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CHAPTER 21
Exploring Our Solar System
LESSON 1
Our Solar System
Key Concepts
What do you think? Read the two statements below and decide
whether you agree or disagree with them. Place an A in the Before column
if you agree with the statement or a D if you disagree. After you’ve read
this lesson, reread the statements to see if you have changed your mind.
Before
Statement
After
1. Our solar system has eight planets.
2. Earth’s atmosphere is mostly oxygen.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Origin and Structure of Our
Solar System
A solar system is a group of objects that revolve around a
star. You might be familiar with our solar system—the eight
planets and other objects that revolve around the Sun. Our Sun
is not the only star in the universe that has a solar system. But
our solar system is the only one scientists can study in detail.
• How do objects in the solar
system move?
• How did distance from the
Sun affect the makeup of
the objects in the solar
system?
• What objects are in the
solar system?
Ask Questions As you read,
write questions you may
have next to each paragraph.
Read the lesson a second
time and try to answer the
questions. When you are
done, ask your teacher any
questions you still have.
Formation of the Solar System
Did you know there are clouds in space? Clouds in space
are made mostly of hydrogen gas. Five billion years ago, our
solar system formed from a spinning cloud of hydrogen gas
and dust. When gravity caused the cloud to collapse, the
cloud began to spin faster. The cloud also got hotter. When
the center of the cloud became hot enough for nuclear
reactions to occur, a star formed. That star was our Sun. The
stars you see in the night sky formed in much the same way
as the Sun formed.
As the cloud containing the Sun continued to spin, it
flattened, with the Sun at its center. Small pieces of ice and
rock orbiting the Sun clumped together and formed small,
rocky or icy bodies called planetesimals (pla ne TE sih mulz).
Gravity pulled some of the planetesimals together. The larger
bodies that formed became planets, asteroids, and other
objects.
Reading Essentials
Reading Check
1. Summarize How did
planetesimals become
planets?
Exploring Our Solar System
377
Solar System
Asteroid belt
Uranus
Earth
Mars
4 AU
Venus
30 AU
Saturn
Jupiter
Sun
5000 AU
Neptune
Mercury
Oort cloud
Inner solar system
Visual Check
2. Locate Which planet is
farthest from the Sun?
3. Recognize How far
does the solar system extend?
Outer solar system
Distances Within the Solar System
The solar system is so large that scientists rarely use
kilometers to measure distances within it. Instead, they
use astronomical units. An astronomical unit (AU) is Earth’s average
distance from the Sun, nearly 150 million km. Scientists often divide
our solar system into regions, depending on distance from
the Sun. Refer to the figure above as you read about regions
in the solar system. The inner solar system extends to about
4 AU from the Sun. The inner solar system contains the
planets closest to the Sun—Mercury, Venus, Earth, and Mars.
It also contains the asteroid belt between Mars and Jupiter,
2–4 AU from the Sun.
The four planets farthest from the Sun—Jupiter, Saturn,
Uranus, and Neptune—are part of the outer solar system.
The outer solar system extends from Jupiter’s orbit to the
Oort (ORT) cloud. The Oort cloud is a large, spherical shell
of icy planetesimals. Evidence suggests the Oort cloud orbits
the Sun from about 5,000 AU to as far as 50,000 AU.
Makeup of the Solar System
You live on a planet with a solid, rocky surface. But the
solar system is made mostly of hydrogen gas. Ices, rocks, and
metals make up less than 2 percent of our solar system’s mass.
Hydrogen gas is the least-dense material in the solar
system. The Sun, Jupiter, and Saturn contain most of the
hydrogen in the solar system. They are the only bodies with
enough mass to have large amounts of hydrogen.
REVIEW VOCABULARY
silicates
compounds composed mostly
of silicon and oxygen, with
smaller amounts of other
atoms
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Exploring Our Solar System
Ices are denser than gases. They are made mostly of
water, carbon dioxide, methane, or ammonia. Rocks are
denser than ices and are made mostly of silicates. Metals,
such as iron, are the solar system’s densest materials.
Reading Essentials
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Reading Check
Planets of the outer solar system
Movement in the Solar System
The Sun contains 99 percent of the solar system’s mass.
Most other objects in the solar system revolve around the
Sun. The Sun’s enormous gravitational pull holds these
objects in orbit.
Revolution is the movement of one object around
another object. The closer an object is to the Sun, the faster
it revolves. While most objects in the solar system revolve,
they also rotate, or spin, on their axes. Both of these
movements—rotation and revolution—are regular and
predictable.
Direction of Motion
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
The motion of an object around the Sun is like the
motion of an object whirled on a string. The string pulls on
the object just as the Sun’s gravity pulls on planets and
objects that revolve around it.
Our solar system formed from a spinning cloud of gas
and dust. Objects that formed from this material spun in the
same direction. Planets and most other solar system objects
still revolve around the Sun in the same direction. If you
were far above Earth’s North Pole and looked down on the
solar system, you would see objects revolving in a
counterclockwise direction. The Sun and six of the eight
planets, including Earth, also rotate in a counterclockwise
direction. Venus and Uranus rotate clockwise.
