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Physics + MathPhysics & Math

The Ninth Planet That Wasn't

An unexplained orbit launched astronomers on a hunt for the mysterious planet Vulcan some 150 years ago.

ByThomas LevensonNOVA NextNOVA Next
The Ninth Planet That Wasn't

Urbain-Jean-Joseph Le Verrier had to know something was amiss. Not in his data or his analysis, but with the planet Mercury itself. It was 1859, and Le Verrier, a giant of French astronomy, was attempting to fine-tune his mathematical model of Mercury—the “theory” of the planet’s motions expressed within mathematical models based on Newtonian gravitation. He had already produced the most accurate theory 16 years earlier, and this time, he expected to do better. And he did. But there was still a discrepancy that couldn’t be explained. Mercury’s track wandered just a little bit more than the theory predicted.

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 La Verrier’s analysis, but rather with something unknown out there, waiting to be discovered.

Decisive Moments

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 discrepancy between the empirical picture and the theoretical one of Mercury’s motion—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.’ "

Le Verrier then took the next step, figuring out how big an intra-Mercurian planet would have to be to drive the advance of the perihelion, the point at which Mercury is closest to the sun. Assuming it lay roughly halfway between Mercury and the sun, he wrote, its mass would have to be about the same as its neighbor. That posed a problem, as he well knew. If it were that big, why hadn’t anyone seen it yet? Even if a Mercury-sized planet in the predicted orbit would usually be hidden within the glare of the sun, “It must be unlikely,” he wrote, that it could avoid detection “during a total eclipse of the sun.” Thus, Le Verrier proposed an alternative: “a group of asteroids [corpuscles] orbiting between the sun and Mercury.”

That conclusion must have seemed a bit deflating to Le Verrier’s readers. Adding to the lengthening list of minor planets, even in such an exotic location, hardly stacked up to the astronomer’s earlier discovery of Neptune. But the stakes of the search were just as high in both cases. Until Mercury’s precession could be accounted for, the anomaly represented a violation of the cosmic order, unthinkable (of course) to all of Newton’s heirs. Hence Le Verrier’s urgency: “It’s likely that some of these [asteroids] will be sufficiently large to be seen on their transits across the disk of the sun. Astronomers, already engaged with all the phenomena that appear on the surface of that star will without doubt find here another reason to track any spot they may see, no matter how small.” In other words: all those sunspots you folks have been tracking? Some of them might be little planets. Go get ’em!

Knowing what you’re looking for is a powerful aid to discovery.

For those unwilling to wade through the long job of sorting sunspots, there was one other way to speed discovery. Le Verrier had published a short form of his Mercury findings in the September 12, 1859 edition of the Académie’s proceedings, Comptes Rendus. In the same issue, the secretary to the Académie, Hervé Faye, wrote that the best chance of seeing Le Verrier’s hypothetical asteroids was during a solar eclipse. By good fortune, the next readily accessible eclipse was almost upon them, to come on July 16, 1860, visible over northern Africa and Spain. During totality, the region closest to the limbs of the sun would suddenly be freed from the brutal glare of the sun, until “at the decisive moment,” Faye wrote, the few minutes of totality “would suffice to explore much of the area designated by M. Le Verrier.”

Faye’s report sparked a wave of preparation. Locations were chosen—near Bilbao, perhaps, or a few miles west of Zaragoza, or maybe across the Mediterranean to a point on the coast around Algiers—wherever each observing team believed they would find the best chance of clear skies on the sixteenth of July. Given Le Verrier’s history, it seemed plausible that one or more little planets might appear, even on a first attempt. And maybe they’d already been seen! Recalling the tally of misidentified sightings of Uranus, Le Verrier’s announcement sent some back to old records, looking for anything that might qualify as an intra-Mercurian body since Galileo had first turned his telescope skyward back in 1609. No persuasive candidates materialized in this first pass—but then again, knowing what you’re looking for is a powerful aid to discovery. On to Spain!

