What Ever Happened to Planet Vulcan?

The story of the planet closest to our sun, aptly named Vulcan, which accidentally existed for half a century.

Thomas Leveson | The Hunt for Vulcan: … And How Albert Einstein Destroyed a Planet, Discovered Relativity, and Deciphered the Universe | Random House | November 2015 | 27 minutes (7,305 words)


The excerpt below is adapted from The Hunt for Vulcan, by Thomas Leveson. In light of recent theorizing about a mysterious new Planet X, this story is recommended by Longreads contributing editor Dana Snitzky.

* * *

Not everyone follows a straight course to the person they might become.

In the 1830s (and still) number 63 Quai d’Orsay turned an attractive face toward the river. In the guidebooks already being read by that novel nineteenth-century species, the tourist, number 63 is described as a “handsome house”—one, the writers warned, that concealed a much more plebeian reality. Visitors—by appointment only, no more than two at a time, welcome only on Thursdays—would be ushered into a courtyard, and then on to the rooms where workers, mostly women, took bales of raw tobacco through every stage needed to produce the finished stuff of habit: hand-rolled cigars, spun strands of chew that became “the solace of the Havre marin,” gentlemen’s snuff. Most of the campus was turned over to laborers serving the machines—choppers, oscillating funnels, snuff mills, rollers, sifters, cutters, and more. By the latter half of the nineteenth century, the works at the Quai d’Orsay would turn out more than 5,600 tons of finished tobacco per year, and was, according to the ubiquitous Baedeker, “worthy of a visit”—though indulging one’s curiosity carried a price: “the pungent smell of the tobacco saturates the clothes and is not easily got rid of.”

A spectacle, certainly, and as an early palace of industry clearly worthy of the guidebooks (themselves novelties). By any stretch of the imagination, though, the Manufacture des Tabacs was an odd place to look for someone who would become the most celebrated mathematical astronomer of his day—but not everyone follows a straight course to the person they might become. Thus it was that in 1833 a young man, freshly minted as a graduate of the celebrated École polytechnique, could be found every working day at the Quai d’Orsay, reporting for duty at the research arm of the factory, France’s École des Tabacs.

No one ever doubted that Urbain-Jean-Joseph Le Verrier had potential: he had been a star student in secondary school, winner of second prize in a national mathematics competition, eighth in his class at the polytechnique. But his early career offered no hints to what would follow. Funneled into the tobacco engineering section in university, he was more or less shunted directly toward the Quai d’Orsay and the task of solving French big tobacco’s problems.

It’s not clear whether Le Verrier actively enjoyed the life of a tobacco engineer—or merely tolerated it. Nothing in his later career remotely suggests he was a born chemist. But he was consistent: if given a task, he got down to it. Never mind all that early training in abstract mathematics; if required, he could be as practical as the next man, and so turned himself into a student of the combustion of phosphorus. That was useful research—tobacco monopolists care about matches. But whether or not he relished his job, he certainly got out as soon as he could. A position back at the École polytechnique opened up in 1836 for a répétiteur— assistant—to the professor of chemistry. Le Verrier applied, and as an until-then almost uniformly successful prodigy, had every hope. . . until the post went to someone else.

Le Verrier would prove to be a man who catalogued slights, tallied enemies, and held his grudges close. But he never accepted a check as a measure of his true worth. A second assistantship became available, this time in astronomy. He applied for that too. Never mind his seven years among the tobacco plants; Le Verrier seems to have believed that he could simply ramp up his math chops to the standard required at the highest level of French quantitative science. As he wrote to his father, “I must not only accept but seek out opportunities to extend my knowledge. [. . . ] I have already ascended many ranks, why should I not continue to rise further?” Thus it was that Le Verrier came into orbit around the great body of work left by that giant of French astronomy, Pierre-Simon Laplace.

* * *

If the planets were a family, Mercury would be the sneaky little sibling.

Laplace had gone to his grave in 1827 convinced that he had solved the core of his great problem. To a pretty good approximation, he was correct. He had shown that the solar system as a whole could be rendered intelligible, its motions accounted for by Newtonian gravitation as expressed within mathematical models—“theories” of the planets. Properly employed, those models could describe the motions of the physical system explicitly, accurately, and indefinitely into the future. If there was some work left to do, new methods to be explored, more observations to be considered, discoveries within the system (like the newly discovered “minor planets”—asteroids—and comets), the basic picture seemed sound.

There were, though, more anomalies than the edifice of Celestial Mechanics acknowledged. Some of the theories of the planets were proving a bit less settled than Laplace had believed, and some, like Mercury’s, were obviously inadequate, unable to predict the planet’s behavior with remotely acceptable precision. Despite such problems (or possibilities), no researcher had yet returned to the whole of Laplace’s program. Several astronomers in France and elsewhere worked on individual questions in planetary dynamics, but none were trying to resolve the system as a whole, to go from a theory of any given planet to one of the solar system, top to bottom.

