Personally, my reaction to someone...anyone...talking about "Planet 9", "Nibiru", or "Planet X" was an eyeroll, and an instant inclusion to the "Flat Earth Asylum".
When asked by (the very rare conspiratorial) friend, "Can you see Nibiru throught that 'giant' telescope you have?", I send them this:
Well, it just got stranger. With some possibly credible scientific paper behind it, a new theory of eccentric orbits and planar angles has popped up:
Not that this is life changing, at least for myself, but it DID give my smug arrogance of 'the way things are' a bit of a rattle. Just a minuscule, "What if...?"