The Country Doctor

Edmond Modeste Lescarbault was a humble, almost diffident man. He lived a small life, confined mostly to a modest compass between the Seine and the Loire rivers, about 70 miles west and a touch south of Paris. He had studied medicine, and in 1848 opened a practice in a little country town, Orgères-en-Beauce. He stayed put there for the next quarter of a century. He died in 1894, 90 years old, locally honored—the street where he kept his surgery is now named rue du Dr. Lescarbault—and generally forgotten.

The country doctor had one great passion. As a boy, he had fallen in love with the night sky. Children grow up, of course, and most put away childish things. Not Lescarbault. Like many before and since, he discovered in astronomy the same consolation that would later comfort Albert Einstein: the contemplation of “this huge world, which exists independently of us,” which, he wrote, serves as “a liberation.”

For Lescarbault, liberating himself from the daily medical round led him to build a genuinely impressive amateur’s observatory: a low stone barn with a modest dome at one end. There he mounted a perfectly competent telescope, a four-foot-long refractor with an objective lens almost four inches in diameter. He would steal time there between patients, just minutes sometimes, sneaking from his office to the dome to look, perhaps to dream, just a little. The discovery of the asteroids in the belt between Mars and Jupiter led him to wonder: where else might such treasures lurk? An answer came to him on the 8 th of May 1845—the day Le Verrier missed the timing of Mercury’s encounter with the sun.

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Lescarbault watched Mercury’s moving dot across the solar face, untroubled by any mathematical subtleties. Instead, he thought not about the planet in transit, but whether there might be other unobserved transits to seek. If a Ceres- or a Pallas-sized asteroid lurked close to our star, its transits would likely be the only opportunity to see it—and the search for such events would be a perfect target for an enthusiastic amateur astronomer, eager for the thrill of finding something in the cosmos that not one other human in all of time had perceived.

He was slow to act on that epiphany. Ordinary life intervened.

His medical practice needed nurturing, for one thing, but more important, he was a true amateur. He lacked both the knowledge and tools to achieve the precision needed to capture a phenomenon as delicate as an asteroid breaching the limb, or edge, of the sun. It took him more than a decade to prepare, but by 1858, he had fitted his telescope with homemade instruments good enough to fix the position of objects within its field of view. He was, at last, ready to go hunting.

A Transit

Saturday, March 26, 1859. Orgères, on the edge of spring, enjoys a sun-warmed afternoon. The flux of patients eases. As is his habit, Dr. Lescarbault takes the opportunity to retreat to his observatory. He turns his telescope toward the sun. An object leaps into view: a small, regular dot, just inside the limb of our star. He makes an estimate of its size: about one quarter the apparent diameter of Mercury. He has just missed its first appearance at the edge of the sun. Working backward from its apparent rate of motion, he estimates the time it crossed the solar limb at almost exactly four o’clock or, to be precise, at 3:59:46 pm., plus or minus five seconds. He writes that down, using a piece of charcoal to scratch on a board. Another patient arrives and, likely with unrecorded frustration, he pulls his eye from his telescope. A few minutes later, he returns. The spot is still there, moving across the face of the sun. He tracks it continuously now, noting its nearest approach to the center of the solar circle, and then the instant and place it disappears over the solar limb. He records the time again: 5:16:55. Total transit duration: one hour, 17 minutes, and nine seconds. If an asteroid were ever to be discovered within the innermost wards of the solar system, this is how it would reveal itself. Lescarbault meticulously transcribes his notes, and then…

Does nothing…

For nine months…

Until, at last, he permits himself to write a letter to be delivered—by hand—to Paris.

He “broke his silence,” Le Verrier later wrote, “solely because he had seen an article in the journal Cosmos on [my] work on Mercury.” Lescarbault described the data he had collected that Saturday in March—and added one bold claim: “I am persuaded also that [the planet’s] distance from the Sun is less than that of Mercury, and that this body is the planet, or one of the planets, whose existence in the vicinity of the Sun M. Le Verrier had made known a few months ago, by that wonderful power of calculation which enabled him to recognize the conditions of the existence of Neptune…”

“M. Lescarbault’s explanations, the simplicity with which he offered them to us gave us total conviction that the detailed observation he had completed must be admitted to science.”