Enter Le Verrier, of whom one of his colleagues would later say, “Laplace’s inheritance was unclaimed; and he boldly took possession of it.” Over his first two years at the polytechnique, Le Verrier surveyed the whole field of solar system dynamics, beginning to suspect that seemingly minor gravitational interactions might matter more than his predecessors had believed—that over time they produce effects that would be noticeable. He seized the opportunity, setting himself as his first major project the goal of recalculating at higher mathematical resolution the motions of the four inner planets—Mercury, Venus, Earth, and Mars. It took him just two years, a phenomenal pace given that he had started from zero as a mathematical astronomer.

Le Verrier presented his results to the French Academy of Sciences in 1839. He came to one striking conclusion: when you take one more term into consideration than prior calculations had attempted, it becomes impossible to say for certain whether or not the orbit of the inner planets would remain stable over the very long haul. Neither he nor anyone else knew how to find a complete solution to the equations that could confirm whether Mercury, Venus, Mars—and Earth—would remain forever on their present tracks.

Crucially, Le Verrier was already showing himself willing to tangle with the acknowledged master of celestial mechanics. Laplace had concluded from his studies of Jupiter and Saturn that the stability of the solar system was proved; here was a young man just two years into the field suggesting otherwise.

It was a fine first effort—good enough to garner attention from the men who could advance his career past an assistantship. At the same time, it was, as Le Verrier knew, still preliminary work, nothing more than recasting an old calculation. But it managed to hook him on celestial mechanics as a life project—and for his next major task, he set himself a problem that no prior researcher had been able to solve: Mercury.


Mercury. Via Wikimedia Commons.

If the planets were a family, Mercury would be the sneaky little sibling: it might be up to something, but it was so good at slipping past any attempt to pin it down it was hard to be sure. But that was no longer quite as true, as Le Verrier’s gift for finding a ripe problem showed itself. Over the preceding decade, advances in instruments and technique made it possible to follow Mercury with a previously unattainable accuracy. He gave credit where it was due: “In recent times, from 1836 to 1842,” Le Verrier reported to the Académie, “two hundred useable observations of Mercury have been carried out” at the Paris Observatory. With these and other records, he was able to construct a better picture of the way Venus influenced Mercury’s orbit as the two planets moved from one configuration to another. That, in turn, led him to a new estimate of Mercury’s mass, with his answer falling within a few percentage points of the modern value.

These were satisfying outcomes—filling in some of the more elusive details of one corner of the solar system. But Le Verrier really wanted a complete account of Mercury, a system of equations encompassing the full range of gravitational tugs that affect its orbit, which can be used to identify planetary positions past and future. Observations constrain such models: any solution to a model’s equations has to at least reproduce what observers already know about a planet’s orbit. More data meant more constraint, and hence a more accurate set of predictions about where the planet would go next. Those predictions, the “table” of the planet, are the test of any planetary theory.

The final exam for Le Verrier’s first version of such a theory for Mercury came in 1845, its next scheduled transit of the sun, best viewed from the United States. Transits are ideal reality checks for such work: mid-nineteenth-century chronometers were accurate enough to note the instant Mercury’s disk would cross the edge of the sun. On May 8, 1845, astronomers in Cincinnati, Ohio, watched as the clock ticked off to the moment Le Verrier had predicted for the start of the event. The astronomer at the eyepiece of the telescope trained on the sun saw “the dark break which the black body of the planet made on the bright disk of the sun.” He called out “Now!” and checked his timepiece. Against Le Verrier’s prediction, Mercury was sixteen seconds late.

This was an impressive result—better by far than any previous published table for Mercury, back to the one prepared by Edmond Halley himself. But it wasn’t good enough. That sixteen-second error, small as it seemed, still meant that Le Verrier had Mercury in transit across the face of the sun in 2006 missed something that kept the real Mercury out of sync with his abstract, theoretical planet. Le Verrier had planned to publish his calculation following the transit. Instead, he pulled the manuscript and let the problem lie for a time. Mercury would have to wait quite a while, as it turned out, for almost immediately he found himself conscripted into a confrontation with what was fast becoming the biggest embarrassment within the allegedly settled “System of the World.”

* * *

Cannonballs flew on courses perfectly described. . . by the exquisite logic of the Principia.

Uranus was the troublemaker, and had been for decades. After Herschel’s serendipitous discovery of the “new” planet, astronomers swiftly realized that others had seen it before, thinking it a star. In 1690, John Flamsteed, the first Astronomer Royal and Newton’s sometime-collaborator, sometime-antagonist, placed it on one of his sky maps as the star 34 Tauri. Dozens of other missed-chance observations turned up in observers’ records, until in 1821, one of Laplace’s students at the Bureau, Alexis Bouvard, combined those historical sightings with the systematic searches that had followed Herschel’s news to create a new table for Uranus, one supposed to confirm that it obeyed the same Newtonian laws that governed its planetary kin.

