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Thread: Planetary rings

  1. #1
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    Planetary rings

    What organizes these rings so well? With chaotic formation of ring+moon systems, one would think that rings would be at some angle to the equator, and be "thick", shaped more like a donut than a plate. Yet they are over the equator and thin layered to within a part per million, IIRC. What makes them so "perfect"?

  2. #2
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    A combination of collisions and the conservation of angular momentum.

    I must have written a one-paragraph description of the process five or six times in different BAUT threads over the years. I'm going to let someone else write it this time.

  3. #3
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    Interesting.
    If the Kessler Syndrome occurs, will the resulting particles fall into a thin, flat ring around Earth's equator?

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    Given enough time, yes. The orbits of the particles might decay due to air drag first, though.

  5. #5
    I thought that part of the reason was because of "herding" or "shepherding" by satellites, in other words that satellites clear out a space and force the material to go into orbits that are outside that zone. Maybe this is a controversial issue?
    As above, so below

  6. #6
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    The "shepherding moons" can cause gaps or sharp edges to appear in a flat disk of material. They aren't responsible for the disk of material being flat.

  7. #7
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    The short answer is gravity. Gravity draws matter into planes, ie rings, due to the fact that it all attracts itself.

    Same reason why the solar system/ecliptic is a disc. The orbitting things attract each other. It's the opposite of a situation like an atom where the orbitting things repel each other.

  8. #8
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    Quote Originally Posted by CaptainToonces View Post
    The short answer is gravity. Gravity draws matter into planes, ie rings, due to the fact that it all attracts itself.

    Same reason why the solar system/ecliptic is a disc. The orbitting things attract each other. It's the opposite of a situation like an atom where the orbitting things repel each other.
    I once thought that, but was corrected on BAUT. Gravity is not the reason stuff ends up in the plane. It's more a kind of friction that averages out the position of the matter. Matter going "up" or "down" relative to the overall plane of rotation of the matter, will collide, evening out over time.
    Thank you, members of cosmoquest forum, you are a part of my life I value.

  9. #9
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    This friction explanation makes no sense to me. Can you explain it better? I'm pretty sure i'm not mistaken about this.

    Friction would push the particles away from a disc.....

  10. #10
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    I tried using the "Search" function within BAUT, and it failed to find any of the many previous messages on this topic. Rats. So, I'll try to type it out again.

    When a big cloud of material begins to collapse, it has some net angular momentum. What that means is that, on average, the material in the cloud is spinning around some particular axis in some particular direction. Yes, some particles may be moving in the opposite direction, but if you average over all the stuff, there will be some particular favored direction. I'm going to re-orient myself, as I describe the cloud, so that this preferred axis runs vertically, up and down. That means that the material will on average be rotating past me, as I watch, in a horizontal direction. Okay so far?

    Now, some of the particles have motions which are mostly vertical. If the particles collide with each other, then, on average, particles travelling upward through the cloud will run into those moving downward through the cloud. When these particles collide, they will lose some of their momentum in the vertical direction. However, since they are both (on average) moving in the same horizontal direction as they rotate around the axis, they will keep most of their horizontal momentum. The result is that particles with large vertical motions gradually lose the vertical components of their velocity.

    On the other hand, since (on average) the particles are all moving horizontally in the same direction around the axis, when two particles collide, they will have only small relative horizontal motions; thus, the collision will not cause much change in the horizontal component of their velocity. They will keep rotating around and around the axis.

    Compare it to a NASCAR race, in which a bunch of cars drive around and around a track, all in the same direction. When a collision occurs, since both cars are moving forward at high speed, the cars may slide up the track, or down the track, a small amount -- but they continue to slide forward. In the same way, two particles which are moving purely in the horizontal direction (i.e., in a disk) will continue to move in the disk, though they may shift inward or outward a little bit. But there is no way for the collision to give the particles any significant vertical velocity.

    So, if you start with a big cloud of many particles, and if the particles do collide with each other, the cloud will eventually settle into a flattened disk. Atoms and molecules and little specks of dust do collide with each other efficiently, so interstellar clouds flatten as they collapse into protostars. Big clouds of gas do collide with each other efficiently, so gigantic collections of enormous clouds of gas will flatten into disk galaxies as they collide and merge. Stars, on the other hand, are so small (relative to their separations) that they do NOT collide efficiently, so a collection of stars will NOT flatten into a disk; it will remain a roughly spherical blob, as a globular cluster or elliptical galaxy or bulge at the center of a spiral galaxy.

