Why Pluto is not a planet? This is a question often asked by fans of this charming dwarf planet on the edge of the Solar System. Pluto lost its planetary status in 2006 when the International Astronomical Union (IAU) redefined the criteria for classifying planets. According to the new definition, an object in the solar system is considered a planet if it fulfills three primary conditions:
- It orbits the Sun. Pluto fulfills this condition because its orbit goes around the Sun.
- It has sufficient mass for its gravity to overcome the structural strength of the material and keep it in a nearly spherical shape. Pluto also fulfills this criterion, as it is massive enough to be spherical.
- It has cleared its orbit of other debris: Pluto does not fulfill this criterion. Pluto shares its orbit with many other objects in the Kuiper belt, meaning it is not the dominant object in its orbit.
Based on this last criterion, Pluto has been reclassified as a "dwarf planet," a new category introduced by the IAU.
The term "clearing its orbit" refers to the ability of a celestial body to become the dominant gravitational force in its orbit around the Sun over time. This means a planet must have enough gravitational pull to either attract and accrete (bind to) smaller objects, push them out of its orbit, or otherwise remove them. In this way, a planet effectively 'clears' the space around its orbit of other smaller bodies and debris.
Classical planets in our solar system, such as Earth, Jupiter, or Mars, have cleared their orbits over billions of years and become dominant objects in their orbital zones. Comparing their mass with the total mass of other objects in their orbits reveals a significant difference — they are far more massive than all other objects in their vicinity.
Pluto, on the other hand, has not managed to clear its orbit because it shares space with other objects in the Kuiper Belt, such as other dwarf planets and smaller icy bodies. Therefore, it is classified as a dwarf planet and not as a full-fledged planet. This is the answer to why Pluto is not a planet.
If the condition of clearing the orbit of other objects did not exist, several of the current dwarf planets could be classified as planets. Here are a few examples:
Eris
Eris is one of the most massive objects in the Kuiper belt, even more massive than Pluto and also almost spherical. If the condition of clearing the orbit were not taken into account, Eris would likely be classified as a planet.
Haumea and Makemake
These two objects are also spherical and are among the largest in the Kuiper Belt. Although they are not as massive as Eris or Pluto, their size and shape would fulfill two of the three main criteria for defining a planet.
So, if the IAU had not included the condition that the orbit must be free in the planet definition, more bodies in our solar system would be recognized as planets. This change in the definition has increased the classification's accuracy and helped distinguish dominant planets from smaller bodies in the solar system.
Astronomers and astronomy enthusiasts have debated whether Pluto is a planet or a dwarf planet. Some experts and enthusiasts believe that the definition of a planet should be revised or expanded to include Pluto again.
Changes to the classification of celestial bodies in astronomy usually result from a consensus in the scientific community based on discoveries and findings. If a sufficient number of members of the International Astronomical Union agreed that a change was needed, they could re-discuss and revise the criteria for classifying planets. Until then, Pluto will continue to be classified as a dwarf planet.