He failed. When he attempted to construct a theory of Uranus that could generate by calculation the positions observers had recorded since Herschel’s night of discovery, he couldn’t make the numbers work. Anything he tried that agreed with observations made since 1781 didn’t line up with the rediscovered positions that had been misidentified as stars before that date. Even worse, when he focused only on the more recent, post-Herschel record, it quickly became clear that the planet was again wandering off course—or rather that reality and calculation diverged.

In the abstract, such uncooperative behavior might point to a very deep problem: if all the gravitational influences on Uranus had been accounted for, the failure to predict its motion would demand a reexamination of the theory behind such analysis. That is: it could threaten the foundations of Newton’s laws themselves. One researcher, the German astronomer Friedrich Wilhelm Bessel, suggested exactly that, wondering if perhaps, just maybe, Newton’s gravitational constant itself might vary with distance.

Such thoughts were thinkable, but horrifying. There was the worshipful awe the man himself inspired, of course, but more to the point, Newton’s physics worked. The tides obeyed its rules; comets were brought to order under its provisions; cannonballs flew on courses perfectly described and explained by the exquisite logic of the Principia. Better, by far, would be any explanation that captured this seeming anomaly within a Newtonian framework.


Representation of the fall of an object from different heights, 1547. Via Wikimedia Commons.

It seems that, privately, Alexis Bouvard was the first to come up with a way to do so. In 1845, his nephew, Eugène Bouvard, reported to the Académie his own, unsuccessful attempt to bring Uranus’s track to mathematical order. Following his uncle, he tried to resolve modern (post-Herschel) observations with older ones. He failed, and admitted as much. But still, he told his learned audience there was a way out, one his uncle had glimpsed two decades earlier. It was not the one Laplace had used to resolve the Jupiter-Saturn mystery. That involved improving the mathematical technique with which he attempted to describe the world out there. Rather, the older Bouvard reasoned, if all the known behaviors of the solar system could not account for the last residue of error—and crucially, if you maintained your faith in Newton—then the only remaining possibility was that something unknown would resolve the matter. Bouvard reminded his readers that if they imagined Uranus had remained undiscovered, then meticulous attention to Saturn would reveal the influence of some more distant unseen celestial body. In exactly the same manner, he wrote, it seemed “entirely plausible [to him] the idea suggested by my uncle that another planet was perturbing Uranus.”

The Bouvards weren’t the only ones to make that leap. By the early 1830s, several researchers began to think about the possibility of an object yet farther from the sun than Uranus. The older Bouvard had shared his notion with correspondents and visitors, one of whom carried the idea across the channel to England. One obvious difficulty kept this widening circle from doing very much with the idea, though. Uranus was too damn slow. Its eighty-eight-(Earth-)year-long period meant that systematic observations since Herschel had followed roughly half of a single journey around the sun. The Astronomer Royal George Biddell Airy conceded the plausibility of the idea of a trans-Uranian planet but quashed the hopes of one inquirer, writing that the mystery would resist solution “till the nature of the irregularity was well determined from several successive revolutions”—which is to say, only in that long run when all those then concerned would be dead.

* * *

If anyone happened to have time to spare on a good telescope, they should find a planet. . . .

Le Verrier disagreed. Or rather, his sometime-mentor, François Arago, the director of the Paris Observatory, thought that Herschel’s planet had embarrassed astronomers long enough. Late in the summer of 1845, Arago pulled the younger man away from a brief dalliance with comets and, as Le Verrier recalled, told him that the growing errors within the theory of Uranus “imposed a duty on every astronomer to contribute, to the utmost of his powers.” Le Verrier began by identifying several errors in the older Bouvard’s sums. Those mistakes did not eliminate the unexplained wobbles in Uranus’s orbit, so Le Verrier instead recalculated the planet’s tables to define those anomalies as precisely as possible. With the intellectual ground thus cleared, he turned into a detective, seeking the as-yet-unidentified perpetrator that could have led Uranus astray.

As a good police procedural would have it, he soldiered on, examining—and eliminating—as many suspects as he could. Historian of astronomy Morton Grosser tallied Le Verrier’s potential culprits: perhaps there was something about the space out by Uranus, some resisting stuff (an ether) that affected its motion. Was there a giant moon orbiting Uranus, tugging it off course? Might some stray object, a comet, perhaps, have collided with Uranus, literally knocking it from its appointed round? Le Verrier even paused on the fraught possibility that Newton’s law of gravitation might need modification. Last: was there some as-yet-undiscovered object, another planet, whose gravitational influence could account for the discrepancies between Uranus’s theoretically predicted and the observed track?

