While the five planets visible to the naked eye have been known for almost all of astronomy’s history, it was not until the discovery of Uranus in 1781 by Sir William Herschel that astronomers began searching for other planets in our Solar System. After Neptune was discovered in 1846, astronomers began turning their telescopes towards the sun in search of another planet, seeking to explain Mercury’s strange behavior. It had been known for some time that Mercury’s orbit precessed slowly over time, and in 1859 French mathematician Urbain Le Verrier predicted that there must be another small planet even closer to the Sun whose gravity caused the odd motion. While many astronomers searched for Vulcan over the coming years, no truly confirmed sightings were ever made (as it does not exist) and Mercury’s orbital precession was eventually explained as a relativistic effect of the Sun.
Vandy Astronomy 