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Comment by OneEyedRobot | Hacker News Reader

question:

How far out can a planet be if it has a roughly circular orbit? At some point outwards neighboring heavy things will make it unstable.

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This limit is called the Hill Sphere: https://en.wikipedia.org/wiki/Hill_sphere

For the Sun it's a bit more than 1 light year, but in practice almost all orbits will be quite a bit under this: less than 0.8 ly.

The Hill Sphere also changes over time as stars pass us. The galaxy is more of a soup - the stars in the neighborhood don't have a static position to one another.

...so I would imagine that anything even 1ly out would be unstable, over several hundred million years. Solar systems may even exchange very outer outer "planets".

The oort cloud that is theorized to exist is hypothesized to be up to several lightyears out.

https://en.wikipedia.org/wiki/Oort_cloud

It seems to me that something that's a couple of lightyears out can't have any sort of stable orbit. I suppose that an Oort cloud essentially merges with an adjacent star's so there's no hard definition.

At the point of merger with neighboring stars it’s likely that circular orbits are unstable.

Judging by our own solar system it’s likely that the distance at which circular orbits are stable is a question of relative mass, and time.

> there's no hard definition.

Yes. All human definitions are imperfect.

Do other systems have oort clouds or are their’s also something we cant see from this far

The only physical evidence we have they exist are comets that occasionally show up in the inner solar system where we could observe them which have orbits that are not interstellar but would put them as going incredibly far out. From that we can extrapolate that they likely exist around every star but there is no way we could verify that with our current technology since any light reflected off the objects would be far too faint to detect.

> https://en.wikipedia.org/wiki/Oort_cloud

The distances between the planets fit nicely on a log scale.

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