25.4 The Outer Solar System
Uranus was
discovered in 1781,
but its rings were not
found until 1977. Like
Uranus itself, the
rings were
discovered by
accident. This falsecolor infrared image
was taken by the
Hubble Space
Telescope.
Red Sea
25.4 The Outer Solar System
Gas Giants
How are the gas giants similar?
The four gas giants are thought to have
small, dense cores and thick atmospheres
that are mostly hydrogen and helium.
25.4 The Outer Solar System
Gas Giants
Jupiter, Saturn, Uranus, and Neptune are very
different from the terrestrial planets.
• They are much colder because of their distance
from the sun.
• All four planets are much larger and more
massive than the terrestrial planets.
• They are often referred to as the gas giants
because they are composed mainly of hydrogen
and helium.
25.4 The Outer Solar System
Gas Giants
None of the gas giants have solid surfaces.
If you tried to land a spaceship on one, you
would sink into the planet until the intense
pressure of the atmosphere crushed the ship.
In fact, the pressure is so great inside each
planet that hydrogen and helium exist mostly
in liquid form.
25.4 The Outer Solar System
Gas Giants
Gas giants have many moons, most of which
revolve in the same direction that the planets
rotate.
Each of the gas giants is surrounded by rings.
A ring is a disk made of many small particles
of rock and ice in orbit around a planet.
Rings are so close to the planet that
gravitational forces are very large, which may
have prevented the ring particles from
clumping together to form moons.
25.4 The Outer Solar System
Gas Giants
The outer planets consist of the four gas
giants. Pluto, formerly considered a planet,
was reclassified as a dwarf planet in 2006.
25.4 The Outer Solar System
Jupiter
What are the characteristics of Jupiter?
Jupiter is the largest and most massive
planet in our solar system.
25.4 The Outer Solar System
Jupiter
Jupiter’s Atmosphere
Jupiter’s clouds move rapidly because Jupiter
itself rotates rapidly—one day on Jupiter is less
than 10 Earth hours.
The brown cloud bands are colder and move
faster around the planet than the white bands.
25.4 The Outer Solar System
Jupiter
Sometimes storms occur at the boundaries
between these brown and white bands.
The Great Red Spot is a huge storm that
rotates around its own center like a hurricane.
It is caught between two bands of winds
blowing in opposite directions.
The Great Red Spot was first observed in
1664 but may be much older.
25.4 The Outer Solar System
Jupiter
This model of
Jupiter is based on
photos taken by
Voyager 1. The
Great Red Spot, is
a giant storm
much larger than
Earth.
25.4 The Outer Solar System
Jupiter
Jupiter’s Moons
Jupiter has at least 63 moons.
• Most of them are small, less than 200 kilometers in
diameter.
• Ganymede and Callisto are similar in size to Mercury.
• Io is covered with actively erupting volcanoes.
• Io and Europa are somewhat smaller, about the size
of our moon.
25.4 The Outer Solar System
Jupiter
• Ganymede, Io, and Europa have metal
cores and rocky mantles.
• Io is covered with actively erupting
volcanoes.
• Ganymede, Europa, and Callisto are
covered with ice.
• On Europa, the icy crust appears to rest on
top of a liquid saltwater ocean. Scientists
hypothesize that Europa is the most likely
place in the solar system, other than Earth,
to support life.
25.4 The Outer Solar System
Jupiter
Io is the most volcanically
active body in the solar
system. Io’s volcanic
eruptions consist mainly
of liquid sulfur.
Europa may have a liquid
water ocean beneath its
icy surface. The dark
lines are wide fractures in
Europa’s crust.
25.4 The Outer Solar System
Saturn
What are the characteristics of Saturn?
Saturn’s rings are the largest and most
visible from Earth.
25.4 The Outer Solar System
Saturn
Saturn, the second largest planet in the solar
system, is best known for its rings.
• The rings are made of particles of ice and icecoated rock.
• These particles are generally between a few
micrometers and 10 meters across, with most
particles being snowball-sized.
• The rings are about 274,000 kilometers in
diameter, yet are only tens of meters thick.
25.4 The Outer Solar System
Saturn
Saturn’s Atmosphere
Like Jupiter, Saturn has colorful banding in its
atmosphere.
