A new theory is also making the rounds...
A micro-black hole.
SciTechDaily had a story just a few weeks ago:
https://scitechdaily.com/bold-plan-to-determine-if-planet-nine-is-a-primordial-black-hole/
A Harvard astronomer suggested we could easily locate the tiny flares that a micro-black hole would generate in the Oort Cloud.
Stephen Hawking theorized the existence of planet-mass black holes 50 years ago.
Don’t think so, if we had one the solar systems that passed near us would have been wreaked. Still possible, but remember that we don’t know if this type of object exists.