For millennia, the planets visible to the naked eye—Mercury, Venus, Mars, Jupiter, and Saturn—filled out the solar system along with the Sun, the Moon, and Earth. But when the scientific revolution led to the discovery of the "invisible" planets Uranus and Neptune in the 19th century, scientists also began to believe that there was at least one more planet lurking around the Sun, and this one was even closer: the planet Vulcan believed to orbit somewhere between Mercury and the Sun.
The search for the planet Vulcan in the 19th century was one of the most stubborn scientific endeavors of the era, but it's one that's been largely forgotten in modern times. What led scientists to go looking for it? Why did they think it was there? And what made them finally give up the hunt?
How Isaac Newton revolutionized the astronomy of the unseen
Prior to Isaac Newton's discovery of the laws of gravitation, astronomers could only identify what they could see with the naked eye or through a telescope.
The existence of the visible planets (that is, those that are visible with the naked eye), has been known about for all of recorded history, and the invention of the telescope (usually credited to Hans Lippershey in 1608, but first pointed skyward by Galileo a year later) revealed even smaller objects like the moons of Jupiter and the largest asteroids in the asteroid belt, like Pallas and Ceres.
But Newton's work turned the solar system and the relationships between the celestial bodies within into mathematical interactions. This allowed astronomers to make predictions about planets, comets, and other objects, using well-defined formulas.
Even better for astronomers, because objects exerted gravity whether you knew they were there or not, instances, when the math didn't add up, became even more important than when it did. If Newton's laws of gravitation told you that you should expect two plus two to equal four, but instead the universe was giving you a five, those same laws meant that something you hadn't accounted for must be contributing a one.
When applied to gravity, if the orbit of a planet or moon is predictable thanks to Newton's laws, but the observations didn't match what you were expecting using those laws, then something else must be exerting additional gravity to the system.
This allowed astronomers to infer the existence of objects they could not see by measuring their observable effects of gravity on some other object. In this way, the eighth planet, Neptune, was discovered in 1846.
Discovery of the planet Neptune
After William Herschel discovered the planet Uranus in 1781 with the aid of a telescope, astronomers used Newton's laws of motion to map out the precise orbit of the new planet. Over the years, though, a discrepancy between the predicted orbit of Uranus and its actual, observed orbit began to emerge.
Either Newton's laws worked differently at great distances from the Sun (an idea that was rejected by nearly everyone at the time) or something was interfering with Uranus's orbit. And, given the immense size of Uranus, that something had to have been quite large.
This kicked off a mathematical hunt for a hidden eighth planet in the vicinity of Uranus. French astronomer Urbain Le Verrier is ultimately credited with working out the position of the then-unknown planet. Eventually, in September 1846, he mailed a letter with the position of the planet to Johann Galle at the Berlin Observatory and then made visual confirmation of planet Neptune using the observatory's refractory telescope.
While there is some controversy over Le Verrier's discovery, English astronomer John Couch Adams independently worked out a less-accurate-but-similar predicted position for the planet at the same time. However, he did not publish his findings until after Galle confirmed Le Verrier's work—even Adams acknowledged Le Verrier's priority in the discovery of Neptune.
This recognition undoubtedly gave Le Verrier's word considerable weight when, in 1859, he used the same mathematical technique to try to explain a similar perturbation in the orbit of Mercury. He proposed that a tiny planet, Vulcan, maybe orbiting close enough to the Sun to have been hidden from view by the Sun's glare, but large enough to disturb Mercury's orbit.
The wobble of Mercury and the search for the planet Vulcan
"Vulcan is remarkable because the idea of this little body inside the orbit of Mercury makes perfect sense," Tom Levenson, professor of science writing at the Massachusetts Institute of Technology, told National Geographic in 2015.
"If you believe Isaac Newton’s theory of gravity, which everyone does at that time, the discovery of a slight wobble in the middle of Mercury’s orbit that can’t be explained by the tug of Venus or Earth has only one interpretation: there has to be an undiscovered planet or flock of asteroids that we can’t see because it’s too close to the sun but must be exerting some gravitational influence on Mercury."
