system
Given the following post, respond as if you're commenting on Reddit:
human
Are moons always smaller than the planets they orbit?

Could there be a planet the size of earth with a moon the size of Jupiter orbiting it? Is it a matter of density or size?
gpt
By definition, a planet is a the dominant body in its orbit around the star. If object A is much bigger than object B, while they share an orbit around their star, then object B can't be a planet, but object A could be.

There's a grey area, where neither A nor B are large enough to have cleared their orbit. This is the case in the Pluto-Charon system. Pluto is clearly the larger of the two, but there are many objects that share the same orbit with Pluto and Charon and Pluto is not big enough to either capture the other objects in its orbit or eject them from the orbit. So Pluto is not a planet.