So Pastor Riley (much as I love him -- I mean, I might not be at this site were it not for a long article I read on 5D in January or February, 1998) gives us no documentation whatsoever.
He writes that unnamed astronomers thirty years ago "learned that the writings of the ancient Sumerians were really true." Though he speaks of "Planet X" rather than "Nibiru" and doesn't mention Zecharia Sitchin by name, the mention of Sumerians in connection with Planet X tells us that's what Pastor Riley is speaking of: the Sumerian destroyer-planet idea Sitchin championed in his books.
Quoting from Dr. Heiser's website:
Is Sitchin correct – Is Nibiru a 12th planet that passes through our solar system every 3600 years? Did the Sumerians know this? Unfortunately for Sitchin and his followers, the answer to each of these questions is no. But how do I know? The cuneiform record in such texts as the one on the left, the astronomical text known as MUL.APIN (The "Plough Star").
Readers can click here for a summary paper I wrote on the word nibiru in cuneiform texts. What follows draws from that paper and, in the case of the video, demonstrates the accuracy of my contention that there isn't a single text in the entire cuneiform record that:
= Has nibiru as a planet beyond Pluto
= Connects nibiru with the Anunnaki
= Has nibiru cycling through our solar system every 3600 years
Searching for Nibiru in Cuneiform Texts
Here is a video that I created showing you where to find the leading dictionary of cuneiform words online (for free). Viewers can find that source and do what I do in the rest of the video: look up the entry for nibiru (spelled neberu in scholarly transliteration) and check to see if any of the above ideas are found in any Akkadian or Sumerian texts that mention nibiru. Spoiler: there aren't any -- but don't take my word for it. Look it up yourself.
In other words, Pastor Riley: " the writings of the ancient Sumerians about Planet X" you refer to don't exist.
If the links in Heiser's text above don't work, you'll find working links on his site:
Fair Use For Educational and Discussion Purposes