### Do moons have moons?

• Have we discovered any natural satellites of natural satellites of planets or dwarf planets? Even very small, or relatively short-lived - e.g. ringlets around Saturn's moons, some meteorites orbiting Jupiter moons, or something to orbit Charon? Or is the Star-Planets-Moons the deepest naturally ocurring orbital recursion level?

I don't have any hard evidence but I think a moon is defined partly by orbiting a planet and might otherwise just be a natural satellite. That's assuming the object isn't pulled into the planets orbit by it's much stronger pull.

I tend to use titles that are more descriptive than factually accurate; I tend to explain more and stick to proper nomenclature in the actual question body. As for "assuming", well, that's what this question is asking about!

Hmmm at what level do you want us to stop? Because I don't think there's any proper size limit or a limit of how many iterations of smaller objects orbiting slightly bigger one there can be. Case in point, Rhea might have its own ring system, which, if true, would mean it has tiny _moonlets_ then. I also remember that antenna cover floating around the ISS, although that's already a dual artificial satellite system LOL. There are also some wacky orbits possible, like the horseshoe orbits that can trap asteroids between two bodies. ;)

@TildalWave: If there is *any* level for *natural* satellites, I'd like to know it. (artificial satellites like that antenna cover don't count). If Rhea has a ring, that would be what I seek. Any *periodic* orbit would work, but please no cheats like two overlapping minimally elliptical orbits that make the bodies move in circular path relative to each other despite not really interacting gravitationally with each other, just following independent path around their planet.)

Well frankly I don't know where to start answering it. It's a bit like asking how many cogs can be in a clockwork and still make it show true time. Planetary systems can be as complex in theory as we are capable of imagining, and as complex in reality as we're able to observe.

@TildalWave: It's more like "does Swatch model 5/1960 use any planetary gears?" - I'm not asking theoretically, I'm asking about actual discoveries.

A more interesting question in this context is why - with the anomolous exception of the Earth - do none of the inner planets of the solar system have any moons? After all, the Earth is only a moon of the Sun, on one measure, so the same question could be posed about our Moon, which is by that reasoning a moon of a moon. Mars has captured two asteroids, so they don't count: they weren't formed by the same processes which created Mars. The processes that gave birth to Mercury, Venus and Mars didn't create any moons, so they are evidently common only to gas giants. Why?

9 years ago

I don't think there are any in the Solar system. We do have around 250 asteroids with moons. Rhea's ring seems to be the only exception.

Edit: Originally I said "a moon with a moon would be an unstable system, due to the gravitational influence of the planet." @Florian disagrees with this. However, the answer is more complex than the Hill sphere alone.

At first approximation, the Hill sphere gives a radius in which orbits around a moon could be stable. Our Moon's Hill radius is 64000 km.

For our own Moon, we know that most low orbits are unstable due to mascons: mass concentrations below the surface which make the Moon's gravitational field noticeably uneven. There are only four inclinations where an object orbiting the Moon avoids all mascons and would be stable: 27º, 50º, 76º, and 86º.

High orbits above the Moon aren't all safe either: above 1200 km and inclinations of more than 39.6º, Earth's gravity disrupts the satellite's orbit. Note that these orbits are comfortably within the Moon's Hill sphere.

There are stable orbits at high inclinations and high eccentricity:

As for other moons in the solar system: most of them are smaller and orbit around larger planets, so their Hill spheres are small, and the planet's gravity will disrupt much of the volume inside the Hill sphere too.

Moons below the limit where their gravity is strong enough to make them spherical, will have problems with uneven gravitational fields. Mascons may also be present.

"a moon with a moon would be an unstable system" - this is incorrect. Orbits are stable within the Hill sphere. BTW, please don't use popsci.com articles as "supporting evidence".

Feel free to write a better answer.

Anyway, the article doesn't mention "unstable systems" :)

Well, 'boulders up to several decimeters in size' within Rhea's ring aren't exactly 'moons' but I wouldn't hesitate to call them 'natural satellites', so, yes, this is clearly a case of a planet's satellite with own natural satellites.