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Chapter 69 Part 2 Formation of Planetary Systems Our Solar System and Beyond

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A young planet's motion can create waves in a planet-forming disk. ... that matter in these waves can tug on a planet, causing its orbit to migrate inward. ... – PowerPoint PPT presentation

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Title: Chapter 69 Part 2 Formation of Planetary Systems Our Solar System and Beyond


1
Chapter 6-9 Part 2Formation of Planetary
SystemsOur 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
Rotation of a contracting cloud speeds up for the
same reason a skater speeds up as she pulls in
her arms.
Collapse of the Solar Nebula
7
Flattening
  • Collisions between particles in the cloud caused
    it to flatten into a disk.

8
Collisions between gas particles in a cloud
gradually reduce random motions.
Formation of Circular Orbits
9
Collisions between gas particles also reduce up
and down motions.
10
The spinning cloud flattens as it shrinks.
Formation of the Protoplanetary Disk
11
Disks Around Other Stars
  • Observations of disks around other stars support
    the nebular hypothesis.

12
Why are there two major types of planets?
13
Conservation of Energy
As gravity causes the cloud to contract, it heats
up.
Collapse of the Solar Nebula
14
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
15
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
16
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.

17
Tiny solid particles stick to form planetesimals.
Summary of the Condensates in the Protoplanetary
Disk
18
Gravity draws planetesimals together to form
planets. This process of assembly is called
accretion.
Summary of the Condensates in the Protoplanetary
Disk
19
Accretion of Planetesimals
  • Many smaller objects collected into just a few
    large ones.

20
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.

21
The gravity of rock and ice in jovian planets
draws in H and He gases.
Nebular Capture and the Formation of the Jovian
Planets
22
Moons of jovian planets form in miniature disks.
23
Radiation and outflowing matter from the Sun
the solar wind blew away the leftover gases.
The Solar Wind
24
Where did asteroids and comets come from?
25
Asteroids and Comets
  • Leftovers from the accretion process
  • Rocky asteroids inside frost line
  • Icy comets outside frost line

26
How do we explain the existence of our Moon and
other exceptions to the rules?
27
Heavy Bombardment
  • Leftover planetesimals bombarded other objects in
    the late stages of solar system formation.

28
Giant Impact
Giant impact stripped matter from Earths crust
Stripped matter began to orbit
Then accreted into Moon
29
Origin of Earths Water
  • Water may have come to Earth by way of icy
    planetesimals from the outer solar system.

30
Odd Rotation
  • Giant impacts might also explain the different
    rotation axes of some planets.

31
Captured Moons
  • The unusual moons of some planets may be captured
    planetesimals.

32
Review of nebular theory
33
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.

34
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.

35
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.

36
Radioactive Decay
  • Some isotopes decay into other nuclei.
  • A half-life is the time for half the nuclei in a
    substance to decay.

37
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

38
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

39
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.

40
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.

41
How do we detect planets around other stars?
42
Planet Detection
  • Direct Pictures or spectra of the planets
    themselves
  • Indirect Measurements of stellar properties
    revealing the effects of orbiting planets

43
Direct Detection
  • Special techniques for concentrating or
    eliminating bright starlight are enabling the
    direct detection of planets.

44
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.

Stellar Motion due to Planetary Orbits
45
Gravitational Tugs
  • Suns motion around solar systems 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.

46
Astrometric Technique
  • We can detect planets by measuring the change in
    a stars position in the sky.
  • However, these tiny motions are very difficult to
    measure (0.001 arcsecond).

47
Doppler Technique
  • Measuring a stars 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
48
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)

49
First Extrasolar Planet Detected
  • The planet around 51 Pegasi has a mass similar to
    Jupiters, despite its small orbital distance.

50
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.

51
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.

52
Transits and Eclipses
  • A transit is when a planet crosses in front of a
    star.
  • The resulting eclipse reduces the stars apparent
    brightness and tells us the planets radius.
  • When there is no orbital tilt, an accurate
    measurement of planet mass can be obtained.

53
How do extrasolar planets compare with those in
our solar system?
54
Measurable Properties
  • Orbital period, distance, and shape
  • Planet mass, size, and density
  • Composition

55
Orbits of Extrasolar Planets
  • Most of the detected planets have orbits smaller
    than Jupiters.
  • Planets at greater distances are harder to detect
    with the Doppler technique.

56
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.

57
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.

58
Surprising Characteristics
  • Some extrasolar planets have highly elliptical
    orbits.
  • Some massive planets orbit very close to their
    stars Hot Jupiters.

59
Hot Jupiters
60
Do we need to modify our theory of solar system
formation?
61
Revisiting the Nebular Theory
  • Nebular theory predicts that massive Jupiter-like
    planets should not form inside the frost line (at
  • The discovery of hot Jupiters has forced a
    reexamination of nebular theory.
  • Planetary migration or gravitational encounters
    may explain hot Jupiters.

62
Planetary Migration
  • A young planets 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.

63
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.

64
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.

65
Chapter 9Asteroids, Comets, and Dwarf Planets
  • Their Nature, Orbits, and Impacts

66
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 wouldnt
    add up to even a small terrestrial planet.

67
Asteroid Orbits
  • Most asteroids orbit in a belt between Mars and
    Jupiter.
  • Trojan asteroids follow Jupiters orbit.
  • Orbits of near-Earth asteroids cross Earths
    orbit.

68
Origin of Asteroid Belt
  • Rocky planetesimals between Mars and Jupiter did
    not accrete into a planet.
  • Jupiters gravity, stirred up asteroid orbits and
    prevented their accretion into a planet.

69
Origin of Meteorites
  • Most meteorites are pieces of asteroids.

70
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
    Earths atmosphere. (
  • Meteorite A rock from space that falls through
    Earths atmosphere and reaches Earth..

71
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.

72
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.

73
Growth of Tail
74
Comets eject small particles that follow the
comet around in its orbit and cause meteor
showers when Earth crosses the comets orbit.
75
Meteors in a shower appear to emanate from the
same area of sky because of Earths motion
through space.
76
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 30100 AU in
disk of solar system
77
How did they get there?
  • Kuiper belt comets formed in the Kuiper belt
    flat plane, aligned with the plane of planetary
    orbits, orbiting in the same direction as the
    planets.
  • Oort cloud comets were once closer to the Sun,
    but they were kicked out there by gravitational
    interactions with jovian planets spherical
    distribution, orbits in any direction.

78
How big can a comet be?
79
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.

80
Discovering Large Iceballs
  • In summer 2005, astronomers discovered Eris, an
    iceball even larger than Pluto.
  • Eris even has a moon Dysnomia.

81
Other Icy Bodies
  • There are many icy objects like Pluto on
    elliptical, inclined orbits beyond Neptune.
  • The largest ones are comparable in size to
    Earths Moon.

82
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?

83
HSTs view of Pluto and moons
84
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?

85
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.

86
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.

87
Iridium Layer
No dinosaur fossils in upper rock layers
Thin layer containing the rare element iridium
Dinosaur fossils in lower rock layers
88
Likely Impact Site
  • Geologists found a large subsurface crater about
    65 million years old in Mexico.

89
Frequency of Impacts
  • Small impacts happen almost daily.
  • Impacts large enough to cause mass extinctions
    are many millions of years apart.
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