Download 4 Inner versus Outer Planets

Unit 7: Solar System
Cheryl Lewis
Ck12 Science
Dana Desonie, Ph.D.
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AUTHORS
Cheryl Lewis
Ck12 Science
Dana Desonie, Ph.D.
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Printed: September 17, 2013
iii
Contents
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Contents
1
Introduction to the Solar System
2
Kepler’s Laws of Planetary Motion
10
3
Planets of the Solar System
14
4
Inner versus Outer Planets
17
5
Asteroids
20
6
Comets
24
7
Meteors
27
iv
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C HAPTER
Chapter 1. Introduction to the Solar System
1
Introduction to the Solar
System
Lesson Objectives
• Describe historical views of the solar system.
• Explain how the solar system formed.
Changing Views of the Solar System
Humans’ view of the solar system has evolved as technology and scientific knowledge have increased. The ancient
Greeks identified five of the planets and for many centuries they were the only planets known. Since then, scientists
have discovered two more planets, many other solar-system objects and even planets found outside our solar system.
The Geocentric Universe
The ancient Greeks believed that Earth was at the center of the universe, as shown in Figure 1.1. This view is
called the geocentric model of the universe. Geocentric means "Earth-centered." In the geocentric model, the sky,
or heavens, are a set of spheres layered on top of one another. Each object in the sky is attached to a sphere and
moves around Earth as that sphere rotates. From Earth outward, these spheres contain the Moon, Mercury, Venus,
the Sun, Mars, Jupiter, and Saturn. An outer sphere holds all the stars. Since the planets appear to move much faster
than the stars, the Greeks placed them closer to Earth.
The geocentric model worked well, by explaining why all the stars appear to rotate around Earth once per day. The
model also explained why the planets move differently from the stars and from each other.
One problem with the geocentric model is that some planets seem to move backwards (in retrograde) instead of in
their usual forward motion around Earth.
A demonstration animation of retrograde motion of Mars as it appears to Earth can be found here:
http://projects.astro.illinois.edu/data/Retrograde/index.html
Around 150 A.D. the astronomer Ptolemy resolved this problem by using a system of circles to describe the motion
of planets (Figure 1.2). In Ptolemy’s system, a planet moves in a small circle, called an epicycle. This circle moves
around Earth in a larger circle, called a deferent. Ptolemy’s version of the geocentric model worked so well that it
remained the accepted model of the universe for more than a thousand years.
An animation of Ptolemy’s system is seen here:
http://www.youtube.com/watch?v=FHSWVLwbbNw&NR=1
The Heliocentric Universe
Ptolemy’s geocentric model worked but it was not only complicated, it occasionally made errors in predicting the
movement of planets. At the beginning of the 16th century A.D., Nicolaus Copernicus proposed that Earth and all the
other planets orbit the Sun. With the Sun at the center, this model is called the heliocentric model or "sun-centered"
model of the universe (Figure below). Copernicus’ model explained the motion of the planets as well as Ptolemy’s
model did, but it did not require complicated additions like epicycles and deferents.
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FIGURE 1.1
Model of a geocentric universe.
This
diagram of the universe from the Middle
Ages shows Earth at the center, with the
Moon, the Sun, and the planets orbiting
Earth.
FIGURE 1.2
According to Ptolemy, a planet moves on
a small circle (epicycle) that in turn moves
on a larger circle (deferent) around Earth.
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Chapter 1. Introduction to the Solar System
Because people were so used to thinking of Earth at the center of the universe, the heliocentric model was not
widely accepted at first. However, when Galileo Galilei first turned a telescope to the heavens in 1610, he made
several striking discoveries. Galileo discovered that the planet Jupiter has moons orbiting around it. This provided
the first evidence that objects could orbit something besides Earth.
An animation of three of Jupiter’s moons orbiting the planet is seen here: http://upload.wikimedia.org/wikipedia/co
mmons/e/e7/Galilean_moon_Laplace_resonance_animation_de.gif.
Galileo also discovered that Venus has phases like the Moon (Figure 1.3), which provides direct evidence that Venus
orbits the Sun.
FIGURE 1.3
The phases of Venus.
Galileo’s discoveries caused many more people to accept the heliocentric model of the universe, although Galileo
himself was found guilty of heresy for his ideas. The shift from an Earth-centered view to a Sun-centered view of
the universe is referred to as the Copernican Revolution.
Watch this animation of the Ptolemaic and Copernican models of the solar system. Ptolemy made the best model he
could with the assumption that Earth was the center of the universe, but by letting that assumption go, Copernicus
came up with a much simpler model. Before people would accept that Copernicus was right, they needed to accept
that the Sun was the center of the solar system (1n - IE Stand.): http://www.youtube.com/watch?v=VyQ8Tb85H
rU&feature=related (0:47).
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MEDIA
Click image to the left for more content.
