Why did astronomers once search for a planet called Vulcan inside Mercury’s orbit?

In the 19th century, astronomers observed that the orbit of Mercury exhibited an anomalous precession, meaning the point of its closest approach to the Sun—its perihelion—shifted over time in a way that could not be fully explained by the gravitational influence of the other known planets.123

Following his successful prediction of Neptune based on irregularities in the orbit of Uranus, the French astronomer Urbain-Jean-Joseph Le Verrier applied similar logic to Mercury. In 1859, he postulated that an undiscovered planet or an asteroid belt existing inside Mercury’s orbit was exerting the necessary gravitational pull to cause the observed shift. When an amateur astronomer reported seeing a dark spot crossing the Sun, Le Verrier concluded it was this missing planet and named it Vulcan.123

However, Vulcan was never confirmed by subsequent observations. The mystery was ultimately resolved in 1915 by Albert Einstein, whose general theory of relativity demonstrated that the precession was caused by the curvature of space-time near the Sun, rendering the hypothesis of a hidden planet unnecessary.134