Since when is a piece of rock called a planet?

Answer

The definition drawn up by the International Astronomical Union in 2006 for a ‘planet’ is as follows:

A planet is a celestial body that
1. orbits the Sun;
2. has sufficient mass so that the self-gravity (the attraction that the planet exerts on itself as a result of its masa) dominates the forces within the planet in such a way that it takes on an almost round shape (a surface where nothing can slide down anymore);
3. has cleaned up her environment (in other words, she rules alone in her job).

It is this last criterion that Pluto does not meet. Since 1992, hundreds of objects, some similar to Pluto in size, have been discovered orbiting Pluto. Rather, Pluto is now regarded as one of the largest objects in a belt of “minor planets” beyond Neptune, as the asteroid belt of minor planets between Mars and Jupiter was previously known. There’s more to it than just nomenclature: in the early solar system, the planets formed precisely by the clumping together of smaller objects, and in that sense Pluto is rather a fragment on its way to form a planet (which will never come).

The minor planets that do not meet criterion 3, but do meet criterion 2 (they are approximately round), have been named ‘dwarf planet’ as consolation prizes. Within the asteroid belt, Ceres complies, beyond Neptune, in addition to Pluto, also Eris, Makemake and Haumea. The many large satellites of some planets, such as ours, are excluded from this definition.

To some extent, the above definition is tampering. According to Criterion 1, the “exoplanets” that we now discover numerous around other stars are not planets. The term ‘almost round’ in criterion 2 does not sound very scientific. And strictly speaking, Criterion 3 excludes even the largest planet, Jupiter, because Jupiter has just trapped a number of minor planets, the Trojans, in its orbit 60 degrees in front of and 60 degrees behind the planet.

Originally ‘planet’ meant wandering star (for the Greeks this also included Sun and Moon), in contrast to the ‘fixed’ stars. Now we know that they are not fixed either, but move very slowly in the sky. The only thing we can’t see moving in the sky is ourselves, the Earth we’re standing on. However, since Copernicus, Earth has also become a planet! You see, it’s a series of crazy situations, which fortunately don’t matter much. It’s not the name that matters, but what it all means.