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The Origin of the Solar System

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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).

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The Origin of the Solar System

Chapter 19

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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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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., Laplace’s 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


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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 Jupiter’s “apple seed”.

Pluto: ~ Speck of pepper.

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Orbits generally inclined by no more than 3.4o

Planetary Orbits

All planets in almost circular (elliptical) orbits around the sun, in approx. the same plane (ecliptic).


Mercury (7o)

Pluto (17.2o)




Sense of revolution: counter-clockwise



Sense of rotation: counter-clockwise (with exception of Venus, Uranus, and Pluto)





(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 Moon’s surface) are common throughout the Solar System.

Not seen on Jovian planets because they don’t 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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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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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


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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 Sun’s 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 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

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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!


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







gravitational collapse

uncompressed density

condensation sequence







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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