Planet X



  • Name used for the undiscovered planet of the solar system in the early decades of the 20th century, the search for which led to the discovery of Pluto. In recent decades the term has been applied to other putative planets of the outer solar system which might be needed to explain anomalous drifts in the movement of Uranus, although it seems that the explanation may instead lie in the use of imprecise figures for the masses of the planets. A wealth of interesting solar system bodies remains to be discovered but none is likely to be of significant mass.