Asked by Steve Hampton
The main reason we don’t get gas moons is that they are simply not big enough. Gas is very volatile, it has a high degree of movement, and this makes it easy for gas to escape. This happens on all the bodies in the solar system that have gas in the atmosphere. Although the rate at which they lose gas tends to be very small. This high movement rate means that objects have to reach a certain size before they are able to hold on to an atmosphere. Most moons don’t reach this minimum size limit.
As the moons are small their gravitational influence usually cannot rival that of their parent planets. This means that the planet usually strips any gas away from the moons around it.
A gas moon, in the style of a gas planet could not exist. The closest you could possibly get would be a binary planet system, where roughly similar size gas planets orbited each other. As of yet, we haven’t found anything like this, but as the search continue, maybe we will.
Answered by Josh Barker from the National Space Centre
Image courtesy of NASA/JPL-Caltech/T. Pyle (SSC)