system
Given the following post, respond as if you're commenting on Reddit:
human
Is there a limit for the size of moons and planets?

Have we ever found a gas planet so big that one of it's moons is another gas "planet"? Is that theoretically possible?
gpt
Even if a moon is the size of a planet, it's still a moon if it's orbiting a planet. 

Ganymede, which is one of Jupiter's moons, is larger than Mercury and would be a planet if the sun, rather than a planet, were its primary.