Astronomers once believed that a planet called Vulcan existed between the planet Mercury and the sun.
Its existence, first proposed by French astronomer Urbain Jean Joseph Leverrier in 1845, was hypothesized to explain a discrepancy in Mercury’s orbit. Vulcan was even reported to have been observed once, but the observation was never confirmed.
Einstein’s general theory of relativity later explained Mercury’s odd orbit, and the existence of Vulcan was discredited.