Vulcan (hypothetical Planet)
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.
Read more about Vulcan (hypothetical Planet): Argument For Vulcan's Existence, The Search For Vulcan, Search Conclusion, Vulcan Revived