Planet Nine is Pluto.

You see how the entire conversation was reframed from demoting Pluto?

Babylonians knew about the 'tenth' planet, everyone and their mom knew about it. And there's a lot more in the ancient histories to learn about it.

So 'scientists' decided Pluto was no longer a planet, then the 'tenth' planet now becomes the 'ninth' planet, and we get articles like this.