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

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Chapter 6-9 Part 2Formation of Planetary
SystemsOur Solar System and Beyond
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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)

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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
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What caused the orderly patterns of motion in our
solar system?
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Conservation of Angular Momentum

• The rotation speed of the cloud from which our
  solar system formed must have increased as the
  cloud contracted.

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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
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Flattening

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

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Collisions between gas particles in a cloud
gradually reduce random motions.
Formation of Circular Orbits
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Collisions between gas particles also reduce up
and down motions.
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The spinning cloud flattens as it shrinks.
Formation of the Protoplanetary Disk
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Disks Around Other Stars

• Observations of disks around other stars support
  the nebular hypothesis.

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

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Tiny solid particles stick to form planetesimals.
Summary of the Condensates in the Protoplanetary
Disk
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Gravity draws planetesimals together to form
planets. This process of assembly is called
accretion.
Summary of the Condensates in the Protoplanetary
Disk
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Accretion of Planetesimals

• Many smaller objects collected into just a few
  large ones.

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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.

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The gravity of rock and ice in jovian planets
draws in H and He gases.
Nebular Capture and the Formation of the Jovian
Planets
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Moons of jovian planets form in miniature disks.
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Radiation and outflowing matter from the Sun
the solar wind blew away the leftover gases.
The Solar Wind
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Where did asteroids and comets come from?
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Asteroids and Comets

• Leftovers from the accretion process
• Rocky asteroids inside frost line
• Icy comets outside frost line

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How do we explain the existence of our Moon and
other exceptions to the rules?
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Heavy Bombardment

• Leftover planetesimals bombarded other objects in
  the late stages of solar system formation.

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Giant Impact
Giant impact stripped matter from Earths crust
Stripped matter began to orbit
Then accreted into Moon
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Origin of Earths Water

• Water may have come to Earth by way of icy
  planetesimals from the outer solar system.

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Odd Rotation

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

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Captured Moons

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

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Review of nebular theory
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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.

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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.

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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.

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Radioactive Decay

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

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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

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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

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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.

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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.

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How do we detect planets around other stars?
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Planet Detection

• Direct Pictures or spectra of the planets
  themselves
• Indirect Measurements of stellar properties
  revealing the effects of orbiting planets

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Direct Detection

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

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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
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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.

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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).

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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
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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)

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First Extrasolar Planet Detected

• The planet around 51 Pegasi has a mass similar to
  Jupiters, despite its small orbital distance.

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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.

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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.

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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.

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How do extrasolar planets compare with those in
our solar system?
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Measurable Properties

• Orbital period, distance, and shape
• Planet mass, size, and density
• Composition

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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.

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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.

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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.

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Surprising Characteristics

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

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

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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.

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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.

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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.

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Chapter 9Asteroids, Comets, and Dwarf Planets

• Their Nature, Orbits, and Impacts

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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.

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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.

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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.

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Origin of Meteorites

• Most meteorites are pieces of asteroids.

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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..

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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.

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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.

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Growth of Tail
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Comets eject small particles that follow the
comet around in its orbit and cause meteor
showers when Earth crosses the comets orbit.
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Meteors in a shower appear to emanate from the
same area of sky because of Earths motion
through space.
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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
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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.

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How big can a comet be?
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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.

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Discovering Large Iceballs

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

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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.

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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?

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HSTs view of Pluto and moons
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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?

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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.

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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.

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Iridium Layer
No dinosaur fossils in upper rock layers
Thin layer containing the rare element iridium
Dinosaur fossils in lower rock layers
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Likely Impact Site

• Geologists found a large subsurface crater about
  65 million years old in Mexico.

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Frequency of Impacts

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