In lots of SF stories, where people are living on lots of planets, Earth is Terra, Latin like Luna and Sol. As for when "Earth" was first employed as the name of our planet, I'd be guessing. I suppose it also dates from after Galileo, when it was realized that what had been "the world" was just one of several planets. Interesting question. Maybe someone has a more authoritative answer than my speculations.
It was probably coined by the same guy who invented the wheel.