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Newtonian mechanics & Oort Cloud Shell

  1. May 16, 2010 #1
    Last week there was a documentary program on Nat Geo I believe having to do with the composition and origins of comets. The narrative seemed to imply that comets originate within & are rather randomly dislodged from a spherical shell around the Solar System called the Oort cloud. This dislodging of the comet supposedly takes place when one of the outer planets such as Pluto or Neptune passes near the shell, haphazardly pulling a random comet out of the Oort cloud causing it to fall towards the sun (!). This explanation also seems to entail that the comets comprising the shell were formed out of the original dust and debris that accumulated into the aggregation and eventual formation of the solar system billions of years ago.

    Do I have this right? Does the above sum up the current thinking as to the origin of Comets?

    Somehow, none of this seems right to me, just intuitively speaking.

    For one thing, isn't it a fact that many comets have orbital trajectory planes highly inclined from the Ecliptic plane of the solar system? How would the outer planets have been able to influence comets say 60 degrees or more inclined from the celestial equator?

    As to how the individual comets in the Oort cloud shell stay suspended there unless influenced by an outer planet controverts my understanding of Newtonian Gravitation. The outer boundaries of the shell are nowhere near the where the influence of the nearest stellar neighbor Proxima Centauri would start pulling things in that direction, are they?

    How does cometary material just stay "suspended" there in the Oort Cloud and not either fall towards the sun or towards the nearest gravitational influences such as nearby stars or perhaps even the galactic center?

    Jim Ostrowski
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  3. May 16, 2010 #2


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    The oort cloud actually extends quite far out (about 1 light year in either direction IIRC). The cloud doesn't all contract inwards probably (and I'm not 100% sure about this) because it's in some sort of hydro-static equilibrium with the solar system. Gravity in is being balanced by gravity out. The nuances with gravity is hard to do exactly, but it's not hard to imagine that things are very delicately balanced. A slight nudge in one direction or another may cause the comet to fall out of its place in the oort cloud and tumble into our solar system.
  4. May 16, 2010 #3
    That sounds highly improbable. Generally "gravity in" is balanced by angular momentum; "gravity out" is balanced by more gravity out (Gauss). (In nature, unstable equilibria do not tend to last at all.)

    Without knowing anything much about the topic of the thread, it does seem that large planets are quite capable of slingshotting any rock or spacecraft that happens by, into a nearly arbitrary trajectory.
  5. May 16, 2010 #4

    D H

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    Comets stay "suspended" the same way that planets do: They are in orbit about the Sun.
  6. May 16, 2010 #5
    Well, that's my point exactly D. H. Very elliptical orbits I might add.

    So then why is it necessary to believe in the idea that they all exist in a spherical shell called the Oort Cloud when the observed situation is that all known comets are in highly elliptical orbits around the sun? Granted they spend long periods of time out in the apogee areas, but isn't that a characteristic of objects having highly elliptical orbits around their primary?

    IOW why bother postulating that comets have to be pulled into the inner reaches of the solar system only by means of the influence of the outer planets?


    PS thanks for the quick responses I wasn't expecting anything on the weeke nd. I figured all you college professors would not be at your computers at all on a weekend!
  7. May 16, 2010 #6
    Firstly, that's pretty much the kind of influence the planets would be calculated to have regardless, not just an arbitrary postulate.
    Secondly, one expects solar system formation to begin with a fairly uniform disc of gas and dust (or rather, one would calculate that collapsing gas/dust interacts randomly to form a fairly uniform distribution), which explains nearly circular orbits as the natural state.
    Also, you seem to be concerned with the influence of distant stars. It's probably better to note that asteroids fall towards them only at about the same rate that the sun and everything else here does.

    Americocentric much?
  8. May 16, 2010 #7

    D H

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    That's only the ones we can see.

