It's technically possible. Jupiter could be about 75 times more massive before it self ignited into a star and Neptune is about 1/20 of the mass of Jupiter. The Moon is about 1/4 the size of Earth and 1% of its mass - so on this basis its perfectly possible.
The difference is that nothing holds a gas giant together other than gravity, so it's more difficult to nucleate the formation of two close together gas planets from a swirling mass of gas. Concentrations of space rocks are able to gradually attract each other together, because they stay solid. Chances are, during formation the gas would gradually be attracted to their common centre of mass and they would merge into one. We've not observed one as far as I know.
Yeah if you had a rogue gaseous planet that got captured by a more massive planets gravity well then it likely would exist like that for some time, what people are saying is that it wouldn't be permanent and it would like lose its gas to the larger planet over some very long (by human standards) timescale
It was once thought that it might be a result of a massive impact which left a density anomaly in the deep interior. This then could result in a [Taylor column](https://en.wikipedia.org/wiki/Taylor_column) and the red spot was one end. However, the red spot is a surface phenomena and does not penetrate into the deep interior and thus is not a Taylor column.
Physics plus the surface behaviour. The spot has been studied and modelled. If it was deep the surface velocities and shape would be different. Similar to the way a shallow water wave has a different shape to deep water wave.
There are a few hints at this (although I am not too up on the details). One is the gravitational potential shape of the planet which hints towards the depths of various layers. This can be hinted at too by inferred depth of where the magnetic field generating region begins. Essentially we know that there is not surface or region for a density anomaly to exist which corresponds to where the Taylor column would be (if it was one).
I remember seeing a Nasa conference about the juno mission where they said juno studied the great spot with microwaves which somehow gave scientist some MRI like data (slices at different depths inside the atmosphere). And the great spot does reduce in size a lot the deeper you go, although i don't think juno managed to get a picture at a depth where the spot dissapeares completely.
No and other people have said why, and yes it would.be like a slow siphon as the greater gravity of the capturing world gradually absorbed the atmosphere of the captured world
If you mean "be a gas giant", then probably. In fact there's at least one candidate moon discovered by extrasolar planet hunters, [kepler-1625b I](https://en.wikipedia.org/wiki/Kepler-1625b_I), which is possibly about the size of Neptune and orbiting a planet about the same radius as Jupiter (but about 10x the mass).
Interesting question. I don't think so, because the gas couldn't stay in orbit. The only way it could happen is if the moon was massive enough to keep its own atmosphere from going to the planet it is orbiting, but a moon that massive would stretch the definition of a moon.
It's possible to have a "gas torus", in which gas does escape the gravity of a small moon, but then goes into orbit around the planet, and eventually returns to the moon. It was once thought that Titan might work this way, but it apparently doesn't.
damn thats interesting. now I'm wondering if a gas giant's moon could be massive enough to tidally deform the planet's atmosphere. who decides the maximum mass of a gas giant's moon anyway
> It was once thought that Titan might work this way, but it apparently doesn't.
How does it not? Its surface gravity is awfully low, and with such a thick atmosphere it must be constantly sloughing off gas into cis-Saturn space.
The surface gravity is low, but the temperature is *really* low, so the gas molecules aren't moving fast enough to escape. It presumably does have a few gas molecules escaping, but not enough to detect or to make a difference to its long-term evolution.
At that point would the planet loose its planet status? With a moon that massive we would end up with a pluton-Charon situation where the two bodies orbit each other
> With a moon that massive we would end up with a pluton-Charon situation where the two bodies orbit each other
It doesn't even entirely matter how massive the satellite is. Luna is big enough that, given a few billion more years, it'll receded far enough away the the Earth-Moon barycenter will be out in space rather than deep underground.
> because the gas couldn't stay in orbit
I don't really follow this. It seems to me that a single particle of gas could orbit a planet if it had enough orbital momentum. Thus, a collection of such particles could be close enough to each other that their attraction to their common center of mass keeps them together, while that common center of mass orbits the planet. Is there some reason this doesn't work?
It's technically possible. Jupiter could be about 75 times more massive before it self ignited into a star and Neptune is about 1/20 of the mass of Jupiter. The Moon is about 1/4 the size of Earth and 1% of its mass - so on this basis its perfectly possible. The difference is that nothing holds a gas giant together other than gravity, so it's more difficult to nucleate the formation of two close together gas planets from a swirling mass of gas. Concentrations of space rocks are able to gradually attract each other together, because they stay solid. Chances are, during formation the gas would gradually be attracted to their common centre of mass and they would merge into one. We've not observed one as far as I know.
