ASTR330

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Solar System Formation

• Lecture 6 Feb 13, 2007

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School closes at 200pm today.
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Homework 2

• HW 2 is on the website.
• Due Feb 20th, beginning of class.
• Probably harder/longer than HW 1, so get a head
  start!
• Some of this material is covered in more detail
  in lecture than the book.sometimes the other way
  around. USE BOTH.

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Formation of the Planetary System

• 1. From what did the Solar System Form?
• 2. How did it form?
• 3. Why are the objects in the Solar System all so
  different?
• 4. Could it have formed differently?
• 5. How long did it take for the planets to
  accrete all their mass?
• 6. How have all the planets evolved since the end
  of their major accretion?
• 7. Is this evolution continuing today?

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Data and evidence

• Unfortunately, in answering these questions we
  have very limited data.
• Looking at our Solar System, we can only see the
  end-point of the evolutionary process the
  observed sizes, orbits and compositions of
  planets and other bodies.
• New technology has allowed us to observe other
  stars with the very beginnings of their own Solar
  Systems, which helps us understand our own.

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Things we know

• More than 99 of the material in the Solar System
  is in the Sun.
• The Sun is almost entirely Hydrogen and Helium.
• Pretty safe to assume that the raw material of
  the Solar System were close to this composition.
• Jupiter and Saturn have similar compositions as
  the Sun
• Smaller bodies are depleted in Hydrogen and
  Helium and other light gases
• Hence the inner planets were probably formed
  without ices or other volatiles.

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Other things we can use

• All the planets have nearly circular orbits
• Mercury is the largest at 0.206
• The orbits are nearly all in the same plane
• Again, Mercury is largest with a 7 degree
  inclinations
• They all travel in the same direction around the
  Sun
• The Sun rotates in the same direction as the
  planets orbit.
• The Suns equator is roughly in the same plane as
  the planets orbits

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Eccentricities
Credit New Horizons Website
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Ecliptic Plane
Mercury is 7 out of ecliptic plane, and Pluto 17

Credit New Horizons Website
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Overview of formation

• On to the details

Figure Universities Corp. For Atmospheric
Research (UCAR)
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The Interstellar Medium

• Within the space between stars, various
  collections of gas and dust exist,
• Giant Molecular Clouds are huge collections of
  gas (molecules, like H2, and other more complex
  molecules) and dust many parsecs across and
  containing up to a million solar masses,
• Small Molecular Clouds, also known as Bok
  Globules, only contain several solar masses of
  gas and dust.
• Interstellar dust scatters short wavelengths,
  creating a reddening effect, and hence excess
  infrared radiation, as well obscuring visible
  wavelengths.
• This is the same effect that gives us nice
  sunsets

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In the beginning

• was a huge cloud of molecular material, known
  as the proto-solar or primordial nebula, similar
  to the Orion Nebula (right).
• This nebula may have only contained only 10-20
  more mass than the present solar system.
• Due to some disturbance, perhaps a nearby
  supernova, the gas was perturbed, causing ripples
  of increased density.
• The denser material began to collapse under its
  own gravity

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Initial Collapse

• The nebula must have possessed some rotation. Due
  to the spin, the cloud collapsed faster along the
  poles than the equator.
• The result is that the cloud collapsed into a
  spinning disk.

• The disk material cannot easily fall all the
  remaining way into the center because of its
  rotational motion, unless it can somehow lose
  some energy, e.g. by friction in the disk
  (collisions).
• Once the collapse is started it takes just a few
  100,000s of years.

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Angular Momentum

• Angular momentum is a conserved quantity in the
  absence of dissipation the total angular momentum
  of the cloud stays the same.
• Angular momentum is the product of three
  quantities mass, size (radius) and rotation
  speed (or velocity)
• L mrv
• If L is constant, then clearly if any one of the
  other quantities decreases, another quantity must
  increase proportionately.
• I.e., if the cloud collapses and becomes smaller
  (r decreases) and the mass stays the same, then
  the rotational speed (v) increases the cloud
  spins up.

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Rotational spin-up

• The spin-up of a shrinking object can be
  demonstrated by a familiar example
• An ice skater performing a spin draws in her arms
  to spin faster without expending any extra
  effort.
• Now lets look at some actual proto-planetary
  disks

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Actual proto-planetary disks

• The images (left) show four protoplanetary disks
  in the Orion Nebula, 1500 light years away,
  imaged by the Hubble Space Telescope (HST).
• The disks are 99 gas and 1 dust. The dust shows
  as a dark silhouette against the glowing gas of
  the nebula.
• Each frame is 270 billion km across about 1800
  AU. The central stars are about 1 million years
  old infants!
• Due to the dust surrounding the star, these young
  objects block most visible light, but can be
  bright in the infrared.

