Vulcan and the Orbit of Mercury

Summary

In 1859 Urbain Le Verrier found that Mercury's orbit turns slightly faster than Newtonian gravity allows, and proposed an unseen planet inside it. The planet, named Vulcan, was reported and hunted for half a century and never found; in 1915 Einstein's general relativity accounted for the excess.

The anomaly was real, and it was small: at the modern figure of 43 arcseconds per century, the excess alone would take about three million years to swing the orbit once around. It was detected in 1859 and has been measured ever since. The planet was never there. What was missing was not an object but a theory of gravity that nobody had yet written.

What is documented

The motion. Mercury's orbit is an ellipse, and the point where it passes closest to the Sun, the perihelion, slowly moves round. Most of that movement is caused by the pull of the other planets, and Newtonian gravity accounts for it: about 530 of the roughly 575 arcseconds per century by which the perihelion advances against the distant stars. An arcsecond is one 3,600th of a degree. The remainder was the problem.

The method had worked before. In 1846 Le Verrier had used irregularities in the motion of Uranus to predict the position of an unseen planet. Johann Galle found Neptune at Berlin on 23 September 1846, about one degree from the predicted place. Le Verrier became director of the Paris Observatory in 1854.

Le Verrier's result, 1859. Working from meridian observations and from timed transits of Mercury across the face of the Sun, the latest from 1848, Le Verrier reported in September 1859, in a letter to Hervé Faye published by the Académie des sciences, that Mercury's perihelion advanced 38 arcseconds per century more than the known planets could explain. He proposed as the cause either an undiscovered planet closer to the Sun than Mercury or a ring of small bodies.

The doctor at Orgères. On 22 December 1859 Edmond Modeste Lescarbault, a physician and amateur astronomer at Orgères-en-Beauce, wrote to Le Verrier. He said that on 26 March 1859 he had watched a small, round, black spot cross the face of the Sun for 1 hour, 17 minutes and 9 seconds. Le Verrier travelled to Orgères on 31 December, questioned him, and was satisfied. He announced the observation to the Académie on 2 January 1860. From Lescarbault's timings he derived an orbit with a period of about 19 days and 17 hours, keeping the planet always within about 8 degrees of the Sun. The planet was named Vulcan in early 1860, and Lescarbault was made a chevalier of the Légion d'honneur that January.

Doubts from the start. Emmanuel Liais, a French astronomer then working in Brazil, published a rebuttal in 1860, stating that he had been examining the Sun at the time Lescarbault gave and had seen nothing cross it. Other amateurs reported dark spots moving across the Sun, and Le Verrier revised the orbit as reports arrived. No transit of Vulcan was ever confirmed.

The eclipse of 29 July 1878. A total eclipse of the Sun crossed the western United States. An eclipse is the one time a body close to the Sun can be seen directly. James Craig Watson, director of the University of Michigan's observatory at Ann Arbor, observed from Separation, a railway station in Wyoming Territory. He reported two objects near the Sun, which he put at magnitudes 4 and 4.5, and concluded that he had seen one, and probably two, unknown planets. Lewis Swift of Rochester, New York, observing near Denver, reported two bright objects about three degrees south-west of the Sun, one of which he identified as the star theta Cancri and the other as a planet.

The objection. Watson had recorded positions with pencil marks on small circles fitted to his telescope. Christian Heinrich Friedrich Peters of Hamilton College, New York, argued in 1879 that the arrangement could not fix a position to better than about 20 arcminutes, and that Watson's two objects sat where two known stars, theta and zeta Cancri, lay. In Peters's words, as reported in Nature: "Watson observed θ and ζ Cancri, nothing else." Peters found Swift's successive published accounts too inconsistent to discuss. Watson moved to the University of Wisconsin in 1879, built an underground observatory to continue the search, and died in November 1880.

Photography closes the search. Lick Observatory photographed the sky around the eclipsed Sun at the eclipses of 1901 (Sumatra), 1905 (Labrador, Spain and Egypt) and 1908 (the South Pacific). The 1908 plates, examined by Charles Dillon Perrine, showed more than 500 known stars down to nearly the ninth visual magnitude, and no planet. In 1909 Lick's director, William Wallace Campbell, wrote that Perrine's work had "brought the observational side of the Intramercurial Problem, famous for a half century, definitely to a close".

