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". Later the theoretical object got attention to explain why is there no mass between the orbit of Mercury and the Sun, but it was not found. Later, the scientists came to understanding, it may be too close to the sun is too small that it can not be observed, or it could of existed at some point in the past and fallen into Sol due to gravitational instabilities.
Small proposed objects that orbit within the orbit of Mercury, or near to the orbit of Vulcan are known as Vulcanoids.
