Why Is Pluto Not A Planet?

For decades, Pluto was celebrated as the ninth planet of our solar system. However, in 2006, the International Astronomical Union (IAU) redefined what constitutes a planet, leading to Pluto’s reclassification as a dwarf planet. This decision sparked widespread debate among scientists and the public alike. So, what changed? Why did Pluto lose its planetary status? Let’s explore the reasons behind this significant shift in astronomy. 

Pluto was discovered on February 18, 1930, by Clyde Tombaugh at the Lowell Observatory in Arizona. Its existence was predicted as astronomers searched for a "Planet X" to explain irregularities in Neptune and Uranus' orbits. At the time, Pluto was assumed to be a planet due to its orbit around the Sun, despite being much smaller than initially believed. 

The name "Pluto" was suggested by Venetia Burney, an 11-year-old girl from England, who named it after the Roman god of the underworld. The name was fitting since Pluto is located in the cold, dark outer reaches of the solar system. 

However, over the years, astronomers learned that Pluto was not unique. It was just one of many small icy bodies in the Kuiper Belt, a region beyond Neptune filled with countless frozen objects. The discovery of several Pluto-like objects led scientists to question whether Pluto should remain classified as a planet. 

Why Pluto Is Not A Planet? 

In 2006, the IAU introduced a new definition of what makes a celestial body a planet. According to this definition, a planet must meet three key criteria: 

• It must orbit the Sun. 
• It must be massive enough for its gravity to pull it into a nearly round shape.
• It must have "cleared the neighbourhood" around its orbit – This means the planet should be gravitationally dominant in its region and not share its orbit with other similar-sized objects. 

Pluto failed the third criterion. Unlike the eight recognized planets, Pluto shares its orbit with many other objects in the Kuiper Belt, meaning it has not "cleared its neighbourhood." Because of this, Pluto no longer fit the definition of a full-fledged planet and was reclassified as a dwarf planet on August 24, 2006. 

Also read: Types of Satellites and their applications

The Role of Eris in Pluto’s Reclassification

In 2005, astronomers discovered Eris, a celestial body in the Kuiper Belt that was initially thought to be larger than Pluto. This discovery created a dilemma: If Pluto was considered a planet, should Eris and other similar objects also be classified as planets? This would have potentially increased the number of planets in our solar system to dozens.

To avoid confusion, the IAU revised its definition of a planet, leading to Pluto’s demotion. Alongside Pluto, Eris was also classified as a dwarf planet under the new criteria.

Scientific and Public Reactions

Pluto’s reclassification triggered an emotional response, especially among those who had grown up with nine planets. Many scientists and space enthusiasts disagreed with the decision, arguing that the IAU’s definition was too strict. 

One of the most vocal critics of Pluto’s demotion was Alan Stern, the principal investigator of NASA’s New Horizons mission, which explored Pluto in 2015. Stern argued that even Earth hasn’t fully cleared its orbit, as asteroids exist in its orbital path. 

Despite the controversy, Pluto remains an object of great scientific interest. The New Horizons mission provided stunning images of Pluto’s surface, revealing glaciers, mountains, and potential subsurface oceans—proving that Pluto is an active and dynamic world, regardless of its classification. 

Pluto's Legacy and the Future of Planetary Science

Though no longer officially a planet, Pluto continues to be a key focus of planetary research. Its reclassification has led to a better understanding of the structure of our solar system and has opened the door for further studies of the Kuiper Belt and other distant objects.

Also read: Types of Orbits and their applications