Jupiter, Saturn, Uranus, and Neptune are the outer planets of our solar system. These are the four planets farthest from the Sun. The outer planets are much larger than the inner planets. Since they are mostly made of gases, they are also called gas giants.
The
gas giants are mostly made of hydrogen and helium. These are the same elements
that make up most of the Sun. Astronomers think that most of the nebula were
hydrogen and helium. The inner planets lost these very light gases. In the
inner solar system, the gases were too hot for the gravity of the inner planets
to keep them. In the outer solar system, it was cold enough for the gravity of
the planets to keep the colder slower-moving hydrogen and helium gas.
All of the outer planets have numerous moons. They also have planetary rings made of ice. Only the rings of Saturn can be easily seen from Earth. Jupiter is truly a giant! The planet has 318 times the mass of Earth and about 1400 times Earth’s 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 beside the Sun. Jupiter is quite far from the Earth.
The planet is more than five times as far from the Sun as Earth. It takes Jupiter
about 12 Earth years to orbit once around the Sun. 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
Jupiter's atmosphere is made mostly of hydrogen and helium gas. There are also small amounts of other gases that contain hydrogen, like methane, ammonia, and water vapor. Astronomers think that clouds in the atmosphere are particles of water, ice, and compounds made of ammonia. Alternating cloud bands rotate around the planet in opposite directions. Colors in these cloud bands may come from complex organic molecules.
The Great Red Spot, shown in the Figure above, is
Jupiter's most noticeable feature. The spot is an enormous, oval-shaped storm.
It can expand to be more than two 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.
Jupiter’s
Moons and Rings
Jupiter has lots of moons. As of 2012, we have discovered over 67 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. The Figure below shows the four Galilean moons and their sizes relative to Jupiter’s 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 moon's 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
differences in Jupiter’s gravity as Europa’s distance changes during an orbit.
These tidal forces are so great that they stretch and squash its moon. This
could produce enough heat for there to be liquid water. Various missions have
been discussed to explore Europa, including the idea to have a probe melt deep
down through the ice into the ocean. However, no such mission has yet been
attempted.
In 1979, two
spacecraft, Voyager 1 and Voyager 2 visited Jupiter and its moons. Photos
from the Voyager missions showed that Jupiter has a ring system. This ring
system is very faint, so it is very difficult to observe from Earth.
2: Saturn
Saturn,
shown in the figure below, is famous for its beautiful rings. Saturn is the
second largest planet in the solar system. Saturn’s mass is about 95 times
Earth's mass. The gas giant is 755 times Earth’s 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.
Saturn
is the least dense planet in our solar system.
Saturn’s
composition is similar to Jupiter's. 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. Saturn's upper atmosphere has
clouds in bands of different colors. These clouds rotate rapidly around the
planet. But Saturn has fewer storms than Jupiter.
Saturn’s
Rings
Saturn's
rings were first observed by Galileo in 1610. He didn't 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 Saturn’s equator. The rings appear tilted. This is because
Saturn’s rotation axis is tilted about 27 degrees from a line perpendicular to
its orbit.
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.
The Cassini spacecraft has been in orbit around Saturn since 2004. From the
Voyager and Cassini data, we learned that Saturn’s rings are made of mostly ice
particles of different sizes with a little bit of dust. There are several gaps
in the rings. The gaps result from gravitational interactions between the ring
particles and Saturn’s moons that orbit outside the ring or by a small moon
orbiting within the gap.
Saturn’s
Moons
As
of 2012, over 62 moons have been identified around Saturn. Only seven of
Saturn’s moons are round. All but one is smaller than Earth’s 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.
Saturn’s largest
moon, Titan, is about one and a half times the diameter of Earth’s moon. Titan
is even larger than the planet Mercury. Scientists are very interested in
Titan. The moon has an atmosphere that is thought to be like Earth’s first
atmosphere. This atmosphere was around before life developed on Earth. Like
Jupiter's moon, Europa, Titan may have a layer of liquid water under a layer of
ice. Scientists now think that there are lakes on Titan's surface. Don't take a
dip, though. These lakes contain liquid methane and ethane instead of water!
Methane and ethane are compounds found in natural gas.
Uranus
Uranus
is the 7th planet out from the Sun. Uranus' rings are almost perpendicular to
the planet. Uranus, shown in the figure above, 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. An Icy Blue-Green Ball
Uranus
is a lot like Jupiter and Saturn. The planet is composed mainly of hydrogen and
helium, but Uranus has a higher percentage of “ices” than Jupiter and Saturn.
These “ices” include water, ammonia, and methane. Uranus is also different
because of its blue-green color. Methane gas absorbs red light so the reflected
light gives Uranus 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 Earth’s 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.
The
Sideways Planet
All
of the planets rotate on their axes in the same direction that they move around
the Sun except for Venus and Uranus. While Venus rotates in the opposite
direction, Uranus is tilted on its side. Its axis is almost parallel to its
orbit. How did Uranus get this way? One possibility is that the planet was
struck by a large planet-sized object as it was forming during the early days
of the solar system.
Rings
and Moons of Uranus
Uranus
has a faint system of rings, as shown in the Figure below. The rings circle the
planet’s equator. However, Uranus is tilted on its side. So the rings are
almost perpendicular to the planet’s orbit. We have discovered 27 moons around
Uranus. All but a few are named for characters from the plays of William
Shakespeare. 4: Neptune
Neptune is shown in the Figure below. 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.
Neptune has a great dark spot at the center-left
and a small dark spot at the bottom center.
Scientists guessed Neptune's 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! 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.
Extremes of Cold and Wind
Like
Uranus, Neptune is blue. The blue color is mostly caused by the absorption of
red light by methane in Neptune’s atmosphere. 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.
Neptune's
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
–218°C (–360°F).
Neptune’s
Rings and Moons
Like
the other outer planets, Neptune has rings of ice and dust. These rings are
much thinner and fainter than Saturn's. Neptune's rings may be unstable. They
may change or disappear in a relatively short time.
Neptune has 13
known moons. Only Triton, shown in the Figure below, has enough mass to be
round. Triton orbits in the direction opposite to Neptune's orbit. Because of
this, scientists think Triton did not form around Neptune. The satellite may
have been captured by Neptune’s gravity as it passed very close to Neptune.
·
The four outer planets — Jupiter,
Saturn, Uranus, and Neptune — are all gas giants made mostly of hydrogen and
helium. Their thick outer layers are gases and have liquid interiors.
·
All of the outer planets have lots of
moons, as well as planetary rings made of dust and other particles.
·
Jupiter is the largest planet in the
solar system. It has bands of different colored clouds, and a long-lasting
storm called the Great Red Spot.
·
Jupiter has over 60 moons. The four
biggest were discovered by Galileo, and are called the Galilean moons.
·
One of the Galilean moons, Europa,
may have an ocean of liquid water under a layer of ice. The conditions in this
ocean might be right for life to have developed.
·
Saturn is smaller than Jupiter but
very similar to Jupiter. Saturn has a large system of beautiful rings.
·
Saturn’s largest moon, Titan, has an
atmosphere similar to Earth’s atmosphere before life formed.
·
Uranus and Neptune were discovered
using a telescope. They are similar to each other in size and composition. They
are both smaller than Jupiter and Saturn, and also have more icy materials.
·
Uranus is tilted on its side, probably
due to a collision with a large object in the distant past.
·
Neptune is very cold and has very strong winds. It had a large
dark spot that disappeared. Another dark spot appeared on another part of the
planet. These dark spots are storms in Neptune’s atmosphere.
