Every schoolchild is now taught that Mercury is the closest planet to the sun in our solar system. Yet for several decades in the 19th century, many of the world’s top scientists had good reason to believe that a planet dubbed “Vulcan” hovered somewhere inside its orbit. A renowned French mathematician first proposed the existence of the phantom planet in 1859, and it remained one of astronomy’s most wanted celestial objects until 1915, when Albert Einstein’s Theory of Relativity finally debunked it for good.

In 1859, the French scientist Urbain-Jean-Joseph Le Verrier went to work on one of the most puzzling problems in astronomy: Mercury’s orbit. For years, astronomers had noted that the solar system’s smallest planet seemed to follow a peculiar course as it circled the sun. In particular, its perihelion—the point where it passed closest to the sun—seemed to shift slightly with each orbit. According to Sir Isaac Newton’s law of gravitation, this discrepancy should’ve been easily explained by the presence of other celestial objects. Yet even after Le Verrier accounted for the gravitational pull of Venus, Earth, Mars, and Jupiter, his predictions of Mercury’s orbit were always slightly off. The planet simply never ended up where it was supposed to be.

After painstakingly checking and re-checking his calculations, Le Verrier offered a groundbreaking hypothesis: some other object, unknown and unseen, was exerting a gravitational pull on Mercury’s orbit. “A planet, or if one prefers a group of smaller planets circling in the vicinity of Mercury’s orbit, would be capable of producing the anomalous perturbation felt by the latter planet,” he wrote. Le Verrier speculated that the sun’s glare had prevented the object from being positively identified in the past. Still, he argued that it should be easily spotted under the proper conditions.

The scientific community welcomed Le Verrier’s theory, and for good reason—he had a proven track record of finding new planets. Thirteen years earlier, the Frenchman had made a similar prediction while trying to account for a gravitational waver in the orbit of the planet Uranus. When astronomers scanned the heavens using his figures, they had found the previously unknown planet of Neptune. The discovery had vaulted Le Verrier to international scientific stardom, winning him admittance to the French Legion of Honor and a post as the head of the Paris Observatory. One fellow mathematician had since described his intellect as “almost superhuman.”

Armed with a fresh prediction from the discoverer of Neptune, astronomers immediately began hunting for Le Verrier’s new planet. As it turned out, however, the breakthrough had already come several months earlier courtesy of an amateur named Edmond Modeste Lescarbault. A physician by trade, Lescarbault was also a passionate stargazer who had built his own makeshift observatory in the rural French town of Orgères-en-Beauce. While staring through his telescope on March 26, 1859, he had watched a small black dot—possibly a planet—drift across the face of the sun. The doctor had told no one of the discovery at the time, but after reading about Le Verrier’s hypothetical planet, he sent him a letter with a full report.

Upon receiving the letter, Le Verrier traveled to Orgères-en-Beauce to question Lescarbault and examine his equipment and notes. He left the meeting more convinced than ever—there was indeed a new ninth planet located even closer to the sun than Mercury. Le Verrier announced the discovery in early 1860. In the tradition of naming the planets for mythological deities, he called the planet “Vulcan” after the Roman god of blacksmithing and the forge.

The discovery of Vulcan was hailed as a great leap forward for science. Lescarbault was admitted to the Legion of Honor, and Le Verrier was once again lauded for his mathematical genius. There was just one problem: the new world was frustratingly difficult to spot. Scattered Vulcan sightings poured in from around the globe, but most were from amateur astronomers. Le Verrier still needed an independent confirmation from a respected professional. In the hope of putting the matter to rest, supporters calculated that the planet would be visible in late March and early April 1860. Astronomers readied their telescopes, yet when the appointed time came, Vulcan failed to show itself. Many soon began to wonder if it existed at all.

Over the next several years, Vulcan became the subject of an international planet-hunt. There were sightings throughout the 1860s, yet for every sky watcher who claimed to have seen it, there were just as many who tried and found nothing. The ranks of the skeptics only grew in 1871, when a team of English astronomers failed to locate Vulcan for the third year running. “It was as if the question of Vulcan had ridden a seesaw since 1859,” author Thomas Levenson writes in his book “The Hunt for Vulcan.” “Occasional sightings and seemingly consistent calculations would propel it up to the top of the ride; hard-nosed attempts to verify its existence sent it crashing back down.”

In 1876, Vulcan’s fortunes seemed to be on the rise. A professional astronomer reported that he watched the planet transit the sun from an outpost in China, and newspapers received a new flurry of amateur sightings. Enthusiasm was so high that the New York Times felt comfortable weighing in. “Vulcan exists, and its existence can no longer be denied or ignored,” read an article from that September. “The Earth must henceforth be ranked as the fourth planet from the sun, and the children in the public schools who have been taught to recite their planets after the old-fashioned order, must be required to commit Vulcan to memory and insert it in its proper place.”

Urbain-Jean-Joseph Le Verrier died in 1877, but the most eventful period in Vulcan’s life was still yet to come. Just a year later on July 29, 1878, a total solar eclipse took place over parts of Russia and North America. The event presented prime viewing conditions for Vulcan, and legions of astronomers set up their telescopes and cameras in the hope of capturing it. Most came up short, but two respected astronomers, James Craig Watson and Lewis Swift, both claimed to have spotted it. Newspapers once again started trumpeting Vulcan’s existence, but the triumph was short-lived. Critics produced evidence that the men had actually seen two well-known stars, and before long, the majority of the scientific community had dismissed their discovery as an error.

After Watson and Swift’s observations were debunked, scientific belief in Vulcan sank to an all time low. The planet became the astronomy equivalent of El Dorado—a myth written off by most but still passionately chased by a select few. Yet if Vulcan didn’t exist, scientists were still left with a nagging question: what was causing the shift in Mercury’s orbit?

The definitive answer to the riddle finally arrived in 1915, when Albert Einstein dropped the scientific bombshell that was his Theory of General Relativity. Unlike Newton’s theories of gravity, which could only explain Mercury’s orbit by positing the existence of an unknown planet like Vulcan, General Relativity argued that a supermassive object—in this case the sun—was capable of bending space and time and altering the path of light. Shortly before announcing his theory, Einstein applied it to Mercury and found that it perfectly accounted for the discrepancy in its orbit. Mercury wasn’t being pulled by some other object, he concluded, it was simply moving through distorted space-time.

As author Isaac Asimov later wrote, in the wake of Einstein’s breakthrough, “Vulcan was hurled from the astronomical sky forever.” Astronomers erased the planet from their charts, and its past sightings were explained as misidentified stars or sunspots. Vulcan has since been cited as one of the most famous blind alleys in scientific history, but its demise didn’t mark the end of the hunt for new worlds within the solar system. In 1930, the dwarf planet Pluto was discovered following a lengthy search. In recent years, meanwhile, scientists have found sufficient evidence that a hypothetical “Planet Nine” might loom somewhere on the outer edge of the solar system.