Vulcan (hypothetical planet)

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 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. Other than Mercury, asteroid 2007 EB26, whose orbit has 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.

In 1840, François Arago, the director of the Paris Observatory, suggested to the French mathematician Urbain Le Verrier that he work on the topic of the planet Mercury's orbital motion around the Sun. The goal of this study was to construct a model based on Sir Isaac Newton's laws of motion and gravitation. By 1843, Le Verrier published his provisional theory on the subject, which would be tested during a transit of Mercury across the face of the Sun in 1843. As it turned out, predictions from Le Verrier's theory failed to match the observations.

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