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1
Note that the following lectures include
animations and PowerPoint effects such as fly ins
and transitions that require you to be in
PowerPoint's Slide Show mode (presentation mode).
2
The Origin of the Solar System

• Chapter 19

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Guidepost
The preceding 18 chapters have described the
origin, structure, and evolution of the physical
universe, but they have neglected one important
class of objects planets. In this chapter, we
can look back on what we have learned and find
our place in the universe. We live on a planet.
What does that mean? Where do we fit? Each time
we have studied a new object, we have asked how
it formed and how it evolved to its present
state. We have done that with stars and galaxies
and the universe, so it is appropriate to begin
our discussion of the solar system by considering
its origin. Another reason for discussing the
origin of the solar system here is to give
ourselves a framework into which we can fit the
planets as we discuss them in the
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Guidepost (continued)
chapters that follow. Without a theoretical
framework, science is nothing but a jumble of
facts. With a good framework in hand, we will be
ready to make sense of the solar system through
the next six chapters.
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Outline
I. The Great Chain of Origins A. Early
Hypotheses B. A Review of the Origin of
Matter C. The Solar Nebula Hypothesis D.
Planet-Forming Disks E. Planets Orbiting Other
Stars II. A Survey of the Solar System A.
Revolution and Rotation B. Two Kinds of
Planets C. Space Debris D. The Age of the Solar
System
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Outline (continued)
III. The Story of Planet Building A. The
Chemical Composition of the Solar Nebula B. The
Condensation of Solids C. The Formation of
Planetesimals D. The Growth of Protoplanets E.
The Jovian Problem F. Explaining the
Characteristics of the Solar System G. Clearing
the Nebula
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Early Hypotheses

• catastrophic hypotheses, e.g., passing star
  hypothesis

Star passing the sun closely tore material out of
the sun, from which planets could form (no longer
considered)
Catastrophic hypotheses predict Only few stars
should have planets!