The Modern Solar System
Today, we know that our solar system is just one tiny part of the universe as a whole. Neither Earth nor the Sun are
at the center of the universe. However, the heliocentric model accurately describes the solar system. In our modern
view of the solar system, the Sun is at the center, with the planets moving in elliptical orbits around the Sun. The
planets do not emit their own light, but instead reflect light from the Sun.
FIGURE 1.4
Relative sizes of the Sun, planets and
dwarf planets.
The relative sizes are
correct and their position relative to each
other is correct, but the relative distances
are not correct.
The Size and Shape of Orbits
Figure 1.5 shows the relative sizes of the orbits of the planets, asteroid belt, and Kuiper belt. In general, the farther
away from the Sun, the greater the distance from one planet’s orbit to the next. The orbits of the planets are not
circular but slightly elliptical with the Sun located at one of the foci (Figure 1.6).
While studying the solar system, Johannes Kepler discovered the relationship between the time it takes a planet to
make one complete orbit around the Sun, its "orbital period," and the distance from the Sun to the planet. If the
orbital period of a planet is known, then it is possible to determine the planet’s distance from the Sun. This is how
astronomers without modern telescopes could determine the distances to other planets within the solar system.
Distances in the solar system are often measured in astronomical units (AU). One astronomical unit is defined as
the distance from Earth to the Sun. 1 AU equals about 150 million km, or 93 million mi. Table 3.2 shows the
distances to the planets (the average radius of orbits) in AU. The table also shows how long it takes each planet to
spin on its axis (the length of a day) and how long it takes each planet to complete an orbit (the length of a year); in
particular, notice how slowly Venus rotates relative to Earth.
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Chapter 1. Introduction to the Solar System
FIGURE 1.5
The relative sizes of the orbits of planets
in the solar system. The inner solar system and asteroid belt is on the upper left.
The upper right shows the outer planets
and the Kuiper belt.
FIGURE 1.6
The planets orbit the Sun in regular
paths.
TABLE 1.1: Distances to the Planets and Properties of Orbits Relative to Earth’s Orbit
Planet
Mercury
Venus
Earth
Mars
Jupiter
Average Distance from
Sun (AU)
0.39 AU
0.72
1.00
1.52
5.20
Length of Day (In Earth
Days)
56.84 days
243.02
1.00
1.03
0.41
Length of Year (In Earth
Years)
0.24 years
0.62
1.00
1.88
11.86
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TABLE 1.1: (continued)
Planet
Saturn
Uranus
Neptune
Average Distance from
Sun (AU)
9.54
19.22
30.06
Length of Day (In Earth
Days)
0.43
0.72
0.67
Length of Year (In Earth
Years)
29.46
84.01
164.8
The Role of Gravity
Isaac Newton was one of the first scientists to explore gravity. He understood that the Moon circles the Earth
because a force is pulling the Moon toward Earth’s center. Without that force, the Moon would continue moving in
a straight line off into space. Newton also came to understand that the same force that keeps the Moon in its orbit is
the same force that causes objects on Earth to fall to the ground.
Newton defined the Universal Law of Gravitation, which states that a force of attraction, called gravity, exists
between all objects in the universe (Figure 1.7). The strength of the gravitational force depends on how much mass
the objects have and how far apart they are from each other. The greater the objects’ mass, the greater the force of
attraction; in addition, the greater the distance between the objects, the smaller the force of attraction.
FIGURE 1.7
The force of gravity exists between all
objects in the universe; the strength of the
force depends on the mass of the objects
and the distance between them.
The distance between the Sun and each of its planets is very large, but the Sun and each of the planets are also very
large. Gravity keeps each planet orbiting the Sun because the star and its planets are very large objects. The force of
gravity also holds moons in orbit around planets.
Formation of the Solar System
There are two additional key features of the solar system:
1. All the planets lie in nearly the same plane, or flat disk like region.
2. All the planets orbit in the same direction around the Sun.
These two features are clues to how the solar system formed.
A Giant Nebula
The most widely accepted explanation of how the solar system formed is called the nebular hypothesis. According
to this hypothesis, the Sun and the planets of our solar system formed about 4.6 billion years ago from the collapse
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Chapter 1. Introduction to the Solar System
of a giant cloud of gas and dust, called a nebula.
The nebula was drawn together by gravity, which released gravitational potential energy. As small particles of dust
and gas smashed together to create larger ones, they released kinetic energy. As the nebula collapsed, the gravity
at the center increased and the cloud started to spin because of its angular momentum. As it collapsed further, the
spinning got faster, much as an ice skater spins faster when he pulls his arms to his sides during a spin.
Much of the cloud’s mass migrated to its center but the rest of the material flattened out in an enormous disk, as
shown in Figure 1.8. The disk contained hydrogen and helium, along with heavier elements and even simple organic
molecules.
FIGURE 1.8
An artist’s painting of a protoplanetary
disk.
Formation of the Sun and Planets
As gravity pulled matter into the center of the disk, the density and pressure at the center became intense. When the
pressure in the center of the disk was high enough, nuclear fusion began. A star was born—the Sun. The burning
star stopped the disk from collapsing further.
