Astronomy 330

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Astronomy 330
http//hubblesite.org/newscenter/newsdesk/archive/
releases/1995/45/image/a

• Lecture 26

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Astronomy 330 Theory of the Formation of the
Solar System

• Summary The S.S. is composed of the Sun,
  planets, their satellites, asteroids, KBOs, and
  comets.
• All these objects have different properties and
  histories (and also similarities) which we have
  discussed.
• How did it get this way? How common are
  planetary systems around other stars? Are there
  other Earths?

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Astronomy 330 Basic properties of the Solar
System

• The oldest stuff in the solar system is less
  than 4.6 billion years old. The universe is much
  older, about 15 billion years.
• The planets move around the sun in the same
  direction, in the direction that the sun rotates,
  and in the same plane.
• The sun has 99 of the mass, the planets have 98
  of the angular momentum (L mrv).

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• The inner planets are composed of cosmically rare
  elements (metals, silicon minerals) and are small
  and dense. The giants are composed of material
  common in the cosmos and the sun (H, He) and
  their satellites are, in general, icy-rock bodies
  with lower densities.
• The asteroids represent a transition between the
  metal-rich interior planets and the volatile-rich
  outer solar system and there is a composition
  gradient in their compositions. Also, they are
  located between Mars and Jupiter.

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• The primitive meteorites are composed of
  materials similar to the solid grains that
  probably formed in a cooling gas cloud which had
  solar or cosmic composition.
• Comets are largely water ice and frozen volatiles
  (e.g. CO2 and methane), silicate dust, and
  carbonaceous material.
• Specific Isotope ratios are preserved in objects
  which formed at different distances from the sun.

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• Volatile compounds are present in the solar
  system even through their bulk composition
  suggests that they formed at temperatures too
  high to allow this. The volatiles came from
  elsewhere.
• Venus, Uranus, and Pluto rotate in a retrograde
  manner.
• All the giants have regular systems of satellites
  and ring systems.

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• The giant planets also have one or more irregular
  satellites.
• All the giant planets have been enriched in heavy
  elements (they have rocky cores) of about 10-15
  Earth masses and atmospheres rich in H and He and
  all radiate heat from their interiors (except
  Uranus).

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• Any model for the formation of the Solar System
  must take into account these observational facts.
• Further, these models need to take into account
  our knowledge of physics and chemistry as well as
  our increasing knowledge, both theoretical and
  observational, of the star formation process.

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Astronomy 330 Overview of Star formation

• Stars form in spinning disks of gas which
  condense out of large clouds of denser gas which
  inhabits the Galaxy.
• Secondly, many planets HAVE been detected around
  other, nearby stars.
• Finally, most stars in the Galaxy ARE double star
  systemsstars dont form alone!

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• The sun is relatively young when compared to the
  age of the Universe.
• However, there are younger stars, the massive
  blue-white stars which live for only a short time
  (a few 10s of millions of years) since they are
  so bright and energetic, they use up their
  nuclear fuel quickly.
• These massive stars blow up in supernovae and
  spread the heavy elements they have created (C,
  N, Si, Fe,.) in their nuclear furnaces back into
  the galaxy. This is the source of all the
  elements heavy than He.

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Astronomy 330

• The Sun and planets must have formed out of a gas
  cloud which had been enriched in heavier elements
  by earlier generations of stars.
• Star formation is taking place today in gas
  clouds which seem to have the same composition as
  the Suns (H, He and other elements).
• Further, these are dense and are composed of
  surprisingly complicated molecules, including
  many organic molecules. Many of the same
  compounds that are found in comets are found in
  these clouds.

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Astronomy 330 Orion Nebula
http//hubblesite.org/newscenter/newsdesk/archive/
releases/1995/45/image/a
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Astronomy 330 A supernova remnant enriching the
galaxy
http//hubblesite.org/newscenter/newsdesk/archive/
releases/2004/29/image/a
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Astronomy 330 The Pleiades, a cluster of young
stars
http//hubblesite.org/newscenter/newsdesk/archive/
releases/2004/20/image/a
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Astronomy 330 A molecular cloud surrounded by
star formation
http//hubblesite.org/newscenter/newsdesk/archive/
releases/1995/44/image/b
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Astronomy 330

• Star formation starts with a slowly rotating
  cloud of gas.
• The cloud is unstable to its own self-gravity and
  collapses. As more gas is added, the gravity
  becomes stronger, attracting even more gas to it
  .
• As the cloud collapses, it spins faster and
  fasterconservation of angular momentum.
• Secondly, the cloud flatens into a disk due to
  this rotation combined with the gas pressure.
• Finally, the gas begins to heat as it collapses.