Lescarbault entrusted it to a M. Vallée, “Honorary Inspector General of Roads and Bridges,” for delivery to the obvious recipient, Le Verrier himself. Dated December 22, 1859, it reached Paris a few days later. Le Verrier’s first reaction—as he told it—was one of doubt. But he was prepared to hope. There was only one way to be sure if Lescarbault could possibly have made the observations he claimed to have achieved: meet the man; inspect his instruments; test him. No matter how unlikely it might be that some rural hobbyist could have plucked such a prize, even the possibility that he might made any delay intolerable. Le Verrier was promised to his father-in-law’s for a New Year’s Day celebration— but the train schedules showed that it was just possible that he could get to Orgères and back to Paris before midnight on the 31 st . He commandeered Vallée to return with him as a witness, and the two men set out to see if Lescarbault’s “planet” might actually exist.

Le Verrier and Vallée arrived at Orgères-en-Beauce unannounced, covering the last 12 miles from the nearest railway station on foot. A few days later, he painted for the Académie a calm, almost placid picture of the encounter: “We found M. Lescarbault to be a man long devoted to the study of science…He permitted us to examine his instruments closely, and he gave us the most detailed explanations of his work, and in particular of all the circumstances of the passage of a planet across the sun.” The two men from Paris made Lescarbault walk them through each phase of his observation until they were convinced that their amateur had in fact seen what he said he had—and, crucially, that his interpretation of the event was correct. “M. Lescarbault’s explanations, the simplicity with which he offered them to us gave us total conviction that the detailed observation he had completed must be admitted to science.”

Urbain-Jean-Joseph Le Verrier

Le Verrier told the story very differently in private. Released from the conventions of scientific discourse, he seems to have composed a hero’s epic. Abbé Moingo, editor of the same journal, Cosmos , in which Lescarbault had first read of the problem of the precession of Mercury, was present at one of these performances. Le Verrier told of setting out for Orgères, Moingo wrote, assuming that no mere rural medico could have both discovered a new planet and kept quiet about it for nine months. Yet he had “a secret conviction that the story might be true.” At the doctor’s house, the astronomer confronted “the lamb” who trembled before the lion from Paris: “One should have seen M. Lescarbault … so small, so simple, so modest and so timid.” Le Verrier roars; Lescarbault stammers—and yet, according to the Abbé, still manages to defend himself at every turn. “You will then have determined…the time of first and last contact?” Le Verrier demanded, noting that measuring first contact is “of such extreme delicacy that professional astronomers often fail in observing it.” Lescarbault admitted that he had missed first contact, but had estimated the timing by checking how long it took for his spot to travel the same distance again it had already passed from the limb. Not good enough, said Le Verrier, and on learning that the doctor’s chronometer lacked a second hand, stormed “What! With that old watch, showing only minutes, dare you talk of estimating seconds? My suspicions are already too well founded.”

Le Verrier transformed the observation into a planetary orbit in less than a week.

Lescarbault rallied from even that devastating assault, though, showing his visitors the pendulum he used to count seconds, and reminding the astronomer that as a doctor “my profession is to feel pulses and count their pulsations…I have no difficulty in counting several successive seconds.” By this point in the remembered (and, to modern ears at least, suspiciously dramatic) account, it’s becoming clear what Moingo (and/or Le Verrier) is doing. The ebb and flow of leonine attack, each swipe seemingly fatal, and yet disarmed by a counter from the charmingly naive lamb, enlarges Lescarbault. The famous astronomer plays the part of the skeptic (never mind how much he may have hungered for one outcome over another), while the country doctor becomes more and more a competent, even an excellent man of science. The interrogation lasted an hour, enough to exhaust Le Verrier’s reservoir of doubt. At the last, he surrendered: “with a grace and dignity full of kindness, he congratulated Lescarbault on the important discovery he had made.” He would lead Lescarbault to a more tangible reward as well, securing within the month the Légion d’Honneur for “the village astronomer” who had, it seemed, discovered the first intra-Mercurian planet.

The next step was all Le Verrier. Lescarbault had none of the mathematical skill needed to transform his observation into a planetary orbit. Le Verrier did so in less than a week. By making the assumption that its orbit was nearly circular, he calculated that the new planet would complete one revolution around the sun in just under 20 days, on a path that never exceeded eight degrees distance from the sun. Such an object would be difficult if not impossible to see directly. But if Le Verrier’s analysis were even close to correct, the proposed planet would repeat its transits two to four times each year.

