Let's think about this logically:
If you know X, then it is either because you have figured out X on your own or someone else has told you X (unless we take into consideration stuff like Jung's 'collective unconscious' or similar ideas, which aren't really relevant for the rest of this post). This gives us two alternatives: The Sumerians either figured about the asteroid belt and the Kuiper Belt Objects on their own (implausible; there are AFAICS no other records ANYWHERE of anybody even observing any Asteroid belt objects before 1801 or Kuiper-Belt obejcts before 1930), or someone else, with technology comparable to what we have today (!) or higher told them. This sounds uber-implausible if we assume terrestrial cvilizations. That only really leaves UFOs, which have earned status as pretty much the #2 hallmark (hearing voices would be #1) that you are just bat-shit insane.
One could potentially argue that UFOs cannot be logically ruled out, but given the sociological nature of UFO belief coupled with the relative lack of hard evidence, it is *very hard* to give very much credence to any theory that involves UFOs.