What planet has a moon with a atmosphere?
The only moon with a fairly dense atmosphere is Titan. That's a moon of the planet Saturn.
Earth is not the only planet that has an atmosphere, it's just the only planet with an atmosphere that is capable of sustaining human life. Even the moon has an atmosphere! The gases in our atmosphere are uniquely combined in a way that is life sustaining, as opposed to mars, for example, which has a very thin atmosphere lacking many of the gases we need to live.