Le Verrier quickly rejected the first three potential candidates. He agreed with virtually every professional astronomer in thinking that modifying or rejecting Newtonian gravitation would be a final, desperate resort. Which meant that after several months of thinking about the problem, he was back to his prime suspect: an as-yet-undiscovered trans-Uranian planet.

With that, his task was sharply defined: once all the known sources of gravitational influence were accounted for, what were the properties—mass, distance, finer details of its orbit—of the object that could account for the remaining anomalies in the motion of Uranus? In that form, the problem resolved down to a conventional problem in celestial mechanics, establishing and then solving a system of equations that described each of the components of the hypothetical planet’s motion. Even so, given how little could be asserted with any confidence about the still hypothetical planet, then familiar or not, the task was deeply fraught.

Le Verrier first set up his calculation with thirteen unknowns—too many for someone with even his gifts to solve in any timely manner. So he simplified his assumptions. He argued that there had to be a sweet spot for at least some of the orbital parameters of the missing planet. As he would later write, it couldn’t be too close to Uranus, for then its effects would have been too obvious. It couldn’t be terribly far away, as that would imply a large enough mass to affect Saturn as well, and no such influence had been detected. He simply guessed that its orbit wouldn’t be too sharply angled to the plane of the rest of the planets. He constructed a few more such arguments to fill in some of the gaps in the observational data from Uranus, which left him with a system of equations with just nine unknowns—which is to say, merely a hugely difficult operation, instead of an impossible one.


Uranus. Via Wikimedia Commons.

Calculating a unique solution within that model—one that would give him a prediction of the mass and position for the planet—proved almost ludicrously laborious. Being clever helped, as when he came up with a way to transform some of the essentially intractable nonlinear equations in the model into a larger set of linear expressions. That made the calculation easier—possible, really—but at the cost of a horrific amount of grunt work to crank through the much greater number of steps the new approach required.

Even so, by the end of May 1846, Le Verrier had advanced to the point where he could report to the Académie that Uranus’s orbit could be exactly described assuming “the action of a new planet”—and that it would be possible to show that “the problem is susceptible to only one solution . . . there are not two regions in the sky in which one can choose to place the planet in a given epoch”—which was his rather grandiose way of saying he was nearing his answer. Near, but not yet all the way there: in this communiqué, he could do no better than suggest that the hypothesized trans-Uranian planet should lie in a region measuring about ten degrees across the sky.

Even that rather loose guidance was subject to a fair amount of uncertainty, too much to help anyone interested take a look. So Le Verrier returned to the mathematical grind, reworked his calculation, and on August 31, 1846, delivered an update: if anyone happened to have time to spare on a good telescope, they should find a planet beyond the orbit of Uranus at a distance of about 36 astronomical units, visible about five degrees east of ∂ Capricorn—a fairly bright star within the Capricorn constellation. Its mass, Le Verrier declared, would be about thirty-six times that of Earth, and to the telescope-aided eye, it would reveal itself not as a point (like a star), but as a clearly discernible disk, 3.3 arcseconds in diameter.

* * *

That star is not on the map!

September 23, 1846, Berlin.

The night is quiet, very dark. Gaslights had come to Prussia’s capital back in 1825, but there still weren’t that many of them, and most were doused by midnight. After that Berlin belonged to those who cherished the night sky—among them, the watchers at the Royal Observatory, near the Halle Gate.

This Saturday, Galle and a volunteer assistant, Heinrich Ludwig d’Arrest, command the main telescope. Galle stands at the eyepiece and guides the instrument, pointing toward Capricorn. As each star comes into view, he calls out its brightness and position. D’Arrest pores over a sky map, ticking off each candidate as it reveals itself as a familiar object. So it goes until, sometime between midnight and 1 a.m., Galle reels out the The “new” Royal Observatory in Berlin, depicted sometime after 1835. numbers for one more mote of light invisible to the naked eye: right ascension 21 h, 53 min, 25.84 seconds.

D’Arrest glances down at the chart, then yelps: “that star is not on the map!”

The younger man runs to fetch the observatory’s director, who earlier that day had only reluctantly given his permission to attempt what he seems to have thought a fool’s errand. Together, the trio continue to watch the new object until it sets at around 2:30 in the morning. True stars remain mere points in even the most powerful telescopes. This does not, showing instead an unmistakable disk, a full 3.2 arcseconds across—just as Le Verrier had told them to expect. That visible circle can mean just one thing: Galle has just become the first man to see what he knows to be a previously undiscovered planet, one that would come to be called Neptune, just about exactly where Urbain-Jean-Joseph Le Verrier told him to look.

