Planet 'O'. Artwork of the hypothetical Planet 'O' which is alleged to lie beyond the edge of the known solar system. Here its surface is a mixture of nitrogen ice, methane ice and dust. The Sun is the bright star at right. Planet 'O' 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. In 1908 he calculated that Planet 'O' had twice the mass of Earth, orbited every 373.5 years and was 51.9 times more distant than Earth from the Sun. Extensive searches have failed to find such a planet, and it is unlikely to exist.

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