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Disk composition

• The central parts of the nebula were very hot
  over 10,000 K.
• Going outwards in the nebula, the temperature
  drops, and different compounds condense out at
  different distances from the protostar
• Calcium, Aluminum oxides first,
• then Iron-Nickel alloys (by 0.2 AU, Mercury),
• Magnesium silicates and oxides next (by 1.0 AU),
• Olivine and Pyroxene (Fe-Si-O compounds),
• Feldspars (K-Fe-Si-O compounds),
• Hydrous silicates,
• Sulfates,
• And finally ices (water ice by 5.0 AU).
• This radial variation in composition in the
  nebula is one cause of the variation in
  composition of the planets with solar orbit
  distance.

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Proto-Sun

• Gravity caused the center of the cloud to
  collapse into a ball the proto-sun. The
  gravitational energy released begins to heat
  things up.
• When the proto-sun became hot and dense enough,
  nuclear fusion was ignited.

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T Tauri phase

• Young solar-type stars are said to be in the T
  Tauri phase (named after the first example), and
  can have wind velocities of 200-300 km/s. This
  phase lasts about 10 million years.

• Once the star begins to shine, the stellar wind
  turns on, and the star begins to blow material
  which has not yet accreted outwards.
• T Tauri stars are characterized by vigorous
  outflows perpendicular to the relatively dense
  disk.
• After 105 or 106 years, the original gas nebula
  has been dissipated.

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Rotation of the Sun

• The sun rotates with a period around 25 hours.
• T Tauri stars rotate much faster, with periods
  near 12 hours.
• Can we think of any reason the Sun might have
  slowed down since it was very young?
• Contracting will just make it spin faster.

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Beta Pictoris Systems

• Beta Pictoris was a relatively normal star with
  an Infrared excess, which was observed to have a
  dust disk in 1984.
• The disk reaches about 400 AU from the star.
• Spectra has matched absorption lines in Beta Pic
  stars, to gas species found in comets in our
  Solar System!
• So the dust and debris in this disk, not only is
  similar in size to the extent of our Solar
  System, but has a similar composition.

Credit Jean-Luc Beuzit, et al. Grenoble
Observatory, European Southern Observatory
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Estimating the mass of the solar nebula

• Terrestrial planets are made of heavy elements,
  i.e. silicon and iron.
• These elements are only a small fraction of the
  basic Solar abundances, 0.0006 of cosmic material
  is Silicon.
• 1 out of every 1,700 grams of interstellar
  material needed to get 1 gram of Silicon.
• To condense Mercury, Venus or Earth, around 400
  times the final mass was originally needed to
  condense enough heavy materials.
• Overall around 0.15 Msun was needed to create all
  the planets and planetesimals.

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Planetesimals

• Dust grains and ices were sticky (not just
  chemically, but electrically and magnetically
  cohesive) and began to clump together
  (accretion), forming small bodies 0.01 to 10 m
  across, all orbiting the proto-star in the same
  direction like Saturns rings.

• As their size grew, gravity began to have an
  effect, and larger bodies around 1 km in size
  called planetesimals formed.
• The details of planetesimal formation are still
  uncertain, but km-sized bodies would have
  appeared by 10,000 years after the disk formed.

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• Planetesimal Growth

• Gravitational interactions between planetesimals
  perturbed their orbits into non-circular,
  collisional trajectories.
• Time passed, and the planetesimals impacted one
  another. In lower energy collisions or where the
  sizes are unequal, the planetesimals merged into
  a new larger object.
• But in higher-energy collisions, two
  similarly-sized original bodies were disrupted
  back into fragments.
• Over time, the larger planetesimals gathered up
  more and more mass from collisions with smaller
  impacting bodies.
• In this way, the cores of the inner and outer
  planets were formed.

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• Inner Planets

• After about 108 years, the solar wind and
  accretion of planetesimals had cleared the inner
  solar system of small debris and gas.

• The inner planets had by then accreted almost all
  their eventual mass.
• A period called the Late Heavy Bombardment,
  around 3.9 billion years ago is associated with
  clearing up the last planetesimals on inclined
  orbits, as inferred from lunar cratering.

• However, the process of collision and
  accumulations continues to the present day e.g.
  meteors, SL-9.

Picture credit AnimAlu Productions
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• Outer Planets

• The outer planets continued to accrete for longer
  than the inner planets, and gathered much more
  ices and volatiles.
• The outer planets are also responsible for the
  asteroid belt and comets.
• Any ideas why there are no major planets between
  Mars and Jupiter (where the asteroid belt lies)?

Picture NASA
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Nice Model

• A relatively new model of early Solar System,
  includes a dramatic shakeup of the planets, 700
  million years after the Earth formed.
• Saturn and Jupiter cross an orbital resonance,
  causing high eccentricities, for all the outer
  planets.
• Similar to the Pluto/Neptune 32 resonance, this
  is a 21 resonance with Jupiter and Saturn.

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Nice model

• However, this resonance is not stable, it causes
  large eccentricities in the Giant Planets.
• These eccentric planetary orbits scatter the then
  organized Kuiper Belt.
• This model might explain,
• Giant Planet orbits (eccentricities and
  semi-major axes)
• Structure of the Kuiper Belt,
• Trojan asteroids of Jupiter,
• Late Heavy Bombardment of the Moon.

