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Why isn't the Oort Cloud shaped like a disk?

  1. Aug 15, 2015 #1
    Why isn't the Oort Cloud shaped like a disk like the asteroid belts and how the planets orbit? Also, why exactly do the planets orbit in that flat disk shape?
  2. jcsd
  3. Aug 15, 2015 #2


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    Who says it isn't? If you dip a stick into warm wax you willl pull out a globular ball of wax. If you then spin the stick on its axis the glob will flatten out into a disc, that looks a lot like a galaxy or solar system. In the case of solar systems and galaxies, the glob of wax is a glob of gas and gravity causes it to flatten out as it collapses to form stars and planets.
  4. Aug 15, 2015 #3


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    This article distinguishes between an overall spherical shape (which dominates beyond 5000 AU) and an inner part which is partially flattened.

    Imagine that the solar system originally formed from a roughly spherical self-gravitating cloud. the question to be asking is "by what processes did a disk structure emerge?

    Think about the conservation of angular momentum law. A cloud could in principle have zero overall angular momentum if every particle's looping around the center of mass was exactly canceled by some other particle looping around in the opposite direction.

    But that exact cancelation is extremely unlikely. After all the averaging together and after all the cancelation there is likely to be some average net rotation, and some AXIS direction. That means when the cloud collapses a preferred plane and a preferred axis and direction of rotation is going to emerge. by conservation of angular momentum.

    Actually collapsing clouds have ways of shedding extra energy and angular momentum, by collision heating and radiating heat, by gravitational boost during near collision that can fling some particles out of the cloud---giving them escape velocity. But overall an axis and plane is going to emerge as long as the cloud is dense enough to begin with.

    Gravitational interaction gives clusters, clouds etc (if they are dense enough to have a lot of interaction) a kind of VISCOSITY.

    So on average stuff gets dragged along in the dominant rotation direction.

    Overall the oort cloud is believed to be SPHERICAL because not dense enough to collapse down to a preferred plane. So far at least. Maybe given a lot more time. And orders of magnitude more interactions.
    But according to that article I linked, a small inward PART of it is believed to be roughly planar, IOW has partially collapsed

    Over longer spans of time GALAXIES collapse from roughly spherical clouds of proto-galaxy globby blobs of stars. And these interact gravitationally and eventually these rough spheres flatten out to be beautiful spirals (still surrounded by still-as-yet-uncollapsed spherical clouds or "halo"s of other matter)

    It seems like a kind of universal process that has to do with the angular momentum conservation law (and various analogs of viscosity) and which happens over a wide range of scales----from solar system size all the way up to spiral galaxy scale
    Last edited: Aug 15, 2015
  5. Aug 16, 2015 #4
    So is the emergence of complex rotating structure rather than just one big disk or a crystal due to "various analogs of viscosity"?
  6. Aug 16, 2015 #5
    Last edited: Aug 16, 2015
  7. Aug 16, 2015 #6
    Cool article. Is the solar system thought to have a spiral arm analog, or the galaxy an Oort Cloud analog?

    In this article it looks like they are modeling the Saturnian Ring density waves (mentioned in the article you linked) as perturbations in object density propagating through the some reference density plane.

    Unravelling Temporal Variability in Saturn's Spiral Density Waves: Results and Predictions
    Matthew S. Tiscareno, Philip D. Nicholson, Joseph A. Burns, Matthew M. Hedman, Carolyn C. Porco
    (Submitted on 8 Sep 2006 (v1), last revised 9 Oct 2006 (this version, v2))
    We describe a model that accounts for the complex morphology of spiral density waves raised in Saturn's rings by the co-orbital satellites, Janus and Epimetheus. Our model may be corroborated by future Cassini observations of these time-variable wave patterns.

    This is arguably off topic at this point. But I am still confused as to what is resonating or transmitting the density wave other than the relative positions of the objects (asteroids, rocks, planets, stars) in question? Is relative position all that is waving or resonating? If so how does geometry (distance) resonate or wave or vary? If it doesn't then what is the medium that does? If it does what gives geometry specific "viscosity"? The actual density of the objects isn't what is varying is it? Or is that indistinguishable from the case where geometry is what's varying?

    Cool wiki page on orbital resonances. https://en.m.wikipedia.org/wiki/Orbital_resonance
  8. Aug 16, 2015 #7
    Density wave also applies to galaxy spiral arms, as well as the Saturn rings. Some of the principles apply to Solar system dynamics though in terms of the hydrodynamics, not the specific model. Think of a gas, or plasma. apply a rotating influence ie from gravity. In the case of the galaxy spiral arms, different mass objects will have gain different rates of momentum from that influence. Larger mass objects will move slower than larger objects. F=ma. Once you have the lighter material complete a rotation, it will eventually catch up to the heavier material creating a sort of traffic jam. Short explanation, but density waves can be applied to numerous rotating fluidic dynamics. Think of the whirl pool in your sink when you pull the plug. Then add sand. The point being with plasma one gets numerous hydrodynamic effects which depends on several factors. The type and nature of influence, the constituents of the plasma, the vorticity and viscosity etc.
    Last edited: Aug 16, 2015
  9. Aug 16, 2015 #8


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    I'm a little surprised at the answers here, but I could be wrong.