With that, planet fever hit the popular press— The Times of London, Popular Astronomy in the United States, The Spectator (which had some very kind words for Dr. Lescarbault). Alternative orbits were proposed: one reexamined the data on the assumption that the new planet traced a highly eccentric ellipse around the sun. Others returned to old records to see if Lescarbault’s planet had been seen and ignored previously—and just as with Uranus and Neptune, candidate objects soon turned up, reaching double figures in a series of sightings stretching back to the mid-18 th century.

It was clear more work needed to be done, beginning with a repeat observation of the mystery object. Nonetheless, the celebrations continued heedless of any lingering uncertainty—and for good reason. The faith in the new planet stood in equal measure on Le Verrier’s own reputation and the rock-solid logic behind the discovery. Mercury’s perihelion precession was and is real. Newtonian gravitation provides an obvious solution to such a problem. The appearance of an object exactly where necessity suggested it ought to be made perfect sense. It fit. It had a moral right to be true.

Celestial facts need labels. In this case, 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: Vulcan.

Preparing for Discovery

Vulcan’s career began happily. Weeks after Le Verrier’s announcement, no less an old rival than the Royal Astronomical Society bowed before the new planet: “The singular merit of M. Lescarbault’s observations will be recognized by all who examine the attendant circumstances; and astronomers of all countries will unite in applauding this second triumphant conclusion to the theoretical inquiries of M. Le Verrier.” More practically, the news evoked the sincerest form of flattery—claims of prior, never recorded encounters with the newcomer. Benjamin Scott, Chamberlain of the City of London and an avid amateur astronomer, wrote to The Times to assert that he had long before found an intra-Mercurian planet: a candidate object the apparent size of Venus glimpsed at sunset “at or about Midsummer 1847.”

Scott’s “discovery,” reported only in a conversation with a fellow of the Royal Astronomical Society, could hardly be taken seriously, but working astronomers wondered if they too had missed the prize. Rupert Wolf, a Zurich-based astronomer long fascinated by sunspots, reviewed his own and other solar observations to find potential mistakes—Vulcan transits he may have mistaken as mere spots—and came up with 21 possibilities that he published, and sent directly to Le Verrier as well, highlighting four that seemed the closest match to Lescarbault’s object.

“The sun’s disk was watched every few minutes from March 27 to April 10.”

Wolf’s list caught the attention of another astronomer, J.C.R. Radau, who used the data from two of Wolf’s candidates to refine what could be extracted from just a single Vulcan sighting. Radau joined other professionals who sniped at “the procrastinated publication of Dr. Lescarbault’s remarkable observation.” But once past his pique, Radau performed his analysis meticulously, generating exactly what astronomers needed to attempt the next phase of Vulcan research: a prediction for an observable transit. With the assumption that Wolf’s two suspects were in fact that same object as the one Lescarbault had seen, Radau published the results in early March: transits of Vulcan could next be expected between March 29 and April 7.

Radau’s transit would be visible in the southern hemisphere, and astronomers there readied themselves for the moment of discovery. The director of the Victoria Observatory, a Mr. Ellery, monitored the sun at half-hour intervals. Major Tennant, head of the Madras station, went one better, reporting that “the sun’s disk was watched every few minutes from March 27 to April 10.” At the Sydney Observatory, Mr. Scott set up a parallel search. Ellery summed up the outcome for all three: the planet hunt performed by multiple observers reached the end of the predicted period for Vulcan transits “without success.”

That was a blow, but hardly a fatal one. It had been obvious from the start that Vulcan would be hard to observe. If it weren’t, any large body—Mercury-sized or thereabouts—would have been seen long since. That was why Le Verrier had thought that an intra-Mercurian asteroid belt was the more likely option until Lescarbault’s report had raised hopes for a singular Vulcan. Still, while Lescarbault’s object appeared to be bigger than most if not all asteroids, his notes suggested it would be as small as one twentieth the diameter of Mercury. At that scale, it could not account for all of the perihelion advance Le Verrier had discovered. Lescarbault himself largely disappeared from view after his sudden burst of fame. The Légion d’Honneur he received in 1860 did not change his habits; he remained a country doctor and amateur astronomer until his death. After Le Verrier’s visit, he made no further claims about any intra-Mercurian objects.

