Planet nine has already been destroyed - see Greek & other Myth - some big smash up of planets and bodies that formed along wiith the solar system 4.5 billion years ago happened. MIT and other boffins say Myth does point to something like that but cannot work it out.
other boffins say Saturn, Jupiter and Mercury are the wrong size to be where they are todat and must have gained mass or lost (Mercury) atmosphere.
Then there the asteroids, now coming from a cluster up to 5 planetoids or a cloud of oids of some type
Saturn and Jupiters rings diminishing along with the red spot on Jupiter (Mars's atmosphere)
See Myth for the answers