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.