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?

  • $\begingroup$ 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 '16 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. $\endgroup$ Feb 2 '16 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 '16 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? $\endgroup$ Feb 2 '16 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 '16 at 6:04

There may be Sednoids there.

Sednoids are a hypothetical class of "inner Oort Cloud objects" named after their prototype, Sedna. Sedna's aphelion is ~936 AU, bringing it close to the inner boundary of the Oort Cloud. Sednoids may have aphelions ranging from about 100 AU to 1,000 AU.

The problem is, only two Sednoids have beet detected to date, 90377 Sedna and 2012 VP113. Brown et al. 2004) suggested that ~500 may be detectable; surveys simply haven't tracked objects in that area.

Why are Sendnoids where they are? Three ideas have but put forth:

  • A planet at ~70 AU scattered these objects into elliptical orbits.
  • A close pass by a nearby star.
  • Interactions with other stars in the Sun's original cluster.

These objects would fill in the space between the Kuiper Belt/scattered disc and the Oort Cloud itself.

  • $\begingroup$ Thanks for the answer, but it doesn't really explain why there is so much "empty space" between the Kuiper belt/scattered disc and Oort cloud. Why is the Oort cloud packed into one region? Shouldn't it fill that gap much more? All we have now are a few potential Sednoids in the thousand AU gap. $\endgroup$ Feb 2 '16 at 16:30
  • $\begingroup$ I can also understand why the Oort cloud was sent so far away in the early Solar System. But still, shouldn't the objects still fill the gap just as much, instead of almost entirely being located at the edge of the Solar System? $\endgroup$ Feb 2 '16 at 16:32
  • $\begingroup$ @SirCumference Some theories state that Oort Cloud objects originated outside the Solar System (e.g. captured comets). This means that there's no reason for them to be as close the Sun as other objects that formed closer in. And regarding the density issue: There may be many, many more Sednoids, but they're simply not easy to detect, like all bodies that far away. $\endgroup$
    – HDE 226868
    Feb 2 '16 at 16:35
  • $\begingroup$ I'm a bit iffy on that theory. How could the Solar System could capture so many comets, especially since the Oort cloud is thousands of AU across? There's also the problem with the inner Oort cloud being planar. Wouldn't this imply it formed with the rest of the Solar System? $\endgroup$ Feb 2 '16 at 16:38
  • $\begingroup$ @SirCumference Yes, the inner Oort Cloud might have. It could be argue dthat Sednoids are simply a transition between Oort Cloud objects an KBOs. As for gravitational capture - well, the original cluster of stars would have been dense enough that capture might have been relatively easy. Read about interstellar comets. $\endgroup$
    – HDE 226868
    Feb 2 '16 at 16:39

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.

  • $\begingroup$ 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. $\endgroup$ Feb 2 '16 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. $\endgroup$ Feb 2 '16 at 5:22

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