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Guess that very much depends on which angle, relative speed and path it comes in.

Based on my very limited astrophysics I'd guess it wouldn't have much of an impact and in most cases would just cruise through the system with a slight gravitational course correction arc and be on it's way again to interplanetary space.

Feel free to correct me.

This concept was considered as early as 1933 in the excellent and influential novel When Worlds Collide (https://en.wikipedia.org/wiki/When_Worlds_Collide), which I highly recommend as well as the sequel After Worlds Collide where they have all sorts of fun on Bronson Beta. Skip the movie, the books are of course more dated but much better.

The chances are incredibly small.

If there are 100,000 free floating planets per star, and it's still an incredibly small chance that one wanders into a solar system it gives you some slight sense of proportion.

I would have imagined that the chance is almost the same as another space object entering a solar system..