The planet Neptune used to be a hypothetical planet—it was predicted to exist but had never been seen. In fact, many other hypothetical planets have been proposed. Some have been ruled out, but others might have actually existed in the past and may even exist now. Here is a list of the most 5 hypothetical planets proposed by scientists
Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the gas giants, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.
Fourteen years after Lowell died, an astronomer at his observatory found Pluto, but it still wasn’t heavy enough to account for the observed motion of Uranus, so people kept looking for Planet X.