Lost world: An ancient ninth planet that went missing

https://www.knowablemagazine.org/article/physical-world/2018/lost-world-ancient-ninth-planet-went-missing

Good overview of the recent history in solar system origins, suggesting a Planet X could be further out in the Kuiper best or even the Oort Comet cloud.

I probably need to re-read it actually; but, I recall the article saying that solar system data and simulations suggested problems that can only be resolved by a PlanetX(Mike Brown likes to call it Planet9 because he demoted Pluto from planethood; i argue otherwise, and so do other astronomers)