The View from Earth
Earth rotates on its axis once every 24 h. But on Earth, it
seems as though you are standing still, and the Sun, the
Moon, and stars move around you. While Earth rotates from
west to east, objects in the sky appear to move from east to
west. The same is true on a merry-go-round. As you move in
one direction on a merry-go-round, objects around you seem
to move in the opposite direction.
Use four sheets of paper to
make an eight-layer book.
Use it to identify and describe
objects in the solar system.
Objects In The Solar System
Sun
Inner and Outer Planets
Dwarf Planets
Asteroids
Natural Satellites
Kuiper Belt Objects
Comets
Meteoroids, Meteors, Meteorites
Key Concept Check
4. Explain Why do most
objects in our solar system
move in the same direction?
Objects in Our Solar System
Our solar system contains billions of objects. Scientists
group these objects into categories based on their makeup,
their size, their distance from the Sun, and whether they
orbit the Sun or another object.
From Gas and Dust to Solids Recall that the solar system
formed from a cloud of gas and dust that was extremely hot
at its center, where the Sun formed. As regions beyond the
Sun cooled, some of the gases solidified into ices, rocks, and
metals.
Reading Essentials
Exploring Our Solar System
379
Key Concept Check
5. Evaluate How did
distance from the Sun affect
the makeup of objects in the
solar system?
Ices, Rocks, and Metals Ices formed far from the Sun, where
temperatures were extremely cold. Closer to the Sun,
temperatures were too high for ices to form. Most gases there
solidified into rocks and metals. The densest matter, the
metals, sank to the centers of the largest objects throughout
the solar system.
The Sun
The Sun is made mostly of hydrogen gas. It also contains
helium and tiny amounts of other elements. The Sun is the
only star in our solar system, and it is the largest object in
the solar system. The Sun’s diameter is 10 times that of
Jupiter and more than 100 times that of Earth.
Planets
A planet orbits the Sun, is large enough to be nearly spherical in
shape, and has no other large object in its orbital path. The figure
below shows the eight planets in their order from the Sun.
Visual Check
6. Differentiate How do
the inner and outer planets
differ?
The four inner planets—Mercury, Venus, Earth, and
Mars—formed from rocks and metals. As shown in the
figure, they are smaller than the outer planets. They have
few or no moons and rotate slowly.
The four outer planets—Jupiter, Saturn, Uranus, and
Neptune—formed mostly from gas and ice. They are large and
have many moons. They rotate quickly and have rings. The
table on the next page describes the planets.
Jupiter
Saturn
Mercury
Neptune
Earth
Sun
Venus
Mars
Uranus
Distances not to scale
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Exploring Our Solar System
Reading Essentials
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The Planets
The Planets of the Solar System
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Planet
Distance from
the Sun and
Diameter
Makeup and Atmosphere
Mercury
0.39 AU from
the Sun
4,900 km
diameter
Mercury has a large metal core under
its small rocky mantle. Its surface is
covered with craters and looks much
like the surface of Earth’s moon.
Mercury has no permanent atmosphere.
Venus
0.72 AU from
the Sun
12,100 km
diameter
Venus is similar to Earth in size and
makeup. Its rocky mantle surrounds a
molten or partially molten metal core.
Its thick carbon dioxide atmosphere
traps thermal energy, making Venus’s
surface the hottest of all the planets.
Earth
1 AU from the
Sun
12,800 km
diameter
Most of Earth is covered by a thin layer
of liquid water. Earth has a rocky
mantle, a molten outer metal core,
and a solid inner metal core. Its
atmosphere is 80 percent nitrogen
and 20 percent oxygen.
Mars
1.5 AU from the
Sun
6,800 km
diameter
Mars has a rocky mantle and a partially
molten metal core. Its thin atmosphere
is mostly carbon dioxide. Iron in its
surface rock gives the planet a reddish
color. The surface of Mars has ice but
no liquid water.
Jupiter
5.2 AU from the
Sun
143,000 km
diameter
Jupiter has more mass than all the
other planets combined. Under its
atmosphere of hydrogen gas is a layer
of liquid hydrogen. Rock and metal
have sunk to its core. A thin, barely
visible ring system surrounds it.
Saturn
9.6 AU from the
Sun
121,000 km
diameter
Saturn’s makeup is similar to that of
Jupiter, but its atmosphere is hazier.
Saturn’s rings, the most distinctive
rings of all the outer planets, are made
mostly of small particles of ice.
Uranus
19 AU from the
Sun
51,100 km
diameter
Uranus has a hydrogen gas outer layer;
a fluid inner layer made of water,
methane, and ammonia; and a rocky
core. Uranus has a blue-green color
because of the small amount of methane
in its cloud layers. It has thin rings.
Neptune
30 AU from the
Sun
49,500 km
diameter
Neptune has more mass than Uranus
but is slightly smaller. Neptune and
Uranus have similar makeup. Neptune
is a deeper blue because it has more
methane in its atmosphere. Like the
other outer planets, it has thin rings.
Reading Essentials
Interpreting Tables
7. Name the largest and
smallest planets, by diameter.
Exploring Our Solar System
381
Dwarf Planets
Visual Check
8. Identify Which is the
largest dwarf planet?
