The amusing part about the story is the two scientists' announcement of the discovery, along with the declaration that they're now going to start looking for some evidence to prove their theory. You'd think it'd be the other way around, right? But then the search for PLANET X has been going on for a long time, whether within the orbit of Mercury ('Vulcan'), or on the other side of the sun (called 'Mondath' in DR WHO; I forget what the real-world scientists who proposed the Earth's dark twin called it). Mostly, though, they look beyond the furthest known planet to see what might be out there on the edges of the solar system, which turns out to be stranger and more interesting all the time.
Historically, it's a search with mixed results. Uranus they found more or less by accident, as (somewhat closer to home) was the discovery of Ceres. Neptune was the great success story: predicted by a French mathematician and discovered (after a search lasting, it is said, only a little over an hour) by a German astronomer using his data (although the waters got muddied a bit by an English claim demanding co-credit on dubious grounds).* The search for Pluto went on for decades, and inspired Lovecraft (who'd been a keen amateur astronomer in his youth) to create the cold, dark, sinister world of Yuggoth, to which he prescribed chaotic properties quite unlike any real-world astronomical phenomenon.
Or, as Bellairs' King Gorm puts it when describing recent events in his all-too-interactive magical planetarium:
"We've been having some trouble with Sector 8," he said . . . "A couple of planets are doing a horn-pipe, and before long --apocalypse! I think we must blame the terrible black planet Yuggoth, which rolls aimlessly in the stupefying darkness. Ooop! Watch out!"
[They] hit the floor as a five-pronged comet . . . came whooshing down at them . . ."
--Jn Bellairs, THE FACE IN THE FROST , p. 50
Perhaps they shd name the mythical Tenth** Planet YUGGOTH -- it's only fitting for a mythical planet to have a fictional name.
*cf. "The Case of the Pilfered Plane" by Sheehan, Kollerstrom, & Waff in SCIENTIFIC AMERICAN (Dec 2004), wh. produces a strong case for deliberate fraud on the part of the English claim -- the English had all the pieces but didn't put them together until after the fact, when they were assembled to create a credible paper trail for their own claim.
**or Ninth, if you don't count Pluto, which I do.