    Once again, those comets with elliptical orbits are the only ones we see. That doesn't mean that all comets have elliptical orbits. It only means that all of the comets we see have elliptical orbits. We can't see the ones in circular orbits. Just because we can't see them doesn't mean they don't exist. It might just mean that we can't see them.
  9. May 16, 2010 #8
    Cesiumfrog wrote:

    What argument is there that circular orbits for comets are their "natural" state if there are so few (or maybe none) of them which can be observed and checked for orbital characteristics out there in the spherical cloud to begin with?
  10. May 16, 2010 #9
    NASA gives us some history about the Oort Cloud that I thought would be helpful for our viewers. I love NASA as some of our experts here already know!:smile: And, it gives me an opportunity to share NASA with our viewers with hope they will explore it's wonderful website and enjoy it as much as we do Physicsforums! :smile:

    Discovery is an uplifting experience for me. The more I learn about a topic, it seems to broaden my understanding about the universe I live in.
    Last edited: May 16, 2010
  11. May 16, 2010 #10
    Wikipedia notes "their volatile composition means that as they repeatedly approach the Sun, radiation gradually boils the volatiles off until the comet splits or develops an insulating crust that prevents further outgassing. Thus, reasoned Oort, a comet could not have formed on its current orbit, and must have been held in an outer reservoir for almost all of its existence." Have you read that page?

    If the Oort cloud hasn't been directly verified, I wonder if it would be possible to detonate a flare out where we expect it to be..?
    Last edited: May 16, 2010
  12. May 16, 2010 #11
    Indeed yes. On that same page it also says:

    What if the real origin of comets was not the hypothesized cloud, because if that were true it would be difficult to account for their volatile compositions and the above mentioned undercount, but rather more likely they were originally launched like rockets from the sun......

    anybody for Protocomets = Coronal Mass Ejections?

    That would have to be one heck of a flare, wouldn't it?

    I recall reading something about back in the days of above ground Nuclear testing there was supposedly a plan in the works to set off a nuclear weapon in outer space, possibly on the moon to help scientists of the day determine the spectral composition of the debris. ...

    I think the Oort cloud should remain just a theory until we figure out something better...

  13. May 16, 2010 #12
    Ho hum. Cesiumfrog and Jim Ostrowski, NASA is backed up by this latest article as well.:smile:

    I should mention that in the realm of SCIENCE, a theory is based on observations.
  14. May 16, 2010 #13
    Are we boring you?

    It sounds like you are alleging that this article contradicts something I or CF said...?

    The assertion that all known comets orbit the sun is indeed based on observation and Newtonian physics. The idea that they exist for long periods of time in areas far beyond the orbit of pluto is not being questioned. What I question is the assertion that the orbits of a majority of comets are "naturally" circular when ALL of the the ones that we can see are elliptical. If you have something relevant which addresses this issue let's hear it.

    Now then, since a "cloud" of circularly orbiting comets has never been observed, the idea that the Oort cloud consists of circularly orbiting comets is a theory that is not backed up by any observations.

    As to your cut and pasted article, the planned observation of stellar occultations by objects in the theorized Oort cloud will prove exactly what? Are they going to be able to prove by these occultations (presuming any occur) that the orbits of the objects are circular? I sorta doubt that Views of Mars. Not enough of an arc present in an occultation to do any tracking.

  15. May 17, 2010 #14
    Jim Ostrowski wrote:

    Oort noted that the number of returning comets was far less than his model predicted, and this issue, known as "cometary fading", has yet to be resolved. No known dynamical process can explain this undercount of observed comets.

    Well ???? Cat got everybody's tongue?

    Y'all know I'm right, don't ya?

    That's ok, everyone have fun with your anonymous little forum, when you kids grow up and are ready to use your real names like the big people, drop me a line.

    Jim Ostrowski
  16. May 18, 2010 #15
    Where did anyone say that the Oort cloud should consist of bodies in circular orbits? The only statements you've provided suggest that it ought to be a roughly spherical shell. But, these are very different statements.

    Consider. The usual estimates suggest that the inner edge of the Oort cloud is somewhere between 2000 and 5000 AU from the sun, while the outer edge is something like 50000 AU out (with some estimates suggesting it's much farther). It seems pretty reasonable to say that any orbit remaining within those bounds is in the cloud. But, even using the most conservative numbers (5000 AU to 50000 AU), orbits remaining entirely within the Oort cloud can have eccentricities up to around .8. (And, of course, if we take less conservative estimates of the bounds of the Oort cloud, the maximum allow eccentricity goes up.)