Working astronomers, though, still had to deal with the problem. Any calculation of Vulcan’s orbit based on one or a few sightings would be an approximation at best, and stood a good chance of being just wrong. For Le Verrier, as for many of his peers, the missing transit expected on the basis of Radau’s calculation only demonstrated, once again, that doing astronomy at the limit of the math and empirical capacity is really, really hard. The necessity of the search hadn’t changed one bit: Mercury still precessed, and whatever was compelling it to do so remained to be found.

As it was, swiftly.

In the middle of the 19 th century, Manchester, England, prided itself on being smart as well as rich. In 1861, the city showcased both its wealth and brains as it hosted Britain’s largest celebration of knowledge, the annual meeting of the British Association for the Advancement of Science. Charles Darwin had published The Origin of Species less than two years before, and that explosion continued to reverberate through every gathering of the learned. At the Manchester meeting Darwin’s defenders prepared to battle religious doubters. One speaker, the “blind economist” Henry Fawcett, made the ultimate claim: Darwin was a true scientific hero, one who solved his problem by the same methods, the same approach to experiment, observation, and generalization that the great Isaac Newton himself had used in his physics.

Much else was discussed, of course—advances in dredging engineering, a report on birds of New Zealand, news from the balloon committee. The astronomy section was relatively quiet, but all in all, the meeting reflected a basic truth about Victorian curiosity: it was ubiquitous, constant, the common passion of both professionals and amateurs. No wonder, then, that Manchester’s citizen-scientists would chase new planets.

So it happened, on the morning of March 20, 1862, a “Mr Lummis, of Manchester” stole a few minutes to peer at the sun through a small telescope. As the formal report in The Astronomical Register told it, Lummis was watching “between the hours of 8 and 9 a.m., when he was struck by the appearance of a spot possessed of a rapid proper motion.” The object was startling enough that Lummis called for a witness, and they “both remarked on its sharp circular form.” Lummis tracked the spot for 20 minutes before being called in to his day’s work. By the time he returned to his telescope, the object was gone, “but he has not the slightest doubt of the matter.” Radau and a colleague repeated the by-now familiar exercise, constructing the elements of an orbit from incomplete observations, and they found that Lummis’s potential Vulcan was at least compatible with Lescarbault’s, even if there wasn’t enough data to settle the matter once and for all.

There were doubters. Two professional astronomers, the American Christian H. F. Peters and the German Gustav Spörer, dismissed Lummis’s “discovery” as a mere sunspot. But for many others, Le Verrier among them, the ongoing identification of plausible Vulcans, in sightings that allowed for at least rough estimates of consistent trajectories, made an ultimate validation seem inevitable. By the mid-1860s, The Astronomical Register itself seemed to view the matter as settled, listing Vulcan (without stating whether it was Lescarbault’s object or some other) as the innermost body in its “Descriptive Account of the Planets.”

Matters soon grew more complicated, though. Reports of sightings continued to arrive, 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 48 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 moment of discovery 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.

Photographic Proof

Benjamin Apthorp Gould had a perfect Boston pedigree: son of the headmaster of the Boston Latin School, grandson of a Revolutionary War veteran, he graduated from Harvard College— where else?—in 1844, all of 19 years old. Then, having paid his debt to ancestry, he kicked over the traces. Heading to Europe, he took work at the Greenwich, Paris, and Berlin observatories just as Neptune made its (perceived) solar system debut. He studied math at the University of Göttingen, and in 1848 became the first American to receive a Ph.D. in astronomy—still only 23! On returning to Boston in 1849, he was appalled by the primitive state of research in his home country, and took it on himself to transform American astronomy. Most important for the future of the discipline as a whole, in the 1860s he became one of the first investigators skilled in the new technique of astrophotography, the marriage of a camera to a telescope.

