A hypothetical planet in the solar system that was thought to orbit closer to the Sun than Mercury. In the early 1800s, there were many reports of black objects passing across the face of the Sun, presumed to be the profiles of small planets. Early 19th-century observations were sufficiently positive to encourage leading French astronomer Urbain-Jean-Joseph Le Verrier to investigate the phenomenon. Le Verrier had attempted to explain the high and unique eccentricity of Mercury’s orbit and concluded that it could be caused by the gravitational attraction of a belt of asteroids between Mercury and the Sun; a small planet was another possible explanation. He looked at over 50 observations, dismissing all but six of them as inadequate. The remaining six observations, made over the 60 years to 1861, could be accounted for in several ways, of which one, a small planet, seemed to fit best. He calculated the orbit of the postulated planet, which he named Vulcan, as having a period of 33 days and as being at a relatively large angle to the plane of Earth’s orbit, the ecliptic. He could now predict the new planet’s behavior and did so: Vulcan would next transit the face of the Sun on March 22, 1877. It did not, nor could Vulcan be seen during solar eclipses. There has been no confirmed sighting of Vulcan since Le Verrier’s announcement.
If a planet did exist in the orbit calculated by Le Verrier or anywhere near, it would be roughly at half Mercury’s distance from the Sun. The temperature on the sunny side of Mercury is about 350°C (about 76°L), above the melting point of lead. The side away from the Sun is near absolute zero. Neither side could support life. The conditions on a planet in the Vulcan position would be much worse. So there is no Vulcan; even if there were, there could be no aliens living on it.