The largest moon in the solar system is Ganymede, one of Jupiter's 79 confirmed moons. Our own Moon is the fifth largest in the solar system and is about 1/4 the size of Earth.
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MOIYA MCTIER,
Astrophysicist, Folklorist, & Science Communicator
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Great question! Moons can form in a couple different ways. Some moons, like ours, form when a giant collision knocks material away from an already formed planet. Others can form by matter slowly building up around a central massive body, kind of just like a smaller-scale version of how planets form from the leftover material after stars form.
I don't know if there's a maximum mass for a moon before it can become a planet, because moons and planets aren't differentiated based on mass. A moon is a moon if it orbits a planet, no matter how big it is.
Is there a typical ratio of total mass of all moons to planet mass? What's the largest a moon can get in terms of % of planet mass before it's not a moon anymore and it's just a... another planet?