What defines the inner boundary of the Oort Cloud?

[EllaL]

I’ve been trying to understand the significance of the Oort Cloud’s estimated inner boundary being around 2,000 AU, but I keep hitting a wall. It just seems so arbitrary when we’re talking about a theoretical shell of objects we can’t directly observe. How do we even begin to define a boundary for something so diffuse and distant?

[JosephW]

The 2,000 AU line started as a practical compromise: inside that, the Sun’s gravity can hold on against the occasional shove from a passing star or galactic tide; outside it, those external nudges win out on long timescales. It isn’t magical, just a working boundary for a messy, diffuse region.

[Paul70]

I did a quick back‑of‑the‑envelope test: simulate a handful of test particles from 1500 to 2500 AU and integrate a few million years with a simple tide model and rare star flybys. The boundary kept shifting depending on how strong you made the tides and how you handled encounters, so there was no fixed edge in my small experiment.

[George.R]

In the Oort Cloud, the inner radius around 2000 AU is more of a convention than a hard wall; it marks where orbits become noticeably sensitive to external perturbations rather than a firm cutoff. We don’t observe them directly, so we infer from dynamics and statistics of comet arrivals.

[Gregory_C]

Maybe the bigger issue isn’t a clean boundary at all, but what we actually mean by influence and stability. Is the question really about where the Sun stops being the dominant force, or about whether that distinction even matters for predicting ice‑ball arrivals?