Meanwhile, the outer parts of the disk were cooling off. Matter condensed from the cloud and small pieces of
dust started clumping together. These clumps collided and combined with other clumps. Larger clumps, called
planetesimals, attracted smaller clumps with their gravity. Gravity at the center of the disk attracted heavier
particles, such as rock and metal and lighter particles remained further out in the disk. Eventually, the planetesimals
formed protoplanets, which grew to become the planets and moons that we find in our solar system today.
Because of the gravitational sorting of material, the inner planets — Mercury, Venus, Earth, and Mars — formed
from dense rock and metal. The outer planets — Jupiter, Saturn, Uranus and Neptune — condensed farther from
the Sun from lighter materials such as hydrogen, helium, water, ammonia, and methane. Out by Jupiter and beyond,
where it’s very cold, these materials form solid particles.
The nebular hypothesis was designed to explain some of the basic features of the solar system:
•
•
•
•
•
The orbits of the planets lie in nearly the same plane with the Sun at the center
The planets revolve in the same direction
The planets mostly rotate in the same direction
The axes of rotation of the planets are mostly nearly perpendicular to the orbital plane
The oldest moon rocks are 4.5 billion years
This video, from the ESA, discusses the Sun, planets, and other bodies in the Solar System and how they formed
(1a, 1d). The first part of the video explores the evolution of our view of the solar system starting with the early
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Greeks who reasoned that since some points of light - which they called planets - moved faster than the stars, they
must be closer: http://www.youtube.com/watch?v=-NxfBOhQ1CY&feature=player_profilepage (8:34).
MEDIA
Click image to the left for more content.
Lesson Summary
• The solar system is the Sun and all the objects that are bound to the Sun by gravity.
• The solar system has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Ceres,
Makemake, Pluto and Eris are dwarf planets.
• The ancient Greeks and people for centuries afterwards believed in a geocentric model of the universe, with
Earth at the center and everything else orbiting our planet.
• Copernicus, Kepler, and Galileo promoted a heliocentric model of the universe, with the Sun at the center and
Earth and the other planets orbiting the Sun.
• Gravity holds planets in elliptical orbits around the Sun.
• The nebular hypothesis describes how the solar system formed from a giant cloud of gas and dust about 4.6
billion years ago.
Review Questions
1. What does geocentric mean?
2. Describe the difference between the geocentric model and heliocentric model of the universe and who is credited
for them.
3. Provide two pieces of evidence Galileo Galilei used to support Copernicus’s model.
4. What keeps planets and moons in their orbits? How does it do this?
5. How old is the solar system? How old is Earth?
6. Use the nebular hypothesis to explain why the planets all orbit the Sun in the same direction.
Further Reading / Supplemental Links
• More information about the solar system from NASA: http://sse.jpl.nasa.gov/planets/index.cfm
• Lots of information about the solar system from the BBC: BBC Explore the solar system http://www.bbc.c
o.uk/solarsystem/
• Information about solar system objects: http://www.solarviews.com/eng/homepage.htm
• A multimedia tour of the solar system: http://www.nineplanets.org/
• Windows to the Universe: http://www.windows.ucar.edu/tour/link=/our_solar_system/formation.html
• Space news: http://www.space.com/
Discussion Questions:
• Would you expect all the planets in the solar system to be made of similar materials? Why or why not?
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Chapter 1. Introduction to the Solar System
Vocabulary
•
– Astronomical Unit - used to measure distance between objects in the solar system; one astronomical
unit (AU) is defined as the distance from Earth to the Sun
– Fusion - The combing of nuclei of lighter elements to form nuclei of heavier elements, such as hydrogen hydrogen nuclei fusing to form helium nuclei; also called nuclear fusion
– Geocentric model - "Earth-centered" model of the Universe
– Gravity - Force of attraction between any two objects
– Heliocentric model - "Sun-centered" model of the Universe
– Nebula - A cloud of gas and dust in space, in which stars form
– Nebular hypothesis - the widely accepted explanation that the solar system formed from the collapse of
a nebula
– Orbit - The path an object in space follows as it revolves around another object, such as Earth around
the sun or a satellte around Earth
– Planetismals - Left over matter that clumped together to eventually form planets and moons
– Solar system - Made up of eight planets and other objects that orbit the Sun
References
1. From Edward Grant, "Celestial Orbs in the Latin Middle Ages". http://en.wikipedia.org/wiki/File:Ptolemaics
ystem-small.png. Public Domain
2. Fastfission. http://en.wikipedia.org/wiki/File:Ptolemaic_elements.svg. Public Domain
3. Nichalp. http://en.wikipedia.org/wiki/File:Phases-of-Venus.svg. Public Domain
4. Courtesy of NASA. http://en.wikipedia.org/wiki/File:UpdatedPlanets2006.jpg. Public Domain
5. Courtesy of NASA/JPL-Caltech/R. Hurt. http://en.wikipedia.org/wiki/File:Oort_cloud_Sedna_orbit.jpg. Public Domain
6. Courtesy of NASA Jet Propulsion Laboratory (NASA-JPL). http://nix.larc.nasa.gov/info;jsessionid=c21rl6i
dgm4cc?id=PIA11800&orgid=10.jpg. Public Domain
7. Xzapro4. http://commons.wikimedia.org/wiki/File:Gravitation.png. Public Domain
8. Courtesy of NASA. http://en.wikipedia.org/wiki/File:Protoplanetary-disk.jpg. Public Domain
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C HAPTER
2
Kepler’s Laws of Planetary
Motion
• Describe Kepler’s three laws.
