PLANET X

supposed planet of the solar system 10th in distance outward from the Sun, surmised on the basis of calculations of its effect on the orbits of Halley's Comet and of the planets Uranus, Neptune, and Pluto. At the time its existence was proposed (1972), Planet X was stated to be located in the direction of the constellation Cassiopeia. The planet was calculated to have a mass approximately three times that of Saturn and a period of revolution around the Sun of about 500 years; its orbit was believed to be inclined to the ecliptic (the plane of the Earth's orbit) at 120. Within a year after the proposal of Planet X, other astronomers demonstrated that a body with the suggested mass and orbit would have perturbed the orbits of the known outer planets by amounts wholly incompatible with the observed paths. In addition, they showed that if the density and the albedo (reflective power) of Planet X had plausible values, it should be visible by standard astronomical techniques. Careful examination of photographic records disclosed no evidence of Planet X. Moreover, the planets Uranus and Neptune show no detectable irregularities, or perturbations, in their orbits that can be attributed to the gravitational influence of an unknown body.