Thanks for your question to us about Planet X…we appreciate it!
I had to research this subject before I could discuss it with you, and I found this information at Wikipedia on the subject which states: “[Percival] Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the gas giants, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.” So, we have this theory that there is another planet on the outskirts of our solar system…but it is as yet only a theory.
The Urantia Book actually teaches us that there are twelve planets in our solar system, instead of the now nine planets that we have discovered. Could planet X be one of the twelve? There is no mention of this, but in this section about our solar system, which is called Monmatia, there is discussion of the gas giants. Maybe by careful reading of this section, one could posit the existence of this outer planet. I did not have the time to do this careful reading today, but maybe you’ll want to, in order to satisfy your curiosity.
Thanks so much for this question. Not everything that is true about the cosmos and about our solar system is contained in the teachings of The Urantia Book. Sometimes, we have to just keep abreast of current discoveries in science and astronomy, and possibly we can corroborate some of these findings. In any event, it is a fascinating subject, and I hope that this reply has been helpful to you.