Here's is one question that keeps bugging me why is Earth called Third Earth? I've looke around the web to see if anyone had come up with a theory for the name of our planet and so far there isn't any. So I decided to do one.
Below are the possiblities of the name of Earth...
1. There are more than one planet called Earth and our Earth is the third best planet 2. It's the third planet in our solar system (very believable) 3. Just the alien inhabitants like to call the planet 'Third Earth' but they know it's not 4. Maybe all the above
Those are the theories that I've come up with for Earth being called Third Earth. My four suggestions should help you writers to come up with an explanation for the two names for one planet if you were to have an explanation in your fic(s).
Third Earth and Earth is something similar in how we talk about Mars. We know that the planet is called Mars but we also like to call it the "Red Planet" since most of the planet is red. Chances are if aliens have landed here and always flying around our orbit, which I strongly believe in, they could call this planet the "Blue Planet" because 80% of the planet is water.