    What makes the Oort cloud a spherical (or nearly so) shell isn't that all orbits in it are circular but that there isn't a preferred direction for the orientation of the orbits. That is to say, the perihelia of the orbits can be in any direction from the sun.

    One final point. If we consider the full class of small objects in closed orbits with semi-major axes in the thousands to tens to thousands of AU, the only ones we will be able to observe easily are those with eccentricity very close to 1, simply because the rest will never get close enough for us to see without great difficulty.
  17. May 18, 2010 #16
    Reply #7 to this thread by "D H":
    See also reply #6 by "cesiumfrog":

    Parlyne writes:
    Did you read any of the previous few posts in order to understand what we are talking about before you decided to chime in Parlyne?

    Let's go over this again.... Cesiumfrog believes:

    "their (a comet's) volatile composition means that as they repeatedly approach the Sun, radiation gradually boils the volatiles off until the comet splits or develops an insulating crust that prevents further outgassing. Thus, reasoned Oort, a comet could not have formed on its current orbit, and must have been held in an outer reservoir for almost all of its existence."

    And I pointed out:

    After which I suggested an alternate explanation of the formation of protocomets:

    Parlyne writes:
    You need to get up to date with the news, Parlyne. It's actually really old news that there are so many sungrazing comets coming in at all kinds of angles off the ecliptic plane that NASA had to enlist the aid of the public to monitor SOHO in order to keep track of them. Google SOHO sun comets sungrazer etc..

    http://sungrazer.nrl.navy.mil/ [Broken]

    "SOHO is the most successful comet discoverer in history, having found over one thousand six-hundred comets in over thirteen years of operation! What's even more impressive is that the majority of these comets have been found by amateur astronomers and enthusiasts from all over the world, scouring the images for a likely comet candidate from the comfort of their own home. Absolutely anyone can join this project -- all you need is an internet connection and plenty of free time! (See the FAQs for more information.) "

    Last edited by a moderator: May 4, 2017
  18. May 18, 2010 #17
    Yes. I did. In particular, I saw that you were responding as if the other comments in this thread were claim that orbits in the Oort cloud are exactly circular rather than the intended meaning from the earlier post that orbits in the Oort cloud are much closer to circular than those of the comets we see in the inner solar system. After all, to pass inside Earth's orbit, a comet originating from the innermost region of the Oort cloud must have an orbital eccentricity of .999 or larger.

    And, exactly what about this contradicts anything I said? The comets we observe in the inner solar system have orbital eccentricity very close to 1. This is true of the sungrazers as well as any others we see here. My point was that those are much easier to observe than objects which orbit entirely in the Oort cloud; but, that these have a comparatively small range of eccentricities, suggesting that there ought to be far more cometary bodies in the Oort cloud than we see passing through the inner solar system. I never in any way implied that there are particularly few comets passing through the inner solar system.

    Now, if what you're meaning to imply is that there's an overpopulation of sun-grazing comets compared to other inner solar system comets, you should probably remember that the closer a comet comes to the sun in the first place, the more likely it is to fragment due to uneven heating and be seen as multiple comets on subsequent passes.

    As for your coronal mass ejection idea, in addition to problems with how the material ejected would manage to coalesce into objects as large as comets, this idea wouldn't be able to explain the composition of comets. Most of the matter in CMEs is in the form of free protons and electrons; but, comets are known (both from spectra features and direct examination) to be compose largely of water ice, rock, and dust with significant quantities of carbon compounds.
    Last edited by a moderator: Apr 25, 2017
  19. Jun 27, 2010 #18
    A new article dated June 11, 2010 from NASA discusses the Oort Cloud.:biggrin:
    http://astrobiology.nasa.gov/articles/most-comets-not-from-this-solar-system/ [Broken]
    Last edited by a moderator: May 4, 2017
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