Vulcan was a theorized 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 French mathematician who had used disturbances in the orbit of Uranus to predict the existence of planet Neptune, Urbain Le Verrier. By 1859 he had confirmed unexplained peculiarities in Mercury's orbit and predicted they had to be the result of gravitational influences 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.
Theories and observations
Celestial bodies interior to the orbit of Mercury had been hypothesized, searched for, and even claimed as having been observed, for centuries.
Claims of actually seeing objects passing in front of the Sun included those made by the German astronomer Christoph Scheiner in 1611 (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.
Theories that there could be planets orbiting inside Mercury's orbit were put forward by British scientist Thomas Dick in 1838 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.
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 astronomer Heinrich Schwabe searched unsuccessfully on every clear day from 1826 to 1843 and Yale scientist Edward Claudius Herrick conducted observations twice daily starting in 1847, hoping to catch a planet in transit. French physician and amateur astronomer Edmond 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.
Le Verrier's prediction
In 1840, François Arago, the director of the Paris Observatory, suggested to mathematician Urbain 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 regarding Mercury's motion, with a detailed presentation published in 1845, 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 any differences between the motion predicted and what was observed would point to the influence of an unknown factor. 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.