Dwarf planets orbit the Sun and
are nearly spherical in shape, but they
share their orbital paths with other
objects of similar size. Eris is the
largest known dwarf planet. All
known dwarf planets, including
Eris and Pluto, are smaller than
Earth’s moon, as shown in the
figure on the right.
Our solar system has at least
five dwarf planets. However,
scientists hypothesize that the
solar system might contain
hundreds of dwarf planets, most
of them orbiting the Sun beyond
Neptune. At least one dwarf
planet, Ceres (SIHR eez), shown
in the figure, orbits the Sun
between the orbits of Mars and
Jupiter, in the asteroid belt.
Dwarf Planets
Earth‘s
Moon
Eris
Pluto
Ceres
Asteroids
9. Locate Where do most
asteroids exist?
Natural Satellites
A satellite is an object that orbits a larger object other than a star.
Natural satellites are also known as moons. The solar system
has over 170 moons. Most of them orbit planets or dwarf
planets. However, some moons orbit smaller objects, such as
asteroids. For example, the asteroid Ida is 50 km long. It is
20 times larger than Dactyl (DAK tul), its moon.
10. Analyze Why are
the moons around the
outermost planets mostly ice
rather than rock?
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Exploring Our Solar System
Just as planets have different makeups, satellites have
different makeups depending on their location. The satellites
of the inner planets are mostly rock. Jupiter’s moons are a
mixture of rock and ice. The moons around the three
outermost planets are mostly ice.
Reading Essentials
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Reading Check
Asteroids are small, rocky or metallic objects that are left
over from the solar system’s formation. There are hundreds
of thousands of asteroids in the asteroid belt, but they are so
small that their total mass is less than the mass of Earth’s
moon. Though most asteroids exist in the asteroid belt,
some exist elsewhere in the solar system. Most asteroids are
irregularly shaped and have craters on their surfaces.
Kuiper Belt Objects
You read that the asteroid belt lies between the orbits of
Mars and Jupiter. The Kuiper (KI pur) belt, shown in the
figure below, is a similar but much larger belt of objects
between 30 and 50 AU from the Sun. Like the asteroid belt,
the Kuiper belt contains objects left over from the solar
system’s formation. While asteroids are mostly rock and
metal, Kuiper belt objects are mostly ice. Pluto is the bestknown object in the Kuiper belt.
Kuiper Belt and Comets
Reading Check
11. Contrast How do
Kuiper belt objects differ
from objects in the asteroid
belt?
Comet
Hyakutake
Kuiper
belt
Visual Check
Jupiter
Saturn
Uranus
Halley’s
Comet
12. Observe Which
direction does a comet’s tail
point?
Neptune
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Comets
Comets are small objects made mostly of ice. Most
comets formed in the Kuiper belt or the Oort cloud. Comets
revolve around the Sun in long, stretched-out orbits, as
shown in the figure above. As a comet’s orbit nears the Sun,
some of the comet’s ice becomes gas and forms the comet’s
tail. The tails of some comets extend millions of kilometers
into space. A comet loses mass with each orbit. After a
certain number of orbits, the comet breaks up.
Meteoroids, Meteors, and Meteorites
A meteoroid is a solar system object that is smaller than an
asteroid or a comet. Some meteoroids are the result of collisions
between asteroids. Some are debris from comets. A meteor is
the streak of light created when a meteoroid enters Earth’s atmosphere.
Most meteoroids are no bigger than a grain of rice. They are
visible only when they get hot and glow as they pass
through Earth’s atmosphere. A few large meteoroids become
meteorites. A meteorite is a meteoroid that strikes Earth.
Reading Essentials
Key Concept Check
13. Name What objects
are in our solar system?
Exploring Our Solar System
383
Mini Glossary
astronomical unit (AU): Earth’s average distance from the
Sun, nearly 150 million km
meteoroid: a solar system object that is smaller than an
asteroid or a comet
dwarf planet: an object that orbits the Sun and is nearly
spherical in shape, but shares its orbital path with other
objects of similar size
meteor: the streak of light created when a meteoroid enters
planet: an object that orbits the Sun, is large enough to be
nearly spherical in shape, and has no other large object in its
orbital path
satellite: an object that orbits a larger object other than a star
Earth’s atmosphere
meteorite: a meteoroid that strikes Earth
1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explains
some differences between a planet and a dwarf planet.
2. Use the descriptions given in the graphic organizer to identify objects in our solar system.
Object
Description
Object
Description
result of collisions between
asteroids or debris from comets
small object made mostly of ice
orbits a larger object other than
a star
a meteoroid that strikes Earth
small, rocky or metallic object
with an irregular shape and
craters
nearly spherical object that orbits
the Sun but shares orbital path
with other similarly sized objects
streak of light created when a
meteoroid enters Earth’s
atmosphere
3. Does the Sun rise in the east or west on Venus? Explain your answer.
What do you think
Reread the statements at the beginning of the
lesson. Fill in the After column with an A if you
agree with the statement or a D if you disagree.
Did you change your mind?
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Exploring Our Solar System
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LESSON
Reading Essentials
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the only star in our solar system