What is Planet - X

Several astronomers have sought dynamical or photographic evidence of a planet beyond Pluto , sometimes called planet-X. Clyde Tombaugh, the discoverer of Pluto , conducted a long search of the region beyond Pluto and ruled out any planet as large as Neptune near the plane of the solar system out to a distance of around 100 AU. However , the 1978 discovery of Pluto's satellite allowed a better mass determination than had been possible earlier for Pluto, and the mass turned out to be too small to account for all of the perturbation on the motions of Neptune.  This finding has prompted a number of astronomers to suspect that other objects may exist, perhaps well out of the plane of the solar system. Naval Observatory astronomer Thomas Van Flandern  (and others,1981) has proposed a search for an object with AZ much as three Earth masses in a highly inclined eccentric orbit. There is growing suspension that the outer solar system may harbor additional small bodies. Comets traverse this region and they may grade upward in size- if not to three Earth masses, then at least to other objects resembling Pluto.