Download PART 1 OBJECTS IN THE SOLAR SYSTEM 4.1 INTRODUCTION

PART 1—OBJECTS IN THE SOLAR SYSTEM
4.1 INTRODUCTION
Besides the Sun, the central object of our solar system, which is a star and will be discussed in more detail in Chapter 11, there are basically three types of objects in our solar system: planets, moons, and debris. Solar system debris is the collective term used
for objects that have not become part of a planet or a moon: asteroids, comets, and meteors. These objects will be discussed in detail in Chapter 6. Moons are objects that orbit planets and there are two types of planet.
4.2 PLANET TYPES
One of the best ways to study planets is to investigate their properties, and based on
these properties compare the objects to one another. This is known as comparative
planetology. The properties of planets are the quantities that we can measure such as
physical properties like size, mass, and density or their orbital properties like distance
from the Sun, orbital or revolution period, and rotational period. Table 4.1 lists the values of these and other properties for known planets and several other objects in our solar system.
Figure 4.1 shows bar graphs or histograms comparing the radii (the radius is the
distance from the center of a planet to its edge) or size of each object listed in Table 4.1,
their mass (of how much matter each is composed), and their density. Density is a combination of mass and size. It is a measure of how much mass per unit volume there is in
something. Solids like rocks and metals are objects of high density, gases like air are of
low density; and liquids like water are in between.
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Table 4.1 䡲 Planetary Data
Radius
Earth=1
Mass
Earth=1
Density
Water=1
Orbital Radius
(AU)
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Eris
0.382
0.949
1
0.533
11.19
9.46
3.98
3.81
0.181
0.183
0.055
0.815
1
0.107
317.9
95.18
14.54
17.13
0.0022
0.0028
5.43
5.25
5.52
3.93
1.33
0.7
1.32
1.64
2.05
2.52
0.387
0.723
1
1.524
5.203
9.539
19.19
30.06
39.48
67.67
Orbital
Period
(Years)
Rotation Period
Earth=1
0.2409
0.6152
1
1.881
11.86
29.42
84.01
164.8
248
561
58.6
243
0.9973
1.026
0.41
0.44
0.72
0.67
6.39
15.8
Number of
Moons
0
0
1
2
67
62
27
13
5
1
Planetary Radii
12
8
6
Radius
4
2
0
1
2
3
4
5
6
7
8
9
10
Object
FIGURE 4.1 Bar graphs comparing planetary radii, masses, and densities.
Created with Graphical Analysis 3 by Vernier Software
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Radius (Earth=1)
1
2
3
4
5
6
7
8
9
10
Object
Name
Chapter 4
Solar System Overview
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Planetary Masses
350
300
200
Mass
150
100
50
0
1
2
3
4
5
6
7
8
9
10
Object
Created with Graphical Analysis 3 by Vernier Software
Mass (Earth=1)
250
Planetary Densities
6
4
3
Density
2
1
0
1
2
3
4
5
6
7
Object
FIGURE 4.1 (cont’d)
8
9
10
Created with Graphical Analysis 3 by Vernier Software
Densities (water=1)
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Part 2 The Solar System
Examination of the radii bar graph shows that there are different-size objects (the
numbers on the bar graphs match the numbers in Table 4.1). Objects 5 and 6, Jupiter
and Saturn are very large compared to objects 7 and 8, Uranus and Neptune that are
more medium in size while Earth and all the others are very small by comparison. The
mass bar graph shows that Jupiter is by far the most massive, then Saturn with Uranus
and Neptune the only others that even register on the graph. Perhaps at this point
Jupiter, Saturn, Uranus, and Neptune could be considered a group of large and massive
planets while Earth and all the others could be called small and less massive planets.
Courtesy of NASA.
FIGURE 4.2 Images of our solar system’s planets.
The density graph shows something different. Now Mercury, Venus, Earth, and
Mars have large values or high densities while the larger, massive planets and Pluto and
Eris all have lower densities. The higher densities are because Mercury, Venus, Earth,
and Mars are all made mostly of rocks and metals; while the lower-density objects,
Jupiter, Saturn, Uranus, and Neptune, are made mostly of gases and liquids. Pluto and
Eris, being so far from the Sun, are composed partly of ice.
