1. Chapter 6 Formation of Planetary Systems Our Solar System and Beyond

2. 6.1 A Brief Tour of the Solar System
Our goals for learning:
What does the solar system look like?

3. The solar system exhibits clear patterns of composition and motion.
These patterns are far more important and interesting than numbers, names, and other trivia.

4. Planets are very tiny compared to distances between them.
Before embarking on the tour of the planets, you might wish to review the overall scale from ch. 1.

5. Sun Over 99.9% of solar system’s mass Made mostly of H/He gas (plasma)
These slides follow the planetary tour pages in ch. 6.
Over 99.9% of solar system’s mass
Made mostly of H/He gas (plasma)
Converts 4 million tons of mass into energy each second

6. Mercury Made of metal and rock; large iron core
Desolate, cratered; long, tall, steep cliffs
Very hot and very cold: 425°C (day), –170°C (night)

7. Venus Nearly identical in size to Earth; surface hidden by clouds
Hellish conditions due to an extreme greenhouse effect
Even hotter than Mercury: 470°C, day and night

8. Earth
Earth and Moon to scale
An oasis of life
The only surface liquid water in the solar system
A surprisingly large moon

9. Mars Looks almost Earth-like, but don’t go without a spacesuit!
Giant volcanoes, a huge canyon, polar caps, and more
Water flowed in the distant past; could there have been life?

10. Jupiter Much farther from Sun than inner planets
Mostly H/He; no solid surface
300 times more massive than Earth
Many moons, rings

11. Jupiter’s moons can be as interesting as planets themselves, especially Jupiter’s four Galilean moons
Io (shown here): Active volcanoes all over
Europa: Possible subsurface ocean
Ganymede: Largest moon in solar system
Callisto: A large, cratered “ice ball”

12. Saturn Giant and gaseous like Jupiter Spectacular rings
Many moons, including cloudy Titan
Cassini spacecraft currently studying it

13. Rings are NOT solid; they are made of countless small chunks of ice and rock, each orbiting like a tiny moon.
Artist’s conception
The Rings of Saturn

14. Cassini probe arrived July 2004.
(Launched in 1997)
Inset art shows the Huygens probe separated from the main spacecraft on its descent to Titan…

15. Uranus Smaller than Jupiter/Saturn; much larger than Earth
Made of H/He gas and hydrogen compounds (H2O, NH3, CH4)
Extreme axis tilt
Moons and rings

16. Neptune Similar to Uranus (except for axis tilt)
Many moons (including Triton)
Note: this is the same image as in the text but rotated 90° to show the axis tilt relative to the ecliptic plane as the horizontal on the page. (The orientation in the book chosen for its more dramatic effect.)

17. Pluto and Eris Much smaller than other planets
Icy, comet-like composition
Pluto’s moon Charon is similar in size to Pluto

18. What have we learned? What does the solar system look like?
Planets are tiny compared to the distances between them.
Each world has its own character, but there are many clear patterns.

19. 6.2 Clues to the Formation of Our Solar System
Our goals for learning:
What features of our solar system provide clues to how it formed?
What theory best explains the features of our solar system?

20. What features of our solar system provide clues to how it formed?

21. Motion of Large Bodies
All large bodies in the solar system orbit in the same direction and in nearly the same plane.
Most also rotate in that direction.

22. Two Major Planet Types
Terrestrial planets are rocky, relatively small, and close to the Sun.
Jovian planets are gaseous, larger, and farther from the Sun.

23. Swarms of Smaller Bodies
Many rocky asteroids and icy comets populate the solar system.

24. Notable Exceptions
Several exceptions to normal patterns need to be explained.

25. What theory best explains the features of our solar system?

26. According to the nebular theory, our solar system formed from a giant cloud of interstellar gas.
(nebula = cloud)

27. What have we learned?
What features of our solar system provide clues to how it formed?
Motions of large bodies: All in same direction and plane
Two major planet types: Terrestrial and jovian
Swarms of small bodies: Asteroids and comets
Notable exceptions: Rotation of Uranus, Earth’s large moon, and so forth

28. What have we learned?
What theory best explains the features of our solar system?
The nebular theory, which holds that our solar system formed from a cloud of interstellar gas, explains the general features of our solar system.

29. 6.3 The Birth of the Solar System
Our goals for learning:
Where did the solar system come from?
What caused the orderly patterns of motion in our solar system?