• evolutionary hypotheses, e.g., Laplaces nebular
  hypothesis

Rings of material separate from the spinning
cloud, carrying away angular momentum of the
cloud ? cloud could contract further (forming the
sun)
Evolutionary hypotheses predict Most stars
should have planets!
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The Solar Nebula Hypothesis
Basis of modern theory of planet formation.
Planets form at the same time from the same cloud
as the star.
Planet formation sites observed today as dust
disks of T Tauri stars.
Sun and our Solar system formed 5 billion years
ago.
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Extrasolar Planets
Modern theory of planet formation is evolutionary
? Many stars should have planets!
? planets orbiting around other stars
Extrasolar planets
Extrasolar planets can not be imaged directly.
Detection using same methods as in binary star
systems
Look for wobbling motion of the star around the
common center of mass.
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Circumstellar Disk
(SLIDESHOW MODE ONLY)
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Evidence for Ongoing Planet Formation
Many young stars in the Orion Nebula are
surrounded by dust disks
Probably sites of planet formation right now!
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Dust Disks Around Forming Stars
Dust disks around some T Tauri stars can be
imaged directly (HST).
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Indirect Detection of Extrasolar Planets
Observing periodic Doppler shifts of stars with
no visible companion
Evidence for the wobbling motion of the star
around the common center of mass of a planetary
system
Over 100 extrasolar planets detected so far.
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Survey of the Solar System
Relative Sizes of the Planets
Assume, we reduce all bodies in the solar system
so that the Earth has diameter 0.3 mm.
Sun size of a small plum.
Mercury, Venus, Earth, Mars size of a grain of
salt.
Jupiter size of an apple seed.
Saturn slightly smaller than Jupiters apple
seed.
Pluto Speck of pepper.
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Planetary Orbits
Orbits generally inclined by no more than 3.4o
All planets in almost circular (elliptical)
orbits around the sun, in approx. the same plane
(ecliptic).
Exceptions Mercury (7o) Pluto (17.2o)
Mercury
Venus
Mars
Sense of revolution counter-clockwise
Earth
Jupiter
Sense of rotation counter-clockwise (with
exception of Venus, Uranus, and Pluto)
Pluto
Uranus
Saturn
Neptune
(Distances and times reproduced to scale)
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Two Kinds of Planets
Planets of our solar system can be divided into
two very different kinds
Terrestrial (earthlike) planets Mercury, Venus,
Earth, Mars
Jovian (Jupiter-like) planets Jupiter, Saturn,
Uranus, Neptune
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Terrestrial Planets
Four inner planets of the solar system
Relatively small in size and mass (Earth is the
largest and most massive)
Rocky surface
Surface of Venus can not be seen directly from
Earth because of its dense cloud cover.
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Craters on Planets Surfaces
Craters (like on our Moons surface) are common
throughout the Solar System.
Not seen on Jovian planets because they dont
have a solid surface.
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The Jovian Planets
Much lower average density
All have rings (not only Saturn!)
Mostly gas no solid surface
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Space Debris
In addition to planets, small bodies orbit the
sun
Asteroids, comets, meteoroids
Asteroid Eros, imaged by the NEAR spacecraft
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Comets
Icy nucleus, which evaporates and gets blown into
space by solar wind pressure.
Mostly objects in highly elliptical orbits,
occasionally coming close to the sun.
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Meteoroids
Small (mm mm sized) dust grains throughout the
solar system
If they collide with Earth, they evaporate in the
atmosphere.
? Visible as streaks of light meteors.
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The Age of the Solar System
Sun and planets should have about the same age.
Ages of rocks can be measured through radioactive
dating
Measure abundance of a radioactively decaying
element to find the time since formation of the
rock
Dating of rocks on Earth, on the Moon, and
meteorites all give ages of 4.6 billion years.
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Radioactive Decay
(SLIDESHOW MODE ONLY)
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Our Solar System
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The Story of Planet Building
Planets formed from the same protostellar
material as the sun, still found in the Suns
atmosphere.
Rocky planet material formed from clumping
together of dust grains in the protostellar cloud.
Mass of more than 15 Earth masses
Mass of less than 15 Earth masses
Planets can grow by gravitationally attracting
material from the protostellar cloud
Planets can not grow by gravitational collapse
Earthlike planets
Jovian planets (gas giants)
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The Condensation of Solids
To compare densities of planets, compensate for
compression due to the planets gravity
Only condensed materials could stick together to
form planets
Temperature in the protostellar cloud decreased
outward.
Further out ? Protostellar cloud cooler ? metals
with lower melting point condensed ? change of
chemical composition throughout solar system
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Formation and Growth of Planetesimals
Planet formation starts with clumping together of
grains of solid matter Planetesimals
Planetesimals (few cm to km in size) collide to
form planets.
Planetesimal growth through condensation and
accretion.
Gravitational instabilities may have helped in
the growth of planetesimals into protoplanets.
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The Growth of Protoplanets
Simplest form of planet growth
Unchanged composition of accreted matter over time
As rocks melted, heavier elements sink to the
center ? differentiation
This also produces a secondary atmosphere ?
outgassing
Improvement of this scenario Gradual change of
grain composition due to cooling of nebula and
storing of heat from potential energy
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The Jovian Problem
Two problems for the theory of planet formation
1) Observations of extrasolar planets indicate
that Jovian planets are common.
2) Protoplanetary disks tend to be evaporated
quickly (typically within 100,000 years) by the
radiation of nearby massive stars.
? Too short for Jovian planets to grow!
Solution Computer simulations show that Jovian
planets can grow by direct gas accretion without
forming rocky planetesimals.
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Clearing the Nebula
Remains of the protostellar nebula were cleared
away by