The anomaly survived the planet. Simon Newcomb, head of the American Nautical Almanac Office, re-derived the excess from transits observed between 1677 and 1881 and in 1882 put it at about 43 arcseconds per century. His later value, in 1895, was 41. Other causes were proposed: a flattened Sun, a slight change to the inverse-square law of gravity, and diffuse matter near the Sun, the material that scatters the zodiacal light. The last, worked out by the German astronomer Hugo von Seeliger, was the explanation Campbell favoured in 1909.

Einstein, November 1915. Einstein treated Mercury as a test. In 1913 he and Michele Besso calculated the perihelion motion predicted by the draft, or Entwurf, version of his theory and obtained 18 arcseconds per century, well short of the observed excess. On 18 November 1915 he presented to the Prussian Academy of Sciences in Berlin the calculation from his revised theory: 43 arcseconds per century, against the astronomers' figure of 45 ± 5. The calculation required no new matter and no adjustable quantity. A week later, on 25 November, he presented the final field equations of general relativity.

Measured since. Radar ranging to Mercury and the tracking of spacecraft have repeated the test many times. The most precise measurement to date comes from radio ranging to NASA's MESSENGER spacecraft, which orbited Mercury from 2011 to 2015. Ryan Park of the Jet Propulsion Laboratory and colleagues, publishing in 2017, found the perihelion's motion consistent with general relativity, and measured the small contribution of the Sun's own flattening at the same time. In 2013 Andrew Steffl and colleagues searched images from NASA's STEREO spacecraft for small asteroids in the stable zone inside Mercury's orbit, the hypothetical "vulcanoids", and found none; their limit excludes any larger than about 5.7 kilometres, assuming dark surfaces.

Next. BepiColombo, the joint European and Japanese Mercury mission, is due to enter orbit around Mercury on 21 November 2026, with science operations planned from April 2027. Its radio science experiment is designed to improve tests of general relativity and to measure the Sun's flattening more precisely.

Leading explanations

General relativity. Established. In Einstein's theory the Sun's mass curves space and time, and a planet's orbit does not close on itself exactly. For Mercury, closest to the Sun and with a markedly elliptical orbit, the extra advance is about 43 arcseconds per century (42.98 in modern calculations). No hidden body is involved. The result has been confirmed by radar and by spacecraft tracking to a precision far beyond anything available in 1915. It was also the theory's first observational success, three and a half years before the 1919 eclipse test of light bending.

An intra-Mercurial planet, Vulcan. Rejected. Le Verrier's hypothesis, and a reasonable one when the same reasoning had found Neptune thirteen years earlier. No sighting survived examination. Photography at three eclipses between 1901 and 1908 found nothing, the 1908 plates reaching stars of nearly the ninth magnitude, and the STEREO search rules out any dark asteroid larger than about 5.7 kilometres in the stable zone inside Mercury's orbit. A 2025 paper by Souren Pogossian in the Journal of Astrophysics and Astronomy suggested that Vulcan might be a planet-mass primordial black hole, which telescopes could not see. The paper notes that such a body would also affect the perihelion motions of the other inner planets. The proposal is untested, and no such body has been detected.

Diffuse matter near the Sun. Superseded. Seeliger's model could be made to fit the observed excess by choosing how much matter there was and how it was spread. General relativity produced the figure without any such choice, and the model was abandoned.

A flattened Sun. Rejected. In 1967 Robert Dicke and H. Mark Goldenberg reported that the Sun's equatorial radius exceeds its polar radius by about 5 parts in 100,000, which implied a discrepancy of 8 per cent in Einstein's figure for Mercury. Later measurements, culminating in the MESSENGER analysis, found the Sun's gravitational flattening far too small to have that effect, and general relativity consistent with Mercury's motion once it is included.

The sightings. Peters's identification of Watson's objects with theta and zeta Cancri is the standard reading. Swift's records were never consistent enough to test. Lescarbault's spot has not been explained and was never seen again.

Current status

Later explained. The excess advance of Mercury's perihelion is a consequence of general relativity. It was computed in 1915 and has been confirmed since to a precision the nineteenth century could not approach. Eclipse photography and spacecraft searches have found no body inside Mercury's orbit, down to their limits. The record would reopen only if Mercury's motion were found to depart from general relativity, which BepiColombo is due to test from 2027, or if a substantial body were found inside Mercury's orbit.

The registry keeps the record because the anomaly was genuine and was recognised from the start, and because the obvious explanation, the one that had worked for Neptune, was wrong. The sightings that seemed to confirm it were a spot nobody else saw, two stars, and a set of accounts that could not be reconciled. The anomaly behind them was real all along.

Last reviewed: September 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.