Isnt it like... Not technically in the solar system at all?
It makes a short, wide turn visit, then leaves since its busy rotating in its own system.
Depends on your definition of where the solar system ends.
Here's a model of the solar system.
Neptune, the farthest out major planet, is 30 AU out from the sun (30 times further from the sun than the Earth is).
Pluto, is 30-50AU out from the sun, depending on it's location in it's orbit. Pluto sits in the Kuiper belt, an Asteroid field.
Planet 9 is estimated to be roughly 600 AU out from the sun.
However, the Oort cloud, which is a sea of matter, where most of our comets come from, ranges from 1000 to 100,000 AU out.
By most definitons the Solar System definitively ends at the end of the Oort cloud, if not earlier.
More conservative definitons define the Solar System as ending at the edge of the Kuiper belt.
So, Planet 9 is potentially part of the Solar System (it's certainly significantly closer to our star than any other), but by other definitions, it's just too far out.