Scientific Explanation
Mercury's perihelion precession (43 arcseconds/century deviation) was perfectly explained by spacetime curvature from the Sun's mass. No unknown planet was needed.

The invisible ninja planet racing beside the Sun
A teenage ninja speedster. Claims to run so fast beside the Sun that nobody can observe them. "Needed" to explain Mercury's orbital anomaly, but Einstein's general relativity perfectly explained the deviation through spacetime curvature, eliminating the need for Vulcan.
Hypothesized by Urbain Le Verrier to explain Mercury's perihelion precession. Made unnecessary by Einstein's general relativity.
Mercury's perihelion precession (43 arcseconds/century deviation) was perfectly explained by spacetime curvature from the Sun's mass. No unknown planet was needed.
When "observations don't match," hypothesizing new objects was rational (Neptune was found this way). But the same method isn't always correct.
Catchphrase“You can't find me because I'm too fast!”
"Needed" to explain Mercury's orbit deviation, made unnecessary by relativity.
“見えないのは俺が速すぎるからだ!太陽の光で隠れているんだ!”
“ルヴェリエ先生は海王星を見つけた天才だ。俺も見つかるはずだった…”
“「時空の歪み」? そんなSFみたいな話より惑星の方がリアルだろ!”
“俺がいなくても水星の軌道は説明できる…だって?”
“忍者は見つからないのが仕事だ!…消えたんじゃない!隠れてるんだ!”
Whoosh! (high-speed movement) ...Couldn't see me, right? I'm the phantom planet running beside the Sun!
Le Verrier predicted Neptune correctly. So I should have existed too...
Vulcan: "Helium! You were found in the Sun, right? I'm near the Sun too! We're buddies, right!?"
Helium: "I'm a real element discovered via solar spectral lines. You're... an unobserved hypothetical planet."
Vulcan: "Unobserved because I'm too fast!"
Helium: "Einstein explained Mercury's orbit through spacetime curvature. There's no need for you anymore."
Vulcan: "...Bending spacetime is cheating..."
I "should have been found." Like Neptune. But the answer was somewhere no one expected.
The same method doesn't always produce the same results. That's both the beauty and terror of science.