Vulcan was a small planet proposed to exist in an orbit between Mercury and the Sun. In an attempt to explain peculiarities of Mercury's orbit, in the 19th-century French mathematician Urbain Jean Joseph Le Verrier hypothesized that they were the result of another planet, which he named Vulcan. No such planet was ever found, and Mercury's orbit has now been explained by Albert Einstein's theory of general relativity.
Argument for Vulcan's existence
In 1840, François Arago, the director of the Paris Observatory, suggested to the French mathematician Urbain Jean Joseph Le Verrier that h...