• Radiation pressure of the sun

• Sweeping-up of space debris by planets

• Solar wind

• Ejection by close encounters with planets

Surfaces of the Moon and Mercury show evidence
for heavy bombardment by asteroids.
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New Terms
passing star hypothesis evolutionary
hypothesis catastrophic hypothesis nebular
hypothesis angular momentum problem solar nebula
hypothesis extrasolar planets terrestrial
planet Jovian planet Galilean satellites asteroid
comet meteor meteoroid meteorite half-life gravita
tional collapse
uncompressed density condensation
sequence planetesimal condensation accretion proto
planet differentiation outgassing heat of
formation radiation pressure heavy bombardment
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Discussion Questions
1. In your opinion, should all solar systems have
asteroid belts? Should all solar systems show
evidence of an age of heavy bombardment? 2. If
the solar nebula hypothesis is correct, then
there are probably more planets in the universe
than stars. Do you agree? Why or why not?
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Quiz Questions
1. What was the major problem for the solar
nebula hypothesis that was proposed by
Pierre-Simon Laplace? a. It did not predict that
inner planets orbit the Sun more quickly than
outer planets. b. The Sun contains little of the
angular momentum of the Solar System. c. It
called for a catastrophic event to produce the
Solar System. d. The Sun spins more rapidly than
is expected. e. All of the above.
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Quiz Questions
2. Why do we reject the formation of planets as
proposed by Buffon (the passing star
hypothesis)? a. Material pulled out of the Sun
would be too hot to condense. b. Planetary
systems are common, whereas nearby star
collisions are rare. c. The angular momentum of
the Sun is too low. d. Both a and b above. e. All
of the above.
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Quiz Questions
3. How do astronomers believe the Sun came to
have less angular momentum than its system of
planets? a. The solar wind mass outflow carries
angular momentum away from the Sun. b. The Sun's
magnetic field drags material out in the Solar
System, transferring angular momentum outward. c.
A large planetesimal impacted the Sun on its
leading hemisphere. d. The planets gain angular
momentum from passing stars. e. Both a and b
above.
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Quiz Questions
4. What is the origin of the atoms of hydrogen,
oxygen, and sodium in the perspiration that exits
your body during an astronomy exam? a. All of
these elements were synthesized inside stars more
than 4.6 billion years ago. b. All of the
elements were produced in the first few minutes
after the Big Bang event. c. The hydrogen nuclei
were produced few minutes after the Big Bang
event 13.7 billion years ago, and the oxygen and
sodium nuclei were synthesized inside stars more
than 4.6 billion years ago. d. They were all
fused deep inside Earth. e. None of the above.
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Quiz Questions
5. What evidence do we have that planets form
along with other stars? a. At radio wavelengths,
we detect cool dust disks around young stars. b.
At Infrared wavelengths, we detect large cool
dust disks around stars. c. At visible
wavelengths, we see disks around the majority of
single young stars in the Orion Nebula. d. Both a
and b above. e. All of the above.
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Quiz Questions
6. How do we know that extrasolar planets are
orbiting other stars? a. We see a star's light
dim as a planet passes in front of the star. b.
We detect alternating Doppler shifts in the
spectra of some stars. c. We see a series of
small faint points in line with stars, much like
Galileo's discovery of the moons of Jupiter. d.
Both a and b above. e. All of the above.
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Quiz Questions
7. What are the general characteristics of the
extrasolar planets discovered so far? a. They
have low mass and orbit close to their stars. b.
They have low mass and orbit far from their
stars. c. They have high mass and orbit close to
their stars. d. They have high mass and orbit far
from their stars. e. These extrasolar planetary
systems are much like the Solar System.
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Quiz Questions
8. Why haven't we detected low-mass planets close
to their stars and high-mass planets far from
their stars? a. Our techniques are not yet
sensitive enough. b. We have not been observing
for a long enough time. c. We have not been
looking at stars similar to our Sun. d. Such
systems cannot form, as the material in dust
disks is densest close to their stars. e. Both a
and b above.
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Quiz Questions
9. How is the solar nebula theory supported by
the motion of Solar System bodies? a. All of the
planets orbit the Sun near the Sun's equatorial
plane. b. All of the planets orbit in the same
direction that the Sun rotates. c. Six out of
seven planets rotate in the same direction as the
Sun. d. Most moons orbit their planets in the
same direction that the Sun rotates. e. All of
the above.
