What got me thinking about deep space wanderers was the creationist thread about a planet with a retrograde orbit. In my opinion there are only two physical likelihoods.
the planet was hit by a very large object that started it to orbiting the opposite direction of the star or it came from deep space from a direction opposite the star’s rotation and was captured. Retrograde orbits aren’t all that unusual and there is at least one moon in our solar system that orbits in a retrograde orbit around its planet.
Who knows, frozen planets between the galaxies may be the norm.
If there’s a planet in retrograde orbit, it is generally considered diagnostic of capture. If a group of moons (for example) are all moving in retrograde (sort of the case with the moons of Uranus, nice example eh?) it probably means the planet rolled over at some point, perhaps due to a large impact.