Vulcan

Vulcan was a hypothetical world that was predicted to be the cause of the drift in Mercury's orbital procession. This theory was put forth by the same guy who predicted to within one degree, Neptune's position. Unfortunately, Such a planet was ether transparent, or never existed in the first place, as many different astronomers from professionals to amateurs never saw anything when several transit windows came and went with no trace of Vulcan. Today, Mercury's orbital procession is explained perfectly by Einstein's general theory of relativity, which explains that gravity is not a force, but rather gravity was a consequence of geometry. Mass bent space, and this bent space changed what a straight line was, in that space. An object moving along said line would appear to slow down, change directions and start falling. Thus: gravity.