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Friction necessary for planet formation?

  1. Feb 21, 2009 #1
    For a gas and dust cloud to collapse under gravity to form a star and planets, is the concept of friction important at all?

    In other words: let's consider a large number of billiard balls, no internal degrees of freedom, participating in completely elastic collisions, with random initial conditions, subject only to their own gravity. Will they eventually clump together into one big "star" and several smaller "planets" surrounding the star?
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  3. Feb 21, 2009 #2


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    Axel, I only have a minute (have to go out) so I hope other people will help answer. Consider ways that energy and angular momentum could be ejected from the system.

    Some energy can be radiated. When things crash and turn red hot you get rid of some energy by thermal radiation. Also debris may acquire escape speed and leave the system taking unwanted energy with it.

    Also there is the gravitational slingshot thing that happens. One billiard has a close encounter with another. One can pick up energy and angular momentum and be flung clear out of the system, the other perhaps is slowed down by the interaction.

    I think your conjecture is right that they will eventually clump into a central body and some smaller planets----but there will also be billiards that you didn't mention that were ejected from the system, taking surplus gravitational energy.
  4. Feb 21, 2009 #3


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    Agreed, there will be a significant portion of matter that is slingshotted out of the system. Also, the collisions can't be elastic or else things wouldn't end up sticking together! What will happen is there will be two "states" if you will, for the original particles:
    1. Initial velocity and position lead to an unstable orbit. This situation has two possibilities:
    a. The ball is ejected out of the system after it falls towards the gravitationally forming center.
    b. The ball collides with the gravitationally forming center and merges with it.

    2. Initial velocity and position lead to a stable orbit. This situation is relatively rare, but when it does happen the stable orbiting "ball" accretes matter until it grows to planetary size.

    I should think friction is important in this model in the sense that depending on the density of the gas and dust cloud, collisions which heat matter cause the particles to lose rotational kinetic energy and fall into the center.
  5. Feb 22, 2009 #4
    Yes, that's the heart of my problem. Assuming totally elastic collisions, the balls will never "stick together", but there could still emerge regions of very high density, where balls are kicking each other like crazy, with the whole region being held together by gravity. And these are the regions I would call "star" and "planets". Possibly though the most likely outcome in this scenario is that almost all balls will eventually leave the system?

    Does anyone know any simulators I could play with?
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