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I would prefer a definition based on how round the object is. If it's round or a rotational ellipsoid close enough to a sphere, it's a planet.

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This is called Hydrostatic equilibrium[1] and is one of the factors at play when determining if a body is a dwarf planet/planemo.

[1] https://en.wikipedia.org/wiki/Hydrostatic_equilibrium

Is the Moon a planet?

I was wondering before if it would count as a planet if it wasn't bound to Earth. It is around 6 times more massive than Pluto. There are also other, bigger moons in our solar system, such as Ganymede, which has about half the mass of Mercury (but also has a bigger diameter, due to being significantly less dense).

One could defend the Earth-Moon system to be a double planet. The Moon is much too large compared to other satellites in the solar system.

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