  11. #11
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    Thank you for your time StupendousMan

    And you even worked in globular clusters...

  12. #12
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    Are disk galaxies disks because of collisions between gas
    molecules, or between dust particles, or for some other reason?
    The scale is so vastly larger than that of planetary rings or a
    protoplanetary disk that I wonder if the mechanism is different.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

  13. #13
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    Collisions between gas particles in the cloud which eventually formed the galaxy.

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    In relation to the OP, would the slightly higher gravity over a planet's equatorial bulge help encourage particles to orbit over the equator rather than anywhere else?

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    Indirectly, yes. If somehow a set of particles entered the space above a planet from a direction far from the equator, and were arranged into a disk which was not coplanar with the planet's equator, gravitational effects of the equatorial bulge of the planet would cause the orbits of the particles to precess. The precession would enhance collisions between particles, leading eventually to the particles settling into an orbit coplanar with the equatorial bulge.

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    Would a ring orbiting the planet in equatorial plane but contrary to the rotation be equally stable as a ring in the forward direction?

  17. #17
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    Cute, Stupendous. I asked if disk galaxies result from collisions
    between gas molecules or dust particles, and you reply that they
    result from collisions between gas particles. Very diplomatic!



    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

  18. #18
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    Quote Originally Posted by chornedsnorkack View Post
    Would a ring orbiting the planet in equatorial plane but contrary to the rotation be equally stable as a ring in the forward direction?
    No it wouldn't, it will be stable enough to exist for a long time, but not as long. If you're patient, and live a very long time, you can watch as Triton forms such a ring around Neptune.
    Forming opinions as we speak

  19. #19
    Quote Originally Posted by Jeff Root View Post
    Are disk galaxies disks because of collisions between gas
    molecules, or between dust particles, or for some other reason?
    The scale is so vastly larger than that of planetary rings or a
    protoplanetary disk that I wonder if the mechanism is different.
    Look at how much of a galaxy is composed of hydrogen/helium and how much is "metals", and the answer will be pretty clear.


    Quote Originally Posted by antoniseb View Post
    No it wouldn't, it will be stable enough to exist for a long time, but not as long. If you're patient, and live a very long time, you can watch as Triton forms such a ring around Neptune.
    Actually, it may be more stable as a ring...less prone to accreting into a moon. This will certainly be so in the case you mention, where the ring forms in the first place because tidal drag pulls the moon in close enough that it can't hold together against tidal forces.

  20. #20
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    One reason I ask about gas vs dust is that some galaxies
    collapse into disks, and some don't. I thought maybe disk
    galaxies collapsed while ellipticals didn't because the disk
    galaxies had more dust, so more collisions. No?

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

  21. #21
    Quote Originally Posted by Jeff Root View Post
    One reason I ask about gas vs dust is that some galaxies
    collapse into disks, and some don't. I thought maybe disk
    galaxies collapsed while ellipticals didn't because the disk
    galaxies had more dust, so more collisions. No?
    Galaxy formation isn't well understood, but some factors are that disk galaxies rotate much faster, possibly resulting from mergers of smaller galaxies, and going the other direction, collisions of disk galaxies can disrupt the disk structure and cancel out much of the overall angular momentum of a galaxy. Age is another factor...the most visible spiral/disc features are young stars recently formed from the gas clouds of the galaxy.

  22. #22
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    Disk galaxies: gas cloud collapses to disk, forms stars. Elliptical galaxy: gas cloud forms stars, can't collapse to disk.

  23. #23
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    Stupendous,

    That sounds like a theory I've come up with once or twice.

    So it sounds very agreeable to me.


    Chris,

    If a disk galaxy rotates faster than an elliptical galaxy of the
    same diameter, doesn't that just mean the disk galaxy is denser?
    Which would be a natural result of collapsing.

    -- Jeff, in Minneapolis
    Last edited by Jeff Root; 2012-May-03 at 05:21 AM. Reason: a teeny tiny typo
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

  24. #24
    Quote Originally Posted by Jeff Root View Post
    If a disk galaxy rotates faster than an elliptical galaxy of the
    same diameter, doesn't that just mean the disk galaxy is denser?
    Which would be a natural result of collapsing.
    Not necessarily. The stars could just be in less circular orbits, mostly moving toward or away from the center of the galaxy, rather than large-scale organized motion leading to a high overall rotation rate for the galaxy. From what I've read, it looks like this is actually the case. Might be the expected result from a galaxy formed from a collision of galaxies that have already consumed much of the available gas, the stars interacting less with each other than the clouds of gas would have.

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