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Vulcan (hypothetical planet)

Vulcan was a small planet proposed to exist in an orbit between Mercury and the Sun in a 19th-century hypothesis. This hypothesis has now been rendered obsolete by Albert Einstein's theory of general relativity.


Argument for Vulcan's existence

Vulcan was proposed to explain a small perturbation in Mercury's orbit from the path predicted by classical mechanics, technically called perihelion precession.

During Mercury's orbit, its perihelion advances by a small amount each orbit. The phenomenon is predicted by classical mechanics, but the observed value differed from the predicted value by the small amount of 43 arcseconds per century.

This idea and the name "Vulcan" was postulated by the French mathematician Urbain Le Verrier in 1859, closely following his spectacular success in "discovering" the planet Neptune in the same way — using only calculus. Various persons and astronomers around the world attempted to prove the existence of the said planet.

The search for Vulcan

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

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 assumed 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’ pulse, he estimated the duration of the transit at 1 hour, 17 minutes and 9 seconds.

Le Verrier was satisfied that Lescarbault had indeed witnessed the transit of a previously unknown planet. On 2 January 1860 he announced the discovery of Vulcan to a 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 witnessed 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" (Popular Science, Volume 13, pages 732-735, 1878).

Based on Lescarbault’s "transit", Le Verrier computed Vulcan’s orbit: it revolved about the Sun in a nearly circular orbit at a distance of 21 million kilometres, or 0.14 astronomical units. The period of revolution was 19 days and 17 hours, and the orbit was inclined to the ecliptic by 12 degrees and 10 minutes. As seen from the Earth, Vulcan’s greatest elongation from the Sun was 8 degrees.

Numerous reports — most of them unreliable — began to reach Le Verrier from other amateurs who claimed to have witnessed 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 new reported sighting reached him. He frequently announced dates of future Vulcan transits, and when these failed to materialize, he tinkered with the parameters some more.

Among the earlier alleged observers of Vulcan, the following are the most noteworthy (Astronomical Register, 1869):

  • 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 eight o’clock on the morning of 29 January 1860, F A R Russell and three other people witnessed an alleged transit of an intra-Mercurial planet from London (Nature, 5 October 1876). 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 (Scientific American, 25 November 1876).

No successful observations of Vulcan were made in 1861. Then, on the morning of 22 March 1862, between eight and nine o’clock, another amateur astronomer, a Mr Lummis of Manchester, England, witnessed a transit. A colleague whom he alerted also witnessed the event. Based on these gentlemen’s reports, two French astronomers, Benjamin Valz and Rodolphe Radau, independently calculated the object’s orbital period, 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 Coumbrary observed an unexpected transit from Constantinople.

Between 1866 and 1878 no reliable observations of the hypothetical planet were made. Then, during the total solar eclipse of 29 July 1878, two experienced astronomers, Professor James Craig Watson, 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, who was 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 6 or 7 minutes from the "planet". Theta Cancri and the planet were very nearly in line with the centre of the Sun.

Watson and Swift were 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.

These are merely the more reliable observations of alleged intra-Mercurial planets. For half a century or more, many other observers tried to find the hypothetical Vulcan. Many false alarms were triggered by round sunspots, that closely resembled planets in transit. During solar eclipses, stars close to the Sun were mistaken for planets. At one point, to reconcile different observations, at least two intra-Mercurial planets were postulated.

Search conclusion

In 1877 Le Verrier died, still convinced of having discovered another planet. With the loss of its principal proponent, the search for Vulcan abated. After many years of searching, astronomers were seriously doubting the planet's existence.

The final act came in 1915, when Einstein's theory of relativity explained the perturbations of Mercury as a mere byproduct of the Sun's gravitational field. His equations predicted slightly different results than classical mechanics, and exactly in the right amount to explain Mercury's actual orbit.

The difference applies to the orbits of all planets, but the magnitude of the effect diminishes as one gets farther out 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.

Vulcan revived

Observing a planet inside the orbit of Mercury would be extremely difficult, since the telescope must be pointed very close to the Sun, where the sky is never black. Also, an error in pointing the telescope can result in damage for the optics, and injury to the observer. The huge amount of light present even quite away from the Sun can produce false reflections inside the optics, thus fooling the observer into seeing things that do not exist.

The best strategy for observations would be to wait for the planet's transit of the Sun's disk. A small, round dark spot might be seen moving, as happens regularly with Mercury and Venus.

In 1915, when Einstein successfully explained the apparent anomaly in Mercury's orbit, most astronomers abandoned the search for Vulcan. A few, however, remained convinced that not all the alleged observations of Vulcan were bogus. Among these was Henry C Courten, of Dowling College, New York. Studying photographic plates of the 1970 eclipse of the Sun, he and his associates detected several objects which appeared to be in orbits close to the Sun (Miami Herald, 15 June 1970). Even accounting for artifacts, Courten felt that at least seven of the objects were real. The appearance of some of these objects was confirmed by another observer in North Carolina, while a third observer in Virginia saw one of them.

Courten believed that an intra-Mercurial planetoid between 130 and 800 kilometres in diameter was orbiting the Sun at a distance of about 0.1 astronomical unit. Other images on his eclipse plates led him to postulate the existence of an asteroid belt between Mercury and the Sun.

None of these claims has ever been substantiated after more than thirty years of observation. It has been surmised, however, that some of these objects - and other alleged intra-Mercurial planets - may exist, being nothing more than previously unknown comets or small asteroids. Today, the search continues for these so-called Vulcanoid asteroids, which are thought to exist in the region where Vulcan was once sought. None have been found yet and searches have ruled out any such asteroids larger than about 60 km.

Vulcan in modern fiction

Ross Rocklynne wrote a short story, "At the Center of Gravity", about two individuals trapped inside a hollow Vulcan. The story was published in the June 1936 issue of Astounding magazine and republished in the 1963 anthology Exploring Other Worlds (ISBN 0-02-023110-5).

A resurge of interest in the theory occurred in the 1960s, and planets called Vulcan began appearing in the science fiction of the time. In the Doctor Who serial entitled The Power of the Daleks the setting is the Earth colony on Vulcan in the early 21st century.

The name was used for the fictional home planet of Mr. Spock and the Vulcan race in Star Trek. Throughout the various series, the fictional planet was portrayed as extremely hot, arid and barren. According to Gene Roddenberry Vulcan orbits either the star 40 Eridani A or Epsilon Eridani. In 1998, scientists indirectly detected a Jupiter-sized planet around Epsilon Eridani. It is not known if this planet is capable of sustaining life or not, but many astronomers point to its highly elliptical orbit as a damaging factor.


  • Richard Baum, William Sheehan (1997). In Search of Planet Vulcan, The Ghost in Newton's Clockwork Machine. ISBN 0-306-45567-6. 

See also

Wikisource has original text related to this article:
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Vulcan_(hypothetical_planet)". A list of authors is available in Wikipedia.
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