Believe it or not, astronomers were sure there actually was a planet called Vulcan somewhere between the planet Mercury and the sun. First proposed by French astronomer Urbain Jean Leverrier in 1854, it was hypothesized to explain a descrepancy Mercury’s orbit. Vulcan was even reported to have been observed once, but the observation was never confirmed. Einstein’s general theory of relativity explained Mercury’s odd orbit, and the existence of Vulcan was discredited. Sad, huh.