system
Given the following post, respond as if you're commenting on Reddit:
human
Is it possible for a planet to have a gas giant as a moon?

Presumably, the planet would need to be incredibly huge and the moon would be relatively small for a gas giant. Has this been observed or speculated before?
gpt
Okay, some of the answers in here are seriously making me facepalm. As someone who studies gas giants, please let me set the record straight.

As far as we know there are no super-gas giants that have smaller gas giants as moons. One of Saturn's moons - Titan - has an appreciable atmosphere that is actually about 50% thicker than Earth's...but that's still a very long way from being a gas giant.

So far as we understand planetary formation, it is *extremely* unlikely to form a gas giant moon. As our solar system formed from the primordial disc of gas and dust orbiting our proto-Sun, dust particles started coalescing to form protoplanets (think of these as asteroid-sized). Certain protoplanets that had a slight edge on size began to dominate their gravitational neighborhood, pulling in ever more dust and eventually gas, as well, to become gas giants.

This created its own gravitational well in the protoplanetary disc, causing material near the newly formed gas giant to form its own little disc orbiting the planet. It's from this material that the largest gas giant moons are formed. However, the mass in the disc is very small mass compared to the planet itself, just as the sum total of all the planets' mass is roughly 1000 times less than the mass of our Sun. As an example, Ganymede, the solar system's most massive moon, has roughly 8000 times less mass than Jupiter (and is still about 25 times less massive than the Earth).

In other words, after a gas giant forms, there's simply not enough material left over to make an appreciably large moon. "Aha!" you say, "but what if the planet were many times larger than Jupiter?" Well, the problem is there's a limit to the size of gas giants. 

If Jupiter were only 13 times more massive - still likely below the mass needed to even have an Earth-sized moon - it would no longer be a planet, but a "brown dwarf". It would begin undergoing fusion of deuterium (a form of heavy hydrogen) near its core. Eventually this gets used up, and the brown dwarf is left to cool as a failed star.

If Jupiter were 80 times more massive - large enough to start to getting Earth-sized moons or even a little larger - then not only will deuterium fuse, but regular hydrogen, as well. At this point, it no longer a brown dwarf, but a full-fledged star that will burn for billions of years.

**TL;DR**: Extremely unlikely. Based on how they're formed, gas giant moons can only be a very small fraction of the host planet's size. If gas giants get too big, they're no longer planets.