1. Chapter 8 Formation of Planetary Systems Our Solar System and Beyond

2. Where did the solar system come from?
According to the nebular theory, our solar system formed from a giant cloud of interstellar gas.
(nebula = cloud)

3. 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

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

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

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

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

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

9. Why are there two major types of planets?

10. Conservation of Energy
As gravity causes the cloud to contract, it heats up.

11. 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

12. 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

13. 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.

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

15. 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

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

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

18. Moons of jovian planets form in miniature disks.

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

20. Where did asteroids and comets come from?

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

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

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

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

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

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

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

28. Review of nebular theory

29. Thought Question
How would the solar system be different if the solar nebula had only been half as hot?
Jovian planets would have formed closer to the Sun.
There would be no asteroids.
There would be no comets.
Terrestrial planets would be larger.

30. 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.

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

32. 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

33. 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.

34. 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.

35. How do we detect planets around other stars?

36. Planet Detection Direct: Pictures or spectra of the planets themselves
Indirect: Measurements of stellar properties revealing the effects of orbiting planets
For more check out this website:

37. Direct Detection
Special techniques for concentrating or eliminating bright starlight are enabling the direct detection of a very few extrasolar planets.

38. Indirect detection : 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.

39. 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.

40. 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).

41. 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!).

42. 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)

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

44. 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…)

45. 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.

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

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

48. 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.

49. 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.

50. Planets: Common or Rare?
Observations indicate ~2-5% of sun-like stars have Jupiter size planets
The others may still have smaller (Earth-sized) planets that cannot be detected using current techniques.
Current estimations range between 20 and 50% of sun-like stars having planets.

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

52. Hot Jupiters

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

54. 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.”

55. 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.

56. 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.

57. 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.

58. Chapter 12 Asteroids, Comets, and Dwarf Planets
Their Nature, Orbits, and Impacts
Fun clip: the trailer from the movie Armageddon, if you can find it. The file was called armageddon.mov in Feb 2004 but it’s scarce now for some reason.
See also for Phil Plait’s review

59. Asteroid Facts Asteroids are rocky leftovers of planet formation.
The largest is Ceres, diameter ~1,000 km.
There are 150,000 in catalogs, and probably over a million with diameter >1 km.
Small asteroids are more common than large asteroids.
All the asteroids in the solar system wouldn’t add up to even a small terrestrial planet.

60. Asteroid Orbits
Most asteroids orbit in a belt between Mars and Jupiter.
Trojan asteroids follow Jupiter’s orbit.
Orbits of near-Earth asteroids cross Earth’s orbit.

61. Origin of Asteroid Belt
Rocky planetesimals between Mars and Jupiter did not accrete into a planet.
Jupiter’s gravity, stirred up asteroid orbits and prevented their accretion into a planet.

62. Origin of Meteorites
Most meteorites are pieces of asteroids.

63. Meteor Terminology
Meteoroid: A small piece of rock or dust in space. Not as big as an asteroid.
Meteor: A meteoroid that is falling through the Earth’s atmosphere. (shooting stars)
Meteorite: A rock from space that falls through Earth’s atmosphere and reaches Earth..

64. Comet Facts
Formed beyond the frost line, comets are icy counterparts to asteroids.
The nucleus of a comet is like a “dirty snowball.”
Most comets do not have tails.
Most comets remain perpetually frozen in the outer solar system.
Only comets that enter the inner solar system grow tails.

65. Anatomy of a Comet Coma is atmosphere that comes from heated nucleus.
Plasma tail is gas escaping from coma, pushed by solar wind.
Dust tail is pushed by photons.

66. Growth of Tail

67. Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet’s orbit.

68. Meteors in a shower appear to emanate from the same area of sky because of Earth’s motion through space.

69. Only a tiny number of comets enter the inner solar system; most stay far from the Sun.
Oort cloud:
On random orbits extending to about 50,000 AU
Kuiper belt:
On orderly orbits from 30–100 AU in disk of solar system

70. How did they get there?
Kuiper belt objects are believed to have formed in the their current location along with the formation of the rest of the solar system. They orbit in a flat plane, aligned with the plane of planetary orbits, orbiting in the same direction as the planets.
Oort cloud objects were once closer to the Sun, but they were kicked out there by gravitational interactions with jovian planets: spherical distribution, orbits in any direction.

71. Is Pluto a Planet? Much smaller than the eight major planets
Not a gas giant like the outer planets
Has an icy composition like a comet
Has a very elliptical, inclined orbit
Pluto has more in common with comets than with the eight major planets.

72. Discovering Large Iceballs
In summer 2005, astronomers discovered Eris, an iceball even larger than Pluto.
Eris even has a moon: Dysnomia.

73. Dwarf Planets
There are many icy objects like Pluto on elliptical, inclined orbits beyond Neptune.
The largest ones are comparable in size to Earth’s Moon.
If the object is massive enough for its own gravity to make it spherical(ish) then it is called a dwarf planet.

74. Kuiper Belt Objects
These large, icy objects have orbits similar to the smaller objects in the Kuiper Belt that become short period comets.
So are they very large comets or very small planets?

75. HST’s view of Pluto and moons

76. 9.4 Cosmic Collisions: Small Bodies Versus the Planets
Our goals for learning:
Have we ever witnessed a major impact?
Did an impact kill the dinosaurs?
Is the impact threat a real danger or just media hype?
How do other planets affect impact rates and life on Earth?

77. Mass Extinctions
Fossil record shows occasional large dips in the diversity of species: mass extinctions.
The most recent was 65 million years ago, ending the reign of the dinosaurs.

78. Iridium: Evidence of an Impact
Iridium is very rare in Earth surface rocks but is often found in meteorites.
Luis and Walter Alvarez found a worldwide layer containing iridium, laid down 65 million years ago, probably by a meteorite impact.
Dinosaur fossils all lie below this layer.

79. Iridium Layer No dinosaur fossils in upper rock layers
Thin layer containing the rare element iridium
Dinosaur fossils in lower rock layers

80. Likely Impact Site
Geologists found a large subsurface crater about 65 million years old in Mexico.

81. Frequency of Impacts Small impacts happen almost daily.
Impacts large enough to cause mass extinctions are many millions of years apart.