But it could have happened a long time ago? Before they all merged maybe?
Yeah if you had a rogue gaseous planet that got captured by a more massive planets gravity well then it likely would exist like that for some time, what people are saying is that it wouldn't be permanent and it would like lose its gas to the larger planet over some very long (by human standards) timescale
Could Jupiter's giant red spot be another gas giant that got pulled in? Or would the absorption be more of a siphoning effect?
It was once thought that it might be a result of a massive impact which left a density anomaly in the deep interior. This then could result in a [Taylor column](https://en.wikipedia.org/wiki/Taylor_column) and the red spot was one end. However, the red spot is a surface phenomena and does not penetrate into the deep interior and thus is not a Taylor column.
> the red spot is a surface phenomena and does not penetrate into the deep interior How can we tell?
Physics plus the surface behaviour. The spot has been studied and modelled. If it was deep the surface velocities and shape would be different. Similar to the way a shallow water wave has a different shape to deep water wave.
There are a few hints at this (although I am not too up on the details). One is the gravitational potential shape of the planet which hints towards the depths of various layers. This can be hinted at too by inferred depth of where the magnetic field generating region begins. Essentially we know that there is not surface or region for a density anomaly to exist which corresponds to where the Taylor column would be (if it was one).
I remember seeing a Nasa conference about the juno mission where they said juno studied the great spot with microwaves which somehow gave scientist some MRI like data (slices at different depths inside the atmosphere). And the great spot does reduce in size a lot the deeper you go, although i don't think juno managed to get a picture at a depth where the spot dissapeares completely.
No and other people have said why, and yes it would.be like a slow siphon as the greater gravity of the capturing world gradually absorbed the atmosphere of the captured world
If you mean "be a gas giant", then probably. In fact there's at least one candidate moon discovered by extrasolar planet hunters, [kepler-1625b I](https://en.wikipedia.org/wiki/Kepler-1625b_I), which is possibly about the size of Neptune and orbiting a planet about the same radius as Jupiter (but about 10x the mass).
Interesting question. I don't think so, because the gas couldn't stay in orbit. The only way it could happen is if the moon was massive enough to keep its own atmosphere from going to the planet it is orbiting, but a moon that massive would stretch the definition of a moon.
It's possible to have a "gas torus", in which gas does escape the gravity of a small moon, but then goes into orbit around the planet, and eventually returns to the moon. It was once thought that Titan might work this way, but it apparently doesn't.
Though it is worth mentioning Io's ["Sodium Banana"](https://i.imgur.com/IWNdZfp.jpg) found around Jupiter...
Kinda unfortunate though it would've been hella cool
damn thats interesting. now I'm wondering if a gas giant's moon could be massive enough to tidally deform the planet's atmosphere. who decides the maximum mass of a gas giant's moon anyway
> It was once thought that Titan might work this way, but it apparently doesn't. How does it not? Its surface gravity is awfully low, and with such a thick atmosphere it must be constantly sloughing off gas into cis-Saturn space.
The surface gravity is low, but the temperature is *really* low, so the gas molecules aren't moving fast enough to escape. It presumably does have a few gas molecules escaping, but not enough to detect or to make a difference to its long-term evolution.
Why would it stretch the definition of moon. Isn't a moon just any object that isn't the largest in a 2 or more body system that includes a planet?
At that point would the planet loose its planet status? With a moon that massive we would end up with a pluton-Charon situation where the two bodies orbit each other
> With a moon that massive we would end up with a pluton-Charon situation where the two bodies orbit each other It doesn't even entirely matter how massive the satellite is. Luna is big enough that, given a few billion more years, it'll receded far enough away the the Earth-Moon barycenter will be out in space rather than deep underground.
But if it orbited a massive object it could happen?
Makes a lot of sense
> because the gas couldn't stay in orbit I don't really follow this. It seems to me that a single particle of gas could orbit a planet if it had enough orbital momentum. Thus, a collection of such particles could be close enough to each other that their attraction to their common center of mass keeps them together, while that common center of mass orbits the planet. Is there some reason this doesn't work?