Planet of the Moms is a planet Timmy has wished up in Invasion of the Dads. At the end of the episode, all the dads started crying because Timmy's mom left. Timmy however wished this planet up to make them happy as well as prevent them from trying to take his mom away again.
After Timmy rescued his mom from the Dad clones, they began to greatly miss Mrs. Turner, so Timmy wishes up the Planet of the Moms to keep them happy and prevent them from kidnapping his mom again. Later the moms invade Earth to steal Dimmsdale's kittens, but were foiled by Mr. Turner and his monkey wrench and forced to leave.