In 1877 Le Verrier died, convinced to the end of having discovered another planet. With the loss of its principal proponent, however, the search for Vulcan abated. After many years of searching, astronomers were seriously doubting the planet's existence.
In 1915 Einstein's theory of relativity, an entirely different approach to understanding gravity from classical mechanics, solved the problem.[1] His equations predicted exactly the observed amount of advance of Mercury's perihelion without any recourse to the existence of a hypothetical Vulcan. The new theory modified the predicted orbits of all planets, but the magnitude of the differences from Newtonian theory diminishes rapidly as one gets farther from the Sun. Also, Mercury's fairly eccentric orbit makes it much easier to detect the perihelion shift than is the case for the nearly circular orbits of Venus and Earth.