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Quiz Questions
10. Which of the following is NOT a property
associated with terrestrial planets? a. They are
located close to the Sun. b. They are small in
size. c. They have low mass. d. They have low
density. e. They have few moons.
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Quiz Questions
11. How do asteroids and comets differ? a.
Asteroids orbit in the opposite direction that
the Sun rotates. b. Comets are younger than
asteroids. c. Asteroids have lower
reflectivity. d. Comets contain ices. e. All of
the above.
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Quiz Questions
12. Where are most of the asteroids located? a.
Inside the orbit of Mercury. b. Between the
orbits of Earth and Venus. c. Between the orbits
of Earth and Mars. d. Between the orbits of Mars
and Jupiter. e. Between the orbits of Jupiter and
Neptune.
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Quiz Questions
13. Radiometric dating of rock samples indicates
that the Solar System formed about 4.56 billion
years ago. Which rock samples have this age? a.
Earth rocks. b. Moon rocks. c. Meteorites. d.
Both a and b above. e. Both b and c above.
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Quiz Questions
14. According to the solar nebula theory, why are
Jupiter and Saturn much more massive than Uranus
and Neptune? a. Jupiter and Saturn formed
earlier and captured nebular gas before it was
cleared out. b. Jupiter and Saturn contain more
high-density planet building materials. c. Uranus
and Neptune have suffered more interstellar wind
erosion. d. Both a and b above. e. All of the
above.
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Quiz Questions
15. How does the solar nebula theory account for
the drastic differences between terrestrial and
Jovian planets? a. The temperature of the
accretion disk was high close to the Sun and low
far from the Sun. b. Terrestrial planets formed
closer to the Sun, and are thus made of
high-density rocky materials. c. Jovian planets
are large and have high-mass because they formed
where both rocky and icy materials can
condense. d. Jovian planets captured nebular gas
as they had stronger gravity fields and are
located where gases move more slowly. e. All of
the above.
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Quiz Questions
16. What is the difference between the processes
of condensation and accretion? a. Both are
processes that collect particles together. b.
Condensation is the building of larger particles
one atom (or molecule) at a time, whereas
accretion is the sticking together of larger
particles. c. Accretion is the building of larger
particles one atom (or molecule) at a time,
whereas condensation is the sticking together of
larger particles. d. Both a and b above. e. Both
a and c above.
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Quiz Questions
17. Which of the following is the most likely
major heat source that melted early-formed
planetesimals? a. Tidal flexing. b. The impact
of accreting bodies. c. The decay of long-lived
unstable isotopes. d. The decay of short-lived
unstable isotopes. e. The transfer of
gravitational energy into thermal energy.
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Quiz Questions
18. How does the solar nebula theory explain the
formation of an asteroid belt between Mars and
Jupiter, rather than a planet at this
location? a. A single planet formed here and was
disrupted by an impact with a large comet from
the outer Solar System. b. Jupiter swept up so
much material that not enough was left to form a
planet. c. Mars was once larger and collided with
a large planetesimal from the inner Solar System
that sent debris outward. d. Jupiter formed
early, and its gravitational influence altered
the orbits of nearby accreting planetesimals such
that their collisions became destructive rather
than constructive. e. The asteroids were
originally moons of the planets that were
perturbed by Jupiter's gravity, and now reside in
the zone between Mars and Jupiter.
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Quiz Questions
19. Which of the following accurately describes
the differentiation process? a. High-density
materials sink toward the center and low-density
materials rise toward the surface of a molten
body. b. Low-density materials sink toward the
center and high-density materials rise toward the
surface of a molten body. c. Only rocky materials
can condense close to the Sun, whereas both rocky
and icy materials can condense far from the
Sun. d. Both rocky and icy materials can condense
close to the Sun, whereas only rocky materials
can condense far from the Sun. e. Small bodies
stick together to form larger bodies.
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Quiz Questions
20. How did the solar nebula get cleared of
material? a. The radiation pressure of sunlight
pushed gas particles outward. b. The intense
solar wind of the youthful Sun pushed gas and
dust outward. c. The planets swept up gas, dust,
and small particles. d. Close gravitational
encounters with Jovian planets ejected material
outward. e. All of the above.
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Answers
1. b 2. d 3. e 4. c 5. e 6. d 7. c 8. e 9. e 10. d
11. d 12. d 13. c 14. a 15. e 16. b 17. d 18. d 19
. a 20. e