Why Is Pluto Not a Planet Anymore? What is the Definition of a Planet Now?

In the past 200 years, the round celestial bodies discovered in the solar system were declared planets. When Pluto was discovered in 1930, one did not know well enough to classify it correctly. Since 2006 Pluto is no longer considered a planet but only a dwarf celestial body. But why is Pluto not a planet anymore? Let’s find out.

A new definition of the word PLANET was decided at the General Assembly of the International Astronomical Union in Prague, and Pluto did not meet its criteria. Since then, it was no longer a planet but a dwarf planet, a term that actually means nothing other than a large asteroid. But what changed, and why is Pluto not a planet anymore?

Why Is Pluto Not a Planet?

When Pluto was discovered in 1930, it was thought to be a continuation of the planetary series and was celebrated as the ninth planet. For a while, it was still thought to be as big as the Earth, and till 1950, it was considered to be only half the size of the Earth.

Over the time, astronomical instruments like telescopes got better and better. The astronomers could now see that Pluto is much smaller than previously thought. Today, its diameter is estimated at around 2,300 kilometres. This would make Pluto even smaller than the Earth’s moon. Together with other criteria, this led to Pluto being renamed a dwarf planet in 2006.

In the end, it’s all a question of definition. Pluto could still be called a Planet. However, there would be many more celestial bodies that also meet the criteria. In which case, there would no longer be just nine planets as long, but ten or even 15. The IAU decided against this and set the criteria so that there are only eight planets.

Pluto orbits around the sun once every 250 years, billions of kilometres from Earth. Astronomers have discovered many celestial bodies similar to Pluto, like Quaoar or Orcus, Sedna and Makemake. They are all pitch black and hardly reflect the light of the sun. Beyond Neptune is a whole ring of dark, icy little bodies that are a part of the Kuiper belt.

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Only the history of Pluto’s discovery distinguishes it from the other objects in the Kuiper belt. It is a major reason for the rectification of the original decision and why Pluto is not a planet anymore.

The Controversial Degradation of Pluto.

The reasons given for why Pluto is not a planet anymore were controversial among the astronomers. It is particularly hotly debated by the public. The United States is particularly annoyed by the degradation of Pluto. After all, Pluto was the only planet discovered by an American, Clyde Tombaugh.

American astronomers keep making new proposals for the definition of a planet through which Pluto can regain its old status.

According to a few definitions proposed at IAU, we would then have to label 100 other celestial bodies in our solar system. Whether a celestial body is spherical or not depends mostly on its size and material.

Whether a celestial body is spherical or not actually says something about its internal structure and can certainly be used as an essential parameter for planetary studies.

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There are many ways to define what a planet is. Still, only one of them will be really satisfactory even if that is the reason why Pluto is not a planet anymore. Nature has not provided rigid boundaries for heavenly bodies.

According to man-made definitions, it makes more sense not to put Pluto and all the other small asteroids in the same category as gas giants in the format of a Jupiter.