Pluto:
Pluto's existence was also proposed to account for deviations in the orbit of Uranus. Even after the influence of Neptune~had been accounted for, an American astronomer Percival Lowell, detected that the orbit of Uranus was still disturbed. Neptune's orbit too showed a similar disturbances. Lowell and Pickering did some calculations to predict the mass and radius of the orbit of Planet X which was supposed to cause these disturbances. In 1916, Pluto was discovered in about the right place in the sky. However, its mass turned out to be much smaller, about that of our Moon. Small, cold and dark, Pluto is about one-fifth of the size of the Earth. Its surface is coated with ,frozen methane. In 1938, a satellite of Pluto was discovered and named Charon. Not much is really known about Pluto. Pluto's orbit crosses that of Neptune's. No other planetary orbits cross in this way, and it is possible that Pluto is an escaped satellite of Neptune. Pluto's discovery had led astronomers to believe that it was the Planet X. But now calculations show that ,the mass of Pluto is too low to cause irregularities in. Uranus' orbit. Thus, the search for the elusive Planet X goes on.