Long before the kerfuffle over Pluto, astronomers–and even Einstein–went on a decades-long hunt for a planet in our solar system that wasn’t even there at all.
On this day in 1859, a French scientist named Urbain le Verrier 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,” writes Marissa Fessenden for Smithsonian.com. Unlike the Vulcan class planets that have been observed with the twenty-first century’s improved telescopes, however, the original Vulcan wasn’t real.
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 legitimized 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,” Fessenden writes.
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.
But the somewhat unlikely hypothesis of a hitherto-unobserved planet was given credence when an amateur astronomer named Edmond Lescarbault sent Le Verrier a letter just months after his paper was presented saying that he had observed Vulcan. This letter, and his subsequent meeting with Lascarbault, prompted Le Verrier to go looking for Vulcan.
“At first things went well,” writes J. Donald Fernie for American Scientist. “In Le Verrier’s expert hands Lescarbault’s observations yielded seemingly reasonable, if somewhat uncertain, parameters for Vulcan’s orbit. Applause was widespread.” Some astronomers thought he was incorrect, but Le Verrier spent the rest of his life defending his position, writes Fernie. To be fair, he did predict the orbit of Neptune, so he wasn’t always wrong.
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.