part 1jobjects in the solar system 4.1 introduction · 63 part 1jobjects in the solar system 4.1...

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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 dis-cussed in more detail in Chapter 11, there are basically three types of objects in our so-lar system: planets, moons, and debris. Solar system debris is the collective term usedfor objects that have not become part of a planet or a moon: asteroids, comets, and me-teors. These objects will be discussed in detail in Chapter 6. Moons are objects that or-bit 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 onthese properties compare the objects to one another. This is known as comparativeplanetology. The properties of planets are the quantities that we can measure such asphysical properties like size, mass, and density or their orbital properties like distancefrom the Sun, orbital or revolution period, and rotational period. Table 4.1 lists the val-ues of these and other properties for known planets and several other objects in our so-lar system.

Figure 4.1 shows bar graphs or histograms comparing the radii (the radius is thedistance 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 com-bination of mass and size. It is a measure of how much mass per unit volume there is insomething. Solids like rocks and metals are objects of high density, gases like air are oflow density; and liquids like water are in between.

ObjectName

RadiusEarth=1

MassEarth=1

DensityWater=1

Orbital Radius(AU)

OrbitalPeriod(Years)

Rotation PeriodEarth=1

Number ofMoons

1 Mercury 0.382 0.055 5.43 0.387 0.2409 58.6 02 Venus 0.949 0.815 5.25 0.723 0.6152 243 03 Earth 1 1 5.52 1 1 0.9973 14 Mars 0.533 0.107 3.93 1.524 1.881 1.026 25 Jupiter 11.19 317.9 1.33 5.203 11.86 0.41 676 Saturn 9.46 95.18 0.7 9.539 29.42 0.44 627 Uranus 3.98 14.54 1.32 19.19 84.01 0.72 278 Neptune 3.81 17.13 1.64 30.06 164.8 0.67 139 Pluto 0.181 0.0022 2.05 39.48 248 6.39 510 Eris 0.183 0.0028 2.52 67.67 561 15.8 1

Table 4.1 � Planetary Data

64 Part 2 The Solar System

FIGURE 4.1 Bar graphs comparing planetary radii, masses, and densities.

Planetary Radii

0

2

4

6

8

10

12

1 2 3 4 5 6 7 8 9 10

Radius

Radi

us (E

arth

=1)

Object

Created with Graphical Analysis 3 by Vernier Software

Chapter 4 Solar System Overview 65

FIGURE 4.1 (cont’d)

Planetary Densities

0

1

2

3

4

5

6

1 2 3 4 5 6 7 8 9 10

Object

Density

Den

siti

es (w

ater

=1)

Planetary Masses

0

50

100

150

200

250

300

350

1 2 3 4 5 6 7 8 9 10

Object

Mass

Mas

s (E

arth

=1)




Examination of the radii bar graph shows that there are different-size objects (thenumbers on the bar graphs match the numbers in Table 4.1). Objects 5 and 6, Jupiterand Saturn are very large compared to objects 7 and 8, Uranus and Neptune that aremore medium in size while Earth and all the others are very small by comparison. Themass bar graph shows that Jupiter is by far the most massive, then Saturn with Uranusand Neptune the only others that even register on the graph. Perhaps at this pointJupiter, Saturn, Uranus, and Neptune could be considered a group of large and massiveplanets while Earth and all the others could be called small and less massive planets.

FIGURE 4.2 Images of our solar system’s planets.

The density graph shows something different. Now Mercury, Venus, Earth, andMars have large values or high densities while the larger, massive planets and Pluto andEris 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 andEris, 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, Ve-nus, Earth, and Mars are all small, of lower mass and higher density while Jupiter, Sat-urn, 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 terrestrialplanets; those grouped with Jupiter are called Jupiter-like or Jovian planets. Notice thatPluto and Eris do not fit with either category.

FIGURE 4.3 The Planets in order of their distance from the Sun.

Courtesy of NASA.

Courtesy of NASA.

Terrestrial Planets Jovian Planets

Members Mercury, Venus, Earth, Mars Jupiter, Saturn, Uranus, NeptuneSize Smaller LargerMass Low mass Great massDensity High LowComposition Rock and Metal Gas and LiquidDistance Close to Sun Far from SunRotation Slower FasterMoons Few or none ManyRings No Yes

Table 4.2 � Properties of Terrestrial and Jovian Planets

4.3 THE KUIPER BELT

As observed several times, Pluto and Eris do not fit into either of the major planet cate-gories 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 char-acteristics of the objects in what is known as the Kuiper belt.

