Chapter 15: The Formation of Planetary Systems The Birth of Our World

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Chapter 15 The Formation of Planetary
SystemsThe Birth of Our World
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Modeling the Solar System

• A complex problem only now becoming understood
• Most of our knowledge of solar system formation
  comes from studies of
• interstellar gas clouds
• fallen meteorites
• Earths Moon
• Other planets and moon of the Solar System
• Earth provides little clue, too much change
• Extra solar planets are forcing a reevaluation of
  once held beliefs

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Modeling the Solar System

• Any model must prescribe to the know facts
• For the Solar system these facts are
• Each planet is relatively isolated in space
• Planetary orbits are almost circular
• Planetary orbits lie nearly in the same plane
• The direction the Sun rotates and planets revolve
  are the same
• The direction most planets revolve and the Sun
  rotates is the same

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Modeling the Solar System

• More Facts
• Most moons rotate and revolve the same direction
  their planets rotate
• The Solar System is highly differentiated
• Asteroids are very old and share little in common
  with the planets or their moons
• Comets are old, icy, and have very elliptical
  orbits

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Nebular Theory

• First proposed by Descartes, formalized by
  Laplace
• The Sun and planets formed from a large cloud of
  interstellar gas
• Conservation of angular momentum formed disk
• The sun formed in the middle and the planets in
  the disk

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Nebular Theory

• Strengths
• Explains planets circular orbit
• Explains why planets orbit in a plane
• Relates planet revolution with Suns rotation
• Weaknesses
• Laplaces ideal of planets formed from rings
  doesnt work
• Interstellar dust is not accounted for

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Condensation Theory

• Keeps the ideal of solar nebula
• Unlike nebular theory, planets formed in a way
  requiring the presence of interstellar dust
• Dust acted as a catalyst for accretion

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Condensation Theory

• Solar nebula contracts to form a spinning disk
• Interstellar dust grains act as condensation
  nuclei allowing accretion of planetesimals
• Solar winds push gas out, outer planets already
  formed
• Inner planets start to form from collisions of
  planetesimals
• Collisions continue over time making the 4 inner
  planets

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Accretion

• Accretion- the slow build up of matter around
  dust grains
• Create millions of small asteroid sized
  planetesimals
• planetesimals collected through collisions
  forming protoplanets which continued to increase
  in size eventually forming the present planets

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Outer Planet Formation

• Two theories
• More material in the outer solar system allowed
  four large protoplanets to form relatively
  quickly, which gravitationally attracted large
  amounts of the solar nebulas gas
• Formed as miniature solar systems, condensing
  from instabilities in the initial solar nebula

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Observational Evidence

• Beta Pictoris has a visible disk of warm gas
  surrounding it
• Star HR4796A evidence of planetessiamal growth

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Temperature and Differentation

• The solar nebula was hot in the middle(1000k)
  and cooler further out (100K near Saturn)
• Far too hot in the inner solar system for
  anything but metals to exist
• This is why the inner planets are so deficient in
  volatiles

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Temperature and the Outer Planets

• Temperature decreased with distance from the
  center of the solar nebula, allowing nonmetals to
  condense
• This provided the Jovian planets with a great
  deal more material to accrete, hence their large
  size
• Carbon, nitrogen, and oxygen were the first
  elements to condense

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Jovian Planets, Comets and the 2 Clouds

• The early outer solar system contained a great
  number of icy planetesimals
• After the Jovian Planets formed, their gravity
  assisted these planetesimals to more distance
  orbits
• This formed the Oort Cloud and left the Kuiper
  Belt just outside Neptunes Orbit

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What happened to all the gas?

• The inner planets were not massive enough to
  accrete the lightest gases of the Solar Nebula
• Just prior to nuclear ignition, a star will enter
  the T Tauri phase, when the solar wind is very
  strong
• This pressure pushed the gas out into space
  leaving the inner protoplanets and the already
  formed outer giants

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Catastrophes

• Condensation Theory explains most of the 9 points
  listed earlier
• It explains some of the anomalies in the Solar
  System as well
• Very large collisions can account for most of
  these aspects
• Uranus tilt
• Earth-Moon system

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Planets Beyond the Solar System

• It has long been thought that there are planetary
  systems besides the Solar System
• It was not until the mid-Nineties that proof was
  provided
• Direct observation not possible
• Doppler wobbles

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Indirect evidence Doppler Shifting

• Extrasolar planets are too faint and close to
  their parent star for any current telescope to
  resolve
• A strong indicator of a large planet orbiting a
  star is an observable wobble about their
  center of mass
• Measurement of the Doppler shift of the light
  gives the planets mass

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Examples
Velocity Curve Star HD130322b
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Solar System Vs. Extrasolar Systems
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Why are these systems so different from us?

• To date only extrasolar planets of sizes
  comparable to Jupiter have been found
• A terrestrial sized planets effect on a star is
  too small to be measured
• Most extrasolar planets have very eccentric
  orbits
• We are not seeing system edge on
• Brown Dwarfs
• Gravitation interactions kept the Jupiter in
  circular, cold orbit
• We only have a small amount of data, and is not
  representative of ALL extrasolar planetary systems

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Finding Earths

• The rare event of a transit of an extrasolar
  planet and its star provides and alternate means
  of identification
• An extrasolar planets spectra would provide
  clues about its atmosphere, and if it was
  Earthlike in nature

An Earth-like Spectra