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• The time for this all to happen is short, less
  than 1 million years.
• Also, a temperature gradient forms in the disk.
  The outer regions of the disk can more easily
  radiate their heat to space. The center of the
  disk grows hotter, while its outer reaches are
  cooler.
• Once the center of the disk reaches 1 million K,
  nuclear fusion begins and collapse stops.
• At 1 AU in the disk, temperatures reach 1500 K.

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• The central concentration in the disk (i.e. the
  proto-star) must have been equal to 1 solar mass
  (roughly).
• The mass of the disk is less certain.
• How much mass would be needed to make all the
  planets we see today with the proviso that they
  would have solar composition?We will do the
  thought experiment of adding H, He to all the
  planets until they have compositions equal to the
  Suns.
• When we do this, the masses of all the planets
  are similar to Jupiter and Saturns.

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• So, the initial disk out of which the planets
  formed, must have had a mass of at least 3 the
  mass of the sun and the entire solar nebula has a
  mass of 1.03 solar masses.
• This is a minimum mass and other processes
  operate which will blow away material such winds
  from the proto-sun and the value is probably
  closer to 1.1 solar masses.

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• In Orion circumstellar disks range in size from
  50 to 1000 AU. This is somewhat larger than the
  Kuiper belt distance from the sun, but probably
  the outer parts of these disks will dissipate.
• These disks have masses of 0.01 - 0.1 solar
  masses.

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Astronomy 330 More disks in Orion being blasted
by UV
http//hubblesite.org/newscenter/newsdesk/archive/
releases/2001/13/image/a
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Astronomy 330

• However, there is a problem with this picture!
• When one measures the angular momentum in a
  typical molecular cloud out of which stars
  presumably form, we see that they contain much
  more angular momentum than is present in the sun.
• However, the total angular momentum in the sun
  and planets together is roughly equal to what is
  observed in clouds.
• Why does the sun contain so little of the solar
  systems angular momentum budget?

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• Note Stars 15 more massive than the sun rotate
  much more rapidly.
• So, some process must have transported this
  angular momentum from the proto-sun to the disk
  forming around it, and hence to the planets we
  see today.

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• Several solutions have been proposed for the
  angular momentum problem
• Magnetic braking - magnetic fields slow rotation
  of proto-star and transfer it to the diskdoes
  not explain why only small stars have slow
  rotation rates.
• Solar wind - a wind from the sun removes its
  excess ang. mom. does explain the difference
  between massive and non-massive stars
• Turbulence - acts like a viscosity and can
  transport angular momentum

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• Once the disk forms around the proto-sun, how
  does it evolve into planets, etc.?
• Remember the disk has a temperature gradient, hot
  near the center, cool at is outer edge.
• Also, the mid-plane of the disk will become
  denser and dust will be concentrated there.
• In regions of the disk, near the proto-sun above
  2000 K this interstellar dust is vaporized.
• This region subsequently cools and allows
  molecules and a new type of dust to form.

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• Also, the new dust grains will be of different
  composition at different distances from sun due
  to the temperature gradient in the diskchemical
  fractionation.
• Calculations indicate that it took about 10
  million years for the Sun to form and begin
  nuclear reactions. This kept the inner disk
  warm and did not allow volatile compounds to
  remain in the inner disk.
• This is consistent with the fact that the inner
  planets do not have a lot of volatile compounds
  and the out planets, their satellites, and the
  comets do.

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Astronomy 330

• Beyond Uranus there is another change in
  composition (the larger satellites of Uranus,
  Triton, and Pluto-Charon have higher densities
  than the satellites of Saturn).
• This fact does not yet have a good explanation,
  but could be due to the chemistry of oxygen and
  carbon.

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• So, the chemical changes (fractionation) we see
  as we travel in radius from the Sun is consistent
  with calculations of the temperature gradient in
  the solar nebular disk.
• But, there also must have been some radial mixing
  of elements since some of the solid, condensed
  material we see in the inner solar system could
  only have formed in the outer, cold regions of
  the solar nebula.

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• So, we now have a disk with dust grains in it and
  those dust grains have been chemically sorted by
  radius from the Sun. How do these dust grains
  accumulate to form the planets?
• The dust grains will be in highly circular
  orbitsthey will not collide with high velocities
  and they can stick together by electrostatic
  forces or if they are fluffy.
• This process would lead to objects of low density
  as large as about 10 kmplanetesimals.

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• At this point the planetesimals are large enough
  to exert a significant gravitational force on
  surrounding material.
• The timescales to form planetesimals are very
  shortabout 1000 years for sizes of 10 km once
  grains of sufficient size have formed or settled
  into the disk.
• These short times are consistent with isotope
  ratios observed in the asteroids which indicate
  that they formed with the first 1 million years
  after the solar nebula collapsed to a disk.