* * *

The bright, beautiful truth that Le Verrier had said go and look—there!—and you will see. . . and someone searched . . . and everyone saw.

Galle’s sighting was the climax of what was almost immediately understood to be the popular triumph of Newtonian science. It’s unsurprising, given the stakes, that the discovery of Neptune produced its share of controversy. The English astronomer John Couch Adams had followed the same reasoning as Le Verrier, performed commensurate feats of calculation, and had come to a very similar prediction at almost the same moment. However, he failed to persuade any of the astronomers at either Cambridge or the Royal Observatory at Greenwich to pursue a rigorous search. Still, a nationalistic priority battle followed, with British scientists pressing the case for Adams to receive co-discoverer credit with Le Verrier. That view held sway for more than a century, at least in the English-speaking world, though current historical analysis reserves pride of place for the Frenchman. A claim of discovery requires both the prediction and the actual measurement made on the basis of that prediction—and by that yardstick Le Verrier got there first.


Neptune. Via Wikimedia Commons.

Still, the fact that the priority battle was so important to the British astronomical establishment tells its own tale. The discovery of Neptune—driven by the mathematical interpretation of fundamental laws, so exactly as to reveal itself within hours of the start of the search—was recognized at once as both a stunning display of individual genius and a triumph for a whole way of knowing the world. In point of fact, Le Verrier (and Adams) had made several arbitrary choices to simplify parts of the problem, most notably guessing how far away the perturbing planet had to be. Those estimates were off by a lot, which would seem to undercut any claim of prescience for their theoretical calculations. And yet even such a miscue actually conveys something of the power of Le Verrier’s reasoning, as he also figured out a way to frame the question to reduce the importance of distance for determining Neptune’s position. It wasn’t luck (or not much) that delivered the new planet; it was the skill with which a very gifted Newtonian scientist had set up a calculation to tolerate a fair amount of wrongness in his assumptions. And in any event, for both the public and the world of professional astronomers, such slips simply disappeared in the glow of the bright, beautiful truth that Le Verrier had said go and look—there!—and you will see. . .and someone searched . . .and everyone saw.

That sequence transformed the discovery of Neptune from being merely spectacular (like Herschel’s stumbling upon Uranus) into something more, a celebration of science as a whole. Le Verrier had confronted an uncomfortable fact, and then subjected it to theory, the theory, Newton’s system of the world, to risk a prediction that then proved true. If ever there was a demonstration of how science is supposed to advance, here it was.

* * *

Something unknown out there in space.

It wasn’t until 1859, sixteen years after his first attempt, that Le Verrier found himself free to return to the problem of Mercury. He was forty-eight years old, at the height of both his fame and, by all witness testimony, his mathematical powers. He had the resources of the Paris Observatory at his disposal. Mercury’s theory should have been a straightforward task.

It was. . . and it wasn’t. The older Le Verrier had one absolute advantage over his younger self: better data. He reexamined the information he had used in 1843—measurements of Mercury’s motion made at the Observatory itself. To that he added the best observations it was possible to make at the current state of astronomical technology: transits, with high-quality records for Mercury extending back to 1697. With a good clock and an accurate fix on where on earth the event was being viewed, timing a planet’s entry or exit from a transit ranked among the most precise measurements available to astronomers.

Le Verrier launched his assault following his usual plan. First he mapped out Mercury’s actual orbit with all of its components of motion as described by the empirical data: direct measurements of Mercury’s behavior. Next came calculation: what do Newton’s laws predict for Mercury, given all the known gravitational contributions of the planets as well as the sun? Any discrepancies—astronomers call them “residuals”—between the empirical picture and the theoretical one must then be explained. If there were none, then the theory of the planet was complete, and the model of the solar system would be one step closer to being done.

But there was a leftover result. It was a small number—tiny, really—but the gap between theory and the data was greater than estimates of observational errors could explain, which meant the problem was real. That settled one matter: it strongly suggested that Mercury’s difficulties almost certainly lay not with flaws in Le Verrier’s analysis, but rather in something unknown out there in space.

The particular anomaly he found is called the precession of the perihelion of Mercury’s orbit. In the squashed circle of an elliptical orbit, the point at which a planet comes closest to its star is called its perihelion. In an idealized two-body system, that orbit is stable and the perihelion remains fixed, always coming at the same point in the annual cycle. Once you add more planets, though, that constancy evaporates. In such a system, if you were to map each year’s track onto a single sheet of paper, you would over time draw a kind of flower petal, with each oval just slightly shifted. The perihelion (and its opposite number, the aphelion, or most distant point in the orbit) would move around the sun. When that shift comes in the direction that the planet moves in its annual journey, the perihelion is said to advance. As every schoolchild confronting geometry knows, a circular (or elliptical) orbit covers 360 degrees. Each degree can be divided up into sixty minutes of arc; each minute into sixty arcseconds. Le Verrier’s analysis told him that this was happening to Mercury: its perihelion advances at a rate of 565 arcseconds every hundred years.


