Every year, Pluto Day on February 18th celebrates the discovery of the former planet. Up until 2006, the dwarf planet was considered one of the nine planets in the solar system.
Clyde Tombaugh, an American astronomer, discovered Pluto. He made the discovery at the Lowell Observatory on February 18th, 1930 84 years after the discovery of Neptune. Pluto was named for the Roman god of the Underworld.
The name also honors Percival Lowell. His initials are the first two letters of Pluto. Lowell was an American astronomer who believed there was another planet beyond Neptune. He initiated the search that led to the discovery of Pluto. On March 19, 1930, Lowell Observatory officially announced the discovery of a 9th planet.
On August 24th, 2006, the International Astronomical Union (IAU) reduced Pluto to a dwarf planet. The IAU determined that Pluto failed to meet certain criteria that defined it as a full-sized planet.
A full-sized planet must:
- Orbit the sun
- Have sufficient mass to assume hydrostatic equilibrium
- Clear the neighborhood around its orbit
Pluto meets the first two criteria, but not the third. In other words, there are no other bodies of comparable size under its gravitational influence.
Pluto Day commemorates the anniversary of its discovery each year on February 18th, 1930.
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