    It was my assumption that the Oort cloud is (essentially) spherical because - initial angular momentum aside - there's too little interaction within the Oort cloud to draw it into a disc.

    Protosystem spheres form into discs as they collapse when interactions between orbiting protobodies pull the more polar-orbiting bodies into line with the disc (or otherwise discourage long-term stable polar orbits). In the Oort cloud, the protobodies are so few and far between that the only influence on them is the sun.

    The "initial angular momentum of the system" mechanism is not enough to explain a disc unless you include this inter-body interaction.

    I state this as an educated assumption, not as an authority on it.
    Last edited: Aug 16, 2015
  10. Aug 16, 2015 #9


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    This is what I'm saying, yes. Oort cloud is essentially unaffected by density waves or physical interference from other bodies/dust/gas, leaving only the sun's gravity.

    (Not to mention the fact that the Oort cloud is probably as much loosely-captured bodies as it is bodies that were originally part of the protosun's rotating system.)

    Again, I don't know this, just always seemed logical to me.
  11. Aug 16, 2015 #10
    I was definitely thinking of the scenario that includes the interbody interactions, just GR. I can visualize the cascade of "pulls" propagating from some unstable initial distribution (the spherical cloud). I can imagine this forming a disk, or a star, a blob, something simple. I can even imagine it condensing somehow into a crystal - all the bits sort of flying at once to some positions of symmetry, or randomness.

    What seems curious to me is what differentiates those outcomes, and what the full chain of the mechanism is? It seems a tad undersold as good old plasma collapse since complex sentient life has been known to emerge from it. That said I really do like the viscosity analogy. So, SM Viscosity? SUSY Viscosity? Lie Group Viscosity? Is that sort of what those do?

    Also, when you see images of star formation there seem to always be these plasma "winds". Why don't we see solar systems that are more "wind-blown" as it were? I mean how probable is a nice spherical cloud as a starting point?
    Last edited: Aug 16, 2015
  12. Aug 16, 2015 #11

    Ken G

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    Actually, it was my understanding that the Oort cloud came from strong perturbations due to close encounters within the Kuiper belt, or other initially flattened disklike distributions, especially involving interactions with Neptune or perhaps large asteroids. So if you imagine that you start out with a disk that is not perfectly flat, and let the objects in the disk have close gravitational encounters, they can pass above or below each other. But a close interaction like that can throw them out of the disk, onto highly inclined orbits. Many of these can extend out a long distance, where there can be interactions with other stars that don't conserve the angular momentum of the solar system. So I think it is those weak external interactions that cause the Oort cloud to be more spherical, moreso than something about the initial formation of the solar system.
  13. Aug 16, 2015 #12
    I do believe I mentioned density waves isnt involved on the Oort cloud

    my response was to this particular question

  14. Aug 16, 2015 #13
    Interesting. I was thinking that there must be a pretty good understanding of the angular momentum environment that a typical stellar disk evolves in? But I have no idea if that is so. I had pictured it as initially quiescent, and spherically symmetric. But maybe that is really wrong.

    I've seen a lot of astronomy magazine picture of stellar nurseries. Doesn't seem like they start off quietly spherical, not at the point of ignition anyway. Is it true that most solar systems are thought or observed to be disk-like? spherical, even symmetric, at all, maybe past a certain point in their lifetimes? I honestly haven't a clue.

    Is the "disturbed and ejected former members of the disk" model you seem to be describing a possible fit for galactic globular clusters also? And is the idea that the solar system starts off more like a highly structured disk due to the highly ordered angular momentum ignition situation in "windy" stellar nurseries, where the direction/angle of the wind is defining the plane of rotation?
    Last edited: Aug 16, 2015
  15. Aug 16, 2015 #14

    Ken G

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    The basic idea, as I recall, is that when the solar system initially forms, the combination of energy loss (acting like viscosity) with having a lot of angular momentum is what forms a disk. This is because if you have a large amount of angular momentum, a disklike configuration has lower energy than a spherical distribution. So you get a disk as the solar system forms. But then you get strong interactions within that disk, because you have planets and large asteroids. That tends to disrupt the disk, and create a second component that is more spherical. Interactions with distant stars can also make it more spherical over time, even though it came from the disk originally. The details must be a bit complicated.
    The whole nebula is not spherical, but the stars themselves are rather spherical. In between there are more and less spherical kinds of structures. For example, a proplyd is usually not too far from spherical, but inside there could be a disklike component, the part that has lost a lot of heat and contracted enormously. That's how you get disks.
    Yes, planetary systems are generally quite disklike, especially around single stars. We often see these forming as what are known as T Tauri stars. The binary situation is much more complicated.
    That's a good question, I think the situation should be pretty similar indeed. There isn't the analog of a giant planet to throw out comets, but perhaps globular clusters can still get a pretty good orbital disruption without being broken apart.

    The disk comes from the presence of angular momentum, even a relatively small amount can make its presence felt. The exact source of that angular momentum I couldn't say, perhaps some kind of turbulence in a "windy" situation, but perhaps other things could do it without any prevailing winds.
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