14-03 Formation of the Solar System

  • Star formation and the solar nebula
    conservation of angular momentum and the protoplanetary disk (proplyd)





     
  • Planets have compositions different from the Sun and the solar nebula:
    1) Planets begin growing by sticking together solid grains.
    2) Only when much more massive than the earth do planets have enough gravity to capture gas directly from the solar nebula.
     
  • Condensation of solid grains
    under low pressure, matter exists either as a solid or as a gas
    the temperature of the original cold isothermal solar nebula is about 50 K
    the original solar nebular consists of mainly hydrogen and helium gases seeded with many solid grains including dusts and ices of water, methane, ammonia etc.
    the protosun during its Kelvin-Helmholtz contraction heated the solar nebula

    [Water, methane, ammonia etc condensate only in the outer part of the solar nebula, while rock-forming substance can also condensates in the inner part.]

     
  • Solid grains sticked together to form larger chunks, and then planetesimals (1 km or so).
    Planetesimals in a rotating disk then coalesced into objects of 100 km or so.
    Large planetesimals further fused together to grow and attract smaller objects.
    They became protoplanets of 1000 km or so.
    Protoplanets then collided, accretted, and coalesced to form planets.



    The process completed within about 108 years, the same as the time for the protosun to ignite its core nuclear burning.
     
  • On planets: heating (by violent impacts, decay of radioactive elements) --> melting --> chemical differentiation
     
  • In the outer region of the solar nebula, more solid grains were available to form planetesimals.
    More massive planets were formed, which could trap gases to form an even larger planet.
    (Uranus and Neptune requireed much longer time to form this way, unless they were formed at a smaller distance from the Sun and then migrated to their current location. An alternative model to this issue is the disk instability model.)
     
  • Even farther away, icy bodies formed as KBOs we see today.
    Gravitational influence of large planets is important on the population properties of KBOs.
     
  • Explaining the characteristics of the solar system:
    disk shape (however, the rotation of Venus and Uranus)
    distinction between the terrestrial and Jovian planets
    a common age
     
  • Clearing the nebula:
    radiation pressure
    solar wind
    sweeping up of space debris by planets
    ejection by close encounter with planets