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Comment by Medox | Hacker News Reader

To the Sun, a moon is like a submoon to the planet, so what would be the difference? With enough distance in between it should be possible. Not probable but possible.

Submoons will probably clump together much easier in the formation phase of any system and a moon will never capture an asteroid before it's planet does.

As the author pointed out, the question remains how stable it is. The almost nonexistent cases (at least in our Solar System) shows how rare it is. Maybe only distance is a factor. Huge Solar Systems with twice the distance between objects will also have more submoons.

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There is a limit to how big planets can get before they become stars, and a limit to how small moons can get before electromagnetism becomes dominant over gravity in determining their motion. So there exists a hard (not just probabilistic) maximum number of levels of submoons. Given other constraints like orbital stability and the Roche radius, it would not surprise me if this number was a low single-digit integer.

I'm curious, how does electromagnetism determine the motion of a planet?

All I mean is that if a "moon" were very small, say a few mm or cm, then it can accumulate enough of a charge imbalance for forces from ambient magnetic fields in a stellar system to push it out of it's nominal gravitational orbit. That size limit might seem trivial, but the ratio of an atom to a paper clip is about the same as a paper clip to the Earth's moon, so it cuts off half of the geometric range.

So it's safe to say that, say, a styrofoam pellet wouldn't make a good moon (not even considering lack of durability against cosmic bombardment).

Yep.

Well the moon has had artificial satellites in the past. I imagine it might not be terribly common, but not impossible either.

>The almost nonexistent cases (at least in our Solar System) shows how rare it is.

It might not exist in our solar system, our solar system isn't a model from which all others operate from. Submoons may be rare, but not because they don't exist in our solar system.

> Submoons may be rare, but not because they don't exist in our solar system

Sure, "submoons are rare because they're rare in our solar system" isn't a proof. (It's a hypothesis.) But the observation is evidence. In the absence of other evidence, it defines the status quo.

> Well the moon has had artificial satellites in the past. I imagine it might not be terribly common, but not impossible either.

There are still artificial satellites around the Moon but the problem as stated in the paper is "long-lived" submoons, on stable orbits, which the Moon can't have.

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