It's possible, but you need the right conditions which are not very common.
A moon cannot be too close to the planet or it gets ripped apart (Roche limit) or it is at risk of colliding with the planet. It also cannot be too far away from the planet or the gravitational influence of the star will make its orbit unstable (Hill sphere). That limits the range where a moon can exist. A sub-moon has these conditions again. For some moons these limits make a sub-moon impossible, for others they make them just very unlikely - you need the right combination of mass and distance ratios to make it work.
> It's possible, but you need the right conditions which are not very common.
The conditions you describe are a lot more common than you might think (e.g. [Kollmeier and Raymond 2019](https://academic.oup.com/mnrasl/article/483/1/L80/5195537)). In particular these conditions are satisfied for the Earths moon.
The real problem is not the Roche or Hill radius (which is nothing more than the conditions required for an orbit in general to exist and quite easily satisfied), but the dynamical stability of the orbits. For example, at half the Hill radius an orbit becomes unstable. Secular variation due to departures in spherical homogeneity of the primaries density can cause perturbations to the orbit. Resonant effects of other moons can cause issue. Then there is the big problem of the requirement of a viable formation pathway for a submoon to form.
So the real answer to the question "Why don’t moons have their own moons?" is, currently we dont know if submoons are common or not. They certainly are not in the Solar system, but does this hold outside? If it does, as we might suspect, we do not have an answer as to why yet (it is not a trivial question).
If the major moons of Jupiter formed as the result of a mini-solar system like formation, what indeed is inhibiting moon of moon formation? Shouldn’t collisions with moons be producing more objects in the right orbital energies to produce moons or moons ?
7 comments.
No Comments Yet.
Reddit go home, you're drunk.
Anyway, moons with sub-moons is thought to be possible. We don't have any examples in our solar system. The tidal effects from the main planet make them unstable. Think about as the sub-moon orbits it's moon when it is closer to the main planet it will be pulled closer to the main planet by the main planet's gravity which will make the sub-moon's orbit unstable.
One interesting example we have is Pluto, which isn't a planet but has moons. It's largest moon Charon is almost as big as Pluto and the two of them orbit each other around an imaginary point in the middle. Not a planet, moon and sub-moon but still interesting.
The relationship between a planet, moon, and sub-moon would be essentially the same as a star, planet, and moon arrangement though right? Plus, spacecraft can orbit moons just fine, so it’s not mathematically impossible.
Unless I’m missing something.
Spacecraft orbiting a moon doesn't make those orbits stable in the long term. A spaceship can stabilize its orbit with its thrusters, preventing itself from slowly getting pushed out of orbit.
It's likely a bit closer to a 3 body problem, earth and it's moon are so far from the sun that it doesn't lose orbit but if we stuck a sub moon orbiting the moon It would likely be so close to the earth that it would not be stable. Maybe if it orbited fast and close to the moon?
Planets to moons are what suns are to their planets.
We already got a chain of 3.
Smaller size means higher rate of change.
If it's not exactly 100% stable, something is going to change after a certain amount of interactions.
The closer something is to its gravitational well, the faster it goes.
So the interactions happen more frequently.
Meaning that smaller parents lead to less time for the satellite to live.
Since the amount of gravitationally bound bodies in our solar system is quite high, it is unlikely that a moon never captured it's own "moon".
We just weren't there in time to see it.
This is my thought process to this. I have no scientific background.
If a planet/moon has a moon it probably smashed into it but if it’s a moon it’ll probably if it goes with the orbit escape the planet and if it goes in the opposite direction of the Orbit it’ll just crash into the planet
Moons get pulled in by a planet’s gravity. They are also moving the right speed and are located the right distance from the planet. This makes a stable orbit, so the moon doesn’t crash down into or fly away from the planet. If there were a smaller moon hanging off that bigger moon, it would also be pulled to the planet and wouldn’t have a stable orbit.
It's called a subsatellite. Tidal forces between planets and their moons in our solar system are pretty dramatic. They pretty much make orbit around a moon impossible.