• Calculate satellite periods given average radius and vice versa.
This is a drawing of our solar system and the elliptical orbits of the planets in the system. If you count the planets you
will recognize that this drawing was made before Pluto was demoted from planet to “dwarf planet.” According to
the astronomical definitions, a dwarf planet is NOT a planet. The lines illustrate the orbit of each planet. Eccentricity
refers to the shape of the orbit an object travels.
Kepler’s Laws
Fifty years before Newton proposed his three laws of motion and his law of universal gravitation, Johannes Kepler
(1571 – 1630) published a number of astronomical papers in which we can find a detailed description of the motions
of the planets. Included in those papers were findings that we now refer to as Kepler’s laws of planetary motion.
These are summarized below.
Kepler’s First Law: The path of each planet around the sun is an ellipse with the sun at one focus.
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Chapter 2. Kepler’s Laws of Planetary Motion
The planet moves with greater speed in its orbit when it is near the sun and slower when it is far away. The planet
is at apogee when it is fartherest from the sun and perigee when it is closest. The reason that a planet moves with
greater speed near the sun is due to the attractive force of gravity.
Kepler’s Second Law: Each planet moves such that an imaginary line drawn from the sun to the planet sweeps out
equal areas in equal periods of time.
(The areas swept out are equal and the times required to sweep out the areas are also equal.)
Kepler’s Third Law: The ratio of the squares of the periods of any two planets revolving around the sun is equal to
the ratio of the cubes of their average distance from the sun.
T1
T2
2
=
r1
r2
3
Example Problem: The planet Venus has a mean distance from the sun of 108.2 ×106 km and a period of 0.615
years. The planet Mars has an average mean distance from the sun of 227.9 ×106 km and a period of 1.88 years. Do
these planets follow Kepler’s third law?
Solution: The average mean distance of Venus divided by the average mean distance of Mars = 0.475. The period
of Venus divided by the period of Mars = 0.327.
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The square of the period ratio is 0.107 and the cube of the mean distance ratio is 0.107. It is clear that these two
planets follow Kepler’s third law.
Summary
• Kepler’s First Law: The path of each planet around the sun is an ellipse with the sun at one focus.
• Kepler’s Second Law: Each planet moves such that an imaginary line drawn from the sun to the planet sweeps
out equal areas in equal periods of time.
• Kepler’s Third Law: The ratio of the squares of the periods of any two planets revolving around the sun is
equal to the ratio of the cubes of their average distance from the sun.
Practice
The following is a video about Kepler’s three laws. Use this resource to answer the questions that follow.
http://www.youtube.com/watch?v=6TGCPXhMLtU
MEDIA
Click image to the left for more content.
1. What is the shape of a planetary orbit?
2. How are the areas swept out by the line able to be equal, when the line is much longer at sometimes than
others?
Review
1. The average mean distance of the earth from the sun is 149.6 ×106 km and the period of the earth is 1.0
year. The average mean distance of Saturn from the sun is 1427 ×106 km. Using Kepler’s third law, calculate
the period of Saturn.
2. Which of the following is one of Kepler’s Laws of Planetary Motion?
(a)
(b)
(c)
(d)
Planets move in elliptical orbits with the Sun at one focus.
Gravitational force between two objects decreases as the distance squared.
An object in motion remains in motion.
Inner planets orbit in a different direction that outer ones.
3. If a planet’s orbital speed is 20 km/s when it’s at its average distance from the sun, which is most likely orbital
speed when it is nearest the sun?
(a)
(b)
(c)
(d)
10 km/s
15 km/s
20 km/s
25 km/s
4. A hypothetical planet orbits the Sun at a distance of 3.0 astronomical units. What is its orbital period?
(a) 21.8 years
(b) 3.0 years
(c) 5.2 years
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Chapter 2. Kepler’s Laws of Planetary Motion
(d) 9.0 years
Vocabulary
Apogee - A point in orbit farthest from the sun
Eccentricity - The shape of an orbit; when two foci are closer together, the eccentricity is smaller
Period - A unit of time
Perigee - A point in orbit closest to the sun
• Kepler’s First Law: The path of each planet around the sun is an ellipse with the sun at one focus.
