Surely, if we're being logical about God we should start not at "god exists, but couldn't have existed on Earth therefore he's an alien" but rather at "God exists as he is presented in the only text that directly references him - the Bible".
The next step would be to disprove that he exists as presented in the Bible.
The step after that is to present what else he could exist as.
After that it's a process of elimination, taking each of those previously presented possibilities and "stress testing" them - the options that come to mind are "he doesn't exist at all", "he was from Atlantis/some other previous super-civilisation" "he's an alien from the planet Gallifrey" or "he's a talking mouse with a time-machine and a zany assistant called Pinky".
Given the only one of those options that doesn't rely on proving something else as yet unproven we're left with the two most likely positions - he either exists as presented in the bible or he doesn't exist at all.
Edited by Wearer of Hats, 21 November 2012 - 09:28 PM.