Is it true that before 2006 Pluto was a planet, and now it no longer is? Or was Pluto never a planet by IAU's post-2006 definition, and still is a planet by the pre-2006 definition? You can't change what something is just by changing a definition right?

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Many concepts in science are at least in part socially constructed. That does not mean that the world is socially constructed, just that our concepts about the world are devised by scientific communities. "Planet" is one of those terms that is partially socially constructed. Over the last 5 years that social construction has become visible in the debate over Pluto's status. "Planet" was first used to mean "object revolving around the sun." But then all kinds of small objects--comets and meteorites--also revolve around the sun, and the decision was made not to call them all planets, but only to call the sizeable ones planets. "Sizeable" reflects our interests as Earth inhabitants in revolving objects of about the same size as we are. But then in the late twentieth century thousands of celestial bodies of the same size or larger than Pluto were found in the Kuiper belt. Scientists could have decided to call them all planets, so that we would have thousands of planets in the solar system, but they prefered (for aesthetic reasons?) a smaller number. So they made up another condition that effectively rules out the thousands of planets in the Kuiper belt, but also ends up making Pluto no longer a planet (the condition was "effectively cleared its neighbouring region of planetismals"). Pluto is the same old lump of rock. It is classified as just one of the planetismals in the Kuiper belt.

The IAU voted on adding this condition, and the majority of voters supported it. There is continuing opposition to the condition from several astronomers who would have preferred to have thousands of planets than to drop Plato for seemingly arbitrary (aesthetic?) reasons.

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