Vulcan (hypothetical planet)

For other uses, see Vulcan.

Vulcan in a lithographic map from 1846

Vulcan /ˈvʌlkən/ was a hypothetical planet that some 19th century astronomers thought existed in an orbit between Mercury and the Sun. Its existence was first proposed by the French mathematician Urbain Le Verrier whose calculations found peculiarities in Mercury's orbit which he thought were the result of gravitational influences of another unknown nearby planet or series of asteroids.

Several searches were done for Vulcan, but no such planet was ever confirmed despite occasional claimed observations. The existence of the planet was later disproved when Einstein's 1915 theory of general relativity showed that the peculiarities in Mercury's orbit were the results of the curvature of spacetime caused by the mass of the Sun.

Proposed existence

In 1840, François Arago, the director of the Paris Observatory, suggested to Le Verrier that he work on the topic of Mercury's orbit around the Sun. The goal of this study was to construct a model based on Sir Isaac Newton's laws of motion and gravitation. By 1843, Le Verrier published his provisional theory on the subject, which would be tested during a transit of Mercury across the face of the Sun in 1848. Predictions from Le Verrier's theory failed to match the observations.

Despite this, Le Verrier continued his work and, in 1859, published a more thorough study of Mercury's motion. This was based on a series of meridian observations of the planet and 14 transits. This study's rigor meant that some unknown factor would cause any differences from observation. Indeed, some discrepancies remained. During Mercury's orbit, its perihelion advances by a small amount, something called perihelion precession. The observed value exceeds the classical mechanics prediction by the small amount of 43 arcseconds per century.

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 size planet or, since it was unlikely that astronomers were failing to see such a large object, an unknown asteroid belt near the Sun.

Le Verrier's contribution in discovering the planet Neptune in 1846 using the same techniques lent veracity to his claim, and astronomers around the world began searching for the proposed object or group of objects near the Sun.

Search

On 22 December 1859, Le Verrier received a letter from French physician and amateur astronomer Edmond Modeste Lescarbault, who claimed to have seen a transit of the hypothetical planet earlier in the year. Le Verrier took the train to the village of Orgères-en-Beauce, some 70 kilometres (43 mi) southwest of Paris, where Lescarbault had built himself a small observatory. Le Verrier arrived unannounced and proceeded to interrogate the man.

Lescarbault described in detail how, on 26 March 1859, he noticed a small black dot on the face of the Sun, which he was studying with his modest 3.75 inch (95 mm) refractor. Thinking it to be a sunspot, Lescarbault was not at first surprised, but after some time had passed, he realized that it was moving. Having observed the transit of Mercury in 1845, he guessed that what he was observing was another transit, but of a previously undiscovered body. He took some hasty measurements of its position and direction of motion, and using an old clock and a pendulum with which he took his patients' pulses, he estimated the duration of the transit at 1 hour, 17 minutes, and 9 seconds.

Le Verrier was satisfied that Lescarbault had seen the transit of a previously unknown planet. On 2 January 1860 he announced the discovery of the new planet, for which he proposed the name "Vulcan" after the god Vulcan from Roman mythology, at the meeting of the Académie des Sciences in Paris. Lescarbault, for his part, was awarded the Légion d'honneur and invited to appear before numerous learned societies.

Not everyone accepted the veracity of Lescarbault's "discovery", however. An eminent French astronomer, Emmanuel Liais, who was working for the Brazilian government in Rio 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".

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 the ecliptic by 12 degrees and 10 minutes (an incredible degree of precision). As seen from the Earth, Vulcan's greatest elongation from the Sun was 8 degrees.

Numerous reports—all of them unreliable—began to reach 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 could not be properly dated. 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.

Among the earlier alleged observers of Vulcan:

• Capel Lofft reported 'an opaque body traversing the suns disc' on 6 January 1818.
• Gruithuisen, on 26 June 1819, reported seeing "two small spots...on the Sun, round, black and unequal in size"
• Pastorff, on 23 October 1822, 24 and 25 July 1823, six times in 1834, on 18 October 1836, 1 November 1836 and on 16 February 1837, also claimed to have seen two spots; the larger was 3 arcseconds across, and the smaller 1.25 arcseconds.

