Oh sure, I'm not suggesting sending probes just to take a look around. But if we do positively identify another planet and its location, sending a probe there would be fascinating. If we could actually land it and do some materials science on a rogue planet, it would basically double our understanding of planetary formation.

The delta-V requirements for landing on an outer solar system body are… intense. New Horizons flew past Pluto at 14 km/s; in order to land, that relative speed would need to become zero. This concept likely requires new propulsion technology.

Agreed on being fascinating, though.

We've already landed on an outer solar system body, Saturn's moon Titan. https://en.wikipedia.org/wiki/Huygens_(spacecraft)

Titan does have the advantage of a thick atmosphere to allow descent by parachute, and a large parent body to capture the spacecraft into orbit.

Well if it is a planet 10x the size if earth it might have an atmosphere so we could aerobrake, no?

It would indeed, but the costs wouldn't make it viable. Spend that same money funding every other effort planetary scientists want to do for years to come. They'll learn far more from those efforts than from the one very expensive probe.

Would they "learn far more"? I am not a xenogeologist or astrophysicist, but landing a probe on a rogue planet seems like a shortcut to visiting another star system.

We have no idea what is the orbit so the distance doesn't matter there is no way to do a flyby unless we know for sure it's there and can map it's orbit nearly fully otherwise regardless of how fast we are going we'll never get to it :)

We actually have a pretty good estimate on the orbit[0], though not necessarily where it is in that orbit (afaik at least). Your point is correct- there's no where to go until we get a fix on it- but your description is technically incorrect.

[0]https://en.wikipedia.org/wiki/Planet_Nine

We don't even know for sure it exists we have very little information on the orbit what exactly is incorrect?