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Planet formation vs Sun collapse

  1. Aug 19, 2013 #1
    I understand how conservation of momentum leads to planet formation and planet rotation. However, after studying this model, I have ran into a point of confusion that I cannot find the answer to:

    Why dont the planets collapse into the sun just as dust particles collapsed inward via conservation of angular momentum to form the planets in the first place? What distinguishes the two situations?

    My two potential theories (pardon any silly answers, I am an amateur):

    * The velocity, size, distance, etc of the planets are just right that they stay in that equilibrium.
    * The accretion process stopped after the sun reached some set stage. Perhaps study the stages of the sun?

    Thanks in advance!!
  2. jcsd
  3. Aug 20, 2013 #2


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    The collapse in radial direction (along the disk radius) stops as soon as the velocity is sufficient to give orbits around the central mass.
    "Every" orbit around a star is stable, if the planet does not hit anything (significant) on its path.
  4. Aug 20, 2013 #3
    Hmm. Im a little confused. Some of the reading that I am doing online is showing me something more like this:

    A protostar accretes mass until it reaches an equilibrium. Equilibrium can be explained in the following manner:

    1. Gravity pulls gas and dust into the core.
    2. The core gets hotter as gas density increases and thereby the rate of gas atom collisions increases.
    3. When the gas pressure = gravity, the accretion process stops.
  5. Aug 20, 2013 #4


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    True, but you're talking about the gas that is collapsing to form the star itself. The gas and dust that forms the planets doesn't collapse into the star.
  6. Aug 22, 2013 #5
    I forget the exact equation, but it basically depends on the kinetic energy of the planet, for which a few scenarios arive:

    1) if it has too little energy it goes into the sun
    2) if it's within a certain range it gains an elliptical orbit (like our planets have)
    3) if it has too much energy it has a hyperbolic orbit and is flung out of the star system

    and our planets have enough kinetic energy to be in the 2nd category, as planets lose energy naturally over time they move into the 1st category from the 2nd and crash into their sun. We just don't see planets like that because they would crash into the sun over the course of millions of years, although there are some examples that you'll occasionally see in the news
  7. Aug 23, 2013 #6


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    Such processes take orders of magnitude longer. Our solar system has been around for over 4 billion years and we're just fine.
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