I'm currently working on the first chapter of a new story and I'm stuck on a word. The story addresses the concept of multiple universes and multiple Earths. I want to write it as Earths because I'm refering to the planet and not types of minerals. Am I correct or should I write it as 'earths' or even possibly 'Earth's'. It's a minor issue but I'd like to get it correct from the beginning before I get to far into the story. Feedback is appreciated.