A small mismatch at the Sun’s edge
Mercury does not trace the same ellipse around the Sun forever. The point where it comes closest to the Sun—its perihelion—shifts slightly with each orbit, so the ellipse slowly turns. Newtonian gravity explains most of that motion: the pulls of the other planets tug Mercury’s path as they circle the Sun.
But in the 19th century, calculations still left a small unexplained remainder. The discrepancy was tiny, yet persistent enough to look like a clue that something was missing from the solar system’s inventory.
Why astronomers looked for Vulcan
French astronomer Urbain Le Verrier proposed that an undiscovered planet inside Mercury’s orbit could supply the extra gravitational pull. The idea had an appealing precedent: Le Verrier had helped predict Neptune’s existence from irregularities in Uranus’s orbit. If an unseen world could explain one planet’s strange motion, perhaps another could explain Mercury’s.
The proposed planet was named Vulcan. Because it would orbit so close to the Sun, it would be difficult to spot in the Sun’s glare. Astronomers searched for it during solar eclipses and watched for a possible silhouette crossing the Sun, but the reported sightings did not establish that such a planet existed.
A different account of gravity
The missing piece was not another planet but a limitation in Newton’s description of gravity. In 1915, Albert Einstein’s general theory of relativity accounted for Mercury’s remaining perihelion shift. In that theory, the Sun’s mass changes the geometry of spacetime, and Mercury follows a path through that geometry that is slightly different from the Newtonian prediction.
Vulcan was a reasonable attempt to make the calculations fit the evidence available at the time. The resolution mattered because it showed that a stubborn orbital discrepancy could reveal not a hidden object, but a need to rethink the rules used to describe the motion.
