Google took us from relying on learned schoolers to research historic writings for our ponderings to "About 126,000,000 results in .25 seconds">>> it's pretty impossible to say exactly who first named the planet 'Earth'. Actually, I really doubt one person really named it intentionally; rather it developed over time as part of the English language. Earth is Old English and German in origin, related to the Old Saxon 'ertha', the Dutch 'aerde', and the German 'erda'. Terra is a French and Latin word, and so isn't part of the 'Earth' etymology. I'm not really an expert on words and word origins, but it seems likely that people used Earth to mean 'land' and then it was the natural thing to refer to all the land and the planet.