1. Our Solar System and Its Origin

2. 6.1 A Brief Tour of the Solar System
Our Goals for Learning
• What does the solar system look like?

3. What does the solar system look like?

4. The planets are tiny compared to the distances between them (a million times smaller than shown here), but they
exhibit clear patterns of composition and motion.
The patterns are far more important and interesting than numbers, names, and other trivia

5. Recall scale of solar system
Before embarking on the tour of the planets, you might wish to review the overall scale from ch. 1.

6. 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.

7. 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

8. 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)

9. Venus
nearly identical in size to Earth; surface hidden by thick clouds
hellish conditions due to an extreme greenhouse effect:
even hotter than Mercury: 470°C, both day and night
atmospheric pressure equiv. to pressure 1 km deep in oceans
no oxygen, no water, …
perhaps more than any other planet, makes us ask: how did it end up so different from Earth?

10. Earth and Moon to scale
Earth
An oasis of life
The only surface liquid water in the solar system; about 3/4 of surface covered by water
A surprisingly large moon

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

12. Jupiter
Much farther from Sun than inner 4 planets (more than twice Mars distance)
Also very different in composition: mostly H/He; no solid surface.
Gigantic for a planet: 300  Earth mass; >1,000  Earth volume.
Many moons, rings…

13. Moons can be as interesting as the planets themselves, especially Jupiter’s 4 large “Galilean moons” (first seen by Galileo)
Io (shown here): active volcanoes all over
Europa: possible subsurface ocean
Ganymede: largest moon in solar system — larger than Mercury
Callisto: a large, cratered “ice ball” with unexplained surface features

14. Saturn
Giant and gaseous like Jupiter
most spectacular rings of the 4 jovian planets
many moons, including cloud-covered Titan
currently under study by the Cassini spacecraft

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

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

17. Uranus
much smaller than Jupiter/Saturn, but still much larger than Earth
made of H/He gas, hydrogen compounds (H2O, NH3, CH4)
extreme axis tilt — nearly tipped on its “side” — makes extreme seasons during its 84-year orbit.
moons also tipped in their orbits…

18. Very similar to Uranus (but much smaller axis tilt)
Neptune
Very similar to Uranus (but much smaller axis tilt)
Many moons, including unusual Triton: orbits “backward”; larger than Pluto.
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.)

19. Pluto
A “misfit” among the planets: far from Sun like large jovian planets, but much smaller than any terrestrial planet.
Comet-like composition (ices, rock) and orbit (eccentric, inclined to ecliptic plane, long years).
Its moon Charon is half Pluto’s size in diameter
Best current photo above; New Horizons mission launch 2006, arrival 2015…

20. This important summary table may be worth some time in class to make sure students understand how to read it…

21. What have we learned? • What does the solar system look like?
Our solar system consists of the Sun, nine planets and their moons, and vast numbers of asteroids and comets. Each world has its own unique character, but there are many clear patterns among the worlds.

22. 6.2 Clues to the Formation of Our Solar Sytem
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?

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

24. The Sun, planets, and large moons orbit and rotate in an organized way
counterclockwise seen from above the north pole)

25. Terrestrial planets are small, rocky, and close to the Sun.
Jovian planets are large, gas-rich, and far from the Sun.
(What about Pluto?)

27. Rocky asteroids between Mars & Jupiter
Icy comets in vicinity of Neptune and beyond
Asteroids and comets far outnumber the planets and their moons

28. A successful theory of solar system formation must allow for exceptions to general rules

29. Summary: Four Major Features of our Solar System

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

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

32. What have we learned?
• What features of our solar system provide clues to how it formed?
Four major features provide clues: (1) The Sun, planets, and large moons generally rotate and orbit in a very organized way. (2) With the exception of Pluto, the planets divide clearly into two groups: terrestrial and jovian. (3) The solar system contains huge numbers of asteroids and comets. (4) There are some notable exceptions to these general patterns.
• What theory best explains the features of our solar system?
The nebular theory, which holds that the solar system formed from the gravitational collapse of a great cloud of gas.

33. 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?

34. Where did the solar system come from?

35. The cloud of gas that gave birth to our solar system resulted from the recycling of gas through many generations of stars within our galaxy.

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

38. As gravity forced the cloud to become smaller, it began to spin faster and faster

39. As gravity forced the cloud to become smaller, it began to spin faster and faster
Conservation of angular momentum

40. As gravity causes cloud to shrink, its spin increases
Conservation of angular momentum
Collapse_of_solar_nebula.swf

41. Collisions flatten the cloud into a disk.
The orderly motions of our solar system today are a direct result of the solar system’s birth in a spinning, flattened cloud of gas.

42. Collisions between gas particles in cloud gradually reduce random motions

43. Collisions between gas particles also reduce up and down motions

44. Spinning cloud flattens as it shrinks

45. We see plenty of evidence for spinning disks of gas andf dust around other stars, especially newly formed stars

46. What have we learned? • Where did the solar system come from?
The cloud of gas that gave birth to our solar system was the product of recycling of gas through many generations of stars within our galaxy. This gas consisted of 98% hydrogen and helium and 2% everything else combined.

