Astronomers in the 19th century believed in a hypothetical planet called Vulcan, closer to the Sun than Mercury, due to discrepancies in Mercury’s orbit. Observations of black spots transiting the Sun were thought to be Vulcan, but Einstein’s theory of relativity explained the anomaly. The search for Vulcanoid objects continues, but the Yarkovsky effect makes their orbit unstable.
The planet Vulcan, in addition to being the fictional planet in Star Trek that Spock hails from, is a hypothetical planet that many astronomers in the 19th century believed might have existed. This planet must have had a low mass and been closer to the Sun than Mercury, so close that daytime telescopes could not resolve it due to the overwhelming brightness of the Sun itself. The planet Vulcan, if it existed, could have surface temperatures even hotter than Mercury’s, which reaches 700 degrees K (801 degrees F), perhaps over a thousand degrees.
The reason astronomers deduced that the planet Vulcan could exist was due to discrepancies in Mercury’s orbit, noted by the French mathematician Urbain Jean Joseph Le Verrier in 1840-1843 when he was trying to predict the planet’s motion based on the theories of Newton. The procession of its perhelion (closest point to the Sun) around its orbit was about 43 arcseconds per century distant from what Newton’s theories would have predicted. Considering that every other planet in the Solar System moved in ways precisely predicted by Newton’s theories, this was puzzling, and Planet Vulcan was invoked as the cause of the discrepancy. In 1846, Le Verrier discovered the planet Neptune based on the same principle, the perturbations in the orbit of Uranus. This galvanized astronomers to search for the planet Le Verrier predicted.
Beginning in 1859, astronomers began reporting small black spots transiting the surface of the Sun, thought to be the planet Vulcan. Over the next six years, about a dozen alleged observations of black dots transiting the Sun were made, but the transits never managed to rematerialize based on past observations. In 1866, observations of the planet Vulcan began to dwindle, but earlier observations were regarded as “proof” of the planet’s existence, as Le Verrier announced his discovery in 1860. In 1867, two reliable astronomers claimed to have seen a planet similar to Vulcan near the Sun during an eclipse, but in hindsight this was probably a star. Le Verrier died in 1877, still convinced that he had discovered two new planets.
In 1915 it became clear that there had never been a planet Vulcan. Einstein’s new theory of relativity precisely explained the anomaly as a byproduct of the sun’s gravitational field. The new numbers predicted by his theory exactly matched the observations. In more modern times, astronomers have intensively searched the region around the Sun for rocky bodies, such as volcanic asteroids, that might be orbiting in a stable gravitational region right next to the Sun. However, the observations have ruled out anything larger than about 60 km (37 mi) wide. Today the search for Vulcanoid objects continues. Many scientists are skeptical, arguing that the Yarkovsky effect, which alters orbits based on the emission of high-energy photons at an asteroid right next to a star, would make any orbit of a volcanoid asteroid unstable, whether it sinks into the Sun or impacting Mercury. .
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