I believe the origin for the idea of Nibaru can be found in Zecharia Sitchin’s books. Another name that’s been assigned by a few renegade astronomers to such a hypothetical planet with an elliptical, cometary-style orbit is “Nemesis.”
If you want to read cosmically catastrophic scenarios for the current configuration of the Solar System, try the ideas of Walt Thornhill and his friends over at Thunderbolts. They’ve updated Velikovskian ideas with more up-to-date electrodynamics. Couple that up with a free hand in the interpretation of legend and you’ll find stuff that can curl your hair.
The idea of another large planet which has yet to be found in trans-Plutonian orbit is more widely accepted; it’s used to explain some of the orbital eccentricities of the outermost observed planets.
All is mist for the grill; I do fall out of lockstep with Sagan, though, on his assertion that extraordinary claims require extraordinary proofs. Its unthinking acceptance can impede the progress of science; witness what happened to Flieschmann and Pons...
Sitchcin RIP was so interesting to read. And it made alot of sense for the whole Missing link argument. I believe there is something out there. What Who knows? But the bible does talk about the annunaki or nephelim in Genisis. And many many times about UFOs , Ezekiel, and the other one where he was taken to heaven and never came back. I forget the name. So if you believe it or not there is a lot of evidence out there. You just have to open your eyes and mind to see it.