Potential trans-neptunian Neptune.
Interesting. I heard about it on the radio earlier. They were calling it "Planet 9", thus sealing Pluto's non-planetary fate
1...)
As touched in the news, Pluto was supposed to be the thing that explained various orbital anomalies, originally, before being fortuitously found
despite not being the answer to the original question. (Although it seems like there would have been any
number of 'fortuitous Plutos', now, depending very much on which of a lot of similar objects had been found first due to the intensive searching.)
This maybe be a
different question, of course, from the one that rustled up Pluto's existence. But it sounds related to the whole "planetary migration" theory, in which Jupiter and Saturn formed elsewhere in the solar system. i.e. They couldn't have formed in the place they were, because that isn't where the 'ice line' orbit that would have seeded their initial creations would have been. And, if that's the case, the early-system tussle probably sent a fellow gas-giant out of the Sun's influence, now thought to be sent zooming across the galaxy...
Or maybe not quite, it seems the theory in this case is. Which could be just trying to shape an existing theory to add grist to their
own theoretical mill, but just because it's shoe-horned in doesn't mean it isn't worth considering. (First let's work out if there is (or was, at the time) enough 'retarding gas' out there to prevent the potentially rogue planet from
actually going rogue.)
1 Of course "Planet X" always had the connotations of "Planet 10 (in Roman Numerals)", for me... Rather than the intended "Planet 'Unknown'", just as X-Rays were coined that as "unknown rays", I'm fairly sure. But when Pluto was #9, the tenth planet (whether that be beyond Neptune/Pluto or the hidden Earthly twin that just happened to be on the far side of the Sun, often for reasons of plot in various Sci-Fi representations...) was handily also labelled as "Planet X".