On another plane
Unfortunately, planets move in mysterious ways, and none more annoyingly so than Mercury. Visible only at sunrise and sunset or during solar eclipses, Mercury's proximity to the Sun makes it difficult to observe.
And then there is the problem of its perihelion. That is the point on an elliptical orbit at which a planet is farthest from the Sun. And in the case of Mercury, the perihelion appears to advance with each circuit. How does the planet get ahead of itself? Is it just the gravitational tug it receives from the other known planets, in line with Newtonian mechanics?
According to the French mathematician Urbain LeVerrier, this was only part of the story, since 43 seconds of advance per century remained unaccounted for. In 1846, LeVerrier had calculated that a yet-to-be-discovered planet disturbed the orbit of Uranus. And sure enough, there was Neptune, just as he had predicted. Applying the same logic to the inner planets, he argued that between Mercury and the Sun there must be another planet, whose gravity caused the discrepancy. Like Neptune, he said, it would show up soon enough. He named it Vulcan, whereupon sightings began pouring in from astronomers everywhere.
With each new claim, LeVerrier refined his calculations and issued fresh statements as to the planet's precise whereabouts. And although it never did show up in the sky when he predicted, for more than half a century Vulcan showed up in all the textbooks.
In 1877, LeVerrier died, an unrepentant Vulcanist. But in 1915, Einstein blew the elusive planet clean out of the sky with his General Theory of Relativity, which neatly accounted for every one of Mercury's missing 43 seconds as a function of curved space.
Which is why, today, Vulcan exists only in the minds of Star Trek viewers, as the home of aliens with pointed ears.