This new theory for the wobble of Mercury's orbit touched off a period of "Vulcan-mania" in the latter half of the 19th century as both professional and amateur astronomers went hunting for the proposed planet and some even claimed to have seen it for themselves.
The first of these "sightings" came from an amateur French astronomer named Edmond Modeste Lescarbault in 1859. Working from his makeshift observatory in a barn in his backyard, Lescarbault trained his telescope on the Sun and saw what looked like a tiny round planet transiting the Sun.
Recording its progress, Lescarbault eventually sent his data to Le Verrier, by then the director of the Paris Observatory, after reading an article by Le Verrier about the problem of Mercury's orbit.
"Le Verrier is at a New Year’s Eve party when he gets the letter and treks out to Lescarbault’s house," Levenson explains, "which involves a train ride and then a 12-mile walk, to interrogate him.
"Le Verrier becomes convinced that Lescarbault really did see what he claims to have seen and that the proper interpretation is that this is a transit of a planet. It’s not clear who first named it, but it quickly became known as Vulcan."
The combination of Le Verrier's reputation and various popular accounts of "sightings" of the non-existent planet convinced just about everyone that not only was Vulcan a real planet but that it had also been confirmed by observation.
“There was a belief in many quarters that there was a planet that was between Mercury and the Sun, but which was usually lost in the sun's glare,” said Daniel Kennefick, a professor of physics at the University of Arkansas and the author of No Shadow of a Doubt: The 1919 Eclipse That Confirmed Einstein's Theory of Relativity.
“Some people had thought they had seen Vulcan during total solar eclipses and it was very common to look for Vulcan at those events because the sun's glare was eliminated so you would have a much better chance of seeing any possible planet.”
Given the era, there was little reason to doubt the discovery since it fits within the prevailing view of the universe provided by Newton's laws of gravitation.
"Mercury’s wobble has to be caused by some source of gravitational energy," Levenson said. "There was no other way to think about it. Facts on their own don’t mean anything unless you have a framework to put them in. And the framework was Newton’s laws."
Facts about Vulcan (even though it doesn't exist)
For a planet that didn't exist, people were committed to some rather concrete ideas about Vulcan in the 19th century.
For his part, it should be noted that Le Verrier never claimed definitively that a planet was disturbing the orbit of Mercury. He actually thought that an asteroid belt or even several smaller planets were as likely, if not more so.
The amount of mass needed to create Mercury's wobble would be nearly equal to Mercury itself, and so it didn't seem likely that astronomers had simply missed another Mercury-sized planet orbiting the Sun. That said, it couldn't be written off either, since its orbit would have been entirely within Mercury's. A planet interior to this constrained orbit would easily be lost against the blinding light of the Sun.
And once Le Verrier saw Lescarbault's data about the supposed transit of Vulcan in 1859, he was convinced enough of the singular planet theory that he announced the "discovery" of Vulcan in 1860, based on his own calculations and the observations made by Lescarbault as confirmation.
Lescarbault's observation and the records he kept of it provided important data for Le Verrier to try to determine Vulcan's orbit, distance from the Sun, and other characteristics.
From that data, Le Verrier calculated a roughly circular orbit for the planet. He put Vulcan's distance from the Sun at about 13 million miles. Mercury has the most eccentric orbit of any of the solar system's planets, but at its closest approach to the Sun at perihelion, it is about 28.5 million miles. This would put Vulcan just under half the distance from the Sun as Mercury's closest approach.
Le Verrier calculated an orbital period of about 19 days and 18 hours, with an orbital inclination of about 12 degrees and 10 minutes relative to the ecliptic. And, according to Le Verrier, Vulcan's furthest elongation was roughly eight degrees. This would not be far enough from the Sun to escape its glare, even at twilight, so the only hope of seeing it would have been during an eclipse or during a transit of the Sun. Given the rapid orbital period, Le Verrier figured that there would be two to four transits of Vulcan a year.
Le Verrier made several stabs at predicting a transit of Vulcan before his death in 1877, but none came to pass. As others claimed to have seen a transit of Vulcan between 1860 and 1877, Le Verrier continued to refine his calculations of Vulcan's orbit, hoping to predict a transit that would conclusively prove that the planet did exist.
