Why is Pluto not a planet any more?

Because the definition of "planet" was written for the first time in 2006, and Pluto fails one of its three conditions — not because anything about Pluto changed, but because what we knew about its surroundings did.

What prompted it. For decades Pluto was an oddity: far smaller than the other planets, on a tilted, eccentric orbit that crosses Neptune's. Then astronomers began finding other objects like it in the same region — the Kuiper belt — and in 2005 discovered Eris, which is comparably sized and more massive.

That created an unavoidable decision: either admit Eris and a growing queue of similar bodies as planets, or define the term in a way that excluded them. The number of "planets" was going to rise into the dozens.

The definition adopted. A planet must:

Orbit the Sun.

Be massive enough for its own gravity to make it roughly round — hydrostatic equilibrium.

Have "cleared the neighbourhood" around its orbit.

Pluto satisfies the first two and fails the third. It shares its orbital region with many other Kuiper belt objects, and is dominated by Neptune's gravity rather than dominating its own surroundings. It was reclassified as a dwarf planet.

What "clearing the neighbourhood" actually means. Not that the orbit is empty — Earth's is not — but that the body is gravitationally dominant, having swept up or ejected most other material. Measures of this show a very large gap between the eight planets and everything else, which is the strongest argument that the criterion identifies something real.

The objections, which are substantive: the definition applies only to our solar system as written; "cleared" is imprecise; and some planetary scientists prefer a geophysical definition based on the body itself rather than its orbit — under which Pluto, and several moons, are planets.

The vote itself was taken by a small proportion of the membership and remains contested.

Pluto is unchanged — and far more interesting since a spacecraft visited it in 2015.