At this point it is possible to distinguish between two types of planets. Mercury, Venus, Earth, and Mars are all small, of lower mass and higher density while Jupiter, Saturn, Uranus, and Neptune are all the opposite—large, higher mass, and lower density.
Collectively the planets that are grouped with Earth are called Earth-like or terrestrial
planets; those grouped with Jupiter are called Jupiter-like or Jovian planets. Notice that
Pluto and Eris do not fit with either category.
Courtesy of NASA.
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FIGURE 4.3 The Planets in order of their distance from the Sun.
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Looking at other data from Table 4.1, all the terrestrial planets are closer to the Sun
and therefore have faster orbital periods while the Jovian planets are the opposite, farther from the Sun with longer orbital periods. Rotational periods do not seem to fit the
categories as well. All of the Jovian planets have rotation periods similar to each other
and all the terrestrial planets have longer periods, but as can be seen from Table 4.1, Venus and Mercury have especially long periods. Again, note Pluto and Eris not fitting in
either category. They are far from the Sun like the Jovian planets but have longer rotation periods like the terrestrial planets. Also, due to their large mass and therefore
greater gravitational pull, the Jovian planets all have many moons and rings. Rings are
tremendous numbers of smaller particles all in similar orbits around a Jovian planet
causing the appearance of a ring around a planet when viewed from a distance. Ring
systems will be discussed in more detail in Chapter 8. Table 4.2 is a comparison of the
properties of the terrestrial and Jovian planets.
Table 4.2 䡲 Properties of Terrestrial and Jovian Planets
Members
Size
Mass
Density
Composition
Distance
Rotation
Moons
Rings
Terrestrial Planets
Jovian Planets
Mercury, Venus, Earth, Mars
Smaller
Low mass
High
Rock and Metal
Close to Sun
Slower
Few or none
No
Jupiter, Saturn, Uranus, Neptune
Larger
Great mass
Low
Gas and Liquid
Far from Sun
Faster
Many
Yes
4.3 THE KUIPER BELT
As observed several times, Pluto and Eris do not fit into either of the major planet categories and could in fact be classified together as very small, low mass, icy-rocky objects
(thus their medium density) that are very far from the Sun. These are precisely the characteristics of the objects in what is known as the Kuiper belt.
First proposed by Gerard Kuiper in 1951, many small icy objects, which have also
been called “trans-Neptunian” objects and “ice dwarfs,” have now been observed beyond the orbit of Neptune. There are thousands of Kuiper belt objects known to exist
including several discovered more recently that rival the size of Pluto such as Eris that
may be as large or larger than Pluto.
In the summer of 2006, Pluto lost its status as one of the solar system’s planets. The
International Astronomical Union (IAU) is a group of astronomers from throughout the
world that meets every third year and makes such decisions. They reclassified Pluto,
along with Ceres, the largest member of the inner solar system asteroid belt located between Mars and Jupiter and several other objects large enough to be spherical, as dwarf
planets. The asteroid belt, Pluto, and other Kuiper belt objects will be discussed in more
detail in Chapter 6.
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CHAPTER 4 PART 1—TERMINOLOGY
Comparative planetology
Debris
Density
Dwarf planet
Kuiper belt
Mass
Moon
Planet
Jovian
Terrestrial
Radius
PART 2—THE FORMATION OF THE SOLAR SYSTEM
4.4 THE SOLAR NEBULA
Courtesy of NASA.
Our Sun was formed by a gravitational collapse within a gigantic cloud of mostly hydrogen gas and dust in the otherwise nearly empty interstellar space between the stars
in our galaxy. The leftover material surrounding the not yet shining protosun was called
the solar nebula. Eventually the protosun accumulated enough material from the nebula to become massive enough to put sufficient pressure on its core to raise the core
temperatures high enough for nuclear fusion to occur. This process provided the energy
necessary for the Sun to give off light and heat or to shine and thus become a star. The
processes of star formation and nuclear fusion will be discussed in more detail in Chapter 11.
FIGURE 4.4 Solar nebulae around protostars.