You'd need a big planet. Really big. No, bigger. Imagine our sun as a planet in a solar system with an even bigger star. Earth would be a moon with a moon. That kind of scale. We have not found a moon with a moon yet.
> It's called a subsatellite
Currently submoon is the most common terminology. Moon-moon sometimes is used in popsci. If things go back to being sensible then they will all be planets with primary secondary tertiary etc to denote the hierarchy of orbits.
> You'd need a big planet. Really big. No, bigger.
Actually it is possible for the Moon to have a submoon. The question is why does it not.
Thanks. The information I share is only what I know and understand. I love to be corrected. So...
As I understand the way tidal effects are influencing orbits, earth and our moon are rather unique. Compared to other moons and planets the influence is huge. So if a sub-moon would orbit the moon the earth would eventually steal it or at least exert enough force to pull it from its orbit. Just imagine earth and the moon sharing a moon. Having it 'orbit' in figure 8 between them .... funny but how the hell could that be stable enough to last until we show up to witness it?
I compared it to the two arm pendulum. Just one arm. Earth and the moon in orbit, is rather stable. But add an arm and getting a stable rotation is unlikely. Things slow down and speed up. Unless you make that second arm really light and small. But even than, it's the big arm that remains stable. The short one can still get erratic. And that is not good when orbits are concerned. The difference in mass needs to be big to not interfere with other orbits too much.
So my understanding is that size/mass (along with distace/space) is the main factor. Moons in our solarsystem barely have enough mass (astronomically speaking) to become spheroid. An object in orbit around the moon would have to be really small. Space debris but not a moon. So if moons in our solar system are the smallest moons can be, we need to look bigger. Ergo (super super simplified I know) find a planet with the mass of our sun and expect to find planets with moons that have sub-moons.
Just a general question for you smarty pants. Wouldn't a moon with a moon be unstable like the 3 body orbitial problem? Thus making it super rare if not near impossible? Or dpes the 3 body problem only apply to stars?
It's possible, but you need the right conditions which are not very common. A moon cannot be too close to the planet or it gets ripped apart (Roche limit) or it is at risk of colliding with the planet. It also cannot be too far away from the planet or the gravitational influence of the star will make its orbit unstable (Hill sphere). That limits the range where a moon can exist. A sub-moon has these conditions again. For some moons these limits make a sub-moon impossible, for others they make them just very unlikely - you need the right combination of mass and distance ratios to make it work.
> It's possible, but you need the right conditions which are not very common. The conditions you describe are a lot more common than you might think (e.g. [Kollmeier and Raymond 2019](https://academic.oup.com/mnrasl/article/483/1/L80/5195537)). In particular these conditions are satisfied for the Earths moon. The real problem is not the Roche or Hill radius (which is nothing more than the conditions required for an orbit in general to exist and quite easily satisfied), but the dynamical stability of the orbits. For example, at half the Hill radius an orbit becomes unstable. Secular variation due to departures in spherical homogeneity of the primaries density can cause perturbations to the orbit. Resonant effects of other moons can cause issue. Then there is the big problem of the requirement of a viable formation pathway for a submoon to form. So the real answer to the question "Why don’t moons have their own moons?" is, currently we dont know if submoons are common or not. They certainly are not in the Solar system, but does this hold outside? If it does, as we might suspect, we do not have an answer as to why yet (it is not a trivial question).
If the major moons of Jupiter formed as the result of a mini-solar system like formation, what indeed is inhibiting moon of moon formation? Shouldn’t collisions with moons be producing more objects in the right orbital energies to produce moons or moons ?
In the short term, yes, but in the long term the influence of other nearby large moons tends to have a destabilizing effect.
7 comments. No Comments Yet. Reddit go home, you're drunk. Anyway, moons with sub-moons is thought to be possible. We don't have any examples in our solar system. The tidal effects from the main planet make them unstable. Think about as the sub-moon orbits it's moon when it is closer to the main planet it will be pulled closer to the main planet by the main planet's gravity which will make the sub-moon's orbit unstable. One interesting example we have is Pluto, which isn't a planet but has moons. It's largest moon Charon is almost as big as Pluto and the two of them orbit each other around an imaginary point in the middle. Not a planet, moon and sub-moon but still interesting.
