Le Verrier renewed 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 as well as 14 transits. The rigor of this study meant that any differences from observation would be caused by some unknown factor. Indeed, there still remained some discrepancy. During Mercury's orbit, its perihelion advances by a small amount each orbit, technically called perihelion precession. 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.
Le Verrier postulated that the excess precession could be explained by the presence of a small planet inside the orbit of Mercury, and he proposed the name "Vulcan" for this object. In Roman mythology, Vulcan was the god of beneficial and hindering fire, including the fire of volcanoes, making it an apt name for a planet so close to the Sun. Le Verrier's recent success in discovering the planet Neptune using the same techniques lent veracity to his claim, and astronomers around the world attempted to observe a new planet there, but nothing was ever found.
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 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 thought he was satisfied that Lescarbault had seen 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 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" (Popular Science, Volume 13, pages 732-735, 1878).
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, 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 (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 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):
Shortly after eight o'clock on the morning of 29 January 1860, F A R Russell and three other people saw 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 "observations" of Vulcan were made in 1861. Then, on the morning of 22 March 1862, between eight and nine o’clock 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.
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, 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, 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 six or sevem minutes from the "planet". Theta Cancri and the planet were very nearly in line with the center of the Sun.
Watson and Swift had the reputation as excellent observers. Watson had already discovered more than twenty asteroids, while Swift had several comets named after him. Both described the color 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.
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.
The best strategy for observations might be to wait for the planet's transit of the Sun's disk, and then observed through a strong optical filter. A small, round dark spot might be seen moving, as happens regularly with transits of 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.
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.