Vulcan (/ˈvʌlkən/)[2] was a proposedplanet that some pre-20th century astronomers thought existed in anorbit betweenMercury and theSun. Speculation about, and even purported observations of, intermercurial bodies or planets date back to the beginning of the 17th century.
The case for their probable existence was bolstered by the support of the French mathematicianUrbain Le Verrier, who had predicted the existence ofNeptune using disturbances in the orbit ofUranus. By 1859, he had confirmed unexplained peculiarities in Mercury's orbit and predicted that they had to be the result of the gravitational influence of another unknown nearby planet or series ofasteroids. A French amateur astronomer's report that he had observed an object passing in front of the Sun that same year led Le Verrier to announce that the long sought after planet, which he gave the name Vulcan, had been discovered at last.
Many searches were conducted for Vulcan over the following decades but, despite several claimed observations, its existence could not be confirmed. The need for the planet as an explanation for Mercury's orbital peculiarities was later rendered unnecessary whenEinstein's 1915 theory ofgeneral relativity showed that Mercury's departure from an orbit predicted byNewtonian physics was explained by effects arising from the curvature ofspacetime caused by the Sun's mass.[3][4]
Celestial bodies interior to the orbit of Mercury had been hypothesized, searched for, and were even claimed to have been observed, for centuries.[citation needed]
Claims of seeing objects passing in front of the Sun included those made by the German astronomerChristoph Scheiner in 1611 (which turned out to be the discovery ofsunspots),[5] British lawyer, writer and amateur astronomerCapel Lofft's observations of 'an opaque body traversing the sun's disc' on 6 January 1818,[6] and Bavarian physician and astronomerFranz von Paula Gruithuisen's 26 June 1819 report of seeing "two small spots...on the Sun, round, black and unequal in size".[7] German astronomerJ. W. Pastorff [de] reported many observations also claiming to have seen two spots, with the first observation on 23 October 1822 and subsequent observations in 1823, 1834, 1836, and 1837; in 1834 the larger spot was recorded as 3arcseconds across, and the smaller 1.25 arcseconds.[7]
Proposals that there could be planets orbiting inside Mercury's orbit were put forward by British scientistThomas Dick in 1838[8]: 264 and by French physicist, mathematician, and astronomerJacques Babinet in 1846 who suggested there may be "incandescent clouds of a planetary kind, circling the Sun" and proposed the name "Vulcan" (after the godVulcan fromRoman mythology) for a planet close to the Sun.[8]: 156
As a planet near the Sun would be lost in its glare, several observers mounted systematic searches to try to catch it during "transit", i.e. when it passes in front of the Sun's disc. German amateur astronomerHeinrich Schwabe searched unsuccessfully on every clear day from 1826 to 1843 andYale scientistEdward Claudius Herrick conducted observations twice daily starting in 1847, hoping to catch a planet in transit.[8]: 264 French physician and amateur astronomerEdmond Modeste Lescarbault began searching the Sun's disk in 1853, and more systematically after 1858, with a 3.75 inch (95 mm)refractor in an observatory he set up outside his surgery.[8]: 146
In 1840,François Arago, the director of theParis Observatory, suggested to mathematician Urbain Le Verrier that he work on the topic ofMercury's orbit around theSun. The goal of the study was to construct a model based on SirIsaac Newton's laws ofmotion andgravitation. By 1843, Le Verrier published his provisional theory regarding Mercury's motion, with a detailed presentation published in 1845, which would be tested during atransit of Mercury across the face of the Sun in 1848.[9][10] Predictions from Le Verrier's theory failed to match the observations.[9]
Despite that, Le Verrier continued his work and, in 1859, published a more thorough study of Mercury's motion. That was based on a series of meridian observations of the planet and 14 transits. The study's rigor meant that any differences between the motion predicted and what was observed would point to the influence of an unknown factor. Indeed, some discrepancies remained.[9] During Mercury's orbit, itsperihelion advances by a small amount, something calledperihelion precession. The observed value exceeds the classical mechanics prediction by the small amount of 43arcseconds per century.[11]
Le Verrier postulated that the excess precession could be explained by the presence of some unidentified object or objects inside the orbit of Mercury. He calculated that it was either another Mercury-sized planet or, since it was unlikely that astronomers were failing to see such a large object, an unknown asteroid belt near the Sun.[12]
The fact that Le Verrier had predicted the existence of the planetNeptune in 1846,[13] using the same techniques, lent veracity to his claim.[non-primary source needed][citation needed]
On 22 December 1859, Le Verrier received a letter from Lescarbault, saying that he had seen a transit of the hypothetical planet on March 26 of that year. Le Verrier took the train to the village ofOrgères-en-Beauce, some 70 kilometres (43 mi) south-west ofParis, to Lescarbault's home-made observatory. Le Verrier arrived unannounced and proceeded to interrogate the man.[14]
Lescarbault described in detail how, on 26 March 1859, he observed a small black dot on the face of theSun.[15] After some time had passed, he realized that it was moving. He thought it looked similar to the transit ofMercury which he had observed in 1845. He estimated the distance it had already traveled, made some measurements of its position and direction of motion and, using an old clock and a pendulum with which he took his patients' pulses, estimated the total duration of the transit (coming up with 1 hour, 17 minutes, and 9 seconds).[14]
Le Verrier was not happy about Lescarbault's crude equipment but was satisfied the physician had seen the transit of a previously unknown planet. On 2 January 1860, he announced the discovery of the new planet with the proposed name from mythology, "Vulcan",[16] at the meeting of theAcadémie des Sciences in Paris. Lescarbault, for his part, was awarded theLégion d'honneur and invited to appear before numerous learned societies.[17]
