Vulcan (hypothetical planet)

• 1. Introduction

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  Vulcan takes its name from the Roman god of fire. The label was first proposed in 1846 by the French physicist Jacques Babinet. He imagined "incandescent clouds of a planetary kind" circling close to the Sun. For decades afterward, professional astronomers treated this planet, supposedly hidden in the Sun's glare, as real, not myth. They charted its orbit, predicted its transits, and waited for it to cross the Sun's face on schedule. Yet no photograph, no confirmed transit, and no lasting trace of Vulcan ever survived close scrutiny. What convinced serious astronomers that an unseen planet was hiding in the Sun's glare? Who claimed to have actually seen it? And what discovery, decades later, finally closed the case for good?

• 2. Early Clues

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  In 1611, the German astronomer Christoph Scheiner reported seeing objects pass across the face of the Sun. His sightings turned out to be sunspots, not a planet, but they opened two centuries of similar claims. The British lawyer and amateur astronomer Capel Lofft described "an opaque body traversing the sun's disc" on the 6th of January 1818. The Bavarian physician Franz von Paula Gruithuisen reported on the 26th of June 1819 that he had seen "two small spots...on the Sun, round, black and unequal in size." A German astronomer recorded further sightings of two spots starting on the 23rd of October 1822, with more in 1823, 1834, 1836, and 1837. In 1834 the larger spot measured 3 arcseconds across and the smaller 1.25 arcseconds. The British scientist Thomas Dick proposed in 1838 that undiscovered planets might orbit inside Mercury's path.

  The German amateur Heinrich Schwabe searched for a transiting planet every clear day from 1826 to 1843, without success. The Yale scientist Edward Claudius Herrick began twice-daily observations in 1847, still hoping to catch the elusive world. The French physician Edmond Modeste Lescarbault started his own search in 1853, working more systematically after 1858 with a 3.75 inch refractor at an observatory he had built outside his surgery. That modest observatory would soon draw an unannounced visitor all the way from Paris.

• 3. The Mathematician's Discrepancy

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  In 1840, the director of the Paris Observatory, François Arago, asked the mathematician Urbain Le Verrier to study Mercury's orbit around the Sun. Le Verrier built his model on Isaac Newton's laws of motion and gravitation. By 1843 he had a provisional theory, followed by a fuller version in 1845. A transit of Mercury across the Sun in 1848 was meant to test his predictions, but the observations did not match.

  Le Verrier returned to the problem and published a far more thorough study of Mercury's motion in 1859. This time his work drew on a series of meridian observations plus fourteen transits of the planet. The rigor of the study meant any gap between prediction and observation would point to something real. That gap showed up in Mercury's perihelion, the point in its orbit closest to the Sun, which advances slightly with each pass. The observed advance exceeded the classical prediction by 43 arcseconds per century.

• 4. The Doctor's Report

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  On the 22nd of December 1859, Le Verrier received a letter from Edmond Modeste Lescarbault describing a transit seen on the 26th of March that year. Le Verrier took the train to Orgères-en-Beauce, a village some 70 km south-west of Paris, and arrived at Lescarbault's home-built observatory unannounced. He proceeded to question the physician in detail.

  Lescarbault described watching a small black dot cross the Sun's disc on the 26th of March 1859. After a while he noticed the dot moving and thought of the transit of Mercury he had watched in 1845. Using an old clock and the pendulum he normally used to time his patients' pulses, he timed the object's crossing at 1 hour, 17 minutes, and 9 seconds.

  Le Verrier was unimpressed by Lescarbault's improvised instruments but left convinced the doctor had genuinely seen an unknown planet cross the Sun. On the 2nd of January 1860, he announced the discovery to the Académie des Sciences in Paris, giving it the name Vulcan. Lescarbault received the Légion d'honneur and invitations to address multiple learned societies.