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. An American observer, Richard Covington, many years later claimed to have seen a well-defined black spot progress across the Sun's disk around 1860 when he was stationed in Washington Territory.

No "observations" of Vulcan were made in 1861. Then, on the morning of 20 March 1862, between 08:00 and 09:00 Greenwich Time, another amateur astronomer, a Mr. Lummis of Manchester, England, saw a transit. His colleague, whom he alerted, also saw the event. Based on these two men's reports, two French astronomers, Benjamin Valz and Rodolphe 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.

On 8 May 1865 another French astronomer, Aristide Coumbary, observed an unexpected transit from Istanbul, Turkey.

Between 1866 and 1878, no reliable observations of the hypothetical planet were made. Then, during the total solar eclipse of July 29, 1878, two experienced astronomers, Professor James Craig Watson, the director of the Ann Arbor Observatory in Michigan, and Lewis Swift, an amateur from Rochester, New York, both claimed to have seen a Vulcan-type planet close to the Sun. Watson, observing from Separation, Wyoming, placed the planet about 2.5 degrees southwest of the Sun and estimated its magnitude at 4.5. Swift, observing the eclipse from a location near Denver, Colorado, saw what he took to be an intra-mercurial planet about 3 degrees southwest of the Sun. He estimated its brightness to be the same as that of Theta 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.

Watson and Swift had reputations as excellent observers. Watson had already discovered more than twenty asteroids, while Swift had several comets 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 approaching superior conjunction.

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 that four objects were observed during the eclipse generated controversy in scientific journals and mockery from Watson's rival C. 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 theory, Peters dismissed all the observations as mistaking known stars as planets.

Astronomers continued searching for Vulcan during total solar eclipses in 1883, 1887, 1889, 1900, 1901, 1905, and 1908. Finally, in 1908, W. W. Campbell, Director, and C. D. Perrine, Astronomer, of the Lick 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."

Existence disproved

In 1915 Einstein's theory of relativity, an approach to understanding gravity entirely differently from classical mechanics, solved the problem. It showed that the peculiarities in Mercury's orbit were the results of the curvature of spacetime caused by the mass of the Sun. 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). 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 of Venus and Earth. Einstein's theory was empirically verified during the solar eclipse of May 29, 1919 when photographs showed the curvature of spacetime was bending starlight around the Sun. Astronomers generally quickly accepted that a large planet inside the orbit of Mercury could not exist, given the corrected equation of gravity.

Today, the International Astronomical Union has reserved the name "Vulcan" for the hypothetical planet, even though it has been ruled out, and also for the Vulcanoids, a hypothetical population of asteroids that may exist inside the orbit of the planet Mercury.

See also

• Fictional planets of the Solar System
• Hypothetical moon of Mercury
• Nemesis (hypothetical star)
• Planet Nine
• Planets beyond Neptune
• John H. Tice, weather forecaster who based predictions on supposed movements of Vulcan
• Tyche (hypothetical planet)
• Vulcan (Star Trek)

Further reading

• Baum, Richard; Sheehan, William (1997). In Search of Planet Vulcan, the Ghost in Newton's Clockwork Machine. New York: Plenum Press. ISBN 978-0-306-45567-4.
• Levenson, Thomas (2015). The Hunt for Vulcan: How Albert Einstein Destroyed a Planet and Deciphered the Universe. London: Head of Zeus. ISBN 978-0812998986. Originally published as The Hunt for Vulcan: ... And How Albert Einstein Destroyed a Planet, Discovered Relativity, and Deciphered the Universe.
• The subject was also featured on an episode of Arthur C. Clarke's Mysterious World entitled "Strange Skies", originally broadcast on November 18, 1980.

External links

• Asimov, Isaac (1975). "The Planet That Wasn't", The Magazine of Fantasy and Science Fiction
• Schlyter, Paul (2006). "Vulcan, the intra-Mercurial planet, 1860–1916, 1971", The Nine8 Planets: A Multimedia Tour of the Solar System (Appendix 7: "Hypothetical Planets") converted to HTML by Bill Arnett.
• "The Planet Vulcan", Scientific American, 31 August 1878, p. 128, columns 2–3