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In the episode of Cosmos dedicated to cometary objects and other things, the Oort cloud was graphically rendered as a spherical distribution of dots.

This surprised me as I thought that it should be somehow a disk or a torus, given the solar system's rotational momentum.

Why is the Oort ensemble thought to be spherical? Is it deduced from the orbits of the various comets? I think so.

Why it does not spread into a more disk like object?

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    $\begingroup$ Are you sure you are not confused with the Oort Cloud in your memory from the Cosmos episode? I think the Oort Cloud is believed to be spherical. $\endgroup$ – gerrit Jan 21 '18 at 12:37
  • $\begingroup$ Thank you @gerrit ! Indeed! But now I have to change Q :) $\endgroup$ – Alchimista Jan 21 '18 at 12:44
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    $\begingroup$ Maybe I'll turn this into an answer (or someone else can if they like), but the relatively flat disk forms by collision. If there's insufficient material to generate enough collisions, the shape remains generally spherical or debris cloud shaped. It may be possible that there's some influence on the shape due to magnetism or possibly by large distant orbiting objects doing some shepherding, but the spherical shape is the simplest and likely best approximation. We've never actually seen or measured the Oort cloud so estimates of its shape should be taken with a grain of salt. $\endgroup$ – userLTK Jan 22 '18 at 0:02
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    $\begingroup$ This video may help you : Why is the Earth Round and the Milky Way Flat? $\endgroup$ – Jaideep Khare Jan 22 '18 at 9:21
  • $\begingroup$ @userLTD yes please do it. Convey why a solid ball tends indeed to flatten to a disk and at least become oblate but not linked objects keep their paths. It will be more general and not only Oort related. Thanks $\endgroup$ – Alchimista Feb 7 '18 at 15:26
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I didn't really want to answer this one because there may be some new information that I'm not aware of and, to my understanding, the shape, mass and content of the Oort cloud is a subject of some ongoing study, so I invite correction.

The formation of a solar-system is pretty complicated and there may still be significant unknowns on the process and with the Oort cloud, uncertainty on the precise shape, mass, density distribution and origin, whether it formed with the solar-system or whether much of it is captured. I wanted to begin with the unknowns.

We can't see the Oort cloud so we can't measure it directly. Estimates can be made by observing very long period comets that fly into the inner solar system and by extrapolating their orbital periods, an estimate of the contents of the Oort cloud can be made. One problem is, that estimate is based on very eccentric orbits only, because those are the only ones we see that enter our telescope's range.

This is one of my favorite minute physics videos for it's simplicity and the fun diagrams they use. The gist of the simple answer is that a cloud or nebulous mass of debris and dust, has a fixed angular momentum and a fixed orbital plane, so as the cloud of debris gets a push (usually from a not too far away supernova), and the debris begins to condense into a proto-solar-system, it maintains the orbital plane and angular momentum and as it condenses, it rotates very rapidly.

The rapid rotation isn't all that relevant to your question, but that's why not all the matter can fall into the star, because there's usually too much angular momentum. The same happens with gas giant planets, which is why Jupiter, for example, has 4 large formation moons (where as the Earth - somewhat smaller region where it formed, no formation moons, but it has an impact moon). I'm making this more wordy than it should be, but the point is, stuff orbits the sun because of angular momentum. The sum of this angular momentum has an orbital plane, and each individual object has an inclination to that orbital plane. When planets absorb the matter in their orbital regions, the up and down directions, or inclinations of the objects in orbit tends to cancel out, but the net angular momentum and orbital plane remain constant. (This is mostly, but not entirely true - for example, Planet 9, if it's eventually discovered, might explain why the 8 inner planets, on average don't line up with the sun's rotational plane. Planet 9 may have taken some of the inclination in one direction with it as it was thrown far outside the solar-system leaving the inner 8 planets with an inclination in the other direction relative to the sun. When (if) planet 9 is discovered, then we can check if it balances out our solar-systems tilted inclination.

But it's the collisions that help the planets line up along the orbital plane of the solar system, because the ups and downs mostly cancel out. If there are no collisions, then there's no cancelling out of the ups and downs and the objects in orbit remain in a kind of nebulous blob assortment - which over time, is probably best represented as a sphere.

That's not the entire answer, however. Take the asteroid belt for example. There was (I think) probably not sufficient collisions to make the asteroid belt flat, and Ceres is probably not a failed planet, because it probably came later. Based on Ceres' density it probably came from further out in the solar-system. It may have started out as a moon (perhaps kicked out of Neptune's orbit by the miss-behaving Triton) or a dwarf-planet originally in the kuiperbelt. It's density is too low to have formed by collisions in the asteroid belt.

So the Asteroid belt is flat (er mostly) likely due to gravitational shepherding from Jupiter and perhaps, by a very strong magnetic field from our young sun,

A proper answer of our estimated shape of the oort cloud would be based on a survey of all the long-period (or highly eccentric) comets that we've observed and I'm not that interested in doing the research on that, but I would guess that there's enough variety of orbital inclination to support the generally circular shape - because I don't think the circular shape would be used so often if it didn't reflect the observation of long period comets.

The Kuiper belt, for example, (and I couldn't find a really specific answer to this), but it appears to be somewhat flat, kind of donut or torus shaped (Pluto has a higher than usual inclination). The proper term, if you want to get technical, is inclination distribution. No distribution = flat. Full or high distribution = sphere.

The Kuiper belt's relative/somewhat flatness may be driven by mostly Neptune sheparding rather than collisions (again I'm not precisely sure). In fact it was the regularity of some inclinations of the most eccentric kuiper belt objects (just passing through in other words) that lead to the planet-9 theory in the first place. If there's large planets around, they assist in the shepherding and flattening of smaller objects in their orbital viscinity.

In the case of the Galaxy, to my knowledge, there's insufficient collisions to explain the flatness of the Milky-way (it's basically pizza shaped, with maybe a small ping pong ball or large marble in the middle). It's my understanding that the Galaxy was flattened by a very strong magnetic field more than by collision (somebody correct me if I'm wrong).

That's the extent of my knowledge at least. I invite someone smarter than me to answer this as well.

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  • $\begingroup$ Accepted because in spite of the premise made by its author, this A explains why a spherical ensemble of orbiting bodies does not necessarily tend to a thorus and finally a disk as for this requires interactions. This latter indeed is the case for a rotating ball of clay or for the asteroids belt. It also explain that indeed our knowledge of Oort belt is extrapolated from cometary obsvs. $\endgroup$ – Alchimista Feb 9 '18 at 15:19

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