Planet 'P'. Artwork of the hypothetical Planet 'P' (red) which is alleged to lie beyond the edge of the known solar system. Here it is comprised of swirling gas, having two moons and a thin ring of ice grains & dust around it. Planet 'P' was one of seven undiscovered planets whose existence was predicted by American astronomer William Henry Pickering during 1908-32. He made his predictions by studying perturbations in the orbits of known planets. By 1931 he calculated that Planet 'P' had 50 times the mass of Earth, orbited every 656 years and was 75 times more distant than Earth from the Sun. Extensive searches have failed to find such a planet, and it is unlikely to exist.