• Kepler’s Second Law: Each planet moves such that an imaginary line drawn from the sun to the planet
sweeps out equal areas in equal periods of time.
• Kepler’s Third Law: The ratio of the squares of the periods of any two planets revolving around the sun is
equal to the ratio of the cubes of their average distance from the sun.
References
1. CK-12 Foundation. . Used under license from Shutterstock.com
2. CK-12 Foundation. . CC-BY-NC-SA 3.0
3. CK-12 Foundation. . CCSA
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C HAPTER
3
Planets of the Solar System
• Define astronomical unit.
• Identify the solar system’s eight planets and their characteristics, including size and length of orbit relative to
Earth.
Who is in the Sun’s family?
The family includes the Sun, its eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune),
and the five known dwarf planets (Ceres, Pluto, Makemake, Haumea, and Eris). In the image above, relative sizes
of the Sun, planets, and dwarf planets and their positions relative to each other are correct, but the relative distances
are not.
Eight Planets
Since the time of Copernicus, Kepler, and Galileo, we have learned a lot more about our solar system. Astronomers
have discovered two more planets (Uranus and Neptune), five dwarf planets (Ceres, Pluto, Makemake, Haumea, and
Eris), more than 150 moons, and many, many asteroids and other small objects.
Although the Sun is just an average star compared to other stars, it is by far the largest object in the solar system.
The Sun is more than 500 times the mass of everything else in the solar system combined! Table 3.1 gives data on
the sizes of the Sun and planets relative to Earth.
TABLE 3.1: Sizes of Solar System Objects Relative to Earth
Object
Mass (Relative to Earth)
Sun
Mercury
Venus
Earth
Mars
Jupiter
333,000 Earth’s mass
0.06 Earth’s mass
0.82 Earth’s mass
1.00 Earth’s mass
0.11 Earth’s mass
317.8 Earth’s mass
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Diameter of Planet (Relative to
Earth)
109.2 Earth’s diameter
0.39 Earth’s diameter
0.95 Earth’s diameter
1.00 Earth’s diameter
0.53 Earth’s diameter
11.21 Earth’s diameter
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Chapter 3. Planets of the Solar System
TABLE 3.1: (continued)
Object
Mass (Relative to Earth)
Saturn
Uranus
Neptune
95.2 Earth’s mass
14.6 Earth’s mass
17.2 Earth’s mass
Diameter of Planet (Relative to
Earth)
9.41 Earth’s diameter
3.98 Earth’s diameter
3.81 Earth’s diameter
Orbits and Rotations
Distances in the solar system are often measured in astronomical units (AU). One astronomical unit is defined as
the distance from Earth to the Sun. 1 AU equals about 150 million km, or 93 million miles. Table 3.2 shows the
distances to the planets (the average radius of orbits) in AU. The table also shows how long it takes each planet to
spin on its axis (the length of a day) and how long it takes each planet to complete an orbit (the length of a year); in
particular, notice how slowly Venus rotates relative to Earth.
TABLE 3.2: Distances to the Planets and Properties of Orbits Relative to Earth’s Orbit
Planet
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Average Distance from
Sun (AU)
0.39 AU
0.72
1.00
1.52
5.20
9.54
19.22
30.06
Length of Day (In Earth
Days)
56.84 days
243.02
1.00
1.03
0.41
0.43
0.72
0.67
Length of Year (In Earth
Years)
0.24 years
0.62
1.00
1.88
11.86
29.46
84.01
164.8
Here is a website that illustrates both the sizes of the planets, and the distance between them: http://www.scalesola
rsystem.66ghz.com/#sun.
How old are you on Earth? How old would you be if you lived on Jupiter? How many days is it until your birthday
on Earth? How many days until your birthday if you lived on Saturn?
Scaling the solar system creates a scale to measure all objects in solar system (1i - IE Stand.): http://www.youtube.c
om/watch?feature=player_profilepage&v=-6szEDHMxP4 (4:44).
Summary
• The planets of the solar system, with increasing distance from the Sun, are Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, and Neptune. The five known dwarf planets are Ceres, Pluto, Makemake, Haumea,
and Eris.
• Solar system distances are measured as multiples of the distance between Earth and Sun, which is defined as
one astronomical unit (AU).
• All planets and dwarf planets orbit the Sun and rotate on their axes.
Making Connections
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MEDIA
Click image to the left for more content.
Practice
Use this resource to answer the questions that follow.
http://www.youtube.com/watch?v=z_RAEESmsrs
MEDIA
Click image to the left for more content.
1. How old is our solar system?
2. How did the planets form?
3. What are the two main regions of the solar system?
4. List the inner planets.
5. List the outer planets.
6. What are the requirements to be a planet?
7. Why was Pluto demoted?
8. What is the Kuiper Belt?
9. What are found in the Kuiper Belt?
10. What is the scattered disk?
11. What is the heliosphere?
Review
1. Why does the number of dwarf planets recognized by astronomers in the solar system periodically increase?
2. What is the order of planets and dwarf planets by distance from the Sun?
3. What is an astronomical unit? Why is this unit used to measure distances in the solar system?
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C HAPTER
Chapter 4. Inner versus Outer Planets
4
Inner versus Outer Planets
• Compare and contrast the inner and outer planets.
