However, a closer inspection of the work shows that they predict a few things that haven’t been observed, including a population of Kuiper belt objects with large inclinations and retrograde orbits, long-period Kuiper belt objects with opposite ecliptic latitudes and longitudes, and infrared data showing the emission from such an outer world. There are many good reasons to be skeptical, and not conclude that there’s a ninth planet without more (and better) evidence.
Image credit: K. Batygin and M. E. Brown Astronom. J. 151, 22 (2016), with modifications/additions by E. Siegel.