Within minutes of my last posting I had half a dozen replies and by the end of the day the number had grown to around fifty. This morning I had another fifteen. I'd like to thank everyone who replied for their speed response and for their articulate explanations as to why 'Earths' is the correct spelling for multiple alternate versions of our planet. I was amazed by detail and effort expended by most responders. You people are amazing. Thanks again for your help.