The planet Mercury is closest to the Sun in the present model of the solar-system. The planet Vulcan was the proposed planet which was assumed to be orbiting around the Sun in the region between Mercury and Sun. Proposal of planet Vulcan was based on the calculations of Le Verrier which was done to explain the peculiarities in the orbit of Mercury. Le Verrier was the same French mathematician who predicted the existence of the planet Neptune by calculations to explain the orbit of Uranus. Le Verrier hypothesized that the peculiarities in the orbit of Mercury is due to some unknown planet orbiting between Mercury and Sun
A large number of investigators became involved in the search of new planet Vulcan but no such planet was ever found. The peculiarities in orbit of Mercury were explained by Albert Einstein’s Theory of General Relativity. Even NASA’s two STEREO spacecraft have failed to detect any vulcanoid asteroid between Mercury and Sun. Other than Mercury, asteroid 2007 EB with as semi-major axis of 0.55 AU has the smallest known semi-major axis orbiting the Sun.
Prediction for existence for Hypothetical Planet Vulcan
In 1840, the director of the Paris Observatory, Francois Arago, suggested to the French mathematician Urbain Le Verrier to work on the topic of orbital motion of the planet Mercury around the Sun. The aim of this study was to propose a model for the planet mercury 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 1843 but his theory failed to match the observation.
Le Verrier again published a thorough study of Mercury’s motion. The rigor of this study meant that any deviation from prediction would be caused by some unknown factor. There was still some discrepancy. During motion of Mercury, its perihelion advances by a small amount each orbit, technically called perihelion precession. The observed value is differing from the value predicted by Classical mechanics by 43 arc seconds per century.
Le Verrier postulated that the excessive 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.
Search for the planet Vulcan
A country doctor, Lescarbault at Orgeres, France, reported Le Verrier seeing an object the size of a planet cross the disc of the Sun on March 26, 1859. Le Verrier went to Orgenes and interrogated the amateur astronomer Lescarbault about the unidentified planetary object. Lescarbault estimated the duration of the transit as 1 hour, 17 minutes and 9 seconds.
Le Varrier was totally convinced by the observation of Lescarbault and believed that he had seen the transit of a previously unknown planet. On 2 January 1860, he announced the discovery of Vulcan to a meeting of the Academie des Sciences in Paris.
Obviously not everyone accepted the discovery of Vulcan. An eminent French astronomer, Emmanuel and Liais, claimed to have been studying the surface of the Sun with a telescope twice as powerful as Lescarbault’s telescope at the same time as reported by him. Liais strongly denied the passage of any planet over the Sun at the time indicated.
Based on Lescarbault’s “transit”, Le Verrier calculated the orbit of Vulcan and found orbiting distance of 21 million km (0.14 AU), orbiting period as 19 days and 17 hours and the orbit inclination to the ecliptic by 12 degrees and 10 minutes. He frequently announced the dates of future Vulcan transits and when these failed to materialize, he tinkered with the parameters some more.
For more than half century, astronomers were engaged in the search of this hypothetical planet Vulcan. Many False alarms were triggered by round sunspots that closely resembled planet transit. The search of Vulcan was not easy because the glare of Sun was harmful for eyes and instruments. The planet can only be observed in absence of glare of the Sun because of closest orbit. So it was believed that during the full solar eclipse, the planet could be observed but unfortunately no such planet was observed in any of the solar eclipse.
Planet Search Result
In 1915, Einstein proposed the Theory of General Relativity which was approaching to gravity differently as compared to classical mechanics. His equations predicted exactly the observed amount of advance of Mercury’s perihelion without any recourse to the existence of a hypothetical planet Vulcan. The new theory predicted the orbits of all the planets but the deviation from the Newtonian theory diminishes rapidly on going away from the Sun.
After the explanation by this theory, most of the astronomers were convinced but few believed that not all the observations of Vulcan were unfounded. Among these was Henry C Courten of Dowling College, studying photographic plates of the 1970 eclipse of the Sun, he and his associates detected several objects which was appearing very close to the Sun. Courten believed that an intra- Mercurial planetoid between 130 and 800 km in diameter was orbiting the Sun at a distance of about 0.1 AU.
None of these claims has ever substantiated but it has been surmised that the other alleged intra- Mercurial objects of smaller size not more than comets or small asteroids, may exist. These are called Vulcanoids. No asteroid larger than about 6 km is found. Neither SOHO nor STEREO has detected a planet inside the orbit of Mercury.