Planet 9 is Pluto.

What they're talking about is 'Planet X', which the Sumerians among others knew about over 3,600 years ago.

So instead of losing face, they claim Pluto isn't a planet, so now they can waffle on about 'Planet 9' when really they mean Nibiru.

It's embarrassing.

You believe in Pluto? The ninth planet is Ceres.

It could be a wormhole exit connecting us to an interstellar highway, it could be anything.

That would be very interesting :)