Page 1 of 5
KEY CONCEPT
Planets orbit the Sun at
different distances.
BEFORE, you learned
NOW, you will learn
• Earth orbits the Sun
• The Moon is Earth’s natural
satellite
• The Moon’s features tell us
about its history
• What types of objects are in
the solar system
• About sizes and distances in
the solar system
• How the solar system formed
VOCABULARY
EXPLORE Planet Formation
astronomical unit (AU)
p. 81
ellipse p. 81
How do planets form?
PROCEDURE
1
Fill the bowl about halfway with water.
2 Stir the water quickly, using a circular
motion, and then remove the spoon.
MATERIALS
•
•
•
•
bowl
water
spoon
wax pieces
3 Sprinkle wax pieces onto the swirling water.
WHAT DO YOU THINK?
• In what direction did the wax move?
• What else happened to the wax?
Planets have different sizes and distances.
MAIN IDEA AND DETAILS
Put sizes and distances
in the solar system into
a chart.
You may have seen some planets in the sky without realizing it. They
are so far from Earth that they appear as tiny dots of light in the darkened sky. If you have seen something that looks like a very bright star
in the western sky in the early evening, you have probably seen the
planet Venus. Even if you live in a city, you may have seen Mars,
Jupiter, or Saturn but thought that you were seeing a star. Mercury
is much more difficult to see. You need a telescope to see three of the
planets in our solar system—Uranus, Neptune, and Pluto.
Like the Moon, planets can be seen because they reflect sunlight.
Planets do not give off visible light of their own. Sunlight is also reflected
by moons and other objects in space, called comets and asteroids.
However, these objects are usually too far away and not bright enough
to see without a telescope.
check your reading
Why do planets look bright?
Chapter 3: Our Solar System 79
E
Page 2 of 5
Objects in the Solar System
The sizes of objects in the solar system range from very small to very large.
asteroids
Sun
On this scale,
the Sun is about
a meter across.
Mars
Saturn
Earth
Venus
Mercury
Sat
’s
urn
mo
ons
Jupiter’s moons
Neptune
Ne
Jupiter
Uranus’s
moons
0
20,000
ptu
s
ne’
mo
Uranus
40,000 kilometers
comets
Objects smaller than about 100 kilometers
are represented as dots.
Pluto
Distances of Planets
Sun Venus Mars
Mercury Earth
0
E
Jupiter
asteroids
2
80 Unit: Space Science
4 AU
Saturn
Uranus
on
s
Page 3 of 5
Objects in the solar system have very different sizes. An asteroid
may be as small as a mountain, perhaps 1/1000 Earth’s diameter.
In contrast, the largest planets are about 10 Earth diameters across.
The Sun’s diameter is about 100 times Earth’s. If the planets were the
sizes shown on page 80, the Sun would be about a meter across.
Distances
The distances between most objects in space are huge in comparison
with the objects’ diameters. If Earth and the Sun were the sizes shown
on page 80, they would be more than 100 meters from each other.
Astronomers understand huge distances by comparing them with
something more familiar. One astronomical unit, or AU, is Earth’s
average distance from the Sun. An AU is about 150 million kilometers
(93 million mi). Mercury is less than 0.5 AU from the Sun, Jupiter is
about 5 AU from the Sun, and Pluto gets nearly 50 AU from the Sun
at times. You can use the diagram at the bottom of pages 80–81 to
compare these distances. However, the planets are not arranged in a
straight line—they move around the Sun.
VOCABULARY
Draw word triangles
in your notebook for
new terms.
You can see that the planets are spaced unevenly. The first four
planets are relatively close together and close to the Sun. They define a
region called the inner solar system. Farther from the Sun is the outer
solar system, where the planets are much more spread out.
Check your reading
What are the two regions of the solar system?
Orbits
More than 99 percent of all the mass in the solar system is in the Sun.
The gravitational pull of this huge mass causes planets and most other
objects in the solar system to move around, or orbit, the Sun.