Beyond Le Verrier's calculations, there wasn't much else anyone could say with any certitude about a planet that, after all, did not actually exist. It would have been hot and rocky, though. While no one at the time even knew how hot Mercury really was at the time, Vulcan was aptly named in any case. Had it existed, it would have been substantially hotter than Mercury owing to being more than twice as close to the Sun as Mercury. This might have been why some observers who claimed to have seen Vulcan during an eclipse claimed it had a reddish tint.
Still, the fact that no one had been able to reliably identify the intermercurial planet, even with some of the most advanced observation equipment of the era, strongly indicated that if such a planet existed, it could not have been as massive as Mercury.
"In our opinion," researchers at the Lick Observatory at Mount Hamilton, California, wrote in 1909, "the work of the three Crocker expeditions, to observe the eclipses of 1901, 1905, and 1908, brings the observational side of the intermercurial planet problem...definitely to a close. It is not contended that no planets will be found in the intermercurial region...but it is confidently believed that their mass would be inadequate to account for the observed disturbances in the motion of Mercury."
With so many opportunities to observe transits proving fruitless or inconclusive at best, and a lack of definitive identification during a multitude of solar eclipses over the years, by 1879, many started to doubt Vulcan's existence, but it would be nearly 40 years before the existence of Vulcan was finally rejected.
Albert Einstein puts Vulcan to rest
As it turned out, the problem was with Newton's laws of motion and gravitation all along.
While Newton's laws were and still are remarkable for their ability to predict gravitational effects, they aren't the last word when it comes to gravity. And when Albert Einstein began developing his theory of general relativity, the elusive planet Vulcan was very much at the front of his mind.
"[For Einstein,] Vulcan had long since drifted to the far penumbra of possibility," Levenson wrote in The Hunt for Vulcan. "But now, Albert Einstein, constructing a cosmos on a foundation of relativity, was taking dead aim at the undiscovered planet. From the beginning of his investigation of gravity, Einstein grasped the crucial either/or of Vulcan's existence or absence."
For more than half a century, the existence of Vulcan was disputed, but it was at least seriously debated. Einstein, therefore, came to understand that Mercury was a crucial proving ground for developing his theory of general relativity. If relativity could explain the wobble of Mercury's orbit without needing to rely on Vulcan, it wouldn't just solve a great mystery in astronomy and physics. It would show that relativity superseded Newton and his laws of motion and gravitation entirely in describing the universe.
The key insight to Einstein's general relativity was conceiving of gravity as more than a force between two bodies, as Newton believed, but instead as a consequence of mass "sinking" into the "fabric" of something called spacetime.
"The core of General Relativity is that space and time are not static, but dynamic and can change," Levenson said. "The way they change is by the presence and motion of matter and energy. A vast mass like the Sun creates curves in spacetime, which means that things don’t go straight. A ray of light passing close to the Sun will travel a curving path."
When it came to Mercury and its orbit, that same curve in a dynamic spacetime exactly accounted for the wobble of the planet's orbit on its own, without requiring another planet between Mercury and the Sun. In honing his theory of general relativity on the problem of Mercury's wonky perihelion, Einstein obliterated the need for a planet Vulcan overnight.
"Disproving Vulcan’s existence was central to Einstein," Levenson said, "because it showed that this radical, bizarre new picture of his, that spacetime is flowing, was actually the right way to see the universe."
"Mercury wobbles because that’s the shortest path it can take through the curved spacetime created by that huge dent imposed by the mass of the Sun. Without curved spacetime, you need some other mass to pull on it. With curved spacetime, Mercury behaves exactly how Einstein’s theory says it should."
In the hundred-plus years since general relativity was published, the planet Vulcan faded from the public consciousness (Star Trek excepted, of course), but its history has resurfaced in recent years as science historians and journalists reflect on the tiny, fiery planet that almost was and its importance in the development of general relativity. More than anything though, it proves to be an important cautionary tale for scientists everywhere.
"Vulcan teaches us how hard it is to understand what nature is telling us, how hard it is to understand when nature says no," Levenson said. "People kept discovering Vulcan because the way they saw the world required Vulcan to be there," he added. "It took Albert Einstein to provide the framework in which Vulcan became not only non-existent but unnecessary."