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How asteroids lost their planethood

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https://astronomy.com/news/2022/12/how-a...planethood

EXCERPTS: It’s been over 16 years since the International Astronomical Union (IAU) changed the definition of a planet to exclude Pluto, igniting a controversy that rages on today. With the icy world struck from planetdom, there are now officially eight planets in our solar system.

But in 1850, astronomers actually counted 20 planets in the solar system. 13 of them were asteroids, newly discovered denizens of the region between Mars and Jupiter. And the number of asteroids — and, therefore, planets — kept rising for decades.

[...] The consensus that asteroids are not planets dates to more than 50 years before the 2006 IAU definition. But pinpointing exactly why astronomers changed their mind is not as obvious as it might seem. Two explanations have been offered for the change in categorizing asteroids...

[...] A more recent explanation comes from planetary scientists Philip Metzger, Mark Sykes, Alan Stern, and Kirby Runyon in a 2019 paper in Icarus. They spent five years diving into more than a century of research papers and other publications, searching for shifts in the words that astronomers used to describe asteroids, dating back to their discovery. What they found was that scientists had followed the data — but also public perception.

[...] As the number of discovered asteroids increased, keeping track of them in some effective way became a serious problem. At first, astronomers used special symbols modeled on the ones for the classical planets. These had been used since at least the times of the ancient Greeks in astrological tables. Astronomers had co-opted them as a sort of shorthand for the planets’ names in their writings and publications.

In 1851 the German astronomer Johann Encke proposed a solution. [...] American astronomer Benjamin Gould, founder and editor of the Astronomical Journal, proposed a modified version...

[...] The new listing scheme was widely accepted, and James Hilton has identified the 1850s to the 1860s as when astronomers began recognizing asteroids as a separate class of objects. ... However, the scientific literature of the time suggests that despite the new classification scheme, astronomers still saw minor planets as planets.

[...] The shift from “planet” to “not-planet” actually began much later. And it happened in two steps, Metzger said to Astronomy in an interview. First, “in the 1920s scientists stopped calling moons ‘planets.’” The term “moon” became the common description. This was not a decision based on science, says Metzger. Instead, in what he calls “a strange turn of events,” it was a reaction to the general public, which over the preceding decades had settled on “moon” because it was simpler and less confusing. “There’s no discussion of this [change] anywhere,” he added. “They just slowly dropped” that categorization for natural satellites. “That only lasted for about 40 years. Then, planetary scientists went back to [calling them] planets again.”

The second step was the publication in the early 1950s of several papers by astronomer Gerard Kuiper. Metzger specifically points to a paper published in 1953. Based on his then-current photometric studies of asteroids, Kuiper suggested, they could well have begun forming through accretion in the early solar system. Collisions would create the myriad of irregularly shaped objects we see today...

[...] Following the publication of Kuiper’s 1953 paper came what Metzger has called “an abrupt disappearance from the literature of papers that called asteroids planets.” In the decades since the quantity and quality of published geophysical data on asteroids has exploded.

Today, almost no one in the astronomical community considers asteroids to be a sub-class of planet. [...] Asteroids ... have been recognized as a separate category of object in the solar system “on the basis of new data showing [their] geophysical differences from large, gravitationally rounded planets, along with the theoretical developments that gave an adequate explanation” for those geophysical differences.” (MORE - missing details)
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