Newtonian mechanics predicted that there should be a planet inside the orbit of Mercury which they named Vulcan to explain Mercury’s anomalous precession of its perihelion, but it turns out that what was really going on was due to the curvature of spacetime predicted by General Relativity.

It would be pretty wild if Planet X didn’t exist at all either and we found out that General Relativity was wrong instead.

Why the long-lost planet Vulcan holds lessons for Planet Nine • 2016 Feb 14 • Elizabeth Shockman • PRI