[PeterWhittaker]

Interestingly enough, the new IAU definition of "planet" makes no reference to origin:

A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

Whether a black hole could be considered a planet would hinge, I believe, primarily on b: Arguably, any black hole has sufficient mass to overcome rigid body forces, but does the notion of hydrostatic equilibrium apply?

More broadly, the definition of planet would allow for bodies of extra-solar origin captured by the sun....