The Solar System Nearly Had 12 Planets Before Pluto Was Demoted
The vote by about 424 astronomers set a three-part definition of planet and left Pluto in a new dwarf planet class.
- Twenty years ago today, on August 24, 2006, the International Astronomical Union reclassified Pluto as a dwarf planet during a General Assembly in Prague, rendering textbooks and classroom materials obsolete by dinner.
- Resolution 5A required planets to 'clear the neighbourhood around its orbit,' but Pluto failed this criterion because it shares its zone with thousands of icy bodies in the Kuiper Belt.
- NASA's New Horizons spacecraft revealed active geology during a 2015 flyby, capturing nitrogen glaciers and the thousand-kilometer heart named Tombaugh Regio, contradicting the frozen-world assumption.
- A 2022 study in Icarus by planetary scientist Philip Metzger argued that planets should be classified by internal makeup and geological behavior rather than orbital dominance, reigniting the debate.
- Twenty years later, researchers remain divided over whether Pluto's physical characteristics or its orbital dominance should determine planetary status, leaving taxonomy unresolved.
13 Articles
13 Articles
Kreiszeitung
Pluto has been a dwarf planet for 20 years. But why was it degraded – and why does the decision continue to cause controversy today?
20 Years Ago Today, 424 Astronomers Voted Pluto Off the Planet List. The Argument Has Never Ended.
The vote was 424 hands in a half-empty hall on the assembly's last day, and the definition's third clause — cleared the neighborhood — is one the critics say Earth itself fails. Twenty years to the day, the full story of the demotion, the revolt, and the world the probe found anyway.
On this day in space! Aug. 24, 2006: Pluto loses its planet status
On Aug. 24, 2006, the International Astronomical Union (IAU) decided that Pluto wasn't a planet anymore. Since that date, Pluto has been officially classified as a dwarf planet along with four other confirmed dwarf planets in the outskirts of the solar system. This decision sparked outrage around the world, and the controversy is still very much alive today. Astronomers in favor of Pluto's demotion argued that a planet by definition should have …
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