I'm willing to assume the planet exists in the BTU if it exists in the real-world solar system.
However, given its location and supposed mass, it will be a wholly uninteresting, dull place, commercially speaking. Uranus and Neptune aren't much mentioned in BT either, primarily because there's simply nothing there.
And then there's the fact that humanity, even during the Terran Hegemony and early Star League era, spread out hundreds of light-years from Terra, discovering, exploring and settling scores if not hundreds of new systems.
There simply is no reason to go back to planet 9 (or Planet X, as it was previously referred to) if those ores can be obtained elsewhere. Not to mention the effort of prospecting a 10g world for ores in the first place.