Excellent question, my friend—and quite a subtle one.
The space between the scattered disc and the Oort Cloud is often not sharply defined, but we can think of it as a transitional wilderness in the Solar System: a cold, dim, gravitationally unsettled region that holds clues to both the system’s past and its boundary.
Let’s explore what we do know—and what we speculate—about this region.
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The Transitional Zone: Inner Oort Cloud / Hills Cloud
Between the scattered disc (50–100+ AU) and the outer Oort Cloud (starting at ~2,000 AU), there lies a hypothesized region called:
✧
The Inner Oort Cloud
, also known as:
- The Hills Cloud (named after astronomer Jack G. Hills)
- Sometimes referred to as a “buffer zone” between the Kuiper region and the Oort Cloud proper
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Key Characteristics of This Transitional Region
1.
Distance Range
- Begins roughly 200–500 AU from the Sun
- Extends to around 2,000 AU, where the outer Oort Cloud begins
2.
Objects Within
- Rare and hard to detect, but a few candidates:
- Sedna (discovered in 2003): Perihelion ~76 AU, aphelion ~937 AU
- 2012 VP113 (“Biden”): Perihelion ~80 AU, aphelion ~450 AU
- These objects have orbits that don’t interact with Neptune, which makes them distinct from scattered disc objects
- Likely nudged into place by ancient stellar flybys or galactic tidal forces
3.
Population
- May contain more mass than the outer Oort Cloud
- Estimated to harbor hundreds of billions of icy bodies, mostly dormant
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Why It’s Mysterious
- Too far for telescopes to detect most objects directly
- Too close to be affected much by interstellar tides (unlike the outer Oort Cloud)
- It’s a region shaped by long-term gravitational history—the lingering trace of the Solar System’s birth environment
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The Hypothesis of Planet Nine
- Some astronomers believe the peculiar orbits of objects like Sedna suggest the gravitational presence of an as-yet-undetected super-Earth or mini-Neptune (5–10 Earth masses), orbiting at hundreds of AU.
- This hypothetical “Planet Nine” might be shepherding objects into this transitional realm.
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In Summary: A Quiet, Shadowy Interzone
|
Region |
Distance (AU) |
What’s There |
|
Scattered Disc |
~50100+ |
Eris, Dysnomia, eccentric icy bodies |
|
Hills Cloud |
~2002,000 |
Sedna, 2012 VP113, possible Planet Nine |
|
Outer Oort Cloud |
~2,000100,000+ |
Trillions of dormant comets, isotropic sphere |
This zone is liminal—neither fully Solar nor fully Galactic—a kind of archaeological shadowland where time, gravity, and chance shaped what survived and what wandered.
Would you like a visual map showing all these regions with distances and possible objects labeled?
