What is a Dwarf Planet?
Pluto was discovered in 1930 by Clyde Tombaugh, a 22-year-old from Kansas working at the Lowell Observatory in Flagstaff, Arizona, USA. After almost a year of searching, he detected its movement based on predictions made by astronomers. The name “Pluto” (its formal designation being 134340 Pluto) was proposed by 11-year-old Venetia Burney, a student at Oxford, who believed that the dark and cold planet was fittingly associated with the Roman god of the Underworld, Pluto. On March 24, 1930, after a vote by members of the Lowell Observatory, the name Pluto was officially chosen.
In Japan, after the discovery of Pluto, a Japanese newspaper suggested translating the planet’s name to “Minh Vương Tinh” (Pluto), a title associated with King Yama, the lord of the underworld in Buddhism. China also referred to the planet as “Minh Vương Tinh.” However, since “Minh Vương” also means “wise king” in Chinese, Vietnamese adopted the name “Diêm Vương” (Pluto), another term for King Yama, instead. Consequently, the names “Diêm Vương Tinh” or “Sao Diêm Vương” have been used for this celestial body.
Why Isn’t Pluto Considered a Planet?
From its discovery until the first half of 2006, Pluto was widely accepted as the ninth planet in the Solar System. Numerous publications and documents over the span of 70 years referred to the Solar System having 9 planets. Although Pluto’s size and mass are much smaller than those of its planetary counterparts, it held its place. Its orbit also stands apart, with an elliptical shape compared to the nearly circular orbits of the other eight planets. Its distance from the Sun ranged from 30 AU (closer than Neptune) to 49 AU (farther than Neptune), with an orbit that was distinctly independent.
However, as technological advancements allowed for more celestial bodies to be observed within the Solar System, astronomers made a significant discovery. They realized that Pluto was part of a group of large bodies located in the Kuiper Belt, a region containing objects that extend from the orbit of Neptune to 44 AU from the Sun. Furthermore, several objects of comparable size and mass to Pluto were found. The introduction of Eris in 2005 posed a challenge, as it had a size and mass 25% larger than Pluto’s. If Pluto were classified as a planet, Eris would also qualify. To address this, the International Astronomical Union (IAU) organized a meeting in August 2006 in Prague, Czech Republic. At the conference, 3,000 astronomers voted to either maintain the existing Solar System model without Pluto or include both Eris and Ceres (the largest asteroid in the asteroid belt between Mars and Jupiter) as planets.
The result was Pluto’s removal from the list of planets in the Solar System. Additionally, the conference established a set of criteria for an object to be considered a planet:
- Orbiting a star or remnants of a star.
- Having sufficient mass for self-gravity to overcome rigid body forces and assume nearly round shape.
- Dominating its orbit region and clearing its orbital path of other debris.
Pluto met the first two criteria but failed to meet the third, as its mass was not significant enough compared to other objects in the Kuiper Belt. Consequently, Pluto was reclassified as a dwarf planet. Dwarf planets are celestial bodies that meet the first two criteria and, following this redefinition, five have been identified: Pluto, Eris, Ceres, Haumea, and Makemake.