A new paper makes a fascinating argument for the existence of a Planet 9 that is largely independent of prior arguments in favor of it, based upon solar activity cycles. The underlying theory it relies upon is controversial and is one of several that has been proposed to explain observed solar activity cycles.
But, if Planet 9 turns out to be where it is supposed to be under this theory with the predicted parameters, this theory of what is driving solar activity is boosted tremendously.
Planet 9 is currently a hypothetical planet the orbital parameters of which are based on anomalous orbits of Kuiper Belt objects.
The orbital parameters are such that, if Planet 9 exists, the theory of solar barycentric dynamics would be profoundly altered. We show that, with Planet 9 included in the solar system, barycentric theory is a much more effective predictor of solar activity on the decadal, centennial and millennium time scales. In particular the most elementary quantity of barycentric theory, the Sun to planet barycentre distance, is more coherent with decadal solar activity cycles and grand solar activity minima than barycentric distance without Planet 9 included. Further, barycentric theory including Planet 9 contains strong components at periods corresponding to the Hallstaatt and Gleissberg cycles whereas barycentric theory without Planet 9 shows no evidence of these cycles.
A challenge that emerged during this study was the absence of the strongest component in barycentric theory, the Jose component at ~ 178 year period, from the spectra of millennium scale solar activity. This conundrum was solved by demonstrating that, during the transformation from solar motion to solar activity, the Jose component in solar motion was, in the spectrum of solar activity, split into multiple sidebands due to phase modulation by lower frequency cycles.
The excellent fit to solar activity at multiple time scales by barycentric theory with Planet 9 included is itself supporting evidence for the existence of Planet 9, specifically by providing an estimate of the current heliographic longitudinal location and orbital period.
Ian R. Edmonds, "The Sun to planetary center of mass distance is coherent with solar activity on the decade, centennial and millennium time scales when Planet 9 is included in the solar system" arXiv:2201.06745 (January 18, 2022).