Inside the Controversial Definition of a Planet

Sharon Renee Universal /
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What makes a planet a planet? It's a question that's been tossed around for years now. But roughly 15 years ago, that same debate led to Pluto losing its "planet" status. Here's what researchers have to say about the controversial topic.

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15 years ago, in August of 2006, the scientific world was left rattled after its ninth planet was officially no longer. After the International Astronomical Union (IAU) had released new criteria for what constitutes a planet, Pluto was consequentially demoted. But according to astronomer Catherine Cesarsky of CEA Saclay in France, the announcement was actually long overdue... 

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"I believe that the decision taken was the correct one," Cesarsky explained. Why? For starters, "Pluto is very different from the eight solar system planets." And while the intention was never to lessen Pluto's importance, Catherine believes it was necessary to "prototype of a new class of solar system objects, of great importance and interest."

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As for what defines a planet, according to the new criteria? According to the International Astronomical Union, a planet must do 3 things: it must orbit a star (which, in our galaxy, would be the sun), it must have a mass large enough to mold it into a spherical shape, and it must have a defined, clear neighborhood within its orbit. That being said, only 8 planets exist today in our solar system.

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Of course, though, as we know, the universe is a vast, mysterious place. And with endless sects of our galaxy yet to be explored? That number might just change sometime down the line.