Vulcan is a small hypothetical planet that was proposed to exist in an orbit between Mercury and the Sun. Attempting to explain peculiarities of Mercury’s orbit, the 19th-century French mathematician Urbain Le Verrier hypothesized that they were the result of another planet, which he named “Vulcan”.

A number of reputable investigators became involved in the search for Vulcan, but no such planet was ever found, and the peculiarities in Mercury’s orbit have now been explained by Albert Einstein’s theory of general relativity. Searches of NASA’s two STEREO spacecraft data have failed to detect any vulcanoids between Mercury and the Sun that might 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. Other than Mercury, asteroid 2007 EB26 with a semi-major axis of 0.55 AU (82,000,000 km; 51,000,000 mi) has the smallest known semi-major axis of any known object orbiting the Sun.