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Pluto stopped being a planet 20 years ago. Why people still care

The 2006 vote followed Eris’s discovery and set a formal planet definition that Pluto still fails because it has not cleared its orbit.

  • On August 24, 2006, the International Astronomical Union reclassified Pluto as a dwarf planet, a decision that remains in effect today and continues to divide the scientific community two decades later.
  • The 2005 discovery of Eris, an icy world beyond Pluto that is even more massive, forced scientists to establish a formal definition of the word "planet," which previously had no official scientific meaning.
  • At its Prague General Assembly, the International Astronomical Union voted that a planet must orbit the Sun, be round, and clear its orbital neighborhood; Pluto failed the third requirement because it shares its path with many Kuiper Belt objects.
  • The reclassification sparked widespread public debate, with students, teachers, and some astronomers objecting to the change after 76 years of learning the nine planets through mnemonics like "My Very Educated Mother Just Served Us Nine Pizzas."
  • As astronomers continue discovering unusual worlds beyond the Sun, the question that divided scientists in 2006 remains difficult to answer, leaving researchers to debate what exactly qualifies as a planet.
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Pluto stopped being a planet 20 years ago. Why people still care

As astronomers continue discovering unusual worlds beyond our solar system, many say the question that divided scientists in 2006 remains difficult to answer: What exactly is a planet?

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The Billings Gazette broke the news in Billings, United States on Monday, September 7, 2026.
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