Vulcan (hypothetical planet)

• 1. Centuries Of Speculation

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  In 1611, the German astronomer Christoph Scheiner reported seeing dark spots on the Sun. He believed these were small planets orbiting closer to the star than Mercury ever could. His discovery turned out to be sunspots instead of a new world. Yet this error sparked decades of similar claims from other observers. On the 6th of January 1818, British lawyer Capel Lofft wrote about an opaque body crossing the Sun's disc. Bavarian physician Franz von Paula Gruithuisen followed with his own report on the 26th of June 1819. He described two round black spots of unequal size moving across the solar surface. These amateur sightings fueled persistent rumors that something lived between Earth and Mercury. By 1838, British scientist Thomas Dick proposed that such bodies might exist in stable orbits near the Sun. French physicist Jacques Babinet added weight to the idea in 1846 when he suggested incandescent clouds circling the star. He even named one potential planet Vulcan after the Roman god of fire.

• 2. Le Verrier's Prediction

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  Urbain Le Verrier began studying Mercury's motion around 1840 under the direction of François Arago at the Paris Observatory. His goal was to build a model using Sir Isaac Newton’s laws of gravity and motion. By 1845, he published a detailed theory that would later face scrutiny during Mercury’s transit in 1848. The predictions did not match what astronomers actually saw. Despite this mismatch, Le Verrier continued his work into 1859 with a more thorough study based on meridian observations and fourteen recorded transits. Discrepancies remained: Mercury’s perihelion advanced by an extra 43 arcseconds per century beyond classical mechanics expectations. He concluded that unseen gravitational forces must be pulling the planet off course. In 1846, Le Verrier had successfully predicted Neptune’s existence using similar methods. That success gave credibility to his new hypothesis about an unknown object inside Mercury’s orbit. He calculated it could be either a Mercury-sized planet or a belt of asteroids near the Sun.

• 3. The Lescarbault Claim

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  On the 26th of March 1859, French physician Edmond Modeste Lescarbault observed what he believed was a small black dot moving across the Sun. He used a modest 3.75-inch refractor telescope set up outside his surgery in Orgères-en-Beauce. After estimating the duration as one hour, seventeen minutes, and nine seconds, he sent a letter describing the event to Urbain Le Verrier. Le Verrier traveled unannounced to Lescarbault’s home observatory to question him directly. Though skeptical of the crude equipment, Le Verrier accepted the account as proof of a previously unknown planet. On the 2nd of January 1860, he announced the discovery at the Académie des Sciences in Paris. He named the hypothetical world Vulcan after Roman mythology. Lescarbault received the Légion d'honneur for his role in the find. Not everyone agreed. Emmanuel Liais, working in Rio de Janeiro with a more powerful telescope, claimed no such transit occurred during that time. Despite this contradiction, Le Verrier computed an orbit for Vulcan: nearly circular, taking nineteen days and seventeen hours to complete each revolution. The path inclined twelve degrees and ten minutes relative to Earth's ecliptic plane.

• 4. Eclipse Observations

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  Astronomers searched for Vulcan during total solar eclipses between 1860 and 1908. On the 29th of January 1860, F.A.R. Russell and three others in London reported seeing an alleged intra-Mercurial transit. Richard Covington later recalled observing a black spot cross the Sun around 1860 while stationed in Washington Territory. In May 1865, Aristide Coumbary from Istanbul also claimed to witness a similar event. During the eclipse on the 29th of July 1878, two experienced observers made notable claims. James Craig Watson of Ann Arbor Observatory saw what he thought was a red planet about 2.5 degrees southwest of the Sun. Lewis Swift from Rochester, New York, independently spotted something similar near Denver, Colorado. Both described their finds as having definite disks unlike stars. Yet when they compared notes, their coordinates failed to match known stars or each other. Skeptic C.H.F. Peters argued that errors in recording devices could easily explain the sightings as misidentified stars. Searches continued through subsequent eclipses in 1883, 1887, 1889, 1900, 1901, 1905, and finally 1908. William Wallace Campbell and Charles Dillon Perrine concluded after reviewing photographic data that no such planet existed.

• 5. Relativity's Solution

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  Albert Einstein published his general theory of relativity in 1915, offering a new way to understand gravity. The theory explained Mercury’s orbital anomalies without requiring any unseen planet nearby. It showed that spacetime curves due to the Sun’s mass, causing Mercury’s perihelion to advance by exactly 43 arcseconds per century. This matched observations perfectly while eliminating the need for Vulcan. The effect diminishes rapidly with distance from the Sun, making it easier to detect around Mercury than Venus or Earth. In May 1919, during a solar eclipse, Arthur Eddington confirmed Einstein’s predictions by photographing starlight bending near the Sun. Most astronomers quickly accepted that no large planet could exist inside Mercury’s orbit given the corrected equations of gravity. Today, the International Astronomical Union reserves the term 'Vulcanoid' for asteroids possibly existing within Mercury’s path. Despite searches using earth-based telescopes and NASA’s Parker Solar Probe, none have been found.