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James Linzel's List: Assignment 4 - Our Solar System

Formation of the Solar System

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- Motion of the planets in
  their orbits:
- 9 more annotations...
- - asteroid belt
  - Kuiper
    Belt
  - Large bodies in the solar
    system have orderly motion.
    
    All planets and most
    satellites have nearly circular orbits, in the same direction, and nearly in
    the same plane. The Sun and most of
    the planets rotate in the same directions as well.
    
    Planets fall into two main catelgories.
    
    Small, rocky terrestrial planets near the Sun and
    large, hydrogen-rich jovian
    planets farther out. The jovian planets have many moons and rings made of rock and
    ice.
    
    Swarms of asteroids and
    comets populate the solar system.
    
    Asteroids are concentrated
    in the asteroid belt, and comets populate the regions known as the Kuiper belt and the Oort cloud.
    
    There are several important
    exceptions to these trends.
    
    Planets with unusual tilts,
    very large moons, or moons with unusual orbits.
  - Large and diffuse, slowly rotating – it had some angular momentum.
    
    This
    property of the cloud primarily accounts for the motions of the planets. Why?
    Think about momentum and energy…
    
    ·
    Composed primarily of hydrogen and helium, but there must have been some heavier elements,
    including metals, since we find them in the terrestrial planets for example.
    
    This
    property of the cloud will dictate how the planets ended up in two main
    types. Think about phases of matter…
  - The collapse is triggered by
    an increase in density, and driven by gravity.
    
    What do we mean by “collapse”?
    
    During the
    collapse…
    
    1. The cloud’s
    rotation rate increases, due to
    conservation of angular momentum.
    
    2. The cloud
    heats up, as compressing the gas
    cause the particles to speed up, increasing the
    temperature of the gas and dust particles.
    
    3. The disk of
    gas and dust flattens, as collisions
    between the particles of the cloud to lose energy in the direction perpendicular
    to the cloud’s rotation.
    
    
    
    So we can now account for one
    of the four challenges: the orderly
    nature of the orbits of planets in the solar system is due to conservation of
    energy and angular momentum during the collapse of the gas cloud from which
    they formed.
    
    The direction of rotation is dictated by the angular momentum of the
    cloud.
    
    The inclination of the planets is due to the flatness of the nebular disk after
    collapse.
  - Why are there two types of planets, terrestrial and
    Jovian?
    
    A. The force of gravity due to the massive Sun draws the
    heavier, dense material of the terrestrial planets closer.
    
    B. Initial orbits of the terrestrial planets bring them
    closer to the Sun where they fall into smaller orbits.
    
    C. Near the Sun, only heavy elements and rocky material
    can condense from the solar nebula.
    
    D. Jovian planets form first and draw much of the gaseous
    material to them via gravity, leaving only the heavier elements and rocky
    material behind.
  - So beyond
    the frost line, which lies between
    the orbits of Mars and Jupiter, temperatures had dropped enough for ices such
    as water, ammonia and methane to condense.
  - Accretion
    
    How to grow planetessimals:
    
    ·
    Initially small
    particles of gas and dust were able to stick together via their electrostatic
    attraction.
    
    ·
    As they grew larger,
    their gravity began to be strong enough to attract particles as well, and their
    growth accelerated.
    
    ·
    Once large
    enough, gravity pulls the planetessimal into a
    spherical shape.
    
    ·
    Once a planetessimal reaches a certain size (around 1 km) this
    process really takes off and it begins to gravitationally dominate everything
    around it.
    
    We can now account for the
    second important challenge of explaining our solar system, the division of
    planets into two basic types.
    
    Rocky, metallic material of
    the terrestrial planets could condense nearer to the Sun than the ices.   Hydrogen and helium gas remained gaseous
    throughout the solar system.
  - Forming the Jovian
    planets through nebular capture
    
    Once accretion finished
    building the seeds of the Jovian planets, their large
    masses meant that their gravity was strong enough to accumulate large amounts
    of the remaining nebular gases – i.e. the force of gravity of the planet was stronger than that
    from the Sun at that point, so that the gas went from orbiting the sun to
    orbiting the planet.
    
    This process proceeded in
    basically the same way as the nebular collapse which formed the solar system,
    forming similar disks of material around the Jovian
    planets. Some of the material
    contributed to the planet, and some to satellite systems through accretion.
    
    The Solar Wind
    blows out the Remaining Nebular Gases
    
    The solar wind is composed of
    charged particles from the Sun’s hot (millions of degrees K) corona which carry
    the Sun’s magnetic field.
    
    We see evidence (T Tauri stars) that this solar wind is very strong in young
    stars. Radiation pressure from the young
    sun is also important – photons have momentum.
    