First proposed by Gerard Kuiper in 1951, many small icy objects, which have alsobeen called “trans-Neptunian” objects and “ice dwarfs,” have now been observed be-yond the orbit of Neptune. There are thousands of Kuiper belt objects known to existincluding several discovered more recently that rival the size of Pluto such as Eris thatmay be as large or larger than Pluto.

In the summer of 2006, Pluto lost its status as one of the solar system’s planets. TheInternational Astronomical Union (IAU) is a group of astronomers from throughout theworld 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 be-tween Mars and Jupiter and several other objects large enough to be spherical, as dwarfplanets. The asteroid belt, Pluto, and other Kuiper belt objects will be discussed in moredetail in Chapter 6.


Looking at other data from Table 4.1, all the terrestrial planets are closer to the Sunand therefore have faster orbital periods while the Jovian planets are the opposite, far-ther from the Sun with longer orbital periods. Rotational periods do not seem to fit thecategories as well. All of the Jovian planets have rotation periods similar to each otherand all the terrestrial planets have longer periods, but as can be seen from Table 4.1, Ve-nus and Mercury have especially long periods. Again, note Pluto and Eris not fitting ineither category. They are far from the Sun like the Jovian planets but have longer rota-tion periods like the terrestrial planets. Also, due to their large mass and thereforegreater gravitational pull, the Jovian planets all have many moons and rings. Rings aretremendous numbers of smaller particles all in similar orbits around a Jovian planetcausing the appearance of a ring around a planet when viewed from a distance. Ringsystems will be discussed in more detail in Chapter 8. Table 4.2 is a comparison of theproperties of the terrestrial and Jovian planets.

Comparative planetology DebrisDensityDwarf planet

Kuiper belt Mass Moon

PlanetJovian Terrestrial

Radius

PART 2—THE FORMATION OF THE SOLAR SYSTEM

4.4 THE SOLAR NEBULA

Our Sun was formed by a gravitational collapse within a gigantic cloud of mostly hy-drogen gas and dust in the otherwise nearly empty interstellar space between the starsin our galaxy. The leftover material surrounding the not yet shining protosun was calledthe solar nebula. Eventually the protosun accumulated enough material from the neb-ula to become massive enough to put sufficient pressure on its core to raise the coretemperatures high enough for nuclear fusion to occur. This process provided the energynecessary for the Sun to give off light and heat or to shine and thus become a star. Theprocesses of star formation and nuclear fusion will be discussed in more detail in Chap-ter 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 solarsystem would form. Initially, temperatures were so hot that most of the solar nebula re-mained gaseous but as the young Sun cooled, temperatures reached a point where solidrocks and metals could begin to condense out of the nebula. Too close to the Sun thetemperatures never cooled enough for this to happen and no planets could form there.

CHAPTER 4 PART 1—TERMINOLOGY


Courtesy of NASA.


However, beyond about 0.3 AU, known as the rock-metal condensation line, the solidobjects could condense and begin gravitationally pulling together. First they formedsmall, 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 mate-rials could condense out of the nebula and due to the higher temperatures the gaseswere very fast moving so the rocky-metal protoplanets were not massive enough togravitationally capture appreciable amounts of these gases. The protoplanets continuedto accumulate more of the rock and metal in their orbits and eventually became theEarth-like or small, low mass, high-density (rocky-metal) terrestrial planets found closeto 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-metalobjects. This is known as the frost line. The cooler temperatures in this region of thenebula, out further from the Sun, slowed the motions of the gases—making them easierfor planets to catch. This factor and the now greater mass of these objects due to the icethey collected allowed them to gravitationally collect large amounts of these gases andgrow to tremendous sizes and become much more massive than their cousins nearer tothe 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 andmelted the ice leaving the basic structure of a Jovian planet as a terrestrial planet–sizerocky-metal core surrounded by a large liquid ocean below a huge, thick atmosphere ofmostly hydrogen and helium gas. The formation of the three-layered Jovian planets wasa 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 therock/metal line.

4.8 THE LEFTOVERS—SOLAR SYSTEM DEBRIS

Some of the planetisimals and even protoplanets did not become part of a terrestrial orJovian planet. Some were gravitationally captured, mostly by the more massive Jovianplanets and became moons. Other small rocky objects of the inner solar system are nowcalled asteroids. Many of the asteroids are concentrated in a “belt” between the orbits ofMars and Jupiter. The material in this belt was never able to pull together and form aplanet due to the gravitational influences of Jupiter. The two small rocky moons of Marswere likely captured from this asteroid belt. Small rocky objects of the cold outer solarsystem were covered by condensing ice and are known as comets. A large group ofcomet-like objects lies beyond the orbit of Neptune. A few of Neptune’s moons mayhave been captured from this population. The orbit of the most well known member ofthe Kuiper belt, the dwarf planet Pluto, crosses Neptune’s orbit. Asteroids, comets, andthe Kuiper belt will be discussed in more detail in Chapter 6.

