April 24, 2025

Shivika Mangaraj

10th Grade

Williamsville East High School

On August 24, 2006, our solar system went from having nine planets to eight. The reclassification of Pluto shocked millions, as it had been considered a planet for over 70 years. But what actually classifies a planet as a planet, and why was Pluto reclassified as a dwarf planet? Because Pluto isn’t capable of controlling its orbit and removing other similarly sized objects near its path due to its small size. 

NASA states, “The definition of a planet adopted by the IAU says a planet must do three things: 

1. It must orbit a star (in our cosmic neighborhood, the Sun). 

2. It must be big enough to have enough gravity to force it into a spherical shape. 

3. It must be big enough that its gravity has cleared away any other objects of a similar size near its orbit around the Sun.” 

Looking at these rules, what makes Pluto incapable of meeting them, despite its appearance as a planet? Pluto does, in fact, orbit a star—it orbits the Sun of our solar system. It also has sufficient mass to take on a spherical shape. Although Pluto fulfills two out of the three criteria for being a planet, it lacks the ability to clear its orbital path of other nearby objects.

This reclassification was brought into focus by the discovery of similar objects. Using advanced technology, astronomers discovered Eris, a dwarf planet located in the Kuiper Belt, a region beyond Neptune filled with icy bodies. This led to the realization that Pluto is not the only “planet” unable to clear its orbital path. Consequently,  Pluto was reclassified as a dwarf planet when the International Astronomical Union (IAU) recognized that other objects in the Kuiper Belt share similar characteristics.

Not only did Pluto’s reclassification alter the number of planets in our solar system, but it also led to the creation of a new category: dwarf planets. Dwarf planets are defined as objects that orbit the Sun, just like regular planets, but are not able to clear their orbital paths, much like Pluto. Currently, five dwarf planets are officially recognized: Pluto, Ceres, Haumea, Makemake, and Eris. These dwarf planets are presently uninhabitable for humans due to their extreme temperatures and harsh conditions.

Pluto’s reclassification altered the way we perceive our solar system and how we study celestial bodies. The category of "dwarf planet" emerged as astronomers redefined what it means to be a planet, especially after discovering objects similar to Eris. Although Pluto is no longer considered a major planet, it remains scientifically significant and continues to contribute to our understanding of the solar system.