Dwarf planet
Why is Pluto a dwarf planet and not a planet?
A planet has to clear its neighbourhood. Pluto shares the Kuiper belt with a great many other bodies, one of them — Eris — more massive than Pluto. The world itself did not get smaller in 2006. The rule about the neighbourhood did. Pluto and its moons is still a place: ice mountains, a nitrogen glacier, five moons.
Three tests, and Pluto fails the third
In 2006 the IAU asked three things of a planet: it orbits the Sun, it is round because of its own gravity, and it has cleared the other things on its path. Pluto does the first two. It fails the third. The Kuiper belt is full of leftover worlds on similar orbits. Ceres, in the asteroid belt, fails the same test.
Calling it a dwarf planet is a statement about the region, not an insult to the ice. New Horizons found mountains, a heart-shaped glacier with no craters on it, and a blue haze in dozens of layers.
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Why the rule exists
If roundness alone made a planet, the solar system would have dozens, and more every time a telescope found another round Kuiper belt object. The neighbourhood test keeps “planet” for the eight that dominate their paths. It is a census rule. It does not change what Pluto is made of, or that Charon makes it a double world.