Valcan,

the planet in our Solar System that would explain,

why Mercury's orbit didn't quite work, yesterday in 1859, a French scientist named Urbain leVerrier published a paper suggesting that the solar system had an additional planet that was closer to the sun than Mercury. Relying on previous naming conventions, he named it “Vulcan” after the Roman fire god–a naming that has resulted in an astronomical tradition of referring to the closest planet to the sun in a given solar system as a “Vulcan planet,” Le Verrier wasn't the first to suggest Vulcan might be there, as the 1846 image at the top of this article suggests, but he legitimised the idea of Vulcan by applying mathematical analysis to the question of why Mercury's orbit didn't quite work, the hypothetical Vulcan would resolve the question of  “peculiarities in Mercury's transit—it didn't move around the sun exactly in the manner predicted based on Newton’s laws,”

it wasn’t the only possible reason for the peculiarities advanced by the astronomer, according to the proceedings of the Glasgow Philosophical Society, Venus being heavier than previously thought might account for the change, the society wrote, but other factors made this highly unlikely, He also suggested that a series of “corpuscles” (asteroids) near the sun would account for the peculiarity–and he even he spent time looking for the asteroid belt, according to St. Andrews University, the search for the planet Vulcan persisted into the twentieth century, writes Simon Worrall for National Geographic, along the way, new astronomical techniques, such as astrophotography, were employed to confirm or deny its existence, according to PBS, but despite the fact that most reputable astronomers couldn’t find a planet or even an asteroid belt where Vulcan should be, the speculation persisted, “Mercury still wobbled, and in Newton’s cosmos, its motion still demanded something like a Vulcan,” PBS writes,

but then along came Albert Einstein, whose theory of relativity accounted for the non-Newtonian physics of Mercury’s transit, in fact, Mercury became a test case for his theory, writes Worrall, He did a calculation to see what his new theory would suggest Mercury’s orbit to be, and it predicted Mercury’s real orbit exactly, “His new theory correctly provides what astronomers call the table for Mercury, accurately describing how it moves around the sun,” author Tom Levenson told Worrall, Einstein later said this accurate prediction gave him heart palpitations, "He was so excited he couldn’t work for three days,” Levenson said, Oh dear, no work for Albert for three days, no home for Dr. Spock, so yesterday a nearly Happy Birthday for Vulcan!