane clouds on Neptune. Neptunes appearance changes due to its turbulent atmosphere. Winds are stronger than on any other planet in the solar system. Wind speeds can reach 1,100 km/h (700 mph). This is close to the speed of sound! The rapid winds surprised astronomers. This is because Neptune receives little energy from the Sun to power weather systems. It is not surprising that Neptune is one of the coldest places in the solar system. Temperatures at the top of the clouds are about 218C (360F).

neptune

Neptune is shown in Figure 25.29. It is the eighth planet from the Sun. Neptune is so far away you need a telescope to see it from Earth. Neptune is the most distant planet in our solar system. It is nearly 4.5 billion kilometers (2.8 billion miles) from the Sun. One orbit around the Sun takes Neptune 165 Earth years. Scientists guessed Neptunes existence before it was discovered. Uranus did not always appear exactly where it should. They said this was because a planet beyond Uranus was pulling on it. This gravitational pull was affecting its orbit. Neptune was discovered in 1846. It was just where scientists predicted it would be! Due to its blue color, the planet was named Neptune for the Roman god of the sea. Uranus and Neptune are often considered sister planets. They are very similar to each other. Neptune has slightly more mass than Uranus, but it is slightly smaller in size.

textbook_image

rings and moons of uranus

Uranus has a faint system of rings, as shown in Figure 25.27. The rings circle the planets equator. However, Uranus is tilted on its side. So the rings are almost perpendicular to the planets orbit. We have discovered 27 moons around Uranus. All but a few are named for characters from the plays of William Shakespeare. The five biggest moons of Uranus, Miranda, Ariel, Umbriel, Titania, and Oberon, are shown in Figure

textbook_image

textbook_image

the sideways planet

All of the planets rotate on their axes in the same direction that they move around the Sun. Except for Uranus. Uranus is tilted on its side. Its axis is almost parallel to its orbit. So Uranus rolls along like a bowling ball as it revolves around the Sun. How did Uranus get this way? Scientists think that the planet was struck and knocked over by another planet-sized object. This collision probably took place billions of years ago.

an icy bluegreen ball

Uranus is a lot like Jupiter and Saturn. The planet is composed mainly of hydrogen and helium. There is a thick layer of gas on the outside. Further on the inside is liquid. But Uranus has a higher percentage of icy materials than Jupiter and Saturn. These materials include water, ammonia, and methane. Uranus is also different because of its blue-green color. Clouds of methane filter out red light. This leaves a blue-green color. The atmosphere of Uranus has bands of clouds. These clouds are hard to see in normal light. The result is that the planet looks like a plain blue ball. Uranus is the least massive outer planet. Its mass is only about 14 times the mass of Earth. Like all of the outer planets, Uranus is much less dense than Earth. Gravity is actually weaker than on Earths surface. If you were at the top of the clouds on Uranus, you would weigh about 10 percent less than what you weigh on Earth.

instructional diagrams

No diagram descriptions associated with this lesson

questions

The heat to create liquid water on Europa is from

a. radioactive decay

b. its molten core

c. the Sun

-->  d. the gravitational pull of Jupiter

Saturn is the only planet with rings.

a. true

-->  b. false

Jupiter may have a small rocky core.

-->  a. true

b. false

What are Saturns rings?

-->  a. ice and dust

b. a broken up moon

c. a magnetic field

d. captured asteroids

Jupiter has fewer than 30 moons.

a. true

-->  b. false

What is the Great Red Spot?

a. Iron oxide in the asteroid belt

-->  b. A giant storm on Jupiter

c. The planet Mars

d. An enormous volcano on Neptune

How was Neptune discovered?

a. through a radio telescope

b. by the Hubble Space telescope

-->  c. Uranus orbit was unexpected

d. because it wanders across the sky like the other planets

Jupiters moon Europa has some conditions suitable for life.

-->  a. true

b. false

Thunderstorms have been observed on Uranus.

a. true

-->  b. false

Why are Uranus and Neptune blue?

a. They are composed of solid water ice.

-->  b. Methane in their atmosphere filters out red light.

c. They are covered by blue oceans.

d. They are covered by blue algae.

Saturns rings were first observed by the Hubble space telescope.

a. true

-->  b. false

Uranus is tilted sideways because of the pull of Neptunes gravity.

a. true

-->  b. false

Neptune is so far from Earth that it can be seen only with a telescope.

-->  a. true

b. false

Scientists think that Neptunes moon Triton was once part of Neptune.

a. true

-->  b. false

largest planet in the solar system

a. Great Dark Spot

b. Saturn

c. Neptune

d. planetary ring

-->  e. Jupiter

f. Great Red Spot

g. Uranus

It takes Jupiter 5 Earth years to make one orbit around the Sun.

a. true

-->  b. false

only planet that is tilted on its side

a. Great Dark Spot

b. Saturn

c. Neptune

d. planetary ring

e. Jupiter

f. Great Red Spot

-->  g. Uranus

Most moons in the solar system are captured asteroids.

-->  a. true

b. false

planet with stronger winds than any other planet in the solar system

a. Great Dark Spot

b. Saturn

-->  c. Neptune

d. planetary ring

e. Jupiter

f. Great Red Spot

g. Uranus

Jupiters four largest moons are larger than the dwarf planet Pluto.

-->  a. true

b. false

gap in the clouds on Neptune

-->  a. Great Dark Spot

b. Saturn

c. Neptune

d. planetary ring

e. Jupiter

f. Great Red Spot

g. Uranus

circular cloud of dust and other small particles that orbit an outer planet

a. Great Dark Spot

b. Saturn

c. Neptune

-->  d. planetary ring

e. Jupiter

f. Great Red Spot

g. Uranus

Astronauts have tested samples from Europa for signs extraterrestrial life.

a. true

-->  b. false

Saturn is unique because it is the only planet with rings.

a. true

-->  b. false

least dense planet in the solar system

a. Great Dark Spot

-->  b. Saturn

c. Neptune

d. planetary ring

e. Jupiter

f. Great Red Spot

g. Uranus

enormous storm on Jupiter

a. Great Dark Spot

b. Saturn

c. Neptune

d. planetary ring

e. Jupiter

-->  f. Great Red Spot

g. Uranus

The outer planets of the eight planets in our solar system include

a. Mars.

-->  b. Jupiter.

c. Pluto.

d. two of the above

Compared with the inner planets, the outer planets

a. are much larger.

b. are much denser.

c. have more moons.

-->  d. two of the above

The planet that has clouds of ammonia is

a. Uranus.

b. Neptune.

-->  c. Jupiter.

d. Saturn.

The biggest moon in the solar system orbits

a. Neptune.

b. Uranus.

c. Saturn.

-->  d. Jupiter.

Scientists think that Saturns moon Titan may

-->  a. have an atmosphere like early Earths.

b. be larger than Earths moon.

c. have lakes of liquid water.

d. all of the above

Which two outer planets are called sister planets because they are very similar?

a. Jupiter and Saturn

b. Saturn and Uranus

-->  c. Uranus and Neptune

d. Jupiter and Uranus

Neptune and Uranus appear to be blue due to the

a. oceans of water on their surface.

-->  b. methane gas in their atmosphere.

c. minerals in their rocky crust.

d. frigid cold of their surface.

diagram questions

No diagram questions associated with this lesson