However, not everyone accepted the veracity of Lescarbault's "discovery". An eminent French astronomer,Emmanuel Liais, who was working for the Brazilian government inRio de Janeiro in 1859, claimed to have been studying the surface of the Sun with a telescope twice as powerful as Lescarbault's, at the very moment that Lescarbault said he observed his mysterious transit. Liais, therefore, was "in a condition to deny, in the most positive manner, the passage of a planet over the sun at the time indicated".[18]
Based on Lescarbault's "transit", Le Verrier computed Vulcan's orbit: it supposedly revolved about the Sun in a nearly circular orbit at a distance of 21 million kilometres (0.14 AU; 13,000,000 mi). The period of revolution was 19 days and 17 hours, and the orbit was inclined to theecliptic by 12 degrees and 10 minutes (an incredible degree of precision). As seen from the Earth, Vulcan's greatestelongation from the Sun was 8 degrees.[14]
Numerous reports reached Le Verrier from other amateurs who claimed to have seen unexplained transits. Some of these reports referred to observations made many years earlier, and many were not dated, let alone accurately timed. Nevertheless, Le Verrier continued to tinker with Vulcan's orbital parameters as each newly reported sighting reached him. He frequently announced dates of future Vulcan transits. When these failed to materialize, he tinkered with the parameters some more.[19]
Shortly after 08:00 on 29 January 1860, F.A.R. Russell and three other people in London saw an alleged transit of an intra-Mercurial planet.[20] Many years later, an American observer, Richard Covington, claimed to have seen a well-defined black spot progress across the Sun's disk around 1860 when he was stationed inWashington Territory.[21]
No observations of Vulcan were made in 1861. Then, on the morning of 20 March 1862, between 08:00 and 09:00Greenwich Time, another amateur astronomer, a Mr. Lummis of Manchester, England, saw a transit. His colleague, whom he alerted, also saw the event.[22] Based on these two men's reports, two French astronomers,Benjamin Valz andRodolphe Radau, independently calculated the object's supposed orbital period, with Valz deriving a figure of 17 days and 13 hours and Radau a figure of 19 days and 22 hours.[8]: 168
On 8 May 1865 another French astronomer,Aristide Coumbary, observed an unexpected transit fromIstanbul,Turkey.[23]
Between 1866 and 1878, no reliable observations of the hypothetical planet were made. Then, during the totalsolar eclipse of July 29, 1878, two experienced astronomers, ProfessorJames Craig Watson, the director of theAnn Arbor Observatory inMichigan, andLewis Swift, fromRochester, New York, both claimed to have seen a Vulcan-type planet close to the Sun. Watson, observing fromSeparation Point, Wyoming, placed the planet about 2.5 degrees south-west of the Sun and estimated itsmagnitude at 4.5. Swift, observing the eclipse from a location nearDenver, Colorado, saw what he took to be an intra-mercurial planet about 3 degrees south-west of the Sun. He estimated its brightness to be the same as that ofTheta Cancri, a fifth-magnitude star which was also visible during totality, about six or seven minutes from the "planet". Theta Cancri and the planet were nearly in line with the Sun's centre.[citation needed]
Watson and Swift had reputations as excellent observers. Watson had already discovered more than twentyasteroids, while Swift had severalcomets named after him. Both described the colour of their hypothetical intra-mercurial planet as "red". Watson reported that it had a definite disk—unlike stars, which appear in telescopes as mere points of light—and that its phase indicated that it was on the far side of the Sun approachingsuperior conjunction.[24]
Both Watson and Swift had observed two objects they believed were not known stars, but after Swift corrected an error in his coordinates, none of the coordinates matched each other, nor known stars. The idea thatfour objects were observed during the eclipse generated controversy in scientific journals and mockery from Watson's rivalC. H. F. Peters. Peters noted that the margin of error in the pencil and cardboard recording device Watson had used was large enough to plausibly include a bright known star. A skeptic of the Vulcan hypothesis, Peters dismissed all the observations as mistaking known stars as planets.[25]: 215–217
Astronomers continued searching for Vulcan during total solar eclipses in 1883, 1887, 1889, 1900, 1901, 1905, and 1908.[25]: 219 Finally, in 1908,William Wallace Campbell, Director, andCharles Dillon Perrine, Astronomer, of theLick Observatory, after comprehensive photographic observations at three solar eclipse expeditions in 1901, 1905, and 1908, stated: "In our opinion, the work of the three Crocker Expeditions ... brings the observational side of the intermercurial planet problem—famous for half a century—definitely to a close."[26]
In 1915Einstein'stheory of relativity, an approach to understanding gravity entirely differently fromclassical mechanics, removed the need for Le Verrier's hypothetical planet.[3] It showed that the peculiarities in Mercury's orbit were the results of the curvature of spacetime caused by the mass of the Sun.[27] This added a predicted 0.1 arc-second advance of Mercury's perihelion each orbital revolution, or 43 arc-seconds per century, exactly the observed amount (without any recourse to the existence of a hypothetical Vulcan).[28]
The new theory modified the predicted orbits of all planets, but the magnitude of the differences from Newtonian theory diminishes rapidly as one gets farther from the Sun. Also, Mercury's fairly eccentric orbit makes it much easier to detect the perihelion shift than is the case for the nearly circular orbits ofVenus andEarth. Einstein's theory was empirically verified in theEddington experiment during thesolar eclipse of May 29, 1919, during which photographs showed the curvature of spacetime was bending starlight around the Sun. Most astronomers quickly accepted that a large planet inside the orbit of Mercury could not exist, given the corrected equation of gravity.[25]: 220
TheInternational Astronomical Union has reserved the name 'Vulcanoid" for asteroids that may exist inside the orbit of the planet Mercury. So far, however, earth- and space-basedtelescopes and theNASAParker Solar Probe have detected no such asteroids.[29]
