Pluto Day is celebrated annually to commemorate the anniversary of the discovery of Pluto in 1930. Although Pluto was discovered in 1930, the story of its discovery started in 1840 after French astronomer Urbain Le Verrier sensed that there was a planet outside of Uranus due to irregularities in its orbit. His intuition led him to develop mathematical calculations to explain the discrepancies in Uranus’s orbit in relation to the laws of planetary motion and gravity, which led to the eventual discovery of Neptune.

After Neptune was discovered, an event widely regarded as a validation of a subset of the astronomy practice called celestial mechanics. It was then realized that there was yet another planet disturbing Uranus’s orbit since the irregularity in its orbit continued. This led to the search for Pluto — initially called Planet X — being headed by Percival Lowell, whose death would later see the search for Pluto passed to Clyde Tombaugh, who eventually discovered it.

The planet, which was named after the Roman god of the Underworld, was considered one of the nine planets in the solar system up until 2006. The International Astronomical Union reduced its status and tagged it a ‘dwarf’ planet due to not meeting the criteria to be considered a full-sized planet and being two-thirds of the size of the earth. It is believed that the first two letters in ‘Pluto’ were in honor of Percival Lowell whose belief that there were other planets beyond Neptune, helped fuel the drive that led to its discovery.