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Chapter 19
The Origin of the Solar System
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
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.
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
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
Early Hypotheses
• catastrophic hypotheses,
e.g., passing star hypothesis:
Catastrophic hypotheses predict:
Only few stars should have planets!
Star passing the sun closely
tore material out of the sun,
from which planets could form
(no longer considered)
• evolutionary hypotheses, e.g.,
Laplace’s nebular hypothesis:
Evolutionary hypotheses predict:
Most stars should have planets!
Rings of material separate from
the spinning cloud, carrying away
angular momentum of the cloud 
cloud could contract further
(forming the sun)
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.
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.
Circumstellar Disk
(SLIDESHOW MODE ONLY)
Evidence for Ongoing Planet Formation
Many young
stars in the Orion
Nebula are
surrounded by
dust disks:
Probably sites of
planet formation
right now!
Dust Disks Around Forming Stars
Dust disks
around
some T
Tauri stars
can be
imaged
directly
(HST).
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.
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
Jupiter’s “apple seed”.
Pluto: ~ Speck of pepper.
Orbits generally
inclined by no
more than 3.4o
All planets in almost
Exceptions:
circular (elliptical)
Mercury (7o)
orbits around the
Pluto (17.2o)
sun, in approx. the
same plane
(ecliptic).
Planetary Orbits
Mercury
Venus
Earth
Sense of revolution:
counter-clockwise
Sense of rotation:
counter-clockwise
(with exception of
Venus, Uranus,
and Pluto)
(Distances and times reproduced to scale)
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
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.
Craters on Planets’ Surfaces
Craters (like on
our Moon’s
surface) are
common
throughout the
Solar System.
Not seen on
Jovian planets
because they
don’t have a
solid surface.
The Jovian Planets
Much lower
average density
All have rings
(not only Saturn!)
Mostly gas; no
solid surface
Space Debris
In addition to planets, small bodies orbit the sun:
Asteroids, comets, meteoroids
Asteroid
Eros,
imaged by
the NEAR
spacecraft
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.
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.
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.
Radioactive Decay
(SLIDESHOW MODE ONLY)
Our Solar System
The Story of Planet Building
Planets formed from the same protostellar material
as the sun, still found in the Sun’s atmosphere.
Rocky planet material formed from clumping
together of dust grains in the protostellar cloud.
Mass of less than ~ 15
Earth masses:
Planets can not grow by
gravitational collapse
Earthlike planets
Mass of more than ~ 15
Earth masses:
Planets can grow by
gravitationally attracting
material from the
protostellar cloud
Jovian planets (gas giants)
The Condensation of Solids
To compare densities of planets,
compensate for compression due
to the planet’s 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
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.
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
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.
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.
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
gravitational collapse
uncompressed density
condensation sequence
planetesimal
condensation
accretion
protoplanet
differentiation
outgassing
heat of formation
radiation pressure
heavy bombardment
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?
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Quiz Questions
19. Which of the following accurately describes the
differentiation process?
a. High-density materials sink toward the center and lowdensity materials rise toward the surface of a molten body.
b. Low-density materials sink toward the center and highdensity 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.
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.
Answers
1.
2.
3.
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7.
8.
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10.
b
d
e
c
e
d
c
e
e
d
11.
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20.
d
d
c
a
e
b
d
d
a
e