Le Verrier pointing to Neptune, in front of the Observatoire de Paris. Via Wikimedia Commons.

Next came a round of celestial bookkeeping: how much of that total could be explained by the influence of the other planets on Mercury. Venus, as Mercury’s neighbor, proved to be doing most of the work. Le Verrier’s sums revealed that it accounted for almost exactly half of the precession, 280.6 seconds of arc per century. Jupiter provided another 152.6 to the total, Earth 83.6, with the rest causing scraps of motion. The total: 526.7 arcseconds per century.

A century and a half later, the one irreducibly extraordinary fact of this work remains how incredibly small an “error” Le Verrier uncovered. The unexplained residue of Mercury’s orbital dance came down to a perihelion that landed just .38 seconds of arc ahead of where it should every year. To put it into the form in which Le Verrier’s number became famous: every hundred years, during which Mercury travels a radial journey of 36,000 degrees, the perihelion of its orbit shifts about 1/10,000th beyond its appointed destination, an error of just 38 arcseconds per century.

Tiny, yes. But the excess perihelion advance of Mercury retained one crucial property: it wasn’t zero. Le Verrier knew what such unreconciled motion must mean. If Mercury moved where no known mass existed to push it, then there was some “imperfection of our knowledge” waiting to be repaired.

* * *

Venus’s husband, the lord of the forge.

Le Verrier was hardly infallible, to be sure, but there were some errors he simply did not commit. Mercury’s orbit does precess around the sun. It does so at a rate that cannot be fully accounted for by any combination of gravitational influences within the solar system. Le Verrier’s number for the residual motion of Mercury—38 arcseconds per century—is a little off the modern value of 43 arcseconds, but he got it as nearly right as anyone could in 1859, given the limitations of the data at his disposal. Le Verrier never doubted the work. Nor did his fellow astronomers. For them, it was in fact fantastic news: the unexplained invites discoveries.

Of all men, Le Verrier knew what came next: in his booklength report on Mercury, he said as much: “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. . . . According to this hypothesis, the mass sought should exist inside the orbit of Mercury.’”

Celestial facts need labels. The common practice held: planets major and minor took their identities from the gods of antiquity. It’s an oddity of history that there is no record of who first fixed on the ultimate choice, but the decision was easy. A body that never escaped the intense fires of the sun had only one real counterpart on Olympus: Venus’s husband, the lord of the forge. By no later than February 1860, the solar system’s newest planet knew its name:


* * *

Everyone with a telescope was looking for Vulcan; some found it.

Matters soon grew more complicated, though. Reports of sightings arrived, some from reputable observers, others from unknowns. In 1865, an otherwise completely obscure M. Coumbary wrote to Le Verrier with a detailed account of an observation he made in the city that he—an unreconstructed Byzantine, apparently—referred to as Constantinople. With his telescope in Istanbul he watched as a black spot separated itself from a group of sunspots and appeared to move independently. He continued to track the object for forty-eight minutes, until it vanished over the limb of the sun. Le Verrier endorsed Coumbary’s report, noting that though he didn’t know his correspondent, his information seemed to him to be marked by a combination of “exactitude and sincerity.” In 1869, a group of four eclipse mavens at St. Paul’s Junction, Iowa (one a lady, as contemporary records took pains to mention), saw “with the naked eye what they termed a little brilliant at a distance about equal to the Moon’s diameter from the Sun’s limb”—an object that at least two others (one equipped with a small telescope) seem to have noted as well.

To those for whom the logical necessity of Vulcan was overwhelming, this spray of messages was comforting, not proof in and of itself, but an ongoing accumulation of information building on an already established pattern. The lack of a pure Neptune moment must have been frustrating, but given the inherent difficulty of the problem, such momentary glimpses gained significance each time another letter from some sincere and precise stranger reached Paris. As The New York Times put it, “a little scrap of positive evidence overbears an immense amount of negative.” But despite a growing heap of such hopeful wisps, Vulcan remained almost maliciously elusive when confronted by a systematic search.

A way out was obvious to the more mathematically sophisticated Vulcan hunters. People simply could have gotten their sums wrong. There were enough imprecise assumptions about the elements of a putative Vulcan’s orbit so that calculations for transits could just be wrong. Princeton’s Stephen Alexander told his fellow members of the National Academy of Sciences that he had reworked Vulcan’s elements to arrive at the conclusion that there should be “a planet or group of planets at a distance of about twenty-one million miles from the sun, and with a period of 34 days and 16 hours.” In other words: we may have been looking in the wrong places, or at the wrong times. Vulcan could be elusive, but not absent.