Asteroid Comet CraterFrost line

ImpactMeteor Planetisimal Protoplanet

Protosun Rock-metal condensation

line Solar nebula

During the formation of the solar system when there were more objects that had notyet become parts of planetary systems many collisions occurred. A large object collidingwith Earth is thought to have been what formed Earth’s Moon. A collision is alsothough to be the reason that the rotational axis of Uranus lies nearly in the plane of itsorbit rather than more upright relative to it like the other planets. When an object isfalling toward an impact with Earth and gives off a bright flash of light due to frictionwith the gases in the atmosphere it is called a meteor. The holes the collisions leave arecalled impact craters. There are many impact craters on the surfaces of the terrestrialplanets, their moons, and Jovian moons from collisions. Over time as collisions occurthe number of objects available for further collisions becomes less and less. Most im-pact craters like those we see on our Moon were formed long ago when the rate of im-pact cratering was higher; but there are still occasional large impacts like the Tunguskaevent on Earth just over a century ago or the collision of a comet with Jupiter in 1994or the Russian meteor impact of 2013. Impacts will also be discussed in Chapter 6.

CHAPTER 4 PART 2—TERMINOLOGY


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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 propertiesof 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 itdoes not fit in.

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 lo-cation? What controls this factor?

11. Many smaller objects that condensed from the solar nebula are leftovers or debris that did not be-come part of a planet. Of what would small objects that formed closer to the Sun be made? What dowe call them? Of what would small objects that formed farther from the Sun largely be made? Whatdo 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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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 smallerobjects 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’smembers 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 incommon.

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?


Student 1: Pluto and Eris should be in a group with Earth and the other objects like Earth (the ter-restrial planets) because they are small, of low mass, do not have very many moons, and have longerrotation periods.

Student 2: No, Pluto and Eris should be in a group with Jupiter and the other objects like Jupiter (theJovian 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 considereda 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 fartherfrom the Sun than the Jovian planets?

CHAPTER 4 PART 2 TUTORIAL—FORMATION OF THE SOLAR SYSTEM

The Sun formed when material at the center of a giant rotating cloud of gas and dust called a nebula grav-itationally pulled together. When this protosun gathered enough material it became massive enough to ex-ert tremendous pressure on its core. This raised the temperature high enough for nuclear fusion to occurthere. This produced the energy necessary for the Sun to begin to shine, or more simply, to give off lightand heat to its surroundings, the leftovers of the original nebula or the Sun’s solar nebula. This solar neb-ula was the material from which the planets of our solar system would form.

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Metals Rock Hydrogen Compounds Gases

Examples Iron, nickel, aluminum Various minerals Water, methane, ammonia Hydrogen, heliumCondensation Temperatures

1,000–1,600 K 500–1,300 K < 150 K DO NOT condense in Nebula

Relative Abundance

0.2% 0.4% 1.4% 98%

Table 1 � Condensation Temperatures of Materials in the Solar Nebula

Courtesy of Lina Levy. © Kendall Hunt Publishing Company

FIGURE 1 The rock-metal line and frost line.

1. How would you expect temperatures in the solar nebula to vary with increasing distance from thesun?

Do Table 2 and Graph 1 agree with what you expect?

2. As the solar nebula cooled, parts of it cooling to as low as 500 K, which type(s) of materials of those shownin 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 plane-tary seeds or protoplanets.

3. About how far from the Sun would be the closest distance that any of the materials from question 2could 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 conden-sation line?

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Distance from Sun(AU)

Temperature(Kelvin)

0.2 20000.5 10001 5002 3005 150

10 10020 5050 20

Table 2 Graph 1


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 (seeTable 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 protoplanetsbeyond 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?


Student 1: The objects forming closer to the Sun will be more massive because they are made ofrock and metal while the objects forming farther will be less massive because they will be madeof ice.

Student 2: No, rock and metal condensed everywhere past the rock-metal line and the objectsforming 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 re-maining 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 inwarmer 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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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?


Student 1: The planets farther from the Sun got more massive by collecting a layer of gases becausefarther out where temperatures are lower the gases don’t move as fast so they were easier to catch thanthey would be closer to the Sun.

Student 2: I think the planets farther from the Sun got more massive because the ice layer that formedon them made them massive enough to gravitationally collect gases while the planets closer of the Sunwere not massive enough to do this.

16. What names have we given to smaller objects, composed of various materials that condensed from thesolar 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 callthem?

Name Date

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