The shape of each orbit is an ellipse —a flattened
circle or oval. A circle is a special type of ellipse, just
as a square is a special type of rectangle. Most of the
planets’ orbits are very nearly circles. Only one
planet—Pluto—has an orbit that looks a little
flattened instead of round.
Neptune
Pluto
comets
Chapter 3: Our Solar System 81
E
Page 4 of 5
Distances
How far apart are the planets?
SKILL FOCUS
Using models
PROCEDURE
1
Mark one sheet from the end of the roll of paper as the location of the Sun.
Mark an X and write the word Sun with dots rather than lines.
2 Use the Distance Table data sheet to mark the distances for the rest of the
solar system. Count sheets and estimate tenths of a sheet as necessary.
Re-roll or fold the paper neatly.
3 Go to a space where you can unroll the paper. Compare the distances of
MATERIALS
• roll of toilet
paper
• felt-tipped pen
• Distance Table
TIME
30 minutes
planets as you walk along the paper and back again.
WHAT DO YOU THINK?
• How does the distance between Earth and Mars compare with the
distance between Saturn and Uranus?
• How would you use the spacing to sort the planets into groups?
CHALLENGE If it took two years for the Voyager 2 spacecraft to
travel from Earth to Jupiter, about how long do you think it took for
Voyager 2 to travel from Jupiter to Neptune?
The solar system formed from a swirling
cloud of gas and dust.
MAIN IDEA AND DETAILS
Remember to take notes
about how the solar system
formed.
The planets orbit the Sun in similar ways. Their paths are almost in a
flat plane, like the rings of a target. They all orbit the Sun in the same
direction—counterclockwise as seen from above Earth’s North Pole.
Most of the planets rotate on their axes in this direction, too. Many other
objects in the solar system also orbit and rotate in this same direction.
These similar motions have given scientists clues about how the solar
system formed.
According to the best scientific model, the solar system formed out
of a huge cloud of different gases and specks of dust. The cloud flattened
into a disk of whirling material. Most of the mass fell to the center and
became a star—the Sun. At the same time, tiny bits of dust and frozen
gases in the disk stuck together into clumps. The clumps stuck together
and became larger. Large clumps became planets. They moved in the
same direction that the flat disk was turning.
Not all the clumps grew big enough to be called planets. However,
many of these objects still orbit the Sun the same way that planets orbit.
Some of the objects close to the Sun are like rocks or mountains in
space and are called asteroids. Other objects, farther from the Sun, are
more like enormous snowballs or icebergs. They are called comets.
E
82 Unit: Space Science
Page 5 of 5
Formation of the Solar System
The Sun and other objects formed out of material in a flat disk.
1
Nebula
Part of a huge cloud of material,
called a nebula, collapsed into
a flattened disk.
2
Disk
The Sun formed at the center
of the disk. Other objects
formed from the whirling
material of the disk.
3
Solar System
Much of the material was
cleared away. The Sun, planets,
and other objects remained.
Some objects orbit planets instead of orbiting the Sun directly, so they
are considered moons. You will read more about asteroids, comets,
and moons in Section 3.4.
You can tell a little bit about the size of an object in space from
its shape. Lumpy objects are usually much smaller than round objects.
As a space object starts to form, the clumps come together from many
directions and produce an uneven shape. The gravity of each part
affects every other part. The pieces pull each other closer together.
When an object has enough mass, this pulling becomes strong enough
to make the object round. Any parts that would stick far out are pulled
in toward the center until the object becomes a sphere.
check your reading
Why do planets and large moons have a spherical shape?
KEY CONCEPTS
CRITICAL THINKING
1. What are the types of space
objects in the solar system?
4. Analyze Why do the planets
all orbit in one direction?
2. Why is the unit of measurement
used for the distances of planets
from the Sun different from
the unit used for their sizes?
5. Infer Which of the two
moons below has more mass?
Explain why you think so.
CHALLENGE
6. Apply Could you model all
the sizes of objects in the solar
system by using sports balls?
Explain why or why not.
3. How did planets and other
objects in the solar system
form out of material in a disk?
Chapter 3: Our Solar System 83
E