Planet 9 is somewhat different to Planet X as it is projected, or was invented, in order to explain anomalies in the orbits of objects beyond Neptune. At https://astrobites.org/2020/04/18/an-alternative-to-planet-9-maybe-there… … an alternative to Planet 9. Maybe there is nothing big out there at all and it is an invented problem. I suspect most people that read the original paper promoting the idea of Planet 9 felt the same way – but detractors of the idea have gone back to the drawing board (their computers) to find a way out – by smoothing the data. A sort of hockey stick tactic in order to elimate some unwelcome facts.
Maybe there is nothing out there but the study has the title, 'Testing the isotropy of the Dark Energy Survey's Extreme Trans Neptunian Objects' by Bernardinelli et al of the University of Pennsylvania. Data from NASAs search for dark energy mission sparked this study which has discovered some more objects in the outer region of the solar system – which means, in their opinion, there must be lots more. Lots lots more. A figure of 40 million is mentioned. I may have got hold of the wrong end of the stick, as far as numbers are concerned, as we are then told modelling has taken place – based on the assumption there is a helluva lot more objects waiting to be discovered by human cameras. Hence, they begin with a simulated population of Trans Neptune objects in order to advance an isotropic study base – or conclusion, to be fair. Then, they say that what has been seen is just a few objects that appear to be anomalous, invoking the idea of Planet 9. When all the simulated objects buzzing around are taken into account the anomaly disappears – and they are left with their desired outcome, an isotropic solar system. Have I missed something.