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• Differentiation

• As the planets accreted, temperature and pressure
  rose in the inner regions.
• Heavier substances fell to the core (e.g. metal
  for the inner planets) and lighter substances
  floated on top.
• This process, called differentiation, occurred in
  all the planets but the end result depended on
  the initial ingredients!

Below proposed Ganymede interior rock core and
ice mantle.
Picture NASA
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• Asteroids

• The major asteroid belt lies between the orbits
  of Mars and Jupiter, at a distance of around 2.7
  AU.

• The asteroids were once thought to be the remains
  of a planet destroyed by a massive impact.

Picture credit NASA GSFC
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• Asteroids

• Current theories hold that the fragmented belt of
  material is the natural consequence of the
  presence of the giant planet Jupiter nearby
  during the planetary accretion phase.
• The massive Jupiter core formed first, and then
  either gobbled up nearby planetesimals, or, in
  the case of the asteroids slightly further away
  Jupiter was able to disrupt any attempts they
  made to cling together into a planet! The
  Asteroids are all less than 1000 km in size.
• Asteroids also exist in groups either preceding
  or trailing Jupiter in its orbit (Jupiter
  Trojans) or Mars (Martian Trojans). There are
  also asteroids which cross the Earths orbit, and
  others.
• Asteroids are important because they are examples
  of the original planetesimals from 4.6 billion
  years ago. We will talk more about asteroids in a
  later lecture.

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• Edgeworth-Kuiper Belt

• The Edgeworth-Kuiper belt is a band of icy
  planetesimals outside the orbit of Neptune
  (40-120 AU), hypothesized in the 1940s.

• These objects are relics from the early formation
  phase of the solar system, which did not manage
  to form into planets.
• The first EKO (or KBO) detected was found in 1992
  (not counting Pluto and Charon!) now over 800
  are known.

Picture Johns Hopkins University
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• EB 313 and Pluto

• The object EB 313, first seen in 2003, caused a
  major upset to astronomy when its size was
  announced in mid-2005 to be larger than Pluto!
  (2400 or 3000 km, according to 2 studies Pluto
  is 2300 km)

• This animation shows EB 313 moving against the
  star background in the upper left.
• Astronomers have been grappling ever since with
  the question of how to define what is a planet!
• A decision in August 2006 has resulted in Pluto
  being downgraded to a new dwarf planet category.

Graphic wikipedia
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• Eris and Dysnomia

• Follow-up observations with the Keck adaptive
  optics system showed that EB 313 was accompanied
  by a small moon.
• Originally dubbed Xena and Gabrielle by the
  discoverers, they gained official names on Sept
  13 Eris and Dysnomia.
• The names mean strife or discord, and
  lawlessness - appropriate to the trouble they
  are causing!

Graphic wikipedia
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Graphic wikipedia
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• KBOs and SDOs

• Kuiper belt objects are actually clustered quite
  closely between 39 and 48 AU - stable orbital
  zones with respect to Neptune.
• Eris lies at a68 AU, but its 557-year orbit is
  highly elliptical, ranging from 38 to 100 AU, and
  inclined at 45 degrees.

• For this reason, Eris is classified as a SDO or
  scattered disk object.

Graphic wikipedia
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• Other Kuiper Belts

• We cannot gain a good view of the Kuiper belt as
  a whole due to our position in the inner solar
  system but, we can look elsewhere.
• These HST images show 2 Kuiper Belts around other
  stars, face on (left) and edge-on (right).

Graphic wikipedia
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• Oort Cloud

• A vast reservoir of icy planetesimals at 100s out
  to 100,000s of AU.
• Named the Oort Cloud, after Jan Oort who guessed
  its existence in 1950, by noting that long-period
  comets came from all directions of the sky.
• Ironically, Oort cloud objects formed closer to
  the Sun the EKOs, but are on extremely eccentric
  orbits.

Graphic SWRI
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• Oort Cloud Formation

• Any planetesimals coming close to mighty Jupiter
  and Saturn were ejected from the solar system
  entirely.
• However, icy bodies coming close to Neptune and
  Uranus were merely flung into very distant and
  eccentric orbits around the Sun.
• These orbits were no longer confined to the plane
  of the solar system and so these icy bodies
  formed a huge spherical cloud around the Sun,
  reaching out to 100,000 AU.
• These objects periodically visit the inner
  reaches of the solar system, and we see their
  long tails of gas and dust as comets.

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Oort cloud comets

• Hyakutake
• A period 72,000 years
• Eccentricity 0.999902
• Semi-major axis 2349 AU
• Hale-Bopp ?
• A period of 2537 years
• Eccentricity 0.995
• Semi-Major axis 186 AU
• Halley (Short period comet)
• A period of 75.3 years
• Eccentricity 0.967
• Semi-Major axis 17.8 AU

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• Summary

Picture credit James Schimbert, U. Oregon, eugene