The Milky Way

							     The Origin
of the Solar System
How did the solar system form?
 Early Hypotheses
                                       • catastrophic hypotheses,
                                      e.g., passing star hypothesis:
                                       Star passing the sun closely
                                        tore material out of the sun,
                                      from which planets could form
                                           (no longer considered)
Catastrophic hypotheses predict:
Only few stars should have planets!
                                       • evolutionaryhypotheses,
                                         e.g., Laplace’s nebular
                                               hypothesis:
                                      Rings of material separate from
                                        the spinning cloud, carrying
                                      away angular momentum of the
Evolutionary hypotheses predict:       cloud  cloud could contract
Most stars should have planets!           further (forming the sun)
The Solar Nebula Hypothesis

             Basis of modern theory
             of planet formation

             Planets form at the
             same time from the
             same cloud as the star.

             Planet formation sites
             observed today as dust
             disks of T Tauri stars

             Sun and our Solar system
             formed ~ 5 billion years ago
Dust Disks




         Many young stars in the Orion
        Nebula show the dust disks out of
         which planetary systems form.
Our Solar System
        How it Started
• Initially there was a very large, low
  density, cold cloud of gas where gravity
  was balanced by motion of the gas
• Some external force (like a supernova
  exploding nearby) caused the cloud to
  begin to collapse
• Three processes occurred in the
  collapsing cloud
  – Heating
  – Spinning
  – Flattening
  Why does it Heat up?
• Energy must be conserved
• If something gets smaller it loses
  gravitational potential energy
• This energy goes into kinetic energy
  of the atoms in the gas (they bump
  into each other more often)
• Thermal energy is just kinetic energy
  of a bunch of particles…the cloud
  heats up
    Why does it Spin?
• Angular momentum must be
  conserved
• It is extremely unlikely that a
  collapsing cloud is not spinning
• As the cloud gets smaller it must spin
  faster
         Why does it Flatten?
• As the cloud grows
  smaller, it spins
  faster causing it to
  flatten

 Movie
1. Patterns of motion among large bodies   
2. Two major types of planets

3. Asteroids and comets


4. Exceptions to the rules
The Story of Planet Building
 Planets formed from the same protostellar material
   as the sun, still found in the Sun’s atmosphere.

     Rocky planet material formed from clumping
   together of dust grains in the protostellar cloud.


 Mass of less than ~ 15        Mass of more than ~ 15
    Earth masses:                 Earth masses:
                                  Planets can grow by
Planets can not grow by         gravitationally attracting
 gravitational collapse.           material from the
                                   protostellar cloud.
    Earthlike planets
                              Jovian planets (gas giants)
The Condensation of Solids
To compare densities of planets,
compensate for compression due
to the planet’s gravity:

Only condensed materials
could stick together to form
planets.

Temperature in the protostellar
cloud decreased outward.

Further out  Protostellar cloud
cooler  metals with lower
melting point condensed 
change of chemical composition
throughout solar system.
Formation and Growth of Planetesimals

    Planet formation
  starts with clumping
  together of grains of
      solid matter:
     Planetesimals

    Planetesimals (few
     cm to km in size)
      collide to form
         planets.



                Planetesimals grow through
                condensation and accretion.

  Gravitational instabilities may have helped in the growth of
                planetesimals into protoplanets.
                                                      Movie
The Growth of
Protoplanets
Simplest form of planet growth:
  Unchanged composition of
  accreted matter over time
   As rocks melted, heavier
 elements sank to the center
        differentiation

     This also produces a
    secondary atmosphere
         outgassing

   Improvement of this scenario:
     Gradual change of grain
   composition due to cooling of
  nebula and storing of heat from
         potential energy
        Inner Planets
• The rocks and metals condensed in
  the inner solar system to create the
  Terrestrial planets.
• Since the solar nebula was mostly
  hydrogen and helium there wasn’t
  much in the way of heavy elements
  for the planets to form from…hence
  the Terrestrial planets are “small”
        Outer Planets
• Beyond the Frost line
  – where it is cool enough for hydrogen
    compounds such as water, ammonia, and
    methane to condense into solid ice grains
• Object were large enough to pull in
  material (via gravity) which had not
  been swept clean
• Mostly hydrogen and helium (the most
  abundant elements in the Solar
  Nebula), which although still gaseous
  were cool enough to accrete onto the
  outer planets as dense, thick
  atmosphere.
The Jovian Problem
   Two problems for the
theory of planet formation:

