Going rogue sounds misleading. Makes it seem the planet moves without external forces. More likely if such a planet exists it is on dramatically different orbit, possibly the way Neptune and Uranus are thought to swap proximities from the sun, as their orbits are much more oblong. It’s possible that a planet 9 (if it exists) also has a more pronounced oblong orbit and being so distant from the sun it has stronger gravitational forces from other planets or even other so load systems at times.
There are probably many ‘rogue’ planets not attached to a particular star. Just wandering around the galaxy like any other debris leftover from novas and supernovas.
That’s the premise for the novel and movie “When Worlds Collide”..............That is one movie I’d like to see re-made with modern technology!................
They don't. Neptune's perihelion is still 900 million miles greater than Uranus's aphelion.
You might be thinking of Neptune and Pluto. From 1979 to 1999, Pluto was closer to the sun than was Neptune. Neptune's orbit is very close to circular, while Pluto's orbit is much more eccentric.