Saturn’s atmosphere is the largest of any planet in
the solar system. Saturn also has the lowest
average density, less than the density of liquid
water.
Saturn’s atmosphere is made mostly of hydrogen
and helium, with small amounts of other elements.
25.4 The Outer Solar System
Saturn
Saturn has a thick atmosphere. It is
surrounded by a spectacular set of rings, the
largest in the solar system.
25.4 The Outer Solar System
Saturn
Saturn’s Moons
Astronomers have found at least 56 moons
orbiting Saturn.
• Most of these moons are very small.
• Titan is larger than Mercury. Titan is the only
planetary moon with a thick atmosphere.
25.4 The Outer Solar System
Uranus
What are the characteristics of Uranus?
The axis of Uranus’s rotation is tilted more
than 90°.
25.4 The Outer Solar System
Uranus
Because Uranus is so far from the sun, it is
very cold.
Uranus’s atmosphere is mostly hydrogen and
helium.
It also has a large amount of methane, which
gives the planet a distinct blue-green
appearance.
25.4 The Outer Solar System
Uranus
Structure of Uranus
Like Jupiter and Saturn, Uranus has a dense core.
This core is thought to be surrounded by a thick
mantle layer of liquid water and dissolved
ammonia.
A layer of liquid hydrogen and helium surrounds
the mantle.
25.4 The Outer Solar System
Uranus
Uranus has rings, although they are not as
visible as Saturn’s.
Uranus has at least 27 moons, though most
are quite small.
25.4 The Outer Solar System
Uranus
A Tilted Planet
The most unusual characteristic of Uranus is that
it lies nearly on its side.
• Uranus rotates in a direction opposite to the direction
of its revolution around the sun. Uranus’s rings and
moons revolve about the same tilted axis.
25.4 The Outer Solar System
Uranus
• Uranus’s northern and southern
hemispheres alternate being exposed to the
sun for many years, which causes immense
storms.
• No one knows for sure why Uranus’s axis is
so tilted.
• Scientists hypothesize that the tilt may have
been caused by a collision with another
large planetary body early in its history.
25.4 The Outer Solar System
Uranus
Uranus is
unusual in
that its axis is
tilted so that
the planet
rotates nearly
on its side.
25.4 The Outer Solar System
Neptune
What are the characteristics of Neptune?
Neptune’s bluish color comes from the
methane in its atmosphere.
25.4 The Outer Solar System
Neptune
Neptune is so far from the sun that it takes a
very long time—165 Earth years—for it to
make one revolution.
Neptune has about the same composition and
is about the same size as Uranus.
25.4 The Outer Solar System
Neptune
Neptune has clearly visible cloud patterns in
its atmosphere.
• Thin, wispy clouds form high within Neptune’s
atmosphere.
• Neptune’s clouds are made of methane ice
crystals.
Neptune also has large storms in its
atmosphere.
One storm, observed by the Voyager 2 space
probe in 1989, was called the Great Dark
Spot, but by 1994, the storm was gone.
25.4 The Outer Solar System
Neptune
This photo of
Neptune was taken
by the Voyager 2
spacecraft. The
Great Dark Spot,
visible near the
center, was similar
in size to Earth.
25.4 The Outer Solar System
Neptune
Neptune has thin rings very similar to the
rings of Uranus.
In addition, Neptune has at least 13 moons.
Triton, the only large moon, has a thin
atmosphere and an icy surface.
25.4 The Outer Solar System
Dwarf Planets
What is a dwarf planet?
A dwarf planet, like a planet, is spherical and
orbits the sun directly. But unlike a planet, a
dwarf planet has not cleared the
neighborhood around its orbit.
25.4 The Outer Solar System
Dwarf Planets
In 2006, astronomers established a new
classification for certain objects in the solar
system that share some, but not all, of the
characteristics of a planet.
Pluto, which for many years had been
considered the ninth and most distant planet
in the solar system, was reclassified as a
dwarf planet.
25.4 The Outer Solar System
Dwarf Planets
A dwarf planet is too small for its gravity to
attract all the nearby debris, so dwarf planets
tend to orbit in zones that contain many other
objects.
Pluto is much smaller than the inner or outer
planets. Its diameter is about half that of
Mercury.
The density of Pluto is similar to that of Triton,
suggesting a mixture of ice and rock.
