As in unknown?
Mike Brown recently published a Planet Nine video about his discovery where he says, "everybody's been wrong for 170 years, but we're right," and I chuckled.
Of course, Planet 9 hasn't been detected directly, even while four candidates turned up in a recent citizen science project. None the less, I couldn't help but revisit some questions and curiosities. Assuming it's there, a large perturber in the outer system would seem to answer or add to multiple questions:
-sun's axial tilt.
-solar system's angular momentum
-long-period cometary "wakes" and periodic bombardment
-retrograde and resonant motions
-Uranus on its side
-Pluto having the same retrograde motion as Triton while also intersecting Neptune's orbit.
There are probably others but these readily come to mind.
I wonder if the putative perturber will be found to have satellites, or assuming there's one, could there be more?