Short Answer: We don't know yet, but linguists are working on that question.
Earth has been around since the Middle Ages and it has cognates in all the Germanic languages. Interestingly, it is supposed that this word came from a Proto-Indo-European root, but the only cognate outside of the Germanic family is "ert" from Middle Irish... which itself could have come from English.
A word that old wasn't borrowed from Arabic anytime recently. However, there is a very controversial and interesting theory that posits Early Germanic explorers had contact with Semitic peoples, resulting in some language exchange. However, there is only one piece of historical evidence and some scant linguistic evidence.
However, it could be coincidental. There are bound to be identical words occasionally... it happens all the time.
Check out "Our Magnificent Bastard Tongue" which is where I read about it. It's interesting to think about... Great question!