You're referring to Ganymede, one of Jupiter's moons. It is indeed larger than the planet Mercury! However, Ganymede is classified as a moon, not a planet, for a few key reasons:

* Orbiting a star: Planets orbit a star. Moons orbit a planet. Ganymede orbits Jupiter, which in turn orbits the Sun.

* Gravitational dominance: Planets are gravitationally dominant in their orbital region. This means their gravity is strong enough to clear out other objects from their path. Ganymede is in orbit around Jupiter, and Jupiter's gravity is much stronger than Ganymede's.

* Formation: Planets form from the accretion of material around a star. Moons are formed by different processes, often from the debris left over from planet formation.

While Ganymede is a giant moon, it meets the criteria for being a moon, not a planet, according to the current definition.