What’s better than a field trip?
All of the inner planets are orbited by man-made satellites. Jupiter and Saturn have man-made satellites too. We
can see what the planets look like from the photos they take. The satellites also carry instruments that collect a lot
of important data. This is better than taking a field trip to such hostile places. You can learn a lot while sitting your
own, very comfortable planet!
The Inner Planets
The four planets closest to the Sun - Mercury, Venus, Earth, and Mars - are the inner planets or terrestrial planets
(see Figure 4.1). They are similar to Earth. All are solid, dense, and rocky. None of the inner planets has rings.
Compared to the outer planets, the inner planets are small. They have shorter orbits around the Sun and they spin
more slowly. Venus spins backwards and spins the slowest of all the planets.
All of the inner planets were geologically active at one time. They are all made of cooled igneous rock with inner
iron cores. Earth has one big, round moon, while Mars has two very small, irregular moons. Mercury and Venus do
not have moons.
The Outer Planets
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. Figure 4.2 shows the relative sizes of the outer planets and the Sun.
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FIGURE 4.1
This composite shows the relative sizes
of the four inner planets.
From left to
right, they are Mercury, Venus, Earth, and
Mars.
FIGURE 4.2
This image shows the four outer planets
and the Sun, with sizes to scale. From
left to right, the outer planets are Jupiter,
Saturn, Uranus, and Neptune.
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 was hydrogen and helium. The inner planets lost these very light gases.
Their gravity was too low to keep them and they floated away into space. The Sun and the outer planets had enough
gravity to keep the hydrogen and helium.
All of the outer planets have numerous moons. They also have planetary rings made of dust and other small
particles. Only the rings of Saturn can be easily seen from Earth.
Summary
• The four inner planets have slower orbits, slower spin, no rings, and they are made of rock and metal.
• The four outer planets have faster orbits and spins, a composition of gases and liquids, numerous moons, and
rings.
• The outer planets are made of hydrogen and helium, so they are called gas giants.
Practice
Use these resources to answer the questions that follow.
http://kids.nineplanets.org/intro.htm
1. Which planets make up the inner solar system?
2. Which planets make up the outer solar system?
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Chapter 4. Inner versus Outer Planets
3. What separates the inner and outer solar system?
http://kids.nineplanets.org/general2.htm
4. Describe the composition of each group of planets.
5. Describe the size of each each group of planets.
Review
1. Describe the characteristics of the four inner planets.
2. Describe the characteristics of the four outer planets.
3. Why are the inner and outer planets so different from each other?
Discussion Question:
1. Given that the outer four planets are made of gases instead of dust and rocks, why is it improbable that life
exists there? What conditions are necessary for materials essential for life to form?
Vocabulary
• inner planets or terrestrial planets: The four solid, dense, rocky planets that are inside the asteroid belt:
Mercury, Venus, Earth, and Mars.
• outer planets or gas giants: The four large outer planets composed of the gases hydrogen and helium that are
beyond the asteroid belt in our solar system: Jupiter, Saturn, Uranus, and Neptune.
• planetary rings: Rings of dust and rock encircling a planet in a thin plane.
References
1. Brian0918, using images courtesy of NASA. . Public Domain
2. Urhixidur, using images from NSSDC. . Public Domain
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C HAPTER
5
Asteroids
• Describe asteroids and the asteroid belt.
• Explain what affects asteroids may have on Earth and other planets.
Is this rock an asteroid?
This rock is somewhat round. It is called asteroid Vesta but it may actually be a protoplanet: too round to be an
asteroid, not round enough to be a dwarf planet. This image was taken of Vesta as a spacecraft orbited it for a year.
Scientists are interested in asteroids for many reasons. They are remnants from the early solar system. Asteroids
sometimes cause mass extinctions.
Asteroids
After the Sun and planets formed, there was some material left over. These small chunks didn’t get close enough to
a large body to be pulled in by its gravity. They now inhabit the solar system as asteroids and comets.
Asteroids are very small, irregularly shaped, rocky bodies. Asteroids orbit the Sun, but they are more like giant
rocks than planets. Since they are small, they have features that are different from planets. Asteroids do not have
enough gravity to become round. They are too small to have an atmosphere. With no internal heat, they are not
geologically active.
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Chapter 5. Asteroids
An asteroid can only change due to a collision. A collision may cause the asteroid to break up. It may create craters
on the asteroid’s surface. An asteroid may strike a planet if it comes near enough to be pulled in by its gravity.
Figure 5.1 shows a typical asteroid.
FIGURE 5.1
Asteroid Ida with its tiny moon Dactyl. The
asteroid’s mean radius is 15.7 km.