25.4 The Outer Solar System
Dwarf Planets
Pluto’s orbit around the sun is more elliptical
than those of the planets. Pluto actually
swings closer to the sun at times than
Neptune.
Like Uranus, Pluto has a rotation axis that is
tilted more than 90°.
Pluto also has three moons, Charon, Nix, and
Hydra.
25.4 The Outer Solar System
Dwarf Planets
The New Horizons space probe will reach the
dwarf planet Pluto and its three moons in the
year 2015.
25.4 The Outer Solar System
Dwarf Planets
Other known dwarf planets include Eris and
the asteroid Ceres.
Many more solar system objects will likely be
named dwarf planets once their shapes can
be verified as being spherical.
Not all astronomers agree on what defines a
planet, and the classification of dwarf planet
may be subjected to further debate.
25.4 The Outer Solar System
Comets and Meteoroids
What are comets and meteoroids?
Comets are dusty pieces of ice and rock that
partially vaporize when they pass near the
sun. Meteoroids are pieces of rock, usually
less than a few hundred meters in size, that
travel through the solar system.
25.4 The Outer Solar System
Comets and Meteoroids
There are many smaller objects moving
through the inner solar system in two forms.
• Comets are dusty pieces of ice and rock that
partially vaporize when they pass near the sun.
• Meteoroids are pieces of rock, usually less than
a few hundred meters in size that travel through
the solar system.
25.4 The Outer Solar System
Comets and Meteoroids
Certain ancient meteoroids have been largely
unaltered since the birth of the solar system.
• These meteoroids are the oldest remnants of
the early solar system.
• Scientists have used radioactive dating to
determine their absolute age and have found
that the solar system is about 4.6 billion years
old.
25.4 The Outer Solar System
The Edge of the Solar System
What lies in the outer solar system beyond
Neptune?
Most of the objects in the Kuiper belt lie in a
doughnut-shaped region close to the
ecliptic.
Beyond the Kuiper belt lies a great reservoir
of comets called the Oort cloud.
25.4 The Outer Solar System
The Edge of the Solar System
Astronomers estimate that thousands of solar
system objects exist beyond Neptune’s orbit.
• These objects are found largely in two regions,
the Kuiper belt and the Oort cloud.
• The Kuiper belt extends from Neptune’s orbit
out to about 100 AU or more from the sun.
• Pluto is an example of a Kuiper belt object.
25.4 The Outer Solar System
The Edge of the Solar System
The solar system extends much farther out
than the Kuiper belt.
The Oort Cloud is a very sparse sphere of
comets thought to encircle the solar system
out to a distance of about 50,000 AU.
Occasionally objects from the Oort cloud
enter the inner solar system, where they
appear as comets.
25.4 The Outer Solar System
The Edge of the Solar System
Located in the outer
reaches of the solar
system, the Oort
cloud is a sphere of
comets surrounding
the sun and planets.
25.4 The Outer Solar System
Assessment Questions
1. The planets referred to as the gas giants are
composed mainly of
a.
b.
c.
d.
nitrogen and oxygen.
hydrogen and carbon dioxide.
hydrogen and helium.
carbon dioxide and fluorine.
25.4 The Outer Solar System
Assessment Questions
1. The planets referred to as the gas giants are
composed mainly of
a.
b.
c.
d.
nitrogen and oxygen.
hydrogen and carbon dioxide.
hydrogen and helium.
carbon dioxide and fluorine.
ANS: C
25.4 The Outer Solar System
Assessment Questions
2. The part of the solar system that is most distant
from the sun is
a.
b.
c.
d.
Neptune.
Pluto.
the Kuiper belt.
the Oort cloud.
25.4 The Outer Solar System
Assessment Questions
2. The part of the solar system that is most distant
from the sun is
a.
b.
c.
d.
Neptune.
Pluto.
the Kuiper belt.
the Oort cloud.
ANS:
D
25.4 The Outer Solar System
Assessment Questions
1. Most meteroids consist primarily of ice and orbit
the sun at a great distance in the Oort cloud or
Kuiper belt.
True
False
25.4 The Outer Solar System
Assessment Questions
1. Most meteroids consist primarily of ice and orbit
the sun at a great distance in the Oort cloud or
Kuiper belt.
True
False
ANS:
F, comets