Galileo was the first to see Neptune, making a note of it in 1613 — but because it appeared nearly stationary in the sky, he didn’t recognize it as a planet. It would be another 233 years before Johann Gottleib Galle rediscovered Neptune, directed by calculations made by Urbain Le Verrier, who had observed irregularities in Uranus’ orbit that could best be accounted for by the existence of another, undiscovered body beyond that planet’s orbit. Galle’s discovery of Neptune in 1846 — less than a degree from the position suggested by Le Verrier’s calculations — would have a profound influence on the search for Planet X some decades later.
The great strangeness of the discovery of Pluto is this: Percival Lowell’s calculations of the irregularities in Uranus’ orbit suggested that a roughly Neptune-sized planet remained to be discovered at the edge of the solar system, beyond Neptune itself (a “Trans-Neptunian Object”). And in 1930, Pluto was indeed discovered very close to the position Lowell predicted for it.
But Pluto is far too small to have introduced any significant wobble into any object’s orbit. So why did Lowell’s math send him right to Pluto’s position?
As it turns out, Pluto’s location was nothing but an amazing, impossible coincidence. Still, it’s hard not to imagine some kind of mystical explanation, as though Lowell had managed to divine some truth about the Solar System simply by sitting in the high desert air for decades.
And as it happens, there aren’t any significant wobbles in Uranus’ orbit; the observations Lowell was using were minutely faulty.
Well, it is a mystery.