The notion of aliens also goes against the word of God (biblical scripture) if you're into that. Demons sure, but God made this plane stationary, with heavenly bodies revolving in our sky.

Now, there is a discussion to be had on what's on the other side of the planet, as Admiral Byrd described. The planet is certainly not exactly what we're told, so it's possible there is a different civilization on the other 3/4ths of the planet that's officially unexplored and uncharted. Perhaps those are the "aliens" as Clif understands them.