Planet X

Planet X was Percival Lowell’s theory to explain apparent discrepancies in the orbits of Uranus and Neptune. Many astronomers and mathematicians had predicted trans-Neptunian planets based on the resonances of known planets. Lowell was motivated because he could see the signature of a distant hidden planet from orbital wobblings and irregularities, and he needed to promote something that was more scientifically credible than his derided belief in canals on Mars. Lowell calculated that Planet X was seven times more massive than Earth and orbited the sun forty-three times more distant.