1860: French mathematician Urbain Le Verrier announces the discovery of Vulcan, a planet orbiting between Mercury and the sun, to members of the Académie des Sciences in Paris.

Le Verrier, who used Vulcan to explain an anomaly in Mercury's orbit, already enjoyed a stellar reputation among astronomers, having discovered Neptune in 1846 using only mathematic principles to detect its presence.

Turns out Le Verrier was a bit hasty this time, not to mention gullible, basing his claim on some pretty dubious observations by one Edmond Modeste Lescarbault, a provincial physician and amateur astronomer working from a homemade observatory. Le Verrier interviewed Lescarbault at length, though, and was convinced that the good doctor knew what he was talking about.

Doubts about this "new" planet surfaced immediately and the professionals set to work attempting to either confirm or debunk Vulcan's existence. Although numerous reports of "transits" by heavenly bodies passing in front of the sun were received, no reliable observation of Vulcan was ever made.

(Le Verrier also theorized the existence of a second asteroid belt in the solar system. He got that one wrong, too.)

Le Verrier steadfastly maintained Vulcan's existence to his dying day in 1877. The hubbub pretty much died with him and the idea was put to rest for good with the publication of Albert Einstein's theory of relativity in 1915, which explained Mercury's eccentric orbit as a byproduct of the sun's gravitational pull rather than the presence of a nearby planet.

(Source: Various)