Planets can revolve around the sun. Moons can revolve around the planets. What keeps a moon from having their own moon?
The term you are looking for is called the Hill Sphere This is the area where a body can orbit and not be perturbed out of said orbit, assuming two gravitational sources and no external factors. The Hill Sphere for the moon, for example. is roughly 35,000 miles. Anything small in that space SHOULD orbit the moon in a stable state for long periods of geologic time. Then you have to factor in the pull of the sun, the push of the solar wind, Jupiter's gravity, Venus etc. And, oh yea the lunar gravity field is 'lumpy' and not smooth; satellites in orbit at low altitude will become unstable quickly. Realistically anything over 200 miles and under 10K miles should be stable over long time frames. That there is nothing there tells us that the sun et. al. helped clean this area of space. Saturn has two moons, Rhea and Iapetus, with equatorial ridges that for all we know about how space works looks like a moon had a slow impact, or that body hit the Roche Limit and became a ring that then spiraled into the moon and left its ridge. Articles have been written about what this would look like, but none have been found. In short, gravity is complex, and other factors work against moons having stable orbiting bodies around them.
Interesting. Is this true for all moons, or just ours? Is it shape dependent, for example are the gravity fields of Mars' potato moons even lumpier? Dala was telling me about binary planets again last night, where the center of gravity is between two planets and not in a planet, like Pluto and its moon. That exo planet and its possible moon are so massive, I wonder if they might actually be binary planets too . . .the lunar gravity field is 'lumpy' and not smooth; satellites in orbit at low altitude will become unstable quickly.
Knowing whether that is true or not may have to wait for discoveries of moons orbiting distant exoplanets. Moons themselves are much harder to detect and only one promising candidate has been found so far – a possible exomoon orbiting the Jupiter-sized exoplanet Kepler-1625b. That possible moon – about the size of Neptune – is large enough and far enough from its planet that submoons should be possible as well. Astronomers will need to verify that primary moon first – if it does exist – before looking for any submoons.
It appears to be the case for all solid bodies. Earth has a lumpy gravity field, as does Mars and Mercury. Not sure about Venus. The outer moons, the ones that are mostly ices and water are probably 'smoother' gravity wise as all the chunks can settle to the center easier. The earth having a non-uniform gravity field is fun. It allows us to do thing like Molniya orbits for Russian communication satellites. It also means that geostationary satellites MUST have fuel or some way of station keeping otherwise they drift to the dips in the gravity field (one such location is the Rocky-Andes Mountains longitude) check the stability tabInteresting. Is this true for all moons, or just ours? Is it shape dependent, for example are the gravity fields of Mars' potato moons even lumpier?
My short no-maths take on it is that It's a very sensitive, dynamically unstable system. It's theoretically possible to have those but, as far as I know, none were ever observed. They could happen on short time-scales, absolutely, but it depends on so many factors that I'm not even sure if we could talk 'weeks'. The interaction between the Sun and hypothetical Moon's moon would probably fling it away, make it collide with Earth or the Moon or it would drop off to find its own orbit around the Sun. francopoli, am_Unition - you guys are better at orbital mechanics than me, and this thread is basically begging for your input.
Maybe moons are too small to have a big enough gravitational force? I know some big moons can have rings : https://en.wikipedia.org/wiki/Rings_of_Rhea How big does a rock that gravitates around a moon has to be, before it's called a "moon" ?
Ask this at a planetary science conference. Go on, I dare you. I've been lucky to see some of the back and forth, and it has been.... spirited. Some of the moons of Jupiter, for example, take 10 earth years to go all the way around Jupiter and are not much more than the size of Mt. Everest. There is a group trying to define what a "moon" is in the same way that "planet" was defined.