Vulcan

Vulcan /ˈvʌlkən/ is a small hypothetical planet that was proposed to exist in an orbit between Mercury and the Sun. The 19th-century Frenchmathematician Urbain Le Verrier hypothesized that peculiarities in Mercury's orbit were the result of another planet, which he named "Vulcan". A number of searches were made for Vulcan, but despite occasional claimed observations, no such planet was ever confirmed. Peculiarities in Mercury's orbit are now explained by Einstein's theory of general relativity. Searches of data gathered by NASA's two STEREO spacecraft have failed to find any vulcanoids that could have accounted for claimed observations of Vulcan. It is doubtful that there are any vulcanoids larger than 5.7 kilometres (3.5 mi) in diameter. There are a number of Mercury-crossing asteroids, but all have a semi-major axis larger than Mercury's.