I had some discussions with a retired French engineer some years ago who had theorised that planets form as spindles, and it was quite interesting to see where that took us. he also considered it possible that a massive Planet X body could arise in this manner,
rather than accreting from a rotating blob of dust and gas. The thing is, the currently accepted theory of planetary formation doesn't preclude the formation of planets way out there, insofar as a solar system could arise from an amorphous nebula with two
or more points of concentration. This would lead to a wide orbit binary system, and if one of the 'stars' just happens to be tiny, then you get the kind of planet we're discussing.
In Darker Stars, I discuss this in some depth, alongside formation of tiny stars/solitary massive planets from globules of dust and gas (and plenty of electrified plasma, too, quite likely). My thinking being that a distant Planet X body would still be enshrouded
in such a dark nebula, never having attained enough stellar power to drive it away. Hence Sitchin's description of Marduk with all its lightning etc. The 'shroud' would partially conceal Planet X because it wouldn't present to astronomers in the clearly
defined way a planet should, but more like a cool, hazy cloud. Easy to misidentify.