  Emmanuel Liais, a French astronomer then working for the Brazilian government in Rio de Janeiro, disputed the claim outright. He had been observing the Sun that same day with a telescope twice as powerful as Lescarbault's. He said, "in a condition to deny, in the most positive manner, the passage of a planet over the sun at the time indicated." Working from Lescarbault's account regardless, Le Verrier calculated Vulcan's orbit. He placed it on a nearly circular path around the Sun, 21 million kilometres out. It completed one revolution every 19 days and 17 hours. The orbit tilted 12 degrees and 10 minutes from the ecliptic. Seen from Earth, Vulcan's greatest distance from the Sun in the sky worked out to 8 degrees. New reports would soon arrive from amateur astronomers scattered across Europe, each sending Le Verrier back to his calculations.

• 5. Sightings Across Decades

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  Shortly after 08:00 on the 29th of January 1860, F.A.R. Russell and three companions in London reported seeing an intra-Mercurial planet cross the Sun. Years later, an American observer named Richard Covington claimed he had seen a similar black spot while stationed in Washington Territory around 1860. No sightings were reported at all in 1861. On the morning of the 20th of March 1862, a Mr. Lummis of Manchester and a colleague both witnessed a transit between 08:00 and 09:00 Greenwich time. Two French astronomers used their reports to calculate different orbital periods. Benjamin Valz worked out 17 days and 13 hours, while Rodolphe Radau calculated 19 days and 22 hours. On the 8th of May 1865, another French astronomer, Aristide Coumbary, reported an unexpected transit from Istanbul.

  Between 1866 and 1878, no reliable sightings were recorded at all. That changed during the total solar eclipse of the 29th of July 1878, when two experienced observers reported seeing a Vulcan-like planet close to the Sun. James Craig Watson directed the Ann Arbor Observatory in Michigan and watched the eclipse from Separation Point, Wyoming. He placed the object 2.5 degrees south-west of the Sun, at a magnitude of 4.5. Lewis Swift, observing near Denver, Colorado, put a similar object about 3 degrees south-west of the Sun. He judged it as bright as the nearby star Theta Cancri. Both men had strong reputations. Watson had already discovered more than twenty asteroids, and Swift had several comets named after him. Both described the object's colour as red.

  Watson also reported that the object showed a definite disk, unlike the points of light typical of stars, suggesting it lay on the far side of the Sun. After Swift corrected an error in his coordinates, none of the two men's readings matched each other or any known star. The mismatch fueled controversy in scientific journals, and Watson's rival, C. H. F. Peters, mocked the claims. Peters pointed out that the margin of error in Watson's pencil-and-cardboard recording device was large enough to include a bright, ordinary star by mistake. Astronomers kept searching during eclipses in 1883, 1887, 1889, 1900, 1901, 1905, and 1908. Finally, after coordinated photographic expeditions to three of those eclipses, William Wallace Campbell and Charles Dillon Perrine of the Lick Observatory delivered their verdict. Their report stated that the three Crocker Expeditions brought "the observational side of the intermercurial planet problem, famous for half a century, definitely to a close." The transit hunt was over, but the anomaly in Mercury's orbit that had started it all still had no accepted explanation.

• 6. Relativity Closes The Case

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  In 1915, Albert Einstein's theory of general relativity offered a completely different account of gravity than classical mechanics. It showed that the peculiarities in Mercury's orbit came from the curvature of spacetime caused by the Sun's own mass. The theory predicted a perihelion advance of 0.1 arcseconds with each orbit, adding up to 43 arcseconds per century. That figure matched the observed anomaly exactly, with no hypothetical planet required.

  On the 29th of May 1919, during a total solar eclipse, the Eddington experiment tested Einstein's theory directly. Photographs from that eclipse showed starlight bending around the Sun, exactly as the curvature of spacetime predicted. The new theory adjusted the predicted orbits of every planet, though the size of the difference from Newtonian predictions shrinks quickly farther from the Sun. Mercury's fairly eccentric orbit also makes its perihelion shift much easier to detect than the nearly circular orbits of Venus or Earth. Most astronomers quickly accepted that no large planet could exist inside Mercury's orbit once the corrected equation of gravity was in place.

  The International Astronomical Union has since reserved the name "Vulcanoid" for any asteroids that might still exist inside Mercury's orbit. Earth- and space-based telescopes, along with NASA's Parker Solar Probe, have searched that region without finding a single one so far.