Download Why are planets round?

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Idea 4
Why are planets round?
There are eight planets in our solar system: Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Some planets
are much larger than others, and some are surrounded by rings or
moons. But all planets have three things in common. They orbit the
sun, they are massive enough that their gravity controls all the
objects in the area around them, and they are spherical.
Although you might not think of “roundness” as a particularly
special or unusual quality, most objects in the solar system are not
round. Millions of asteroids between the orbits of Mars and Jupiter
are irregularly shaped chunks of rock. Planets are round because of
their large mass and gravitational force.
Mercury
Mars
Jupiter
e
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Weekly Question
Venus
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WEEK 3
Vocabulary
spherical
SFEER-ih-kul
round, ball-shaped;
having a surface
equally distant
from the center
at all points
Uranus
Saturn
Neptune
Earth
A. Check the box next to the object that is spherical.
!paper tube
!egg
!football
!basketball
B. What are three things that all planets have in common?
1.
3.
2.
Talk
Pluto would be classified as the ninth planet if it were not for the
fact that there are millions of other space rocks within its orbit that
its gravity doesn’t control. What are two examples of objects that
are controlled by a planet’s gravity?
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Da
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Weekly Question
Why are planets round?
Most scientists today believe that the solar system formed when
a giant cloud of gas and dust contracted into a rotating disk of
matter. Most of this material coalesced in the center of the disk and
became our sun. The rest formed small, rocky bodies or balls of gas.
Over time, gravitational force allowed these objects to attract and
collect other chunks of rock or molecules of gas in their regions of
space. As these objects grew in size, their gravity increased. This
allowed them to attract and gather even more material. This
process, called accretion, is believed to have formed the planets,
asteroids, comets, and other bodies in our solar system.
Idea 4
WEEK 3
Vocabulary
accretion
uh-CREE-shun
the process of
becoming larger
by adding more
material
coalesce
koh-uh-LESS
to unite; to grow
together
Formation of the Solar System
A. Write true or false.
1. Our solar system began as a rotating disk of gas and dust.
2. As the gravity of objects in the solar system increased,
the objects became bigger.
3. Most of the gas and dust in the early solar system became
the planets.
B. Use the vocabulary words to complete the sentences.
1. Dust and other particles of matter can
to form
the beginning of a planet.
2. Through the process of
, an object grows in size
and increases its gravitational force.
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Weekly Question
Idea 4
Why are planets round?
Gravity acts to pull everything toward an object’s center of mass.
The center of mass is a single imaginary point inside an object from
which gravity seems to act. As an object acquires more mass, its
gravity increases and more matter is pulled toward the object’s
center. When an object is massive enough, gravity forces it into a
round shape. This is because the only way to get all of the object’s
matter as close to its center of mass as possible is to form a sphere.
Every point on the surface of a sphere is equally distant from its center.
Objects such as asteroids or small moons are irregularly shaped
because they have much less mass than planets. Therefore their
gravitational force is not strong enough to form a sphere. Unlike
planets, whose center of mass is also the center of the sphere,
asteroids have a center of mass that may actually be located
off-center, depending on where the most mass has accumulated.
WEEK 3
center of mass
center of mass
planet
asteroid
A. Check the box next to the analogy that is not correct.
!Center is to sphere as center of mass is to planet.
!Asteroid is to irregular-shaped as planet is to spherical.
!Center of mass is to planet as axis is to Earth.
!Planet is to asteroid as round is to irregular.
B. Explain in your own words why an object with a lot of mass
takes the shape of a sphere.
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Da
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4
i l y S c i e nc
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Big
Weekly Question
Why are planets round?
Even though Earth’s gravity is strong enough to pull all of the
planet’s matter evenly toward its center, Earth is not a perfect
sphere. This is because Earth’s rotation creates centrifugal force.
Centrifugal force is the same force that causes you to slide to one
side when you go around a curve on a roller coaster. On Earth,
centrifugal force pushes out the planet’s surface at the equator,
causing it to bulge slightly around the middle. In fact, Earth’s
diameter is 26 miles wider when measured at the equator than when
measured from the North Pole to the South Pole. Believe it or not,
this means that you weigh slightly less at the equator than you do
at the poles! This is because at the equator, you are a little farther
away from Earth’s center of mass than you are at the poles.
Idea 4
WEEK 3
Vocabulary
centrifugal force
sen-TRIF-uh-gul
forss
the force that tends
to push an object
outward when it
rotates around a
center
diameter: 7,900 miles
diameter: 7,926 miles
Answer the questions.
1. Why does Earth’s surface bulge at the equator? Use the vocabulary word
in your answer.
2. What would have to happen in order for Earth to become a perfect sphere?
3. Why does being farther from Earth’s center of mass cause you to weigh less?
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Weekly Question
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Idea 4
Why are planets round?
A. Use the words in the box to complete the sentences.
WEEK 3
center of mass
accretion
centrifugal force
coalesced
spherical
1. Earth is not a perfect sphere because
causes
the equator to bulge out slightly.
2. Planets are
so that all of their matter can
as possible.
be as close to the
3. Our solar system formed when gas and dust
into bodies that grew larger and larger over time through the process
of
.
B. Explain how mass and gravity cause planets to be round.
C. What two qualities, besides roundness, must an object have in order
to be considered a planet?
1.
2.
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