30. Where did the solar system come from?

31. Galactic Recycling
Elements that formed planets were made in stars and then recycled through interstellar space.

32. Evidence from Other Gas Clouds
We can see stars forming in other interstellar gas clouds, lending support to the nebular theory.
The Orion Nebula with Proplyds

33. What caused the orderly patterns of motion in our solar system?

34. Orbital and Rotational Properties of the Planets

35. Conservation of Angular Momentum
The rotation speed of the cloud from which our solar system formed must have increased as the cloud contracted.

36. Rotation of a contracting cloud speeds up for the same reason a skater speeds up as she pulls in her arms.
Collapse_of_solar_nebula.swf
Collapse of the Solar Nebula

37. Flattening
Collisions between particles in the cloud caused it to flatten into a disk.

38. Collisions between gas particles in a cloud gradually reduce random motions.
Formation of Circular Orbits

39. Collisions between gas particles also reduce up and down motions.
Why does the Disk Flatten?

40. The spinning cloud flattens as it shrinks.
Formation of the Protoplanetary Disk

41. Disks Around Other Stars
Observations of disks around other stars support the nebular hypothesis.

42. What have we learned? Where did the solar system come from?
Galactic recycling built the elements from which planets formed.
We can observe stars forming in other gas clouds.
What caused the orderly patterns of motion in our solar system?
The solar nebula spun faster as it contracted because of conservation of angular momentum.
Collisions between gas particles then caused the nebula to flatten into a disk.
We have observed such disks around newly forming stars.

43. 6.4 The Formation of Planets
Our goals for learning:
Why are there two major types of planets?
Where did asteroids and planets come from?
Do we explain the existence of the Moon and other exceptions to the rules?
When did the planets form?

44. Why are there two major types of planets?

45. Conservation of Energy
As gravity causes the cloud to contract, it heats up.
Collapse of the Solar Nebula

46. Inner parts of the disk are hotter than outer parts.
Rock can be solid at much higher temperatures than ice.
Temperature Distribution of the Disk and the Frost Line

47. Fig 9.5
Inside the frost line: Too hot for hydrogen compounds to form ices
Outside the frost line: Cold enough for ices to form

48. Formation of Terrestrial Planets
Small particles of rock and metal were present inside the frost line.
Planetesimals of rock and metal built up as these particles collided.
Gravity eventually assembled these planetesimals into terrestrial planets.

49. Tiny solid particles stick to form planetesimals.
Summary of the Condensates in the Protoplanetary Disk

50. Gravity draws planetesimals together to form planets.
This process of assembly
is called accretion.
Same as previous
Summary of the Condensates in the Protoplanetary Disk

51. Accretion of Planetesimals
Many smaller objects collected into just a few large ones.

52. Formation of Jovian Planets
Ice could also form small particles outside the frost line.
Larger planetesimals and planets were able to form.
The gravity of these larger planets was able to draw in surrounding H and He gases.

53. The gravity of rock and ice in jovian planets draws in H and He gases.
Nebular Capture and the Formation of the Jovian Planets

54. Moons of jovian planets form in miniature disks.

55. Radiation and outflowing matter from the Sun — the solar wind — blew away the leftover gases.

56. Where did asteroids and comets come from?

57. Asteroids and Comets Leftovers from the accretion process
Rocky asteroids inside frost line
Icy comets outside frost line

58. Heavy Bombardment
Leftover planetesimals bombarded other objects in the late stages of solar system formation.

59. Origin of Earth’s Water
Water may have come to Earth by way of icy planetesimals from the outer solar system.

60. How do we explain the existence of our Moon and other exceptions to the rules?

61. Captured Moons
The unusual moons of some planets may be captured planetesimals.

62. Giant Impact Giant impact stripped matter from Earth’s crust
Stripped matter began to orbit
Then accreted into Moon

63. Odd Rotation
Giant impacts might also explain the different rotation axes of some planets.

64. Review of nebular theory

65. Thought Question
How would the solar system be different if the solar nebula had cooled with a temperature half its current value?
Jovian planets would have formed closer to the Sun.
There would be no asteroids.
There would be no comets.
Terrestrial planets would be larger.

66. Thought Question
How would the solar system be different if the solar nebula had cooled with a temperature half its current value?
Jovian planets would have formed closer to the Sun.
There would be no asteroids.
There would be no comets.
Terrestrial planets would be larger.

