Planet Nine started off as a hypothetical planet.

In the early 2000s, a number of astronomical discoveries added credence to the idea that there was a far-flung unknown planet in our solar system. This planet had been known as Planet X, but around 2006 when Pluto was demoted to a planetesimal, we had to reevaluate the name, and we now call it Planet Nine . If you're into space, I suggest a read on Planet Nine, it's pretty interesting. If you're not, here's the tl;dr: there's a bunch of stuff out past Neptune that doesn't go around the sun the way it should. One possible explanation is a body of around the mass of Neptune (that is a planet-sized mass), affecting their orbits. That planet-sized mass would be Planet Nine. There are other possible explanations, but none that include a celestial body that pertains to The Outer Space Treaty. Now if Planet Nine is out there, chances are that it's a frozen gas giant just like Neptune. There's no real reason to assume it could be anything else, but it could be something more exotic like a primordial black hole, or some strangely giant dense diamond planet. But the thing with not knowing is that we can speculate. Speculation isn't science. This isn't a hypothesis or a theory. It's just the musings of some dude on the internet. And I say Planet Nine is a spaceship.