    These effects work to clear
    out the remaining gases, before they cool enough for ices to condense in the
    inner solar system.
The Origin of the Solar System

# A cloud of interstellar gas and/or dust (the "solar nebula") is disturbed and collapses under its own gravity. The disturbance could be, for example, the shock wave from a nearby supernova.

# As the cloud collapses, it heats up and compresses in the center. It heats enough for the dust to vaporize. The initial collapse is supposed to take less than 100,000 years.

# The center compresses enough to become a protostar and the rest of the gas orbits/flows around it. Most of that gas flows inward and adds to the mass of the forming star, but the gas is rotating. The centrifugal force from that prevents some of the gas from reaching the forming star. Instead, it forms an "accretion disk" around the star. The disk radiates away its energy and cools off.

# First brake point. Depending on the details, the gas orbiting star/protostar may be unstable and start to compress under its own gravity. That produces a double star. If it doesn't ...

# The gas cools off enough for the metal, rock and (far enough from the forming star) ice to condense out into tiny particles. (i.e. some of the gas turns back into dust). The metals condense almost as soon as the accretion disk forms (4.55-4.56 billion years ago according to isotope measurements of certain meteors); the rock condenses a bit later (between 4.4 and 4.55 billion years ago).

# The dust particles collide with each other and form into larger particles. This goes on until the particles get to the size of boulders or small asteroids.

# Run away growth. Once the larger of these particles get big enough to have a nontrivial gravity, their growth accelerates. Their gravity (even if it's very small) gives them an edge over smaller particles; it pulls in more, smaller particles, and very quickly, the large objects have accumulated all of the solid matter close to their own orbit. How big they get depends on their distance from the star and the density and composition of the protoplanetary nebula. In the solar system, the theories say that this is large asteroid to lunar size

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solarsystem planets on 2008-04-21
- A cloud of interstellar gas and/or dust (the "solar nebula")
  is disturbed and collapses under its own gravity. The disturbance
  could be, for example, the shock wave from a nearby supernova.
  
  As the cloud collapses, it heats up and compresses
  in the center. It heats enough for the dust to
  vaporize. The initial collapse is supposed to take
  less than 100,000 years.
  
  The center compresses enough to become a protostar
  and the rest of the gas orbits/flows around it. Most
  of that gas flows inward and adds to the mass of the
  forming star, but the gas is rotating. The centrifugal
  force from that prevents some of the gas from reaching
  the forming star. Instead, it forms an "accretion disk"
  around the star. The disk radiates away its energy and
  cools off.
- The gas cools off enough for the metal, rock and (far
  enough from the forming star) ice to condense out into
  tiny particles. (i.e. some of the gas turns back into dust).
  The metals condense almost as soon as the accretion disk
  forms (4.55-4.56 billion years ago according to isotope
  measurements of certain meteors);
  the rock condenses a bit later (between
  4.4 and 4.55 billion years ago).
  
  The dust particles collide with each other
  and form into larger particles. This goes on
  until the particles get to the size of boulders
  or small asteroids.
  
  Run away growth. Once the larger of these
  particles get big enough to have a nontrivial
  gravity, their growth accelerates. Their gravity
  (even if it's very small) gives them an edge over
  smaller particles; it pulls in more, smaller particles,
  and very quickly, the large objects have accumulated
  all of the solid matter close to their own orbit.
  How big they get depends on their distance from
  the star and the density and composition of the
  protoplanetary nebula. In the solar system, the
  theories say that this is large asteroid to lunar
  size in the inner solar system, and one to fifteen
  times the Earth's size
  in the outer solar system. There
  would have been a big jump in size somewhere
  between the current orbits of
  Mars and Jupiter:
  the energy from the Sun would have kept
  ice a vapor at closer distances, so the solid,
  accretable matter would become much more common
  beyond a critical distance from the Sun. The
  accretion of these "planetesimals" is believed
  to take a few hundred thousand to about twenty
  million years, with the outermost taking the longest
  to form.
- 1 more annotations...
- - At this point, the solar system is composed
    only of solid, protoplanetary bodies and gas
    giants. The "planetesimals" would slowly collide
    with each other and become more massive.
    
    Eventually, after ten to a hundred million years,
    you end up with ten or so planets, in stable orbits,
    and that's a solar system. These planets and their surfaces
    may be heavily modified by the last, big
    collision they experience (e.g. the largely
    metal composition of Mercury or the
    Moon).
Understanding Solar System Dynamics: Orbits and Kepler’s Laws — NASA Science

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on 2009-12-17
What is A Planet? — NASA Science

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on 2009-12-17