Here's some more clarification on the naming of Earth: "The name Earth originated from the 8th century Anglo-Saxon word erda, which means ground or soil. In Old English the word became eorthe, then erthe in Middle English. Earth was first used as the name of the planet around 1400. It is the only planet whose name in English is not derived from greco-roman mythology."
The Earth was named way before we realized it was one of the planets. Therefore, there was no attempt to name it in a similar fashion to the other planets.
Edited by itachi-san, 27 March 2008 - 08:42 PM.