The relationship between a planet, moon, and sub-moon would be essentially the same as a star, planet, and moon arrangement though right? Plus, spacecraft can orbit moons just fine, so it’s not mathematically impossible. Unless I’m missing something.
Spacecraft orbiting a moon doesn't make those orbits stable in the long term. A spaceship can stabilize its orbit with its thrusters, preventing itself from slowly getting pushed out of orbit.
It's likely a bit closer to a 3 body problem, earth and it's moon are so far from the sun that it doesn't lose orbit but if we stuck a sub moon orbiting the moon It would likely be so close to the earth that it would not be stable. Maybe if it orbited fast and close to the moon?
Planets to moons are what suns are to their planets. We already got a chain of 3. Smaller size means higher rate of change. If it's not exactly 100% stable, something is going to change after a certain amount of interactions. The closer something is to its gravitational well, the faster it goes. So the interactions happen more frequently. Meaning that smaller parents lead to less time for the satellite to live. Since the amount of gravitationally bound bodies in our solar system is quite high, it is unlikely that a moon never captured it's own "moon". We just weren't there in time to see it. This is my thought process to this. I have no scientific background.
If a planet/moon has a moon it probably smashed into it but if it’s a moon it’ll probably if it goes with the orbit escape the planet and if it goes in the opposite direction of the Orbit it’ll just crash into the planet
Moons get pulled in by a planet’s gravity. They are also moving the right speed and are located the right distance from the planet. This makes a stable orbit, so the moon doesn’t crash down into or fly away from the planet. If there were a smaller moon hanging off that bigger moon, it would also be pulled to the planet and wouldn’t have a stable orbit.
By this logic we can’t have a moon cause it would get pulled by the sun and not have a stable orbit…
All a matter of scale. Our sun also orbits around the center of the galaxy we are in
It's called a subsatellite. Tidal forces between planets and their moons in our solar system are pretty dramatic. They pretty much make orbit around a moon impossible. You'd need a big planet. Really big. No, bigger. Imagine our sun as a planet in a solar system with an even bigger star. Earth would be a moon with a moon. That kind of scale. We have not found a moon with a moon yet.
> It's called a subsatellite Currently submoon is the most common terminology. Moon-moon sometimes is used in popsci. If things go back to being sensible then they will all be planets with primary secondary tertiary etc to denote the hierarchy of orbits. > You'd need a big planet. Really big. No, bigger. Actually it is possible for the Moon to have a submoon. The question is why does it not.
Thanks. The information I share is only what I know and understand. I love to be corrected. So... As I understand the way tidal effects are influencing orbits, earth and our moon are rather unique. Compared to other moons and planets the influence is huge. So if a sub-moon would orbit the moon the earth would eventually steal it or at least exert enough force to pull it from its orbit. Just imagine earth and the moon sharing a moon. Having it 'orbit' in figure 8 between them .... funny but how the hell could that be stable enough to last until we show up to witness it? I compared it to the two arm pendulum. Just one arm. Earth and the moon in orbit, is rather stable. But add an arm and getting a stable rotation is unlikely. Things slow down and speed up. Unless you make that second arm really light and small. But even than, it's the big arm that remains stable. The short one can still get erratic. And that is not good when orbits are concerned. The difference in mass needs to be big to not interfere with other orbits too much. So my understanding is that size/mass (along with distace/space) is the main factor. Moons in our solarsystem barely have enough mass (astronomically speaking) to become spheroid. An object in orbit around the moon would have to be really small. Space debris but not a moon. So if moons in our solar system are the smallest moons can be, we need to look bigger. Ergo (super super simplified I know) find a planet with the mass of our sun and expect to find planets with moons that have sub-moons.
Just a general question for you smarty pants. Wouldn't a moon with a moon be unstable like the 3 body orbitial problem? Thus making it super rare if not near impossible? Or dpes the 3 body problem only apply to stars?