As the search for planets beyond our solar system continues, researchers are calling for a clearer definition of what constitutes a planet, and the role of Pluto in astrology plays a surprising role in this debate.
In a recent study published in the journal Icarus, a team of scientists argues that the International Astronomical Union’s (IAU) 2006 decision to demote Pluto from planetary status was influenced by folklore and astrology.
As a result, they recommend revising the current definition.
The IAU’s Definition and Its Flaws
The IAU’s current definition requires a planet to clear its own orbit, meaning it must be the largest gravitational force in its orbit and not share or cross paths with other celestial bodies.
Because Neptune’s gravity influences Pluto and the latter shares its orbit with objects in the Kuiper belt, Pluto lost its planetary status under the IAU’s definition.
The researchers propose that the orbit-clearing requirement should be removed.
Instead, they say, a planet should be defined by its geological activity, a criterion that has been overshadowed in the current definition.
Pluto in Astrology: A History of Planetary Definitions
The study’s authors conducted a five-year review of 400 years of literature on the topic of planets. They found that Galileo’s geophysical definition, which posited that a planet is a geologically active body in space, was used in the scientific literature for most of that period.
This definition was widely accepted from the 1600s, based on Galileo’s observations of mountains on the Moon, until the early 1900s.
But things began to change when there was a marked decline in the number of scientific papers on planetary science from the 1910s to the 1950s. The study suggests that during this period of neglect, the pragmatic taxonomy established by Galileo was interrupted.
Furthermore, the popularity of almanacs, which often relied on astrological factors and required an orderly, limited number of planets for predictions, influenced the understanding of planets.
The researchers argue that astrological views, such as the idea that moons are not planets, started to infiltrate scientific literature.
This shift effectively undermined the central idea of planets as complex, geologically active bodies and led to the redefinition of planets based on their idealized paths around the Sun.
Reigniting Interest and the IAU Decision
In the 1960s, space missions renewed interest in and research on planets and objects in the solar system. Some scientists returned to using Galileo’s geophysical definition, while others clung to the belief that there are a limited number of planets.
The latter belief surfaced when the IAU decided to vote on the definition in 2006, leading to the orbit-clearing requirement.
The study’s lead author, Philip Metzger, a planetary scientist at the University of Central Florida’s Florida Space Institute, argues that the orbit-clearing characteristic is a description of a planet’s current trajectory, but does not provide any insight into the inherent nature of the object.
In fact, research shows that it was never really a criterion scientists used for classifying planets in the past.
Metzger likens the situation to defining mammals, explaining that their classification should be based on intrinsic characteristics rather than their location.
He hopes the IAU will rescind the current definition, encouraging scientists to return to the geophysical definition of planets, and that textbooks will reflect this change.
A Call to Reevaluate the Definition
Charlene E. Detelich, a geologist and researcher with the Johns Hopkins University Applied Physics Laboratory and a study co-author, supports revising the definition.
She and many other planetary scientists consider round icy moons to be planets, as they all have active geologic processes driven by internal mechanisms, much like any celestial body with enough mass to reach hydrostatic equilibrium. Detelich believes that it is far more useful to classify planets based on their intrinsic characteristics rather than their orbital dynamics.
By reconsidering the current definition of a planet, the researchers aim to not only vindicate Galileo’s original insight but also provide a clearer understanding of the diversity of celestial bodies in the universe.
The ongoing discovery of exoplanets and advancements in technology, such as the James Webb Space Telescope, make reevaluating the definition even more critical to accurately classify our new discoveries and expand our knowledge of the cosmos.
A More Inclusive Definition of Celestial Bodies
The study’s authors emphasize the importance of not limiting our understanding of planets to a small, orderly number for the sake of easy memorization.
Detelich suggests that a full understanding of the universe’s diversity and our place within it would be far more beneficial. Instead of viewing ourselves as one of eight planets, we should recognize that we are one of potentially hundreds of celestial bodies with unique characteristics.
As the search for planets beyond our solar system intensifies, revising the current definition of a planet to better reflect its intrinsic characteristics and geological activity is crucial to advance our understanding of the cosmos and ensure that new discoveries are accurately defined.
Article: “Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science“
Authors: Philip T. Metzger, W.M. Grundy, Mark V. Sykes, Alan Stern, James F. Bell III, Charlene E. Detelich, Kirby Runyon, and Michael Summers
Published in: Icarus
Date: October 28, 2021
DOI: 10.1016/j.icarus.2021.114768
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