Why is Planet Earth called Earth?

[KukriKhan]

The word Earth has cognates in many modern as well as defunct - including ancient - languages. Examples in modern tongues include aarde in Dutch, Erde in German, and aard in Arabic, all of which mean 'land', or in some cases, the entire earth. The root also has cognates in extinct languages such as ertha in Old Saxon and ert (meaning 'ground') in Middle Irish. It is derived from Old English eorðe. Taking into account metathesis, we can find cognates of the word Earth in the Latin terra and in the modern Romance Languages (i.e. tierra in Spanish).

Although a link to such Indo-European languages has not been proved, several Semitic languages have similar-sounding words for Earth: irtsitu in Assyrian, araa in Aramaic, erets in Phoenician (which appears in the Mesha Stele), and ארץ (arets, or erets when followed by a noun modifier) in Hebrew.

From a Wikipedia article - extracted from an OED entry. So I guess when Adam fell down one time, and got a mouthful of dirt, his explanation to Eve was "Ur-t-t-t". :)