Well.... as reluctant as I am to disagree with the estimable Q, I thnk perhaps I have a different take on the poster's actual intent...
If I'm correct (always debatable) claymore is asking when the "planet first became known as (the "English word") Earth." If that's the case, then our friends at "Online Etmology Dictionary" trace our modern word "Earth" thusly:
"earth"
O.E. eorþe "ground, soil, dry land," also used (along with middangeard) for "the (material) world" (as opposed to the heavens or the underworld), from P.Gmc. *ertho (cf. O.Fris. erthe "earth," O.S. ertha, O.N. jörð, M.Du. eerde, Du. aarde, O.H.G. erda, Ger. Erde, Goth. airþa), from PIE base *er- "earth, ground" (cf. M.Ir. -ert "earth"). The earth considered as a planet was so called from c.1400.