Retrograde planets?

[Red Iron Crown]

I may be wrong, and I probably am, but isn't it all about just getting to planet's/moon's SOI, in prograde direction (it is possible, right? Just timing must be perfect), then waste a lot fuel to break from freakin fly-by velocity. Right?

That's one approach, and likely the simplest to actually perfom. Another would be to reverse the spacecraft's orbital direction before reaching it, possibly cheaply with gravity assists or bielliptically.

[genericeventhandler]

It could be something from interstellar space, or a very massive object from interstellar space could do funny things to the planets already there... but that would screw up all the other planets (in much the same way that Triton basically destroyed Neptune's original moon system)

The Kerbol system shows no signs of perturbation from a close encounter with another interstellar neighbor. I can't imagine how we could have a retrograde orbiting planet and still have the regular system of planetsthat we have.

Or it might have been the Hypothetical fifth giant planet that was ejected from our solar system. https://en.wikipedia.org/wiki/Hypothetical_fifth_giant_planet