Why isn’t Pluto a planet?
What changed in 2006
Nothing about Pluto itself changed — our map of the solar system did. Through the 1990s and 2000s astronomers began finding more and more icy worlds beyond Neptune, in the Kuiper Belt. The turning point was Eris, discovered in 2005 and at first thought to be larger than Pluto (it is fractionally smaller, but about 27% more massive). That forced a decision: either the solar system had a tenth planet — and probably many more to come — or Pluto belonged to a new category. In 2006 the IAU voted for a formal definition of "planet", and Pluto didn’t make the cut.
The three tests for a planet
"Clearing the neighbourhood" means a planet is gravitationally dominant — it has swept up, flung away or captured the other bodies near its orbit. Earth and Jupiter have done this; Pluto, one icy body among thousands sharing the Kuiper Belt, has not. It isn’t about size alone: Pluto is round and geologically fascinating, but it simply doesn’t rule its zone.
The decision is still debated: some planetary scientists argue the "cleared its orbit" test is unfair to distant worlds. But under the current definition, the count stands at eight planets and five recognised dwarf planets. Meet the reclassified world itself on the Pluto page, or see it in the interactive view.
Keep going
Every figure on this page is a real, rounded value checked against primary sources. Found something out of date? See how we keep it accurate.