Asked by: HN, Toronto
Just like gas planets, a gas moon would need to be a lot bigger than its rocky counterparts – if it were too small then it wouldn’t have enough gravity to hold on to large amounts of light gases such as hydrogen and helium. However, this means that a ‘gas moon’ would likely be of comparable size to its parent planet, and so would more appropriately be called a ‘binary planet’. Such gaseous binary planets are possible, but probably extremely rare because planet formation would usually result in the two objects merging or deflecting each other to separate orbits. So we haven’t spotted one yet!