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Why are their Galactic Plains?

  1. Mar 18, 2004 #1
    What causes Galactic Plains to form? Is it the same reason there is a planetary plain?
  2. jcsd
  3. Mar 18, 2004 #2


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  4. Mar 18, 2004 #3
    so what causes the planetary plain?
  5. Mar 18, 2004 #4


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    I want to understand your question better

    first of all, the plane of the earth's orbit is not quite the same as the plane of jupiter's orbit
    or the plane of another planet
    or the plane of the moon's orbit

    all these planes are slightly tilted with respect to each other and do not exactly coincide

    but I think you are asking why do they so NEARLY coincide
    mostly they are tilted only a few degrees from each other

    what evened them out?
    if you started with a central star and 100 planets orbiting every which way around it
    so the orbit-planes of the 100 planets are initially all tilted randomly by as much as 90 degrees from each other or even more
    then what would cause those 100 planets to gradually
    work out an approximate common orbit plane?

    wolram mentioned collisions
    and there is gravitational interaction which can ressemble a soft-core collision and happen repeatedly over millions of years
    maybe it gives a kind of "viscosity" to a system of 100 planets
    so that eventually their angular momentums boil down and settle down to one basic orientation that they all more or less agree on
    (because otherwise they keep bumping and perturbing each other)

    a lot of energy must get lost and a lot of angular momentum (in opposed directions) must get canceled out and it must be a long process (wouldnt necessarily be pleasant to witness either)
    but something like viscosity keeps dragging on them and bugging them until they work out an approximate common plane

    Let us try to get Phobos or Integral or some such astronomy honcho to pronounce the word on this
  6. Mar 18, 2004 #5
    My guess would be that galaxies are planar for the same reason as star systems - they formed out of large rotating clouds of gas and dust. As the gas condenses, it retains its angular momentum and "settles down" to a plane. It's similar to the oblateness of the Earth due to its rotation, but taken to an extreme due to the gaseous nature of the cloud and the speed of rotation (Earth's surface rotates at about 1 kps; gas at 1AU goes around at about 30kps). I believe we are going around the center of the galaxy at about 200 kps, so despite the massive size it would still flatten out on a timescale of billions of years.

    As a side note, how tilted is the ecliptic with respect to the galactic plane? What about for the orbits of extrasolar planets? I suspect that planetary orbits would be, for the most part, similar to the galactic plane, differences being due to localized turbulence in the original gas cloud as it was condensing.
    Last edited: Mar 18, 2004
  7. Mar 18, 2004 #6
    Ok, I understand it now. Thanks for the information guys:smile:
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