Why Pluto is not a planet
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Why Pluto is not a planet

Author: MozaicNook

Losing its planetary status in 2006 occurred when International Astronomical Union (IAU) redefined the criteria for classifying a planet. Therefore, an object in the solar system becomes a planet if it meets three main conditions recently defined:

  • It must orbit around the Sun. Pluto fulfills this criterion because its trajectory goes round the Sun.
  • The force of gravity causes objects to be pulled towards one another and due to their weight they tend to form spherical shapes and become planets. As such, Pluto fits in here as well since it is big enough to have formed a sphere-like shape.
  • No other celestial body should share the same orbit with it. Pluto does not satisfy this condition. In other words, there are numerous other heavenly bodies within Pluto’s orbital plane in Kuiper Belt.

The last principle has led to Pluto being demoted to a “dwarf planet,” a new category introduced by IAU.

By clearing an orbit, this means that over time it will become gravitationally dominant body in its orbital path around the Sun among others that either get attracted or accreted (bound) smaller items, push them out of their own orbits and otherwise remove them from their paths. This way “a planet effectively ‘clears’” all the minor masses and debris near where it revolves.

Classical planets like Earth, Jupiter or Mars have cleared up their orbits during billions of years and became dominant objects in corresponding orbital zones. The difference between their mass accumulated by all other objects located nearby was significant – these were much more massive than any other objects situated within their spheres of influence.

However, unlike these two dwarf planets, Pluto has not been able to clear up its orbit completely since there are numberless small icy bodies including dwarf planets moving along with it inside Kuiper Belt. Thus, it is referred as a dwarf planet but not as a real 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 is one of the largest objects in Kuiper Belt, even larger than Pluto and also nearly spherical. Eris would most likely be referred to as a planet if only clearing the orbit premise were left out.

Haumea and Makemake

These two items too contain rounded shapes and possess considerable sizes among all things found in Kuiper Belt. Use of this criterion will therefore indicate that they fail to satisfy some criteria for being regarded as either Eris or Pluto, but they can easily fit into another two.

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 has made it possible to classify more accurately and distinguish between major members and minor heavenly bodies within our Solar System.

Whether Pluto should retain its former status of a planet or become a dwarf has always been argued by astronomers and enthusiasts on astronomy. By including Pluto again there are some experts who are optimistic that there is need for changing or broadening such definitions regarding what a planet is.

Changes in astronomy’s classifications of celestial bodies come about through a consensus of the scientists based on discoveries and findings. In case enough members of the International Astronomical Union (IAU) agreed that changes were necessary, they could re-open deliberations regarding criteria for planet classification. Until then, Pluto will remain classified as a dwarf planet.