67. Thought Question
Which of these facts is NOT explained by the nebular theory?
There are two main types of planets: terrestrial and jovian.
Planets orbit in the same direction and plane.
Asteroids and comets exist.
There are four terrestrial and four jovian planets.

68. Thought Question
Which of these facts is NOT explained by the nebular theory?
There are two main types of planets: terrestrial and jovian.
Planets orbit in the same direction and plane.
Asteroids and comets exist.
There are four terrestrial and four jovian planets.

69. When did the planets form?
We cannot find the age of a planet, but we can find the ages of the rocks that make it up.
We can determine the age of a rock through careful analysis of the proportions of various atoms and isotopes within it.

70. Radioactive Decay Some isotopes decay into other nuclei.
A half-life is the time for half the nuclei in a substance to decay.

71. Thought Question
Suppose you find a rock originally made of potassium-40, half of which decays into argon-40 every 1.25 billion years. You open the rock and find 15 atoms of argon-40 for every atom of potassium-40. How long ago did the rock form?
1.25 billion years ago
2.5 billion years ago
3.75 billion years ago
5 billion years ago

72. Thought Question
Suppose you find a rock originally made of potassium-40, half of which decays into argon-40 every 1.25 billion years. You open the rock and find 15 atoms of argon-40 for every atom of potassium-40. How long ago did the rock form?
1.25 billion years ago
2.5 billion years ago
3.75 billion years ago
5 billion years ago

73. Dating the Solar System
Age dating of meteorites that are unchanged since they condensed and accreted tells us that the solar system is about 4.6 billion years old.

74. Dating the Solar System
Radiometric dating tells us that the oldest moon rocks are 4.4 billion years old.
The oldest meteorites are 4.55 billion years old.
Planets probably formed 4.5 billion years ago.

75. What have we learned? Why are there two major types of planets?
Rock, metals, and ices condensed outside the frost line, but only rock and metals condensed inside the frost line.
Small solid particles collected into planetesimals that then accreted into planets.
Planets inside the frost line were made of rock and metals.
Additional ice particles outside the frost line made planets there more massive, and the gravity of these massive planets drew in H and He gases.

76. What have we learned? Where did asteroids and comets come from?
They are leftover planetesimals, according to the nebular theory.
How do we explain the existence of Earth’s moon and other exceptions to the rules?
The bombardment of newly formed planets by planetesimals may explain the exceptions.
Material torn from Earth’s crust by a giant impact formed the Moon.
When did the planets form?
Radiometric dating indicates that planets formed 4.5 billion years ago.

77. 6.5 Other Planetary Systems
Our goals for learning:
How do we detect planets around other stars?
How do extrasolar planets compare with those in our own solar system?
Do we need to modify our theory of solar system formation?

78. How do we detect planets around other stars?

79. Planet Detection Direct: Pictures or spectra of the planets themselves
Indirect: Measurements of stellar properties revealing the effects of orbiting planets

80. Gravitational Tugs
The Sun and Jupiter orbit around their common center of mass.
The Sun therefore wobbles around that center of mass with the same period as Jupiter.
Stellar Motion due to Planetary Orbits

81. Gravitational Tugs
Sun’s motion around solar system’s center of mass depends on tugs from all the planets.
Astronomers who measured this motion around other stars could determine masses and orbits of all the planets.

82. Astrometric Technique
We can detect planets by measuring the change in a star’s position in the sky.
However, these tiny motions are very difficult to measure (~0.001 arcsecond).

83. Doppler Technique
Measuring a star’s Doppler shift can tell us its motion toward and away from us.
Current techniques can measure motions as small as 1 m/s (walking speed!).
Oscillation of a Star's Absorption Line

84. First Extrasolar Planet Detected
Doppler shifts of star 51 Pegasi indirectly reveal planet with 4-day orbital period
Short period means small orbital distance
First extrasolar planet to be discovered (1995)

85. First Extrasolar Planet Detected
The planet around 51 Pegasi has a mass similar to Jupiter’s, despite its small orbital distance.