The Asteroid Belt
Hundreds of thousands of asteroids have been found in our solar system. They are still being discovered at a rate
of about 5,000 new asteroids per month! The majority are located in between the orbits of Mars and Jupiter. This
region is called the asteroid belt, as shown in Figure 5.2. There are many thousands of asteroids in the asteroid belt.
Still, their total mass adds up to only about 4 percent of Earth’s moon.
Asteroids formed at the same time as the rest of the solar system. Although there are many in the asteroid belt, they
were never were able to form into a planet. Jupiter’s gravity kept them apart.
Near-Earth Asteroids
Near-Earth asteroids have orbits that cross Earth’s orbit. This means that they can collide with Earth. There are
over 4,500 known near-Earth asteroids. Small asteroids do sometimes collide with Earth. An asteroid about 5–10 m
in diameter hits about once per year. Five hundred to a thousand of the known near-Earth asteroids are much bigger.
They are over 1 kilometer in diameter. When large asteroids hit Earth in the past, many organisms died. At times,
many species became extinct in a mass extinction. Astronomers keep looking for near-Earth asteroids. They hope
to predict a possible collision early so they can to try to stop it.
Asteroid Missions
Scientists are very interested in asteroids. Most are composed of material that has not changed since early in the
solar system. Scientists can learn a lot from them about how the solar system formed. Asteroids may be important
for space travel. They could be mined for rare minerals or for construction projects in space.
Scientists have sent spacecraft to study asteroids. In 1997, the NEAR Shoemaker probe orbited the asteroid 433
Eros. The craft finally landed on its surface in 2001. The Japanese Hayabusa probe returned to Earth with samples
of a small near-earth asteroid in 2010. The U.S. Dawn mission spent a year in orbit around Vesta ending in August
2012.
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FIGURE 5.2
The asteroid belt is between Mars and
Jupiter.
KQED: Asteroid Hunters
Thousands of objects, including comets and asteroids, are zooming around our solar system; some could be on a
collision course with Earth. QUEST explores how these Near Earth Objects are being tracked and what scientists
are saying should be done to prevent a deadly impact. Learn more at: http://science.kqed.org/quest/video/asteroid
-hunters/
MEDIA
Click image to the left for more content.
Summary
• Asteroids are small rocky bodies that orbit the Sun.
• Most asteroids reside in the asteroid belt, between Mars and Jupiter.
• Near-earth asteroids are the ones most likely to strike Earth.
Practice
Use these resources to answer the questions that follow.
Asteroids at http://www.youtube.com/watch?v=LXQt4eCCiU4 (1:32)
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Chapter 5. Asteroids
MEDIA
Click image to the left for more content.
1.
2.
3.
4.
5.
What percentage of asteroids have been identified?
Why is this identification important?
How many asteroids are there according to the new model?
How many asteroids still need to be found?
Why is the infrared telescope more efficient?
Review
1. What is the reason there is a belt of asteroids between Mars and Jupiter?
2. Why do scientists look for asteroids that might strike our planet?
Vocabulary
• asteroid: Rocky objects larger than a few hundred meters that orbit the Sun.
• asteroid belt: Region between the orbits of Mars and Jupiter where many asteroids are found.
• near-Earth asteroid: Asteroids that orbit relatively close to Earth.
References
1. Courtesy of NASA/JPL, modified by Chzz. . Public Domain
2. Mizusumashi and Autiwa. . Public Domain
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C HAPTER
6
Comets
• Describe the characteristics of comets.
• Explain where comets come from.
Is this comet going to hit?
Astronauts on board the International Space Station witness incredible things. This photo of Comet Lovejoy was
taken in December 2011. Although it looks like its going to strike Earth, it was not even visible from our planet by
the naked eye. The comet is now traveling far out into space.
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Chapter 6. Comets
Comets
Comets are small, icy objects that have very elliptical orbits around the Sun. Their orbits carry them from the outer
solar system to the inner solar system, close to the Sun. Early in Earth’s history, comets may have brought water and
other substances to Earth during collisions.
Comet tails form the outer layers of ice melt and evaporate as the comet flies close to the Sun. The ice from the
comet vaporizes and forms a glowing coma. Particles streaming from the Sun push this gas and dust into a long tail.
The tail always points away from the Sun. The coma is bright because it reflects light from the Sun (Figure 6.1).
Comets appear for only a short time when they are near the Sun. They seem to disappear as they move back to the
outer solar system.
FIGURE 6.1
Comet Hale-Bopp, also called the Great
Comet of 1997. The comet has two visible tails: a bright, curved dust tail and
a fainter, straight tail of ions (charged
atoms) pointing directly away from the
Sun.
The time between one appearance of a comet and the next is called the comet’s period. Halley’s comet, with a period
of 75 years, will next be seen in 2061. The first mention of the comet in historical records may go back as much as
two millennia.
