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As far as I understand, a moon is an object in permanent orbit around a planet, dwarf-planet, asteroid, etc. If there was another object permanently orbiting this moon, would that be a moon-moon?

Is there any natural object of such kind that we know of?

If no, why are such objects so unlikely?

Edit: this is question is different from Do moons have moons? because I explicitly ask for a reason. Also, the terminology is included here.

Everyday Astronaut
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    Similar, not exactly the same question here: https://astronomy.stackexchange.com/questions/856/do-moons-have-moons – userLTK Feb 17 '19 at 11:31
  • @userLTK thanks for this hint, the answer given there is very comprehensive. – Everyday Astronaut Feb 18 '19 at 11:35
  • I reckon the most likely candidates for moons to be orbited by a sub-moon would be Titan, Luna, and Triton. Perhaps also Callisto, but despite the high mass not Ganymede, because of its quite close orbit around the massive Jupiter and strong gravitational interaction with Europa. – leftaroundabout Feb 18 '19 at 12:43
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    I'm voting it's a duplicate, as the answers there are comprehensive and address all the questions asked here (except the one about calling it a moon-moon, which I'm taking as rhetorical humour). rehctawrats, your edit changes nothing. – Chappo Hasn't Forgotten Feb 18 '19 at 21:54

2 Answers2

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No. One explanation for the equatorial ridge on Iapetus involves a now-lost subsatellite, so perhaps such objects existed in our Solar System's past.

Subsatellites tend to get rapidly removed by tidal forces causing changes to the orbit. Depending on the parameters of the system, the orbit may decay until the subsatellite collides with the moon or breaks up at its Roche limit. Alternatively it may migrate outwards until the subsatellite becomes unbound: at this point it would become a moon of the planet, though likely on an unstable orbit that would lead to scattering or a collision. The orbital evolution tends to be faster for more massive subsatellites. See Kollmeier & Raymond (2018) for details, who note that any subsatellites orbiting the candidate Neptune-sized exomoon in the Kepler-1625 system would need to be smaller than Ceres to survive.

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    It is ironic that the larger bodies (the ones you'd expect to have stronger structural integrity) are more susceptible to such forces than smaller ones. But then, astrophysics has known to be non-intuitive on more than one occasion. – corsiKa Feb 18 '19 at 03:52
  • I think there was a planet in Mass Effect like that... – Cloud Feb 18 '19 at 07:35
  • Why is it the case that these 'subsatellites' are less stable than the satellites of a planet? If a planet was as massive as the sun, and had a satellites the same size and distance as earth, would its equivalent moon be inherently less stable than our own? because it's a 'subsatellite'? – JeffUK Feb 18 '19 at 10:59
  • @JeffUK No. I think it's a scale effect. The answer to https://astronomy.stackexchange.com/questions/856/do-moons-have-moons (linked by userLTK) explains it pretty well. – Everyday Astronaut Feb 18 '19 at 11:33
  • We've got reasonably stable artificial moons of moons all over the Solar System. Granted on a million-year scale they probably are goners. – Carl Witthoft Feb 18 '19 at 18:56
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I would imagine that in most cases the sub satellite would be inside the instability limit of the planetary system in question and that such a scenario would be unstable. It would be interesting to model this with a simple 3-body system to see how the sub-satellite behaved over time (assuming a rigid body to start out). Someone else mentioned tidal perturbations and surely those would play a significant role too.

PSR-1937-21
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