There have been a couple older thread on this - one of them (with links to the others) can be located here.

A basic summary of those threads is that "Earth" is only the name of the planet in English - each language has its own name. Within English, the word can be traced back to the language's earliest Anglo-Saxon roots, its origins before that foggy. The early form of "Earth" basically had the meaning of "ground" or "land" - it wasn't thought of as a planet like the others until well after the word was entrenched (something that is found in many other languages as well).

Earth is a word that goes right back to Old Anglo-Saxon. The OECD presents this piece of the West Saxon translation of Matthew, "Me is geseald ælc anweald on heofonan & on eorðan." That's Earth in a metaphysical sense, as opposed to other supernatural realms, like heaven or hell.

From there we get Earth as the sphere we know and love. The OECD gives a tentative reference in the 1300s. Certainly this bit of Chaucer, "Erthe that bitwixe is sett The sonne and hir."