No. 19: Jan-Feb 1982
Neptune is an undisciplined member of the solar system. No one has been able to predict its future course accurately. Already this maverick planet is drifting off the orbit predicted just 10 years ago using the best data and solar-system models. All of the outer planets, in fact, confound predictions to some degree. In addition, some long-period comets have anomalous orbits. Astronomers have been aware that something was wrong for decades and anticipated finding a trans-Neptunian planet large enough to perturb the outer solar system. The discovery of Pluto did not help matters; it is much too small. The most popular explanation of the orbital anomalies relies on a large, still-undetected planet, possibly 3-5 times the mass of the earth, swinging sround the sun at some 80-100 Astronomical Units. But many have searched and no one has found anything. Planet-X, as it is often called, is just another bit of "missing mass." Thomas C. Van Flandern and Robert Harrington propose that all the obvious orbital damage in the outer solar system is the result of a single encounter between Neptune and another body, call it Planet X if you wish, that was passing through the outer reaches of the solar system.
(Frazier, Kendrick; "A Planet beyond Pluto," Mosaic, 12:27, September/October 1981.)
Comment. It is rather ironical that the proposed encounter with an invading planet so closely parallels some longcondemns scenarios of astronomical catastrophism.
Reference. Astronomers have searched for Planet X for many years. See Chapter AX in our Catalog: The Sun and Solar System Debris. This book is described at: here.