The presence of the planet was postulated on the basis of the perturbation of the orbit of Uranus by Le Verrier and was observed for the first time only two years earlier.
In 1889, Le Verrier was succeeded by Amédée Mouchez who set to work to bring astrophysics into the mainstream by hiring Deslandres.
Noted Planet X sceptic Brian Marsden of Harvard University's Minor Planet Center pointed out that these discrepancies were a hundred times smaller than those noticed by Le Verrier, and could easily be due to observational error.
About that time, Le Verrier's work on the outer planets was revised and expanded by Gaillot.