ASTR 330: The Solar System

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ASTR 330 The Solar System

• Lecture 5 Review Quiz

• Distinguish between remote sensing and in situ
  sensing, and give examples.
  
• What is meant by an atmospheric spectral window?
• What information can we tell about a planet from
  infrared spectral lines?
  
• What are the two main types of telescopes, and
  name some recent advances in telescope technology.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Announcements

• Yellow forms still required for students Gilkey,
  Snyder Talebi!
  
• E-mail anyone not getting e-mail on the class
  list?
  
• Homework 1 returned.
• Standard was high mean42.5.
• Question 4 problems.
• Materials on-line HW 1 solutions. Lectures 6
  7.
  
• Calculators.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Lecture 5
• Formation of the
• Planetary System

Dr Conor Nixon Fall 2006
Picture from emuseum_at_Minnisota State Univ, Mankato
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ASTR 330 The Solar System

• Questions For This Class

• From what did the solar system form?
• How did it form?
• Why are the objects in the solar system all so
  different?
  
• Could it have formed differently, e.g. with a
  binary star?
  
• How long did it take for the planets to accrete
  most of their mass?
  
• How have the planets evolved since the end of
  their major accretion?
  
• Is this evolution continuing today?

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Data and Evidence

• In trying to answer these questions we have
  limited evidence at our disposal today.
  
• In our own solar system, we have only the
  end-point of a complex evolutionary process to go
  on the observed sizes, orbits, compositions etc
  of the planets and other bodies.
• It is somewhat like trying to deduce the
  childhood and experiences of a human, having only
  a picture of the adult.
  
• As astronomical technology has progressed, in the
  last decade we have been able to now view the
  beginnings of solar systems around other stars a
  valuable insight into our own history.
• However, many aspects of solar system formation
  at this stage are still not certain.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Facts We Can Use Chemical Composition

• More than 99 of the material in the solar
  system is in the Sun.
  
• The Sun is composed almost entirely of Hydrogen
  and Helium.
  
• Hence the initial raw material must have had
  close to this composition.
  
• Jupiter and Saturn have almost the same
  composition as the Sun.
  
• The smaller bodies are depleted in H, He and
  other light gases.
  
• Hence the inner planets were probably formed
  without ices or other volatiles.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Facts We Can Use Orbits and Rotations

• All the planets have orbits which are
  approximately circular.
  
• These orbits all lie roughly in the same plane.
• The planets all rotate in the same direction
  around the Sun.
  
• The Sun rotates in the same direction as the
  planets orbit.
  
• The Suns equator lies essentially in the same
  plane of the planets orbits.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Early Theories

• The first serious speculations about the
  formation of the solar and planetary system using
  the laws of gravity and physics were due to
  Pierre-Simon, Marquis de Laplace (1796).

• Laplace envisioned a vast rotating interstellar
  gas cloud which collapsed under its own
  gravity, to form a disk.
  
• These ideas were improved on by Roche (1854) and
  are still valid today, though with many changes
  in the details!

Dr Conor Nixon Fall 2006
Picture creditUniv. St. Andrews
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ASTR 330 The Solar System

• Overview Of Formation

Now lets look at the individual stages in more
detail
Dr Conor Nixon Fall 2006
Figure Universities Corp. For Atmospheric
Research (UCAR)
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ASTR 330 The Solar System

• 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

Dr Conor Nixon Fall 2006
Picture from stardate.org
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ASTR 330 The Solar System

• 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).
• The initial collapse takes just a few 100,000s of
  years.

Dr Conor Nixon Fall 2006
Picture credit AnimAlu Productions
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ASTR 330 The Solar System

• Rotation and 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.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Rotational Spin-Up

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

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Actual Protoplanetary 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!

These image are visible composites from red,
green and blue light.
Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• 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.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• 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.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• The 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 protosun became hot and dense enough,
  nuclear fusion was ignited.

Dr Conor Nixon Fall 2006
Picture credit AnimAlu Productions
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ASTR 330 The Solar System

• 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.

Dr Conor Nixon Fall 2006
Picture credit James Schimbert, U. Oregon, Eugene
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ASTR 330 The Solar System

• 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.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Inner Planets

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

• 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.

Dr Conor Nixon Fall 2006
Picture credit AnimAlu Productions
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ASTR 330 The Solar System

• 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.

Dr Conor Nixon Fall 2006
Picture NASA
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ASTR 330 The Solar System

• 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.
Dr Conor Nixon Fall 2006
Picture NASA
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ASTR 330 The Solar System

• 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.

Dr Conor Nixon Fall 2006
Picture credit NASA GSFC
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ASTR 330 The Solar System

• 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.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• 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 detected was found in 1992 (not
  counting Pluto and Charon!) now over 800 are
  known.

Dr Conor Nixon Fall 2006
Picture Johns Hopkins University
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ASTR 330 The Solar System

• 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.

Dr Conor Nixon Fall 2006
Graphic wikipedia
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ASTR 330 The Solar System

• 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!

Dr Conor Nixon Fall 2006
Graphic wikipedia
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ASTR 330 The Solar System
Dr Conor Nixon Fall 2006
Graphic wikipedia
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ASTR 330 The Solar System

• 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.

Dr Conor Nixon Fall 2006
Graphic wikipedia
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ASTR 330 The Solar System

• 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).

Dr Conor Nixon Fall 2006
Graphic wikipedia
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ASTR 330 The Solar System

• 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.

Dr Conor Nixon Fall 2006
Graphic SWRI
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ASTR 330 The Solar System

• 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.

Dr Conor Nixon Fall 2006
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ASTR 330 The Solar System

• Pictorial Summary

Dr Conor Nixon Fall 2006
Picture credit James Schimbert, U. Oregon, eugene
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ASTR 330 The Solar System

• Quiz - Summary

• Describe the conditions which existed in our
  part of the Milky Way prior to the birth of the
  solar system.
  
• Why did the gas cloud collapse to a disk and not
  a point why did everything not fall into the
  Sun?
  
• Describe how planets formed from the disk.
• Describe the early history (pre-main sequence)
  of the Sun.
  
• Why are the inner planets volatile-poor while
  the outer planets are volatile-rich?

Dr Conor Nixon Fall 2006