Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary scienceOPEN ACCESS 

Philip T. Metzger, W.M. Grundy, Mark V. Sykes, Alan Stern, James F. Bell III, Charlene E. Detelich, Kirby Runyon, Michael Summers

Available online 28 October 2021



• Vital issues were not sorted out before the rushed planet definition in 2006.
• Pragmatic science since the Copernican Revolution has included moons as planets.
• The concept that moons are not planets came from 1800s astrology and teleology.
• Planets in any orbital state are unique as engines of complexity in the cosmos.
• Defining planets this way aligns demonstrably with both historic and modern usage.”

“We argue that taxonomical concept development is vital for planetary science as in all branches of science, but its importance has been obscured by unique historical developments. The literature shows that the concept of planet developed by scientists during the Copernican Revolution was theory-laden and pragmatic for science. It included both primaries and satellites as planets due to their common intrinsic, geological characteristics. About two centuries later the non-scientific public had just adopted heliocentrism and was motivated to preserve elements of geocentrism including teleology and the assumptions of astrology. This motivated development of a folk concept of planet that contradicted the scientific view. The folk taxonomy was based on what an object orbits, making satellites out to be non-planets and ignoring most asteroids. Astronomers continued to keep primaries and moons classed together as planets and continued teaching that taxonomy until the 1920s. The astronomical community lost interest in planets ca. 1910 to 1955 and during that period complacently accepted the folk concept. Enough time has now elapsed so that modern astronomers forgot this history and rewrote it to claim that the folk taxonomy is the one that was created by the Copernican scientists. Starting ca. 1960 when spacecraft missions were developed to send back detailed new data, there was an explosion of publishing about planets including the satellites, leading to revival of the Copernican planet concept. We present evidence that taxonomical alignment with geological complexity is the most useful scientific taxonomy for planets. It is this complexity of both primary and secondary planets that is a key part of the chain of origins for life in the cosmos.”