Vulcan (hypothetical planet)

thumb|Vulcan in a lithographic map from 1846

Vulcan () was a proposed planet that some pre-20th century astronomers thought existed in an orbit between Mercury and the Sun. 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 mathematician Urbain Le Verrier, who had predicted the existence of Neptune using disturbances in the orbit of Uranus. 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 of asteroids. 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 when Einstein's 1915 theory of general relativity showed that Mercury's departure from an orbit predicted by Newtonian physics was explained by effects arising from the curvature of spacetime caused by the Sun's mass.

Hypotheses and observations

Celestial bodies interior to the orbit of Mercury had been hypothesized, searched for, and were even claimed to have been observed, for centuries.

Claims of seeing objects passing in front of the Sun included those made by the German astronomer Christoph Scheiner in 1611 (which turned out to be the discovery of sunspots), British lawyer, writer and amateur astronomer Capel Lofft's observations of 'an opaque body traversing the sun's disc' on 6 January 1818, and Bavarian physician and astronomer Franz von Paula Gruithuisen's 26 June 1819 report of seeing "two small spots...on the Sun, round, black and unequal in size". German astronomer 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 3 arcseconds across, and the smaller 1.25 arcseconds. and by French physicist, mathematician, and astronomer Jacques Babinet in 1846 who suggested there may be "incandescent clouds of a planetary kind, circling the Sun" and proposed the name "Vulcan" (after the god Vulcan from Roman mythology) for a planet close to the Sun. Predictions from Le Verrier's theory failed to match the observations.

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.

The fact that Le Verrier had predicted the existence of the planet Neptune in 1846, using the same techniques, lent veracity to his claim.

Claimed discovery

right|thumb|Edmond Modeste Lescarbault's observatory

On 22 December 1859, Le Verrier received a letter from Lescarbault, saying that he had seen a transit of the hypothetical planet on 26 March of that year. Le Verrier took the train to the village of Orgères-en-Beauce, some south-west of Paris, to Lescarbault's home-made observatory. Le Verrier arrived unannounced and proceeded to interrogate the man.

Lescarbault described in detail how, on 26 March 1859, he observed a small black dot on the face of the Sun. After some time had passed, he realized that it was moving. He thought it looked similar to the transit of Mercury 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). 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.

However, not everyone accepted the veracity of Lescarbault's "discovery". 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 . 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.

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. 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 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.

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 hypothesis, Peters dismissed all the observations as mistaking known stars as planets. Finally, in 1908, William Wallace Campbell, Director, and Charles Dillon 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 problemfamous for half a centurydefinitely to a close."

Hypothesis disproved

In 1915 Einstein's theory of relativity, an approach to understanding gravity entirely differently from classical mechanics, removed the need for Le Verrier's hypothetical planet. 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 in the Eddington experiment during the solar 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.

References

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 <s>Nine</s>8 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