Most of what you’ll read on the internet about Nibiru comes from Zecharia Sitchin’s noggin and those of his cyber-acolytes. In other words, most of it’s nonsense. Here is an exception. I’ll have to admit some of it is over my head (I’m not an astronomer), but this strikes me as an earnest attempt to make sense of the Mesopotamian material on Nibiru (read: the real stuff, not what Sitchin invents in his books). It may be helpful for readers to have these other links handy:

MUL.APIN

Enuma Anu Enlil