In the outer solar system, a moon orbits Jupiter that carries within it the hallmarks of planetary complexity — a layered interior, a vast water reservoir, auroras, and a magnetic field born from its own depths. Ganymede is larger than Mercury yet bound to a gas giant, a world that defies easy categorization not because our definitions are wrong, but because nature rarely honors the boundaries we draw. It reminds us that the universe produces complexity wherever conditions allow, indifferent to the labels we assign. A European spacecraft is already en route to ask, in earnest, what such a worl