Vulcan’s Forge, Giorgio Vasari, 1567-8. Via Wikimedia Commons.

That claim seemed to be confirmed when Heinrich Weber— for once, an actual well-trained professional astronomer—sent word from northeast China that he had seen a dark circular shape transit the sun on April 4, 1876. Sunspot expert and Vulcan devotee Rupert Wolf passed word of his colleague’s sighting on to Paris.

The news enthralled Le Verrier—and energized yet another corps of planet seekers more eager than expert. As historian Robert Fontenrose put it, “everyone with a telescope was looking for Vulcan; some found it.” For a time, Scientific American eagerly trumpeted each new “discovery”: from “B. B.” in New Jersey to a Samuel Wilde in Maryland, to W. G. Wright in San Bernardino, to witnesses from beyond the grave, in the form of a minister who remembered that Professor Joseph S. Hubbard “had repeatedly assured him he had seen Vulcan with the Yale College Telescope.” New Vulcans kept turning up that autumn in seemingly every mail delivery, until at last Scientific American cried “Uncle!” and, following its December 16, 1876, issue, declined to publish any more such happy memories. It was as if the question of Vulcan had ridden a seesaw since 1859. 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. Now, for all that the editors of Scientific American had tired of the flood of anecdotes, the teeter-totter was pointing up: between the one seemingly authoritative report from China and the sheer number, if not the quality of sky-gazer accounts, the matter of Vulcan seemed just about settled.

The popular press certainly thought so. In late 1876, The Manufacturer and Builder said, “Our text books on astronomy will have to be revised again, as there is no longer any doubt about the existence of a planet between Mercury and the sun.” That autumn, The New York Times was even less bashful, interrupting its coverage of the Hayes-Tilden presidential election to assert that any residual doubts about the intra-Mercurian planet could be put down to simple professional jealousy: “‘Vulcan may possibly exist,’ said the conservative astronomers, ‘but Professor So and So never saw it. . .’”—pure us-against-them nastiness, according to the Times, adding “they would hint, with sneering astronomic smiles, that too much tea sometimes plays strange pranks with the imagination.”

Now, such too-smart fellows were about to receive their due, the newspaper proclaimed. Why? Because, in the wake of Weber’s report, the grand old man himself, Urbain-Jean-Joseph Le Verrier, had roused himself. “The man who untied Neptune with his nose—so to speak—cannot be accused of confounding accidental flies with actual planets. When he firmly asserts that he has not only discovered Vulcan, but has calculated its elements, and arranged a transit especially for its exhibition to routing astronomers. . .” the Times wrote, “there is an end of all discussion. Vulcan exists. . .”

The Times got at least one thing right. After shifting his attention to other problems for a few years, Le Verrier had indeed returned to the contemplation of Vulcan. Wolf’s news had fired his passion for the planet, and he began a comprehensive reexamination of everything that might bear upon its existence. Starting with yet another catalogue of claimed sightings dating back to 1820, he identified five observations spread from 1802 to 1862 that seemed to him most likely to represent repeat glimpses of a single planet. That allowed him to construct a new theory for the planet, complete with the prediction the Times had rated so high: a transit that could perhaps be observed, Le Verrier suggested, on October 2nd or 3rd.

The headline writers would be disappointed. Vulcan did not cross the face of the sun in early October. More confounding, Weber’s revelation from China was debunked: two photographs made at the Greenwich Observatory clearly revealed his “Vulcan” to be just another sunspot. Scientific American called this the “coup de grace” for this latest “discovery,” but, as usual in the annals of Vulcan, its real impact was more deflating than destructive. Le Verrier’s calculation turned on earlier observations, not Weber’s, and there was a way to explain away the missed transit, by positing an orbit for Vulcan that was much more steeply inclined than previously assumed. Thus Le Verrier hedged his bets: there might be a chance to see Vulcan against the face of the sun in the spring of 1877, but given the full range of possible orbits this insufferably errant planet might occupy, it might be five years or more before the next transit would occur.

* * *

The physical sciences’ crazy uncle in the attic.

No transits occurred that March. Le Verrier said nothing more in public about Vulcan. He had turned sixty-six on March 11, and he was tired to the bone. As the year advanced, he found he couldn’t drag himself to the weekly meetings of the Académie, nor to his daily post at the Observatory. Time off seemed to help—he returned to his desk in August—but fatigue masked his real trouble: liver cancer.

On the evidence, Le Verrier was not a religious man. He did accept communion in late June on the urging of a much more committed Catholic colleague, but that seems to have been the limit of his willingness to acknowledge conventional pieties. By summer’s end, he could no longer mistake his illness. The end came on September 23rd—forty-one years to the day since young Johann Gottfried Galle had sought and found Neptune in the night sky above Berlin.


