Vulcan was a proposed planet in the Solar System, closer to the Sun than Mercury. It was proposed by the fluctuations in the timing of Mercury transits that was later solved by Einstein’s theory of general relativity. Vulcan was proposed by Urbain Le Verrier, a French mathematician. Later, NASA’s STEREO spacecraft searched for an asteroid type called “Vulcanoids,” or asteroids closer to the Sun than Mercury, which might have accounted for proposed sightings of Vulcan. None were discovered, however.