The idea that there's another planet out in the Kuiper belt has shown evidence and then been dismissed multiple times.
Now the theory is back in fashion - and what they're predicting is out there is big. Or to be more precise, what Dr. Brown, who was responsible for demoting Pluto, is predicting is out there. I suppose he owes us a planet.
Ten times the size of Earth big. Very small gas giant big.
And it gets even more interesting. If the math is right, then Planet Nine (We can't call it Planet X any more and we can't name it until we find it) takes between 10 and 20 thousand Earth years to orbit the sun, has a highly elliptical orbit. It's closest approach is about 7 times as far as Neptune and its furthest is outside the Kuiper Belt.
And until somebody finds it, that's all we know. The assumption is it was thrown way out there in early planet formation. It's also possible, given the distance and bizarre orbit, that it's a captured rogue.
Or, given the distance and time scales involved it might not actually be captured at all, but just wandering by? (I don't know the math, but...)
If there is a ninth planet, then I might suggest that Momus, the Greek god of satire, might be a suitable name, given he was exiled from Olympus for stirring up too much trouble.