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Could a gaseous moon exist in the same way as a giant gas planet? All the moons in the solar system are rocky, or icy. Why shouldn't gas planets have gas moons?

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    $\begingroup$ Have you heard the one about how one must require mass first in order to attract gas then? Or else it flies away gas as it is, and that such initial mass is formed by accumulating a core of dust/rock that can stick together chemically and physically. Hydrogen and helium don't spontaneously form compact spheres, unless there's a rocky mass to attract them gravitationally. $\endgroup$ – LocalFluff Nov 13 '15 at 23:06
  • $\begingroup$ @LocalFluff Of course not. But there should be more than enough rocky mass to form gaseous moons on other planets, right? $\endgroup$ – Sir Cumference Nov 13 '15 at 23:33
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    $\begingroup$ Oh I see now, not purely gaseous, but like a gas giant moon. I don't see why not. A Jupiter could capture a Neptune as its moon for all that I know. Titan is outgassing a thick atmosphere, I suppose it's a matter of definition of what a gas moon is. One moon does have an atmosphere, so the phenomenon is certainly real. (I love all these one offs in the Solar System, because it must be so hard for astronomers to construct a general theory about it). $\endgroup$ – LocalFluff Nov 14 '15 at 0:11
  • $\begingroup$ @LocalFluff Then why haven't we found any moons that are mostly gas, just like Jupiter and Saturn are? $\endgroup$ – Sir Cumference Nov 14 '15 at 0:31
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    $\begingroup$ HDE Posted this, I'll just drop the picture here en.wikipedia.org/wiki/… You need a very low temperature for a moon to retain it's hydrogen and helium. It's very difficult to do that near a sun. It might be possible in deep space, like an escaped rogue moon might be cold enough. $\endgroup$ – userLTK Nov 14 '15 at 3:26
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LocalFluff's comment is spot-on. You need mass to have gas, and moons just don't have enough.

It is thought that gas giants gathered the gas they have today by accreting large amounts of it from the protoplanetary disk in the early days of the Solar System. At first, they were only gas-less cores (not rocky, exactly, but not gaseous), but they quickly became the most massive objects in their immediate vicinities, and thus gobbled up all the gas nearby them.

Now, less massive bodies, like Earth, can still accrete gas. However, they have difficulty retaining it. Lammer et al. (2014) calculated that under conditions like those experienced in the young Solar System, planets of one Earth mass or less could retain captured hydrogen envelopes for no more than ~100 million years - a long time for us humans, but shorter for astronomical objects.

The hydrogen escapes via atmospheric escape, which happens when particles in the atmosphere have velocities higher than escape velocity. This phenomenon is known as Jeans escape. Atmospheric escape happens more often with lighter gases, such as hydrogen. It is also influenced by stellar winds, which can cause even gas giants (generally close to the star, such as hot Jupiters) to lose some or all of their atmospheres.

I wouldn't call gas moons impossible, but certainly very unlikely.

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