Gould brought his cameras with him when he traveled to observe the same 1869 eclipse at which the amateurs had spied a possible Vulcan. He set up in the town of Burlington, Iowa, working on the right bank of the Mississippi River. His goal: to study the solar corona—the sun’s atmosphere, visible only during totality—and to survey the region close to the sun as precisely as possible, looking for whatever might reveal itself within the orbit of Mercury. He and his assistants made 42 photographs during the eclipse. Gould also examined many of what he estimated were 400 images made by others along the path of totality. In all those pictures, he saw—nothing.

Gould sent his findings to Yvon Villarceau at the Paris Académie. He began with a baseline estimate: in the shadow of the eclipse, a planet or planets substantial enough to account for Mercury’s motion should shine about as brightly as Polaris, the North Star, a second magnitude object—easily seen by the naked eye. His photographic equipment, Gould wrote, was sensitive enough to detect any object down to the limit of unaided human perception, well below what he considered the plausible threshold for the discovery of Vulcan. Thus, he concluded, “I am convinced that this investigation dispenses with the hypothesis that the movement of the perihelion of Mercury results from the effects of one or many small interior planets.” I’ve looked, he said, and Vulcan ain’t there.

Not so fast, though: Villarceau added a note of his own to the published version of Gould’s letter. It wasn’t necessary to accept the American’s conclusion as absolute, he argued. There were configurations of asteroids, for example, that could both provide the necessary gravitational influence on Mercury and evade detection. In other words: the problem remained. Mercury still wobbled, and in Newton’s cosmos, its motion still demanded something like a Vulcan. Absence of evidence, to invoke what has become a cliché, could not be taken as evidence of absence.

Others agreed. William F. Denning was by general agreement Victorian Britain’s greatest amateur astronomer. He had made his reputation with the first comprehensive analysis of the motion of the Perseid meteor shower, still to be seen from late July to its peak in mid-August, and meteors remained his primary obsession. Vulcan, though, was a sufficiently pressing problem to draw his attention. He was an obligate organizer, and he used his influence to launch a systematic search for solar transits during the next likely window: March and April of 1869. He persuaded 15 other sky-watchers to put the sun “continually under observation, when visible…with a view of rediscovering the suspected intra-Mercurial planet Vulcan.”

Vulcan obstinately refused to appear.

Glimpse after glimpse of possible candidate planets offered tantalizing hints.

Denning tried again the next year, recruiting a team of 25 to chase the elusive planet during the spring transit season in 1870, and yet once more with a plea to collaborators in 1871. As he gathered his volunteers, he had declared that his aim was to settle the issue once and for all. “There is every reason,” he wrote, “to suppose that the search will end satisfactorily, if not successfully.” End it did. After three conscientious attempts at locating the missing planet, he seems to have concluded that there was nothing more to be done. He did not repeat his call for aid on the search, and those fellow amateurs of the sky who had responded to him were released to their prior ambitions.

After what was to that point the largest systematic search for the object since word of Lescarbault’s sighting first spread, Denning’s null result left Vulcan in a predicament. An explanation for Mercury’s errant motion remained necessary. On one side of the ledger, there was the blunt fact of Le Verrier and his genuine abilities. No one doubted his calculation, and no one should have—a restudy of Mercury’s perihelion advance in the 1880s confirmed and slightly enlarged the very real anomaly he identified. Glimpse after glimpse of possible candidate planets offered tantalizing hints—yet a decade into the search, the most rigorous observers kept coming up empty. What could be done?

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.

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, taking a bit of a victory lap as he did so. He told Le Verrier that “the interval between Lescarbault’s observation and Weber’s amounts to exactly one hundred and forty eight times the period” that Wolf had calculated so many years before.

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 2 nd or 3 rd .

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.

To the End

No transits occurred that March. Le Verrier said nothing more in public about Vulcan. He had turned 66 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 Paris 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 23 rd .

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.


From the book The Hunt for Vulcan:…And How Albert Einstein Destroyed a Planet, Discovered Relativity, and Deciphered the Universe by Thomas Levenson. Copyright (c) 2015 by Thomas Levenson. Reprinted by arrangement with Random House, a division of Penguin Random House LLC. All rights reserved.

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