We are going to discuss the origin of our solar system

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Chapter 19

• We are going to discuss the origin of our solar
  system
  
• Pierre-Simon de Laplace in 1796 brought forth the
  idea of the origin of the solar system that is
  the basis of the modern theory
  
• He thought that a cloud of dust and gas
  contracted under its own gravity

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• It flattened into a disk and became known as the
  nebular hypothesis
  
• The modern idea is called the solar nebula theory
  which says that the planets formed from the disk
  of dust and gas that surrounded the Sun
  
• Our solar system is mostly empty space
• The planets orbit the Sun in orbits that lie
  close to a common plane
  
• Mercurys orbit is tipped 7 to the Earths orbit

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Formation of the solar system
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• As seen from above the solar system the common
  motion appears to be counterclockwise
  
• The planets orbit the Sun in that direction and
  they rotate in that direction (mostly)
  
• There are 2 basic kinds of planets the
  terrestrials and the jovian planets
  
• Mercury, Venus, Earth, and Mars are all
  terrestrial planets
  
• Jupiter, Saturn, Uranus and Neptune are the
  jovian planets

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• There is a large amount of debris in the solar
  system
  
• Asteroids are found throughout the solar system
• We know of over 100,000 of them and 2000 of them
  cross the inner solar system
  
• 200 of them are bigger than 100 km and probably a
  million are over 1 km in size
  
• Most are irregularly shaped like the asteroid
  Eros
  
• Most are found between Mars and Jupiter

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• In 1992 bodies were discovered beyond Neptune
• These are called the Kuiper Belt Objects
• Some of them are over 1000 km in diameter, but
  most are much smaller
  
• Also found in the solar system are the comets
• Comets are thought to be a dirty ice ball
• They are frozen gasses and bits of debris
• As a comet comes into the inner solar system the
  begin to warm up
  

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• When this happens they begin to show off the
  beautiful tail
  
• They follow a long elliptical orbit around the
  Sun
  
• One thing you will find is that the tail always
  points away from the Sun due to the solar wind
  
• The most famous of all is Halleys Comet
• It comes around every 75 79 years
• It has been found in Chinese records back as far
  as about 240 BC

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Comet Enke
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Comet Ikeya-Seki, 1965
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Comet West, 1976
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Comet Hyakutake, 1995
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Comet Hale-Bopp, 1995
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3 galaxies and Comet McNaught, 2007
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Comet Holmes, 2007
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• If you camping away from the city lights at night
  you may see little flashes of light in the sky
  
• These are meteors or shoot stars
• They are simply small pieces of debris entering
  our atmosphere
  
• They travel so fast that friction heats them up
  and burn up in the atmosphere
  
• If they are in space they are called meteoroids,
  when they enter the atmosphere they are called
  meteors and if they hit the ground they are
  meteorites

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• The age of the solar system is thought to be
  about 4.48 billion years old
  
• This was determined from Moon rocks
• We look at the half life of radioactive materials
  to determine this
  
• Uranium has a half life of 4,5 billion years
  which means if you had 1 pound of uranium, in 4.5
  billion years you would have .5 pounds of uranium
  and .5 pounds of lead

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• In this way we can tell the age of the rocks
• Some of the oldest rocks are the meteorites which
  are all about 4.5 billion years old
  
• The oldest dated was 4.56 billion years old
• Lets talk about how the planets formed
• A small interstellar cloud would have been the
  beginning
  
• There would have been hydrogen, helium and some
  trace elements and this is just what our Sun shows

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• The planets formed the way they did due to the
  temperature in the solar system
  
• Planets closer to the Sun contain higher density
  material than do the outer planets
  
• At some point away from the Sun you find the ice
  line
  
• This is where the lighter material could condense
  out
  
• Particles grew by the process of condensation
• This is where the matter was added 1 atom at a
  time
  

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• Smaller particles would stick together to form
  larger particles in a process called accretion
  
• Larger bodies formed called planetesimals
• These are bodies larger than about 1 km
• These bodies settled into the plane of the
  spinning nebula
  
• As these bodies became larger than 100 km new
  processes began to alter them
  
• Planetesimals began to stick together to form
  planets

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• Eventually protoplanets were formed
• As they got bigger, new processes caused them to
  get bigger faster
  
