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?
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.
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.