The Origin of the Solar System

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• The Origin of the Solar System

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In the beginning, we started out looking like
this, just a huge cloud of gas in space.
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Solar Nebula Theory

• A rotating cloud of gas contracts and flattens.
• to form a thin disk of gas and dust around the
  forming sun at the center.
• Planets grow from gas and dust in the disk and
  are left behind when the disk clears.

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Dust Disks Around Stars

• Very cold, low density disks observed (in the
  infrared) around stars.
• Debris left over from comets or collisions
  between small bodies (like asteroids).
• Evidence of planetary systems which have already
  formed.

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Dust Disks Around Stars

• Very cold, low density disks observed (infrared)
  around stars.
• Debris left over from comets or collisions
  between small bodies (like asteroids).
• Evidence of planetary systems which have already
  formed.
• Dense disks of gas and dust observed (visible
  radio) orbiting young stars.
• Stellar systems are too young for planets to have
  formed yet.
• Probable sites of ongoing planetary formation.

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Examples of the Dust Disks around stars
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Planet Buildingthe Condensation of Solids
First Important step in Planet formation

• Different materials condense from the gas cloud
  onto grains of elements (atoms of different
  gasses) at different temperatures.
• The temperature due to the Sun varied with
  distance, so different materials condensed at
  different distances from the Sun.
• Close to the Sun (1200-1500K) metal oxides and
  pure metals.
• Farther out (700-1200K) silicates and rocky
  material.
• Outer regions (50-200K) ices (water, methane
  ammonia).

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Planet Buildingthe Formation of Planetesimals

• Planetesimals small bodies on the order of
  kilometers in size.
• Condensation atoms of gas hit dust grains and
  stick, adding mass to the particle.
• Accretion solid particles collide and stick to
  one another.
• Once particles were massive enough, the settled
  down into a disk rotating around the protosun
  (its not quite a star yet).

Second Important step in Planet formation
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Accretion Taking Place
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Planet Buildingthe Growth of Protoplanets

• As planetesimals grew, they became more massive,
  and therefore had stronger gravitational fields.
• At a certain point, they were able to
  gravitationally hold an atmosphere.

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Planet Building

• Planetesimals contain both rock and metal.
• A planet grows slowly from the uniform particles.
• The resulting planet is of uniform composition.
• Heat from radioactive decay causes
  differentiation.
• The resulting planet has a metal core and
  low-density crust.

• The first planetesimals contain mostly metals.
• Later the planetesimals contain mostly rock.
• A rock mantle forms around the iron core.
• Heat from rapid formation can melt the planet.
• The resulting planet has a metal core and
  low-density crust.

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Planet-building processes

• Dust grains stick together ? planetesimals
• Planetesimals stick together ? protoplanets
• Terrestrial
• metallic / rocky
• but small not much material
• Jovian
• LOTS OF ICES, so quickly grew more massive
• When 15 x Earths mass, gravity strong enough to
  attract lots of H/He from solar nebula
• got really really big but not dense

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The planets eventually formed and differentiated
into Terrestrial vs. Jovian Planets
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Four stages of terrestrial planetary development

• 1. Differentiation
• early planet was molten
• heavy elements sunk, light elements rose
• On Earth
• Dense metal core
• Less dense rocky mantle
• Low-density rocky crust
• (outgassing made primitive
• atmosphere more on that later)

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Four stages of terrestrial planetary development

• 2. Cratering
• heavy bombardment period (first 0.5 billion
  years)
• many impacts with rogue planetesimals
• craters made (some huge)
• On Earth
• many craters later covered by ocean or erased by
  erosion)

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Four stages of terrestrial planetary development

• 3. Flooding
• lava from below
• rain from atmosphere
• On Earth
• made oceans

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Four stages of terrestrial planetary development

• 4. Slow surface evolution
• On Earth
• wind / water erosion
• plate tectonics moving sections of crust

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Clearing of solar nebula

• Sun pushed away remaining debris
• radiation pressure (light)
• solar wind (particles)
• Planets
• swept up debris (craters)
• ejected debris

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Clearing the Solar Nebula

• Around 4.6 billion years ago, the cloud of gas
  (the solar nebula) vanished due to four effects
• Radiation Pressure light from the Sun exerted
  pressure on the particles, pushing them out of
  the solar system.
• The Solar Wind a flow of atoms from the Suns
  upper atmosphere also helped push particles out
  of the solar system.
• As planets moved through their orbits, they swept
  up any material in their paths.
• Gravitational effects due to massive planets
  ejected particles out of the solar system.

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Stellar Debris

• Asteroids rocky objects, mostly found between
  Mars and Jupiter (in the Astreroid Belt 2.8
  AU).
• Range in size up to 100 km in diameter.
• Irregularly shaped, and cratered.
• Remnants of planet formation.
• Comets small icy bodies (dirty snowballs).
• Large elliptical orbits can bring comets in close
  to the Sun.
• Recent studies suggest they are at least 50 rock
  and dust.
• Meteoroids specks of dust and rock which
  encounter Earths atmosphere and either burn up
  or fall to the ground. (Most only about 1g in
  mass).
• Meteors Flash across the sky as the meteoroid
  burns up.
• Meteorite remnant of a meteoroid that reaches
  the ground.

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A Comet
Up close and personal with an asteroid
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Stellar Motions Due to Planets

• Technically, planets dont orbit around a star,
  but around the common center of mass.
• If planets are massive enough, the center of mass
  is not located at the center of the star, and the
  star orbits around this point as well.
• This motion can be detected through Doppler
  shifts in the stars spectrum.

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Using Radioactive Dating, Weve Discovered

• Approximately the same age
• Earth rocks
• Moon rocks
• Martian meteorites
• asteroidal meteorites
• 4.6 billion years
• Determined by radioactive dating
• compare original amount of radioactive element
  with an amount present now

half-life time it takes for ½ of radioact.
elem. to decay into non-radioact. elem.
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Explaining the Solar System

• Terrestrial small, dense, low mass
• Jovian large, low density, high mass
• Condensation sequence and accretion
• Terrestrial heavy gas atmospheres
• Jovian lighter elements
• Jovian planets can gravitationally hold onto
  lighter gas
• Terrestrial few satellites, no ring system
• Jovian many satellites, planetary rings
• Jovian planets gravitationally stronger
• Existence of comets and asteroids
• Leftover material from the formation of the solar
  system.

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Evidence of Extrasolar Planets

• Two methods which suggest the existence of
  extrasolar planets
• Detection of dust which accompanies planets
  around stars.
• Detection of stellar motions due to the presence
  of orbiting planets.

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Known Extrasolar Planets

• Most known extrasolar planets are high-mass and
  low-period planets. (Selection effect)
• High-mass the greater the mass, the greater the
  wobble produced in the stars motion.
• Low-period the lower the period, the shorter
  the period over which the wobble occurs.
• How can high-mass, low-period planets form?
• In dense disks, friction may slow the planets
  down, causing them to spiral inward.