By the first half of 2006, people still held the belief that the Solar System consisted of 9 planets, listed in order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. However, Pluto was later “demoted” from this list and reclassified as a dwarf planet.
What is Pluto?
Pluto, also known as Sao Diêm Vương, was discovered in 1930 by Clyde Tombaugh, a 22-year-old from Kansas working at the Lowell Observatory in Flagstaff, Arizona, USA. After nearly a year of searching based on astronomers’ predictions, he observed the movement of this celestial body. Subsequently, the name Pluto (the formal designation of Pluto is 134340 Pluto) was proposed by an 11-year-old girl named Venetia Burney, who was studying in Oxford. She suggested the name Pluto because she thought this dark and cold planet was a fitting abode for 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 into Japanese as “Minh vương tinh” (Minh vương is a common reference to Yama, the Lord of Hell in Buddhism). China also referred to this planet as “Minh vương tinh” from 1933. From then on, both “Diêm vương tinh” and “Sao Diêm Vương” have been used to refer to this celestial body.
Why is Pluto not considered a planet?
From its discovery until the first half of 2006, Pluto was widely accepted as the ninth planet in the Solar System. Books and documents published during more than 70 years explicitly mentioned that the Solar System had 9 planets. Although this ninth planet was much smaller in size and mass compared to its planetary “colleagues,” its orbit was significantly different: the other eight planets had nearly circular orbits, whereas Pluto had a flat elliptical orbit. Pluto’s farthest distance from the Sun is 49 astronomical units (AU), where 1 AU is the distance from Earth to the Sun, and its closest distance is only 30 AU, closer than Neptune. However, because Pluto had an independent orbit, lying beyond Neptune’s orbit and being significantly larger than the other known small celestial bodies, Pluto continued to be considered a planet for many years.
In the late 20th century, as science and technology advanced, more celestial objects within the Solar System were observed. Astronomers noticed something crucial: Pluto was just one of many large objects within the Kuiper Belt, a region containing objects ranging from the orbit of Neptune (about 30 AU from the Sun) to 44 AU from the Sun. Furthermore, as researchers explored the Kuiper Belt further, they discovered several objects with masses comparable to or even greater than Pluto’s. The key turning point came in 2005 when the object 2003 UB313, later named Eris, was discovered. Eris had a mass 25% greater than Pluto’s. If Pluto were a planet, then Eris would also have to be considered one. Moreover, smaller objects would also qualify as planets. To address this issue, 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 traditional Solar System model without Pluto or accept both Eris and Ceres (the largest asteroid in the asteroid belt between Mars and Jupiter) as planets.
The Kuiper Belt consists of celestial objects located beyond the orbit of Neptune.
As a result, Pluto was removed from the list of Solar System planets. The conference also established a general definition of a planet, which includes three criteria:
- It must orbit the Sun or another star.
- It must have sufficient mass for its self-gravity to overcome rigid body forces and assume a nearly round shape (hydrostatic equilibrium).
- It must clear its orbit of other debris and objects, demonstrating its dominance in its orbital neighborhood.
Pluto met the first two criteria but failed the third one because its mass is significantly smaller than the combined mass of the objects in the Kuiper Belt along its orbit.
Consequently, Pluto was reclassified as a dwarf planet. Dwarf planets are celestial bodies that meet the first two criteria but do not dominate their orbital regions. In total, five dwarf planets have been identified, including Pluto, Eris, Ceres, Haumea, and Makemake.