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• After 50 millions years, the Earth and the
  planets had completely formed.
• It is thought that the inner planets and cores of
  the outer planets formed by collisions between
  the planetesimals.
• After a short period, a few thousand large (100 -
  1000 km) planetesimals formed and contained most
  of the mass of the original, smaller
  planetesimals.
• These are called planetary embryos.

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• At this point, collisions would become much rarer
  (there are simply fewer things to collide).
• Collisions are this point are also much more
  energetic since these embryos are more massive.
• This probably lead to fragmentation as well as
  accumulation, when a collision occurred.
• Calculations show that a few large proto-planets
  would form as well as many moon sized bodies.

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• Large collisions would be the norm at this point
  and explains some of the peculiarities of some of
  the planets.
• These collisions helped to heat the forming
  planets and lead to their differentiation.
• In the outer solar system the bodies contained
  much ice and also their collisions were less
  violent. Also, here the evolution was slower.
• Within 10 million years cores as large as 10
  Earth masses could form and start to accumulate H
  and He from the surrounding nebula (since they
  are massive and cold).

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• In the final stages the sun underwent a period
  where it had an intense wind which cleared the
  solar system of any remaining gas.
• This is observed in young stars.

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Astronomy 330
http//yso.mtk.nao.ac.jp/kokubo/planet/acc2.gif
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Astronomy 330 Impacts

• After the inner planets formed they continued to
  undergo a period of intense impactsthe late
  heavy bombardment period.
• This lasted about 600 million years.
• This late impact period contributed only a small
  amount of mass to the planets, but it is
  responsible for shaping their surfaces which we
  see today (especially the Moon).

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Astronomy 330 The Asteroids

• Left over material from the formation of the
  planet formation process.
• Several possible sources
• Leftover planetesimals
• C-type asteroids might be remnants of giant
  comets.
• Fragments from giant collisions

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Astronomy 330 The Late Heavy bombardment

• Where did the projectiles of the late bombardment
  period come from if most of the planetesimals had
  been used up to form the planets?
• Probably they originated from the Uranus-Neptune
  region.
• The timescales in the outer solar system suggest
  that there could have been much debris left
  there.
• They would have been gravitationally scattered by
  Uranus and Neptune into the inner solar system.

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• Further, these objects would have been icy and
  delivered volatiles with them to form atmospheres
  and oceans. In addition they may have brought
  organic compounds and led to life on Earth.
• Also, these objects would have been scattered
  outwards to form the Oort cloud.

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Astronomy 330 Formation of the giant planets,
their rings and moons

• Remember, these systems are almost like mini
  solar systems.
• The proto-giant planets were massive enough to
  attract enough gas and dust which itself formed a
  sub-nebula around it.
• These planets rocky-ice cores apparently formed
  first and them attracted gas to them.
• The atmospheres of these planets are a mixture of
  heavy elements from these cores and the
  accumulated H and He. We have seen that methane
  is enhanced on these planets relative to solar
  values.

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Astronomy 330

• On Uranus and Neptune the methane is enhanced
  above solar values even more. Apparently they
  did not accumulate as much H and He as Jupiter
  and Saturn.
• The accumulation of gas formed a disk of material
  in an analogous way to the solar nebula.
• Again, in such a disk, we would expect a
  temperature gradient which would have an effect
  on the compositions of any satellites forming
  there.

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• At the distance of Jupiter water ice is stable
  and solidwe expect half rock, half ice
  satellites to form.
• This is what we see for Callisto and Ganymede.
• Io and Europa, being closer to Jupiter have high
  densities (3 and 3.6 g/cm3).
• This can be understood as being due to higher
  sub-nebula temperatures nearer to Jupiter.

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• The satellite systems of Saturn and Uranus do not
  show such a composition and density gradient.
• This is probably due to the lower temperatures
  and lower masses of the planets there.
• Also, the irregular satellites are probably
  captured, icy planetesimals.

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• Rings are thought to be the result of large scale
  collisions and fragmentation which occurred near
  the gas giants. The debris of such collisions
  could not reform into a satellite due to the
  tidal effect of the planet.
• A large fraction of the icy planetesimals in the
  outer solar system went on to form the comets in
  the Oort cloud by being perturbed by the gas
  giants into orbits with high eccentricity.

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Astronomy 330

• The KBOs on the other hand seem to have formed
  where they are now detectedbeyond the orbit of
  Neptune.
• They are the outer-fringe planetesimals.
• Collisions here are less frequent and could not
  form a large planet sized body.

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Astronomy 330

• Disks are seen around other stars as are planets.
• We will figure this out eventually!