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If we take a look at a logarithmic scale of the Solar System, there appears to be a large gap between the heliopause and the Oort cloud: enter image description here

Why is that? There are some objects in the gap, but why are most of the Oort cloud objects in one area? Shouldn't it also fill the large gap in the picture just as much?

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  • $\begingroup$ Great question! I can understand that the outer edge of the oort Cloud is defined by neighboring stars. But the inner edge should be defined by Neptune and be inside of the Heliopause. Maybe the pure scale of the volumes is an explanation? Non-periodic comets with aphelion of 1,000 to 100,000 AU are so much more common than those at 100-1,000 AU, that the latter never have been observed and thus it would be considered unscientific to assume that they exist? Anyway, I think the models of this part of the Solar System are about to change. $\endgroup$
    – LocalFluff
    Feb 2, 2016 at 5:07
  • $\begingroup$ @LocalFluff Er, sorry...I really don't understand what you mean. The heliopause lies about 120 AU away from the Sun, whereas the inner edge of the Oort cloud lies 2,000-5,000 AU away. $\endgroup$
    – Sir Cumference
    Feb 2, 2016 at 5:16
  • $\begingroup$ Anything beyond Neptune, say 50 AU, is what should matter. 120=100=50 in astronomic hand waving. I don't immediately find a distribution of non-periodic comet's aphelion, but the average seems to be tens of thousands of AU. The volume from 1,000 to 100,000 AU is a million times larger than the volume within 1,000 AU. So the comets you are looking for are at least extremely rare, which I guess could explain why they are modelled to not exist at all. $\endgroup$
    – LocalFluff
    Feb 2, 2016 at 5:33
  • $\begingroup$ @LocalFluff So why is the Oort cloud so packed then, if objects in between it and the heliopause are so rare? $\endgroup$
    – Sir Cumference
    Feb 2, 2016 at 5:35
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    $\begingroup$ Maybe because it is a cartoon illustration on a log scale that only excludes one millionth of the comets? For sure, the Oort Cloud gets thinner and thinner further out. A proper distribution of hyperbolic comets' semi-major axes would be needed, but most seem to be reported to have eccentricity of 1, which means infinity. Just wait until someone knowledgable posts a real answer. $\endgroup$
    – LocalFluff
    Feb 2, 2016 at 6:04

2 Answers 2

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There is no "gap" other than an apparent one caused by the use of logarithmic axes.

The density of Oort cloud objects is thought to decrease with increasing distance from the Sun. However, the volume of a shell in the base 10 logarithm of radius increases by roughly 1000 for every order of magnitude increase in radius.

That means although there is a higher density of objects at smaller radii, they are fewer in absolute number. i.e. There are roughly 1000 times more objects between 10000 au and 100000 au than between 100 au and 1000 au, but with about one thousandth the volume density.

However, there is a dearth of asteroidal bodies between 100 au and 10000 au with high inclinations. i.e. The objects that are there are still confined to a broad disc. That is because most of the Oort cloud objects are ejected from a disc in the inner Solar System and then only the outer objects have their inclinations randomised by the tidal field of the Galaxy. See What's the reason for the hollow region in Oort cloud? .

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The Kuiper belt and the Scattered disk are widely believed to lie in the space between the outer planets and the Oort cloud, but not to reach all the way out to the Oort cloud (apparently due to resonances with Neptune and a scarcity of sighted object much outside the 1:2 resonance orbit). The various dwarf planets of the outer solar system are sometimes referred to as Kuiper belt objects.

As I understand it the Kuiper belt is expected to be distinct from the Oort cloud in being at least somewhat planar and aligned with the ecliptic whereas the Oort cloud is essentially spherical. If Pluto can be taken as a guide then we expect considerably larger inclinations from Kuiper belt objects than we see from large bodies in the inner solar system, but still a nod toward the ecliptic.

I've less familiarity with the scattered disc and the Wikipedia articles indicate that the usage is not very consistent. It seems to mean highly eccentric objects beyond the orbit of Neptune.

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  • $\begingroup$ Still confused on why there's such a big space in between the OC and heliopause. Also, the inner Oort cloud (aka Hills cloud) is planar, and only the outer is spherical. +1 though for detailed response. $\endgroup$
    – Sir Cumference
    Feb 2, 2016 at 5:17
  • $\begingroup$ Well, I only know this stuff up to the standard required to do a one week unit on the solar system for a GenEd class I teach occasionally. I've learned a couple of things from the Wikipedia articles referenced herein. This mostly doesn't address your question because the Kuiper belt seems to be assigned an outer boundary roughly coincident with the Heliopause (something I hadn't realized). The scattered disk objects seem to go a bit further, but it does still leave quite a large gap. I can't think of a reason for expecting a Kuiper belt cut-off that doesn't rule out the Oort cloud. I'm lost. $\endgroup$
    – dmckee --- ex-moderator kitten
    Feb 2, 2016 at 5:22
  • $\begingroup$ @SirCumference The query about "gaps" is now answered here: astronomy.stackexchange.com/questions/55039/… $\endgroup$
    – Nilay Ghosh
    Oct 18, 2023 at 18:01

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