86. Thought Question
Suppose you found a star with the same mass as the Sun moving back and forth with a period of 16 months. What could you conclude?
It has a planet orbiting at less than 1 AU.
It has a planet orbiting at greater than 1 AU.
It has a planet orbiting at exactly 1 AU.
It has a planet, but we do not have enough information to know its orbital distance.
Rocky material condensed everywhere.
B. The students often get confused by the differentiation ideas they’ve just
heard about in planet formation.
D. If a passing star ripped off asteroids, it would have stripped away comets
and planets too. (That may be one explanation for the truncation of the
Kuiper belt…)

87. Thought Question
Suppose you found a star with the same mass as the Sun moving back and forth with a period of 16 months. What could you conclude?
It has a planet orbiting at less than 1 AU.
It has a planet orbiting at greater than 1 AU.
It has a planet orbiting at exactly 1 AU.
It has a planet, but we do not have enough information to know its orbital distance.
Rocky material condensed everywhere.
B. The students often get confused by the differentiation ideas they’ve just
heard about in planet formation.
D. If a passing star ripped off asteroids, it would have stripped away comets
and planets too. (That may be one explanation for the truncation of the
Kuiper belt…)

88. Transits and Eclipses
A transit is when a planet crosses in front of a star.
The resulting eclipse reduces the star’s apparent brightness and tells us the planet’s radius.
When there is no orbital tilt, an accurate measurement of planet mass can be obtained.
Planetary Transits

89. Direct Detection
Special techniques for concentrating or eliminating bright starlight are enabling the direct detection of planets.

90. How do extrasolar planets compare with those in our solar system?

91. Measurable Properties
Orbital period, distance, and shape
Planet mass, size, and density
Composition

92. Orbits of Extrasolar Planets
Most of the detected planets have orbits smaller than Jupiter’s.
Planets at greater distances are harder to detect with the Doppler technique.

93. Orbits of Extrasolar Planets
Most of the detected planets have greater mass than Jupiter.
Planets with smaller masses are harder to detect with the Doppler technique.

94. Planets: Common or Rare?
One in ten stars examined so far have turned out to have planets.
The others may still have smaller (Earth-sized) planets that cannot be detected using current techniques.

95. Surprising Characteristics
Some extrasolar planets have highly elliptical orbits.
Some massive planets orbit very close to their stars: “Hot Jupiters.”

96. Hot Jupiters

97. Do we need to modify our theory of solar system formation?

98. Revisiting the Nebular Theory
Nebular theory predicts that massive Jupiter-like planets should not form inside the frost line (at << 5 AU).
The discovery of “hot Jupiters” has forced a reexamination of nebular theory.
“Planetary migration” or gravitational encounters may explain “hot Jupiters.”

99. Planetary Migration
A young planet’s motion can create waves in a planet-forming disk.
Models show that matter in these waves can tug on a planet, causing its orbit to migrate inward.

100. Gravitational Encounters
Close gravitational encounters between two massive planets can eject one planet while flinging the other into a highly elliptical orbit.
Multiple close encounters with smaller planetesimals can also cause inward migration.

101. Thought Question
What happens in a gravitational encounter that allows a planet’s orbit to move inward?
It transfers energy and angular momentum to another object.
The gravity of the other object forces the planet to move inward.
It gains mass from the other object, causing its gravitational pull to become stronger.
Rocky material condensed everywhere.
B. The students often get confused by the differentiation ideas they’ve just
heard about in planet formation.
D. If a passing star ripped off asteroids, it would have stripped away comets
and planets too. (That may be one explanation for the truncation of the
Kuiper belt…)

102. Thought Question
What happens in a gravitational encounter that allows a planet’s orbit to move inward?
It transfers energy and angular momentum to another object.
The gravity of the other object forces the planet to move inward.
It gains mass from the other object, causing its gravitational pull to become stronger.
Rocky material condensed everywhere.
B. The students often get confused by the differentiation ideas they’ve just
heard about in planet formation.
D. If a passing star ripped off asteroids, it would have stripped away comets
and planets too. (That may be one explanation for the truncation of the
Kuiper belt…)

103. Modifying the Nebular Theory
Observations of extrasolar planets have shown that the nebular theory was incomplete.
Effects like planet migration and gravitational encounters might be more important than previously thought.

104. What have we learned? How do we detect planets around other stars?
A star’s periodic motion (detected through Doppler shifts) tells us about its planets.
Transiting planets periodically reduce a star’s brightness.
Direct detection is possible if we can block the star’s bright light.

105. What have we learned?
How do extrasolar planets compare with those in our solar system?
Detected planets are all much more massive than Earth.
Most have orbital distances smaller than Jupiter’s, and have highly elliptical orbits.
“Hot Jupiters” have been found.
Do we need to modify our theory of solar system formation?
Migration and encounters may play a larger role than previously thought.