47. What have we learned?
• What caused the orderly patterns of motion in our solar system?
A collapsing gas cloud naturally tends to heat up, spin faster, and flatten out as it shrinks in size. Thus, our solar system began as a spinning disk of gas. The orderly motions we observe today all came from the orderly motion of this spinning disk of gas.

48. 6.4 The Formation of Planets
Our Goals for Learning
• Why are there two types of planets?
• Where did asteroids and comets come from?
• How do we explain the existence of our Moon and other “exceptions to the rules”?
• When did the planets form?

49. Four Unexplained Features of our Solar System
√ Why do large bodies in our solar system have orderly motions?
--> 2) Why are there two types of planets?
3) Where did the comets and asteroids come
from?
4) How can we explain the exceptions the the ‘rules’ above?

50. Why are there two types of planet, when all planets formed from the same nebula?

51. As gravity causes cloud to contract, it heats up
Conservation of energy

52. Inner parts of disk are hotter than outer parts.
Rock can be solid at much higher temperatures than ice.

53. 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.

54. Tiny solid particles stick to form planetesimals.

55. Gravity draws planetesimals together to form planets
This process of assembly
is called accretion
Same as previous

56. Gravity of rock and ice in jovian planets draws in H and He gases

57. Moons of jovian planets form in miniature disks

58. Why are there two types of planets?
Outer planets get bigger because abundant hydrogen compounds condense to form ICES.
Outer planets accrete and keep H & He gas because they’re bigger.

59. Four Unexplained Features of our Solar System
√ Why do large bodies in our solar system have orderly motions?
√ Why are there two types of planets?
--> 3) Where did the comets and asteroids come from?
4) How can we explain the exceptions the the ‘rules’ above?

60. Comets and asteroids are leftover planetesimals.
• Asteroids are rocky because they formed inside the frostline.
• Comets are icy because they formed outside the frostline

61. Outflowing matter from the Sun -- the solar wind -- blew away the leftover gases

62. Four Unexplained Features of our Solar System
√ Why do large bodies in our solar system have orderly motions?
√ Why are there two types of planets?
√ Where did the comets and asteroids come
from?
--> 4) How do we explain the existence of our Moon and other “exceptions to the rules”?

63. Earth’s moon was probably created when a big planetesimal slammed into the newly forming Earth.
Other large impacts may be responsible for other exceptions like rotation of Venus and Uranus

64. Review of nebular theory
Fig 6.27

65. Four Features of our Solar System - Explained
√ Why do large bodies in our solar system have orderly motions?
√ Why are there two types of planets?
√ Where did the comets and asteroids come
from?
√ How do we explain the existence of our Moon and other “exceptions to the rules”?
Add ‘what have we learned’ slide here

66. When did the planets form?

67. 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

68. The decay of radioactive elements into other elements is a key tool in finding the ages of rocks

69. Age dating of meteorites that are unchanged since they condensed and accreted tell us that the solar system is about 4.6 billion years old.

70. What have we learned? • Why are there two types of planets?
Planets formed around solid “seeds” that condensed from gas and then grew through accretion. In the inner solar system, temperatures were so high that only metal and rock could condense. In the outer solar system, cold temperatures allowed more abundant ices to condense along with metal and rock.

71. What have we learned?
• How do we explain the existence of our Moon and other “exceptions to the rules”?
Most of the exceptions probably arose from collisions or close encounters with leftover planetesimals, especially during the heavy bombardment that occurred early in the solar system’s history. Our Moon is probably the result of a giant impact between a Mars-size planetesimal and the young Earth.
• Where did asteroids and comets come from?
Asteroids are the rocky leftover planetesimals of the inner solar system, and comets are the icy leftover planetesimals of the outer solar system.

72. What have we learned? • When did the planets form?
The planets began to accrete in the solar nebula about 4.6 billion years ago, a fact we determine from radiometric dating of the oldest meteorites.

73. 6.5 Other Planetary Systems
Our Goals for Learning
• How do we detect planets around other stars?
• What have other planetary systems taught us about our own?

74. How do we detect planets around other stars?

75. We detect planets around other stars by looking for a periodic motion of the stars they orbit.
We measure the motion through the Doppler shift of the star’s spectrum

76. The size of the wobble tells us the planet’s mass
The period of the wobble tells us the radius of its orbit (Kepler’s 3rd law)

77. We can also detect planets if they eclipse their star
Fraction of starlight blocked tells us planet’s size

78. What have other planetary systems taught us about our own?

79. Over 120 known extrasolar planets as of 2004
Most are more massive than Jupiter and closer to their star than Earth is to Sun
Revisions to the nebular theory are necessary! Planets can apparently migrate inward
from their birthplaces.

80. Is Earth Unusual? No Earth-like planets found yet.
Data aren’t good enough to tell if they are common or rare
Available methods can only detect BIG planets.

81. What have we learned? • How do we detect planets around other stars?
So far, we are only able to detect extrasolar planets indirectly by observing the planet’s effects on the star it orbits. Most discoveries to date have been made with the Doppler technique, in which Doppler shifts reveal the gravitational tug of a planet (or more than one planet) on a star.

82. What have we learned?
• What have other planetary systems taught us about our own?
Planetary systems exhibit a surprising range of layouts, suggesting that jovian planets sometimes migrate inward from where they are born. This lesson has taught us that despite the successes of the nebular theory, it remains incomplete.