4.5 THE ROCK-METAL CONDENSATION LINE
The leftovers of the solar nebula were the material from which the planets of our solar
system would form. Initially, temperatures were so hot that most of the solar nebula remained gaseous but as the young Sun cooled, temperatures reached a point where solid
rocks and metals could begin to condense out of the nebula. Too close to the Sun the
temperatures never cooled enough for this to happen and no planets could form there.
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Solar System Overview
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However, beyond about 0.3 AU, known as the rock-metal condensation line, the solid
objects could condense and begin gravitationally pulling together. First they formed
small, rocky-metal objects called planetisimals and then larger protoplanets.
4.6 FORMATION OF THE TERRESTRIAL PLANETS
Within about 5 AU of the Sun temperatures remained high enough that no other materials could condense out of the nebula and due to the higher temperatures the gases
were very fast moving so the rocky-metal protoplanets were not massive enough to
gravitationally capture appreciable amounts of these gases. The protoplanets continued
to accumulate more of the rock and metal in their orbits and eventually became the
Earth-like or small, low mass, high-density (rocky-metal) terrestrial planets found close
to the Sun.
4.7 THE FROST LINE AND THE JOVIAN PLANETS
Beyond 5 AU from the Sun temperatures cooled enough for water, methane, and
ammonia to condense from the solar nebula and form layers of ice on the rocky-metal
objects. This is known as the frost line. The cooler temperatures in this region of the
nebula, out further from the Sun, slowed the motions of the gases—making them easier
for planets to catch. This factor and the now greater mass of these objects due to the ice
they collected allowed them to gravitationally collect large amounts of these gases and
grow to tremendous sizes and become much more massive than their cousins nearer to
the Sun. They became the Jupiter-like or large, massive, low-density (gas and liquid)
Jovian planets that are found farther from Sun.
The pressure from the large mass of gas above the icy layers likely warmed and
melted the ice leaving the basic structure of a Jovian planet as a terrestrial planet–size
rocky-metal core surrounded by a large liquid ocean below a huge, thick atmosphere of
mostly hydrogen and helium gas. The formation of the three-layered Jovian planets was
a three-step process while the terrestrial planets were basically formed in only one step,
the gravitational accumulation of rocks and metal inside the frost line but beyond the
rock/metal line.
4.8 THE LEFTOVERS—SOLAR SYSTEM DEBRIS
Some of the planetisimals and even protoplanets did not become part of a terrestrial or
Jovian planet. Some were gravitationally captured, mostly by the more massive Jovian
planets and became moons. Other small rocky objects of the inner solar system are now
called asteroids. Many of the asteroids are concentrated in a “belt” between the orbits of
Mars and Jupiter. The material in this belt was never able to pull together and form a
planet due to the gravitational influences of Jupiter. The two small rocky moons of Mars
were likely captured from this asteroid belt. Small rocky objects of the cold outer solar
system were covered by condensing ice and are known as comets. A large group of
comet-like objects lies beyond the orbit of Neptune. A few of Neptune’s moons may
have been captured from this population. The orbit of the most well known member of
the Kuiper belt, the dwarf planet Pluto, crosses Neptune’s orbit. Asteroids, comets, and
the Kuiper belt will be discussed in more detail in Chapter 6.
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During the formation of the solar system when there were more objects that had not
yet become parts of planetary systems many collisions occurred. A large object colliding
with Earth is thought to have been what formed Earth’s Moon. A collision is also
though to be the reason that the rotational axis of Uranus lies nearly in the plane of its
orbit rather than more upright relative to it like the other planets. When an object is
falling toward an impact with Earth and gives off a bright flash of light due to friction
with the gases in the atmosphere it is called a meteor. The holes the collisions leave are
called impact craters. There are many impact craters on the surfaces of the terrestrial
planets, their moons, and Jovian moons from collisions. Over time as collisions occur
the number of objects available for further collisions becomes less and less. Most impact craters like those we see on our Moon were formed long ago when the rate of impact cratering was higher; but there are still occasional large impacts like the Tunguska
event on Earth just over a century ago or the collision of a comet with Jupiter in 1994
or the Russian meteor impact of 2013. Impacts will also be discussed in Chapter 6.
