Why Is Pluto Not A Planet Anymore?
QUICK ANSWER
Pluto was reclassified from planet to dwarf planet on August 24, 2006, after a vote by the International Astronomical Union. The decision came after astronomers discovered Eris, a Kuiper Belt object similar in size to Pluto, forcing a debate about what counts as a planet. The new rules excluded Pluto.
Pluto lost its planet status in 2006, not because anything about Pluto changed, but because astronomers kept finding more Pluto-like objects in the Kuiper Belt. The IAU had to decide: keep adding planets to the list, or tighten the definition. They tightened the definition, and Pluto didn't make the cut.
When did Pluto stop being a planet?
On August 24, 2006, at a meeting of the International Astronomical Union (IAU) in Prague. Astronomers voted on three new criteria for what counts as a planet, and Pluto failed one of them. The vote was contentious, with only about 400 of the IAU's 9,000+ members present. The decision was final, though, and Pluto has been officially classified as a dwarf planet ever since. The reclassification was 76 years after Pluto's discovery in 1930.
What triggered the reclassification?
The discovery of Eris in 2005. According to NASA, astronomers found Eris in the Kuiper Belt and initially thought it might be larger than Pluto (more recent measurements suggest they're roughly the same size, with Pluto slightly larger). Either way, Eris forced a question: if Eris is a planet, then so are many other large Kuiper Belt objects, and the solar system suddenly has dozens of planets. The IAU chose to tighten the definition instead of expanding the planet list.
What are the three new planet criteria?
A planet must orbit the Sun, be round under its own gravity, and have cleared its orbital neighborhood. Pluto meets the first two but fails the third. The Kuiper Belt is full of icy objects similar in size to Pluto, none of which Pluto has dispersed or absorbed. Earth, by contrast, has cleared the inner solar system of comparable objects. The third criterion is the one most scientists argue about, since it depends on how strictly you define neighborhood and clearing.
Was there opposition to the change?
Yes, immediately and persistently. Alan Stern, the lead scientist on NASA's New Horizons mission, was one of the most vocal critics. Stern and others argued that the orbital-clearing criterion was ad hoc and that a planet should be defined by its own properties rather than its environment. Some states passed symbolic resolutions declaring Pluto a planet. The IAU has held its ground, and the dwarf planet category has gradually become accepted in scientific contexts, though the public hasn't entirely moved on.
Pluto stopped being a planet in 2006 because the IAU adopted a new definition specifically to exclude it, after the discovery of Eris made it clear there could be many similar objects. The change wasn't about Pluto itself, just about how to handle the growing list of similar bodies in the Kuiper Belt. Whether the decision was right or wrong is still debated, but the official classification hasn't changed.
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