Following the discovery of Neptune in 1846, there was much speculation that another planet might exist beyond its orbit.

Over a century ago, astronomers noticed a discrepancy in the gravitational field that exist beyond Pluto.  They suggested that the weird gravitational pull observed to be acting on distant bodies in the Solar system were being caused by another planetary body.

And the existence of Planet Nine was speculated.

The best-known theories predicted the existence of a distant planet that was influencing the orbits of both Uranus and Neptune.

After extensive calculations Percival Lowell (1855 – 1916) predicted the possible orbit and location of the hypothetical trans-Neptunian planet.

400 to 800 times as far from the Sun as Earth

Lowell began an extensive search for it in 1906.

He called it Planet X.