Planet Nine is in the news again – a new paper in the Astrophysical Journal by Kenyon and Bradley (see http://phys.org/print381496454.html) that uses computer simulation to winkle out plausible scenarios. The presence of Planet Nine is yet to be determined but it does not stop people playing around on their computers. The most likely explanation, it seems, is that it formed a great distance away but was nudged by a 'passing star' and captured by the solar system, albeit on the outer side of the solar system (if it exists that is). This is faintly reminiscent of the Rupert Holms theory – a passing supernova segment (remnant) might do the same thing (although Holms favours the supernova remnant as the mysterious Planet Nine, or something like that) (I stand to be corrected). Holms theory is interesting in the light of the renewed focus on Planet Nine (which was out of favour for some years).
Kenyon and Bradley also say that Planet Nine's wide orbit would leave it prone to ejection from our solar system and therefore it must be a latecomer.