Planet Formation

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Planet Formation
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Outline for Today

• How old is the solar system?
• Observed characteristics of the solar system
• Formation of the solar nebula
• Temperature pressure of the solar nebula
• Formation of planets
• Evolution of Terrestrial planets
• Observations of disks around other stars

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The Age of the Solar System

• We can estimate the age of the Solar System by
  looking at radioactive isotopes. These are
  unstable forms of elements that produce energy by
  splitting apart (i.e., fission).
• The radioactivity of an isotope is characterized
  by its half-life the time it takes for half of
  the parent to decay into its daughter element.
  By measuring the ratio of the parent to daughter,
  one can estimate how long the material has been
  around.

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Radioactive Elements
Each of these isotopes spontaneously decays into
its daughter. In each case, the daughter weighs
less than the parent energy is produced.
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Age of the Solar System

• When rocks are molten, heavier elements (such as
  uranium) will separate out from other elements.
  (In liquids, dense things sink, light things
  rise.) Once the rocks solidify, radioactive
  decay will then take over.
• On Earth, the oldest rocks have ages of 3 billion
  years
• The oldest asteroids have ages of 4.5 billion
  years
• Rocks from the plains on the Moon have ages of
  about 3 billion years. The oldest Moon rocks
  have ages of 4.5 billion years.
• The solar system is therefore 4.5 billion years
  old.

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Characteristics of the Solar System

• Any theory for the formation of the Solar System
  must explain
• The flatness of the Solar System
• All of the planets orbit in the same direction
• The separation of Terrestrial and Jovian planets
• The decrease in planet densities with distance
  from the Sun

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1 light year
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Formation of the Solar Nebula
In a large, slowly rotating cloud of cold gas

• Self gravity begins to collapse the cloud
• As the cloud gets smaller, it begins to rotate
  faster, due to conservation of angular momentum.

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Formation of the Solar Nebula
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Formation of the Solar Nebula
In a large, slowly rotating cloud of cold gas

• Self gravity begins to collapse the cloud
• As the cloud gets smaller, it begins to rotate
  faster, due to conservation of angular momentum.
• Centripetal force prevents gas from collapsing in
  the plane of rotation
• Gas falling from the top collides with gas
  falling from the bottom and sticks together in
  the ecliptic plane

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Formation of the Solar Nebula
In the flat solar nebula

• The densest region of the disk (the center)
  becomes the Sun. Eventually, fusion in the Sun
  will occur.
• Atoms orbiting in the disk bump together and form
  molecules, such as water. Droplets of these
  molecules stick together to form planetesimals.
• Over time, the planetesimals grow as more
  molecules condense out of the nebula

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Formation of the Solar Nebula
Planetesimals grow

• Differential rotation (due to Keplers laws) will
  cause particles in similar orbits to eventually
  meet up. One will accrete onto the other,
  forming a bigger body.
• The bigger the body, the greater its
  gravitational force, and the more attraction it
  has for other bodies. Further accretion will
  occur. Protoplanets form.

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Formation of the Solar Nebula
Material begins to evaporate

• While protoplanets are forming, the Suns
  luminosity is growing, first due to gravitational
  contraction, then due to nuclear ignition.
• Regions of the nebula close to the Sun will get
  hot the outer regions will stay cool. In the
  hot regions, light elements will evaporate only
  heavy elements will condense out of the nebula

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Temperature of the Solar Nebula

• Inside the orbit of the Earth, only metals can
  condense out of the solar nebula. Rocky
  (silicates) can condense near Mars. In the outer
  solar system, water and ammonia ice can survive.

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Temperature of the Solar Nebula

• Inside the orbit of the Earth, only metals can
  condense out of the solar nebula. Rocky
  (silicates) can condense near Mars. In the outer
  solar system, water and ammonia ice can survive.

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Radiation Pressure and the Solar Wind

• Two other processes are also important for
  driving light gases from the inner part of the
  solar system.

Radiation pressure Photons act like particles
and push whatever particles and dust they run
into.
Solar wind The Sun constantly ejects (a little)
hydrogen and helium into space. This solar wind
pushes whatever gas and dust it runs into.
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Accretion

• Once the major bodies of the solar system were
  formed, most of the remaining debris was either
  ejected out of the solar system or accreted onto
  other bodies by gravitational encounters.

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Differentiation
Early in the history of the solar system, planets
would be molten due to
Continuous accretion of left over material from
the solar system formation.
Energy from the fission of radioactive isotopes.
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Evolution of Terrestrial Planets

• After the condensation and accretion phases of
  planet formation, terrestrial bodies can go
  through 4 different stages of evolution. (The
  rates of evolution can vary greatly.)
• Differentiation in a molten planet, heavy
  materials sink

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Evolution of Terrestrial Planets

• After the condensation and accretion phases of
  planet formation, terrestrial bodies can go
  through 4 different stages of evolution. (The
  rates of evolution can vary greatly.)
• Differentiation in a molten planet, heavy
  materials sink

• Cratering left over bodies impact the planets
  surface

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Evolution of Terrestrial Planets

• After the condensation and accretion phases of
  planet formation, terrestrial bodies can go
  through 4 different stages of evolution. (The
  rates of evolution can vary greatly.)
• Differentiation in a molten planet, heavy
  materials sink
• Cratering left over bodies impact the planets
  surface

• Flooding water, lava, and gases trapped inside
  the planet come to the surface and cover the
  terrain.

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Evolution of Terrestrial Planets

• After the condensation and accretion phases of
  planet formation, terrestrial bodies can go
  through 4 different stages of evolution. (The
  rates of evolution can vary greatly.)
• Differentiation in a molten planet, heavy
  materials sink
• Cratering left over bodies impact the planets
  surface

• Flooding water, lava, and gases trapped inside
  the planet come to the surface and cover the
  terrain.

• Erosion surface features are destroyed due to
  running water, atmosphere, plate tectonics, and
  geologic motions

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Observations of Protostellar Disks

• The solar nebula theory states that young stars
  should be surrounded by a disk consisting of
  molecular gas and dust. These are now being
  observed.