Formation of the Solar System

1
Formation of the Solar System
2
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.

3
Radioactive Elements
Isotope protons neutrons Daughter Half-life (years)
Rubidium-87 37 50 Strontium-87 47,000,000,000
Uranium-238 92 146 Lead-206 4,510,000,000
Uranium-235 92 143 Lead-207 710,000,000
Potassium-40 19 21 Argon-40 1,280,000,000
Aluminum-26 13 13 Magnesium-26 730,000
Carbon-14 6 8 Nitrogen-14 5,730
Each of these isotopes spontaneously decays into
its daughter. In each case, the daughter weighs
less than the parent energy is produced.
4
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, the material can
  no longer differentiate. Lighter elements (made
  from radioactive decay) stay in the same location
  as they form.
• On Earth, most old 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.

5
Keys to Solar System Formation

• Any theory for the formation of the Solar System
  must explain
• The flatness of the Solar System, and orbital
  similarities
• The separation of Terrestrial and Jovian planets
• The decrease in planet densities with distance
  from the Sun
• Bodes Law

6
Star/Planet Formation

• The story of planet formation is in large part,
  the story of star formation. Inside dense
  interstellar clouds of gas and dust, the
  temperature is just a few degrees above absolute
  zero. Since the temperature is so low, there is
  no gas pressure to resist gravity. The cloud
  collapses.

7
Initial Collapse
Dark clouds are much denser in their center than
on the outside, so their inner regions collapse
first.
Also, since the clouds are lumpy to begin with,
the collapse process causes the clouds to
fragment.
Each fragment is a protostar.
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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

9
Formation of the Solar Nebula
In the flat solar nebula

• The densest region (the center) becomes the Sun.
  Friction in the disk causes the Sun to accrete
  matter and grow in mass. Eventually, fusion
  occurs.
• Atoms orbiting in the disk bump together and form
  molecules, such as water. Droplets of these
  molecules stick together to form planetesimals.

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Formation of the Solar Nebula
Planetesimals grow

• Over time, the planetesimals grow as more
  molecules condense out of the nebula
• Differential rotation (due to Keplers laws)
  cause particles in similar orbits to meet up.
  They stick together forming a bigger body.
• The bigger the body, the greater its gravity, and
  the more attraction it has for other bodies.
  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

12
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.

13
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.
14
The Pre-Main Sequence Sun

• As the Sun formed, it generated its energy via
  gravitational contraction. During this time, it
  was a lot brighter than it is today. The
  radiation pressure in the inner solar system was
  greater.
• In addition, due to conservation of angular
  momentum, the young Sun was also spinning faster
  than it is today. This caused the solar wind to
  be stronger.

15
The Pre-Main Sequence Sun

• As the Sun formed, it generated its energy via
  gravitational contraction. During this time, it
  was a lot brighter than it is today. The
  radiation pressure in the inner solar system was
  greater.
• In addition, due to conservation of angular
  momentum, the young Sun was also spinning faster
  than it is today. This caused the solar wind to
  be stronger.

Radiation pressure and the solar wind blew out
the light material from the inner part of the
solar system.
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The Protoplanetary Disk
17
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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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.

19
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.

20
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.

Unless a body is well-separated from everything
else, or its orbit is in a resonance, its orbit
will be chaotic. Eventually, it will either
crash into something, or leave the solar system
completely.
21
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.

22
Formation of the Solar System
From interstellar cloud to planetary system
23
Observations of Protostellar Disks

• Our technology is just beginning to be able to
  resolve the proto-planetary disks around stars.

24
Observations of Protostellar Disks

• Our technology is just beginning to be able to
  resolve the proto-planetary disks around stars.

25
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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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.
27
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

28
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.

29
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