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Short answer

In 2006 the International Astronomical Union (IAU) reclassified Pluto as a dwarf planet. A full planet must orbit the Sun, be nearly round, and have cleared its orbit of similar objects. Pluto meets the first two but not the third, because it shares the Kuiper Belt with many similar worlds. [S1][S2]

Key points

• Our solar system has five recognized dwarf planets: Ceres, Pluto, Haumea, Makemake and Eris. [S2]
• Pluto is about 1,400 miles wide, about half the width of the United States. [S1]
• New Horizons flew past it in 2015 and found mountains, plains, craters and glaciers. [S1]

Claims

• id: C1 claim: Pluto was reclassified as a dwarf planet in 2006 by the IAU. source: S1 evidence: "Pluto was reclassified as a dwarf planet in 2006 by the International Astronomical Union"
• id: C2 claim: The solar system has five dwarf planets. source: S2 evidence: "Our solar system has five dwarf planets."
• id: C3 claim: Dwarf planets have not cleared their orbits of debris. source: S2 evidence: "but have not been able to clear their orbit of debris"
• id: C4 claim: Pluto is about 1,400 miles wide. source: S1 evidence: "Pluto is only about 1,400 miles wide."

Sources

• [S1] Pluto: Facts. NASA. https://science.nasa.gov/dwarf-planets/pluto/facts/ . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Pluto & Dwarf Planets. NASA. https://science.nasa.gov/dwarf-planets/ . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_fca0dmvx,
  author = {{iinventedlight}},
  title = {{Why Pluto is a dwarf planet}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/pluto-and-dwarf-planets/},
  note = {Permanent ID iil:note:fca0dmvx. License: CC-BY-4.0.},
}

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