A new model that is designed to fit existing observations that don't match our expectations based on the old model, without it correctly predicting new unexpected observations, seems like a rather weak proposition to me.
In all seriousness, I think the bar is higher than just “here is an explanation that makes sense”. The new model has to explain stuff better than existing established models.
Are you referring to what he published in 2016? Because it's much older than that, we used to call it Planet X.
"Planet X" refers specifically to a theory proposed by Percival Lowell in 1906 about the orbit of Uranus which was disproven in 1993 by Voyager 2 flyby data.
They are both more specific theories than just "a trans-Neptunian planet we haven't discovered" which is overly broad.
On "Planet X" specifically, the effect it was offered to explain was determined to have been measurement error after recalculation of Neptune's mass and consequently effect on Uranus's orbit in 1993 based on Voyager 2 data.
> In 1894, with the help of William Pickering, Percival Lowell (a wealthy Bostonian) founded the Lowell Observatory in Flagstaff, Arizona. In 1906, convinced he could resolve the conundrum of Uranus's orbit, he began an extensive project to search for a trans-Neptunian planet,[18] which he named Planet X, a name previously used by Gabriel Dallet.[11] The X in the name represents an unknown and is pronounced as the letter, as opposed to the Roman numeral for 10 (at the time, Planet X would have been the ninth planet).
https://en.wikipedia.org/wiki/Planets_beyond_Neptune#Planet_...
- "no dark matter" in dense ellipticals and lenticular
- external field effect (including keplerian descent for the milky way)
- early galaxies
This description roughly applies to General Relativity; there are clearly merits to both approaches.