CHAPTER 4 PART 2—TERMINOLOGY
Asteroid
Comet
Crater
Frost line
Impact
Meteor
Planetisimal
Protoplanet
Protosun
Rock-metal condensation
line
Solar nebula
Name
Date
CHAPTER 4 PART 1—REVIEW QUESTIONS
1. Write a definition of each type of solar system object: planet, moon, asteroid, comet, meteor.
2. Name the two types of planets found in our solar system.
3. List the planets that are members of each group.
4. List the properties that define each group of planets.
5. What one word describes how the properties of the planets in one group compare to the properties
of the planets in the other group?
6. Give an example of a solar system object that does not fit into either planet group. Explain why it
does not fit in.
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CHAPTER 4 PART 2—REVIEW QUESTIONS
7. What type of planets formed inside (closer to the Sun than) the rock-metal condensation line?
8. What type of planets formed beyond the rock-metal condensation line, but inside the frost line?
9. What type of planets formed beyond the frost line?
10. What is the single most important factor in determining which type of planet will form at a given location? What controls this factor?
11. Many smaller objects that condensed from the solar nebula are leftovers or debris that did not become part of a planet. Of what would small objects that formed closer to the Sun be made? What do
we call them? Of what would small objects that formed farther from the Sun largely be made? What
do we call them?
12. What do we call an object that is captured into the orbit of a larger object like a planet?
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Name
Date
CHAPTER 4 PART 1 TUTORIAL—COMPARATIVE PLANETOLOGY
1. Examine the data for the objects in Table 4.1 and the bar graphs in Figure 4.1 of this chapter.
2. Examine the bar graph comparing the radius (or size) of the objects. Which planets are large?
Which planets are small?
3. Examine the bar graph comparing the mass of the objects. Which objects are massive?
Which objects are lighter?
4. Examine the bar graph comparing the density of the objects; which are more dense?
Which are less dense?
What can density tell you about an object?
5. Based on the comparisons you have made, have any objects been grouped together every time?
How many groups are there?
Which objects are in which groups?
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6. State whether or not you agree with each student and why or why not.
Student 1: Jupiter, Saturn, Uranus, and Neptune are larger, more massive, and more dense than Earth,
Venus, Mars, and Mercury.
Student 2: No, Jupiter, Saturn, Uranus, and Neptune are larger and more massive than Earth, Venus,
Mars, and Mercury but they are LESS dense, being made of mostly liquid and gas, while the smaller
objects are made of mostly rock and metal.
7. Examine the data in Table 4.1 comparing the number of moons orbiting each object. Which group’s
members have many moons?
Which group’s members have few (or no) moons?
On what does the number of moons orbiting an object likely depend?
8. Examine the data in Table 4.1 comparing the orbital radius (distance from the Sun) of each object.
Which group is closer to the Sun?
Which group is farther from the Sun?
9. List the members of each of your object groups.
List what properties from the data table and bar graphs that the objects in each of your groups have in
common.
10. What objects do not seem to fit into a group?
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Name
Date
Have you heard about a fairly recent decision made about one of these objects?
Did this exercise help clarify the reason for the decision?
11. State whether or not you agree with each student and why or why not.
Student 1: Pluto and Eris should be in a group with Earth and the other objects like Earth (the terrestrial planets) because they are small, of low mass, do not have very many moons, and have longer
rotation periods.
Student 2: No, Pluto and Eris should be in a group with Jupiter and the other objects like Jupiter (the
Jovian planets) because they have low density and are far from the Sun.
Student 3: Maybe Pluto and Eris do not fit into either category and maybe they should be considered
a different type of object than the terrestrial or Jovian planets? Perhaps a group of small, low mass,
lower density objects with few or no moons and longer rotation periods because they are even farther
from the Sun than the Jovian planets?
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Name
Date
CHAPTER 4 PART 2 TUTORIAL—FORMATION OF THE SOLAR SYSTEM
Courtesy of Lina Levy. © Kendall Hunt Publishing Company
The Sun formed when material at the center of a giant rotating cloud of gas and dust called a nebula gravitationally pulled together. When this protosun gathered enough material it became massive enough to exert tremendous pressure on its core. This raised the temperature high enough for nuclear fusion to occur
there. This produced the energy necessary for the Sun to begin to shine, or more simply, to give off light
and heat to its surroundings, the leftovers of the original nebula or the Sun’s solar nebula. This solar nebula was the material from which the planets of our solar system would form.