Le Verrier’s tomb. Via Wikimedia Commons.

Le Verrier left the solar system larger than he found it—one both better and less completely understood. Of Vulcan itself, though—surely, given all the fully satisfactory explanations for the behavior of every other astronomical object derived from the Newtonian synthesis, the fault, it seemed so nearly certain, must lie not in the stars, but in some human failure to crack this one particular mystery.

Vulcan itself dwindled into a mostly forgotten embarrassment, the physical sciences’ crazy uncle in the attic. There it sat (or rather, didn’t), hooting in the rafters. No one seemed to hear. Mercury’s perihelion still moved. The gap between fact and explanation remained.

That would change—but only after a young man in Switzerland started to think about something else entirely, nothing to do with any confrontation between a planet and an idea. There was a question he’d begun to ask. One way we now reframe his problem is to ask how fast gravity travels from here to there, from the sun, say, to Earth. But that’s not the way it struck him on an autumn afternoon in 1907 as he stared out his window on the top floor of the patent office in Bern.

* * *

Space by itself and time by itself are doomed.

There is an idea—utterly strange at the time—shot through the fabric of special relativity. In the century since it was first revealed, it has woven itself through the warp of popular culture as much as it flows through formal cosmology. When he first encountered it, though, Albert Einstein was unimpressed. “Now that the mathematicians have seized on relativity theory,” he declared, “I no longer understand it myself.” The offending mathematician? Einstein’s former teacher, Hermann Minkowski. The offending idea? In Minkowski’s own words:

“Gentleman, the concepts of space and time which I wish to present to you have sprung from an experimental physical soil and therein lies their strength. They are radical. Henceforth space by itself and time by itself are doomed to fall away into mere shadows, and only a kind of union between the two will preserve an independent reality.”

We now call that union “space-time.” The old notion that space occupies three dimensions—our familiar height, width, and depth—and time ticks on regardless, Minkowski argued, could no longer hold, not if you take seriously the discovery that one’s state of motion affects measurements of both. His response: to propose a world that exists in four dimensions, three of space, one of time, all intertwined with each other.

* * *

No undiscovered planet, no asteroid belt, no dust, no bulging solar belly, nothing at all. . . .

November 18, 1915.

Masking his emotions behind the required decorousness of scientific communication, Einstein revealed almost no sign of any excitement in his presentation to the Prussian Academy. “The calculation for the planet Mercury yields,” he told his audience, “a perihelion advance of 43 arc minutes per century, while the astronomers assign 45″ +/- 5″ per century as the unexplained difference between observations and the Newtonian theory.” Belaboring the obvious, he added that “this theory therefore agrees completely with the observations.”

Such neutral tones could not conceal the explosion thus detonated. Decades of attempts to save the Newtonian worldview were at an end. Vulcan was gone, dead, utterly unnecessary. No chunk of matter was required to explain Mercury’s track, no undiscovered planet, no asteroid belt, no dust, no bulging solar belly, nothing at all—except this new, radical conception of gravity. The sun with its great mass creates its dent in space-time. Mercury, so firmly embraced by our star’s gravitational field, lies deep within that solar gravity well. Like all objects navigating space-time, Mercury’s motion follows that warping, four-dimensional curve. . . until, as Einstein finally captured in all the abstract majesty of his mathematics, the orbit of the innermost planet precesses away from the Newtonian ideal.

It was said of Newton that he was a fortunate man, because there was only one universe to discover, and he had done it. It had been said of Le Verrier that he discovered a planet at the tip of his pen. On the 18th of November, 1915, Einstein’s pen destroyed Vulcan—and reimagined the cosmos.

* * *

The years of searching in the dark. . . . are known only to him who has experienced them.

In private, among friends, Einstein allowed himself to feel his victory. The equations themselves had simply cranked out the correct orbit. Put the numbers in, and out pops Mercury—as if, to use his own word, by magic. Einstein felt all the pure wonder of that perfect match between theory and reality. Working at his desk, some time in the week before he rose before the Academy, the correct answer appeared as he cranked through the final steps. That was when, he told a friend, his heart actually shuddered in his chest—genuine palpitations. He wrote that it was as if something had snapped within him, and told another friend that he was “beside himself with joy.”

Much later, Einstein tried again to describe what he felt at that first, private instant of great discovery. He couldn’t. “The years of searching in the dark for a truth that one feels but cannot express, the intense desire and the alternations of confidence and misgiving until one breaks through to clarity and understanding,” he wrote, “are known only to him who has experienced them.”

* * *

From the Book: THE HUNT FOR VULCAN…And How Albert Einstein Destroyed A Planet, Discovered Relativity, and Deciphered the Universe by Thomas Levenson
Copyright © 2015 by Thomas Levenson
Published by arrangement with Random House, a division of Penguin Random House LLC