1) Observations of extrasolar planets indicate that
          Jovian planets are common.
2) Protoplanetary disks tend to be evaporated quickly
 (typically within ~ 100,000 years) by the radiation of
                  nearby massive stars.
        Too   short for Jovian planets to grow!
                       Solution:
 Computer simulations show that Jovian planets can
 grow by direct gas accretion without forming rocky
                   planetesimals.
                 Moons
• Shortly after the formation of the planets there
  were a lot of these leftover planetesimals.
• Many of the collided with the newly formed
  planets, creating a period of “heavy
  bombardment” that left visible scars on the
  terrestrial planets.
• Some of these leftovers became captured
  moons.
• Since they weren’t formed with their parent
  planet, captured moons can have different
  compositions and different orbital properties
  from the planet they orbit.
      The Earth’s Moon
• Earth’s moon is far too large to have been a
  captured moon.
• Its composition is different enough from the
  Earth that it couldn’t have formed with the
  Earth.
• Its most likely origin is a collision between a
  Mar-sized planetesimal and the newly formed
  Earth.
• The collision caused surface material from
  the Earth to be dislodged which then
  reaccreted in an orbit around the Earth.
What about the jovian rings and
           moons?
• The gas that was accreted by the forming planets
  would follow the same process as the solar nebula
  itself.
• As the gas is accreted it will heat up, spin and flatten
  into a disk
• This disk then gives rise to many moons and rings
  (remember that the rings are made of small particles).
• Most of the leftover gas was never accreted into any
  planet.
• Once the Sun started shining the solar wind swept the
  remaining gas out into interstellar space.
• Had this occurred earlier or later than it did the planets
  would have looked quite different.
1. Patterns of motion among large bodies   
2. Two major types of planets   
3. Asteroids and comets

4. Exceptions to the rules
What about the asteroids and
         comets?
• They are leftovers from the planet formation process.
• Asteroids are simply pieces of rock from the inner solar
  system that were never incorporated into any of the terrestrial
  planets.
• Comets are the pieces of ice that didn’t make it into one of
  the jovian planets.
• Since they have undergone very little processing since they
  formed, studying them can tell us about the conditions in the
  early solar system.
• The majority of asteroids reside in the asteroid belt located
  between Mars and Jupiter.
• Due to the gravitational pull from Jupiter the asteroids could
  never coalesce into an actual planet.
• Comets are found in the outer parts of the solar system, flung
  there by gravitational encounters with the jovian planets.
• Most comets remain far from the Sun but a few occasionally
  pass through the inner solar system where we can observe
  them.
                                       
1. Patterns of motion among large bodies


2. Two major types of planets   
3. Asteroids and comets
4. Exceptions to the rules
What’s the deal with Pluto?
• Pluto’s characteristics – its
  composition, orbital properties and
  location in the solar system – match
  up much better with the characteristics
  of comets than with planets.
1. Patterns of motion among large bodies   
2. Two major types of planets    
3. Asteroids and comets      
4. Exceptions to the rules
                             
Explaining the Characteristics
of the Solar System
      The Solar Nebula Hypothesis explains:

 1) Disk shape and common sense of revolution:
 Inherited from the disk shape and rotation of the
                   Solar Nebula
     2) Division of Terrestrial / Jovian Planets:
 Result of decreasing temperature throughout the
  Solar Nebula → Further out, lighter elements
 condensed out to form heavier, gaseous planets

                   3) Icy Comets:
Originating in the Oort Cloud, very far away from the
    sun, in the coldest parts of the Solar Nebula
Clearing the Nebula
Remains of the protostellar nebula were cleared away by:
• Radiation pressure of the sun • Sweeping-up of space debris by planets

• Solar wind                     • Ejection by close encounters with planets




       Surfaces of the Moon and Mercury show
    evidence for heavy bombardment by asteroids.
   The Sun Turns On!!
• Planet formation stops when the sun
  turns on
• At the moment of ignition, a strong
  solar wind blows through the solar
  system and blows away most of the
  remaining light particles
Evidence for Ongoing Star Formation:
Dust Disks Around Forming Stars


                           Dust disks
                           around
                           some T
                           Tauri stars
                           can be
                           imaged
                           directly
                           (HST).
Extrasolar Planets
  Modern theory of planet formation is evolutionary
       Many stars should have planets!
 planets   orbiting around other stars = “Extrasolar planets”

                                            Extrasolar planets
                                                can not be
                                             imaged directly.

                                            Detection using
                                           same methods as
                                             in binary star
                                               systems:
                                          Look for “wobbling”
                                           motion of the star
                                          around the common
                                            center of mass.
Indirect Detection of Extrasolar Planets
                                  Observing periodic
                                  Doppler shifts of
                                  stars with no
                                  visible companion:

                                  Evidence for the
                                  wobbling motion of
                                  the star around the
                                  common center of
                                  mass of a planetary
                                  system
 Indirect evidence through
  wobbling motion of stars        Over 100
detects primarily Jupiter-like,   extrasolar planets
        large planets             detected so far
Evidence for “Extrasolar Asteroids”
                        Large amount of dust around young
                          planetary systems might provide
                            evidence for the presence of
                       asteroids, producing dust in collisions.




       This might
  currently still be
   going on in our
own solar system.
Direct Detection of Extrasolar Planets
     Only in exceptional cases can extrasolar
           planets be observed directly.




           Preferentially in the infrared:
    Planets may still be warm and emit infrared
      light; stars tend to be less bright in the
             infrared than in the optical.