FIGURE 1 The rock-metal line and frost line.
Table 1 䡲 Condensation Temperatures of Materials in the Solar Nebula
Examples
Condensation
Temperatures
Relative
Abundance
Metals
Rock
Hydrogen Compounds
Gases
Iron, nickel, aluminum
1,000–1,600 K
Various minerals
500–1,300 K
Water, methane, ammonia
< 150 K
0.2%
0.4%
1.4%
Hydrogen, helium
DO NOT condense in
Nebula
98%
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1. How would you expect temperatures in the solar nebula to vary with increasing distance from the
sun?
Do Table 2 and Graph 1 agree with what you expect?
Table 2
Graph 1
Distance from Sun
(AU)
Temperature
(Kelvin)
0.2
0.5
1
2
5
10
20
50
2000
1000
500
300
150
100
50
20
Created with Graphical Analysis 3 by Vernier Software
2. As the solar nebula cooled, parts of it cooling to as low as 500 K, which type(s) of materials of those shown
in Figure 1 and Table 1 would you expect to condense (become solid) out of the solar nebula first?
As these materials began to gravitationally pull together and form larger objects they became planetary seeds or protoplanets.
3. About how far from the Sun would be the closest distance that any of the materials from question 2
could condense (see Table 2 and Graph 1)?
This distance from the Sun is called the rock-metal condensation line.
4. Can any of the materials in Table 1 condense inside (closer to the Sun than) the rock-metal condensation line?
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Name
Date
Therefore can planets form inside this line?
5. About how far from the Sun would you expect temperatures to cool down as low as 150 K or less (see
Table 2 and Graph 1)?
This distance from the Sun is called the frost line.
6. What materials in Table 2 could then condense (become solid) on the already formed protoplanets
beyond the frost line, creating a second layer of material on these objects?
What common name do we give to these materials when they condense on something? (Remember,
they are beyond the frost line.)
7. So where, relative to the Sun, are larger objects found?
Where are smaller object found?
Where are no objects at all found?
8. State whether or not you agree with each student and why or why not.
Student 1: The objects forming closer to the Sun will be more massive because they are made of
rock and metal while the objects forming farther will be less massive because they will be made
of ice.
Student 2: No, rock and metal condensed everywhere past the rock-metal line and the objects
forming beyond the frost line ALSO got a coating of ice so they are more massive.
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9. Which materials in Figure 2 do not condense (do not become solid) anywhere in the solar nebula?
Why can’t they?
10. Which planetary objects, the larger or smaller ones, are now likely to collect large amounts of the remaining uncondensed gases from the solar nebula? Give two reasons for your answer.
HINT: Keep in mind that temperature is a measure of the energy of molecular motion, so molecules in
warmer regions are moving much faster that those in cooler regions.
11. Now describe the smaller objects that have formed. Of what materials are they mostly composed?
Where did they form relative to the Sun and to the larger objects?
In how many layers (or steps) did they form?
What type of planet that you are familiar with are these?
12. Describe the larger objects that have formed.
Where did they form relative to the Sun and to the larger objects?
In how many layers (or steps) did they form?
What type of planet that you are familiar with are these?
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Name
Date
13. Which type of planet is more evolved (has gone through more steps in its formation)?
14. Based on the investigation you have just undertaken:
What is the single most important factor in determining what kind of planet will form at a given location?
What controls this factor?
15. State whether or not you agree with each student and why or why not.
Student 1: The planets farther from the Sun got more massive by collecting a layer of gases because
farther out where temperatures are lower the gases don’t move as fast so they were easier to catch than
they would be closer to the Sun.
Student 2: I think the planets farther from the Sun got more massive because the ice layer that formed
on them made them massive enough to gravitationally collect gases while the planets closer of the Sun
were not massive enough to do this.
16. What names have we given to smaller objects, composed of various materials that condensed from the
solar nebula, the leftover debris that did not become part of a planet?
Of what would small objects that formed closer to the Sun be made? What do we call them?
Of what would small objects that formed farther from the Sun largely be made? What do we call
them?
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