Where Comets Come From
Short-period comets have periods of about 200 years or less. These comets come from a region beyond the orbit
of Neptune called the Kuiper belt (pronounced “KI-per”). The Kuiper belt also contains asteroids and at least two
dwarf planets.
Comets with periods as long as thousands or even millions of years come from a very distant region of the solar
system. This region is called the Oort cloud. It is about 50,000 — 100,000 AU from the Sun (50,000 – 100,000
times the distance from the Sun to Earth).
Summary
• Comets are icy objects that have very elliptical orbits around the Sun.
• Comet tails form as ice vaporizes and glows in the Sun’s light.
• Short-period comets come from the Kuiper belt beyond Neptune, and long-period comets come from the Oort
cloud far out away from the Sun.
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Practice
Use these resources to answer the questions that follow.
What is a Comet? at http://www.videojug.com/film/what-is-a-comet (4:28)
MEDIA
Click image to the left for more content.
1.
2.
3.
4.
5.
6.
7.
8.
What are comets called?
What are comets?
Where are most comets found?
Explain the composition of comets.
What happens to comets as they approach the sun?
When was Halley’s comet last seen?
How are comets named?
Which comet collided with Jupiter?
Review
1. Why do comets only have tails when they are near the Sun?
2. Where is the Kuiper belt and what is found in it?
3. Why do most comets appear in regular predictable time periods?
Vocabulary
• comet: A small, icy, dusty object with a bright tail in orbit around the Sun.
• Kuiper belt: A region beyond the orbit of Neptune that contains millions of frozen objects.
References
1. Andy Roberts. . CC-BY 2.0
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C HAPTER
Chapter 7. Meteors
7
Meteors
• Define and describe meteors, meteoroids, and meteorites.
• Explain what makes a meteor shower.
Is this a comet?
When a meteor shoots through the atmosphere it burns and glows. When we look up and see one, we call it a
shooting star. In this image, the meteor looks like a comet. No need to go anywhere to see this. Just try to catch one
of these meteor showers: the Perseids in mid-August or the Geminids in mid-December. Look for information on
the dates and whether the moon will not be lighting up the sky.
Meteors
If you look at the sky on a dark night, you may see a meteor, like in Figure 7.1. A meteor forms a streak of light
across the sky. People call them shooting stars because that’s what they look like. But meteors are not stars at all.
The light you see comes from a small piece of matter burning up as it flies through Earth’s atmosphere.
Meteoroids
Before these small pieces of matter enter Earth’s atmosphere, they are called meteoroids. Meteoroids are as large as
boulders or as small as tiny sand grains. Larger objects are called asteroids; smaller objects are interplanetary dust.
Meteoroids sometimes cluster together in long trails. They are the debris left behind by comets. When Earth passes
through a comet trail, there is a meteor shower. During a meteor shower, there are many more meteors than normal
for a night or two.
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FIGURE 7.1
Meteors burning up as they fall through
Earth’s atmosphere.
http://www.youtube.com/watch?v=9UwJFWCLzS4
MEDIA
Click image to the left for more content.
Meteorites
A meteoroid is dragged towards Earth by gravity and enters the atmosphere. Friction with the atmosphere heats
the object quickly, so it starts to vaporize. As it flies through the atmosphere, it leaves a trail of glowing gases.
The object is now a meteor. Most meteors vaporize in the atmosphere. They never reach Earth’s surface. Large
meteoroids may not burn up entirely in the atmosphere. A small core may remain and hit the Earth’s surface. This
is called a meteorite.
Meteorites provide clues about our solar system. Many were formed in the early solar system (Figure below). Some
are from asteroids that have split apart. A few are rocks from nearby bodies like Mars. For this to happen, an asteroid
smashed into Mars and sent up debris. A bit of the debris entered Earth’s atmosphere as a meteor.
Summary
• A meteor that strikes Earth’s surface is a meteorite.
• Many meteorites are remnants of the earliest material that formed in the solar system.
• Shooting stars are meteors that burn up in Earth’s atmosphere.
Practice
Use these resources to answer the questions that follow.
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Chapter 7. Meteors
What is A Meteor at http://www.videojug.com/film/what-is-a-meteor (3:27)
MEDIA
Click image to the left for more content.
1.
2.
3.
4.
5.
6.
7.
8.
What are meteoroids?
What are meteors?
What happens to most meteors in the atmosphere?
What is a fireball?
What are meteorites?
Explain the difference between sporadic meteors and meteor showers.
What causes meteor showers?
When does the Perseid meteor shower occur?
Review
1. Compare and contrast meteoroid, meteor, and meteorite.
2. What are meteorites important to scientists?
3. Why are meteors known as shooting stars?
Vocabulary
•
•
•
•
meteor: Material from outer space that burns up as it enters Earth’s atmosphere.
meteor shower: A time of frequent meteors.
meteorite: Meteors that strike Earth.
meteoroid: A small rock in interplanetary space that has not yet entered Earth’s atmosphere.
References
1. Jared Tennant. . CC-BY 2.0
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