• Since they were all travelling In the same
  direction, they bumped into each other and
  condensed together
  
• As they grew, their gravity pulled in more
  material
  
• As more material accumulated, radioactive
  elements caused the interior to heat up
  
• When this happened they melted

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• Differentiation occurred at this point
• This means that the heaviest material settled to
  the center, then the next heaviest, and so on
  
• If this is true, then our atmosphere is not our
  first
  
• It would have consisted of lighter gases such as
  hydrogen and helium
  
• They would have been driven off by the heat from
  the Sun
  
• Gases released from the interior would have
  formed the secondary atmosphere

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• That process is called outgassing
• As the nebula cooled, the first particles would
  have been the metals
  
• The next material formed would have been the
  silicates

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Solar System Formation
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• For years it was thought that the outer planets
  formed when the gas from the inner solar system
  was blown outward
  
• It appears this idea may be wrong
• When our Sun was very young it was quite violent
• It seems that the disks we talk about would have
  been destroyed by the intense radiation and solar
  wind
  
• The disks could be destroyed in about 1 million
  years

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• That isnt long enough to form the outer planets
• The new model shows the outer planets formed by
  direct gravitational collapse
  
• That would mean that planets like Jupiter would
  form from the gas without first accreting a dense
  core
  
• Jupiter sized planets formed within a few hundred
  years in the computer simulations
  
• Thishas caused a major controversy

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• With the discovery of extrasolar planets we see
  that Jupiter sized planets are common
  
• It appears that through gravitational
  interactions these planets can migrate
  
• If it migrates as it forms, then it moves into
  areas with more matter and they can grow
  
• A Jupiter sized planet could form in a million
  years or so
  
• Migration also helps explain how Uranus and
  Neptune could have formed

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• As far out as they are it is unlikely that they
  could have accreted as much gas as they have now
  
• Looking at all of this makes it seem like the
  solar system should have formed just like it is
  
• But there are exceptions
• Venus rotates clockwise and Uranus rotates on its
  side
  
• These are probably due to some catastrophic event

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• One idea is an off-center collision with another
  body
  
• For Uranus another possibility is that tidal
  interaction with Saturn could have pulled it over
  on its side
  
• Both Jupiter and Saturn release more energy than
  they receive from the Sun
  
• The asteroid belt is a consequence of Jupiters
  gravity
  
• These bodies could never have formed a planet due
  to the intense gravity

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• The comets are the last of the icy planetesimals
• Many could have formed out with the jovian
  planets and gravity threw them outward in the
  solar system
  
• The Kuiper Belt Objects formed out in the colder
  outer solar system
  
• The moons of the outer solar system may have 2
  origins
  
• The bigger moons may have formed with the planet,
  but the smaller ones are probably captured

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• All 4 of the outer planets have ring systems
• From all the evidence (meteorites, etc) it seems
  the ages agree with a common age for the solar
  system
  
• So what cleared the nebula?
• It appears the radiation from the new Sun would
  have been intense enough evaporate away the
  material
  
• The solar wind would have blown away particles

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• The solar wind is a stream of high energy
  particles coming off the Sun
  
• Much of the debris could have been swept up by
  the orbiting planets
  
• You can see the results as craters on the solid
  surfaces
  
• Some of the material could have been ejected due
  to gravitational interactions with the planets
  

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• One of the hot things right now are the
  extrasolar planets
  
• These are planets around other stars
• The Orion Nebula is a great place to look
• At least 50 of the stars in the nebula are
  surrounded by dense disks of dust and gas
  
• Most of these planets have been discovered by
  their tug on their host star
  

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• The first planet discovered in this manner was
  around the star 51 Pegasi
  
• By watching how the star came closer and farther
  from us we were able to determine the orbit and
  mass of the planet
  
• It has a mass of about .5 Jupiters and orbits .05
  AU
  
• One star, Upsilon Andromedae, has 3 planets

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• Another way to search is to look for a regular
  dimming of a star as the planet goes between the
  star and us
  
• This would be a very small drop in brightness
• Currently there are over 250 extrasolar planets
  known today
  
• These planets are all massive
• This is because our technology has not yet
  allowed us to detect Earth sized planets

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