Why Is Pluto No Longer A Planet?

By now, you must have probably heard that Pluto is no longer a planet. But have you ever wondered why and how did that happen? Well, let’s get to it!

Pluto: Basic Facts

First, some basics. Pluto came to be discovered in the year 1930 by the astronomer called Clyde Tombaugh from the United States. The planet got its name in the same year from an 11-year-old girl named Venetia Burney who was from Oxford, England. And if you thought she named it after the Disney character Pluto, you are wrong. She actually named it after the Roman God of the Underworld.

How Did It All Begin?

Over the years, astronomers began finding more and more icy objects in the Kuiper Belt (where Pluto is located) that had the same composition as Pluto. In 2005, an astronomer, Mike Brown, discovered a new object beyond Pluto which appeared to be larger than Pluto. He named the object Eris. It had approximately 25% more mass than Pluto. This shook the very foundation on which a planet came to be ‘defined’. And hence began the controversy of what would actually constitute a planet?

Definition of a Planet

In August 2006, The International Astronomical Union (IAU) set up a committee to define a ‘planet’ and put a final draft proposal before the General Assembly in Prague. On the last day of the assembly, the members voted to adopt a new definition for naming a planet. For an object to be classified as a planet it had to meet three requirements:

  • It must orbit the sun.
  • It had sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
  • It had ‘cleared the neighborhood’ around its orbit.

Now, if you wonder what ‘cleared the neighborhood’ means, we don’t blame you. As planets form, they become the dominant gravitational body in their orbit. When they interact with other objects, they either gain their mass or sling them away with their gravity.

Pluto meets the first two criteria of the definition. It orbits around the sun and it has a nearly round shape. But it fails to meet the third criteria and so it was dropped as a planet. Until Pluto interacts with more objects and gains their mass, it will remain a dwarf planet, which is neither a planet nor a natural satellite.

So our Solar System now has eight planets and for any other celestial body to qualify as a planet it will now have to meet the new definition parameters.

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