Planetology: Using the history of science against Pluto’s “degradation”

Even more than 15 years after Pluto was rededicated as a dwarf planet, the debate about the underlying decision is not over. A research group has now submitted a detailed study to prove that it was a mistake. After analyzing hundreds of scientific articles, they came to the conclusion that the definition of the term “planet” had changed over and over again in the past few years – but not in the way generally assumed. The idea that planets are a small group of objects orbiting a common center is based on a popular judgment that has been adopted by science. The true history of the concept is forgotten.

With the study The group around the planetary scientist Philip Metzger is trying to add new arguments to the debate about Pluto and the definition of planets that has been going on for years. Once the ninth planet in the solar system, it lost its status in August 2006 when the International Astronomical Union adopted a new scientific definition. Since then, our solar system has only eight planets. Hundreds of lexicons, school and textbooks had to be changed – but the dispute about the “degradation” of Pluto continues to smolder among experts. Since then, it has only been a dwarf planet, and their number continues to grow unabated.

According to the IAU definition that has been in effect since then, a planet is a celestial body that orbits the sun, is not a moon, has an approximately spherical shape – the technical term for this is hydrostatic equilibrium – and has cleared its surroundings of other celestial bodies. This definition is far too narrow and is based primarily on an unscientific classification that only took hold in science in the 20th century – when the focus there was on other topics. According to the analysis, this popular narrow definition was motivated, among other things, to preserve elements of the previously popular geocentric worldview. The popularity of astrology also contributed to the desire to have as few planets as possible that are clearly defined.

The complex conceptual history can be illustrated by way of example on the dwarf planet Ceres. After its discovery in 1801, it was considered a planet for a long time. When more and more large objects were found in the asteroid belt between Jupiter and Mars, it was generally assumed that it lost this name. Scientific articles would prove, however, that Ceres was seen as a planet well into the 20th century, sometimes with restrictions such as “secondary planet”. According to the team, it was only the discovery of the various shapes of the objects in the asteroid belt that contributed to the fact that all objects there were no longer indiscriminately referred to as planets.

Pluto’s surface
(Bild: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

Moons, in turn, were referred to as planets (or “secondary planets”) until the 1920s. In both cases, the experts had lost interest and did not prevent the popular classification from becoming established, writes Metzger and his team now. You are asking once again to reconsider the decision made in 2006 and want to do so with yours now published study argue that this would return to a planetary definition that had lasted for centuries. You therefore advocate adopting the so-called “geophysical planetary definition”. According to this, planets are substellar objects in which a nuclear fusion has never ignited and which have a hydrostatic equilibrium.

This means that it is no longer the environment of an object that determines its status, but rather intrinsic features of the celestial body itself. In the solar system, adopting this definition would drastically increase the number of planets, they admit. In addition to the eight classical planets and Pluto, this would not only include Ceres, but also a whole series of moons, including that of Earth. But that is not problematic and, on the contrary, it could rekindle the waning interest in discovering new celestial bodies in the solar system, Metzger recently told Science News. And if kids can remember hundreds of dinosaur or Pokémon names, so can many more planets.


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