Context

If Dr. Seuss had been an astronomer, Horton the Elephant (who heard a Who) would have said “a planet’s a planet, no matter how small.”

Even Pluto.

But don’t quote Dr. Seuss to the International Astronomical Union. In 2006, the IAU declared Pluto a planet not. 

IAU Resolution B5 (not to be confused with Le Petit Prince’s asteroid B 612) declared that in order to be considered a planet, a body must clear the neighborhood around its orbit. Pluto, then, doesn’t qualify, because its “neighborhood” (way out beyond the orbit of Neptune) is populated by other bodies referred to as trans-Neptunian or Kuiper Belt objects. Two of them, Haumea and Makemake, have been recognized as “dwarf planets,” the same designation that the IAU now applies to Pluto.

This demotion of Pluto to dwarf status (no offense intended to dwarfs) makes sense, IAU defenders contend, because the asteroids (orbiting the sun mostly between Mars and Jupiter) aren’t planets, either — no one of them has cleared out the orbital neighborhood. After all, nobody would call an asteroid a PLANET. Except actually, nearly everybody called them planets for 150 years after they were discovered. Only half a century or so ago did astronomers stop considering most asteroids to be planets. And that shift had nothing to do with clearing out any neighborhoods, Philip Metzger of the University of Central Florida and colleagues point out in a new paper.

“The planetary science community did not reclassify asteroids on the basis of their sharing of orbits, which had been known … since the mid-19th century,” write Metzger and coauthors (including Alan Stern of the Southwest Research Institute in Boulder, Colo.). “Rather, they were reclassified beginning in the 1950s on the basis of new data showing asteroids’ geophysical differences from large, gravitationally rounded planets.”

When astronomers first discovered asteroids (Ceres and Pallas, in 1801 and 1802), the famous astronomer William Herschel did not consider them planets. He called them asteroids because they were “starlike,” too small to appear through his telescope as bigger than a point. All previously known planets (Mercury, Venus, Mars, Jupiter, Saturn and Uranus) appeared as discernible disks. Those (except Uranus) had been known since ancient times; the Greeks called them “planets,” the word for “wanderers,” because they moved through the constant background patterns of the “fixed” stars.

After Herschel, though, everybody else generally referred to the asteroids as planets, since they orbited the sun (unlike moons, which orbited other planets). In 1845, for instance, the prominent scientist Alexander von Humboldt wrote in his masterwork Kosmos that the solar system consisted of 11 “primary planets,” five of them asteroids. (In the 1858 English version, a translator’s note updated that number to 16, adding four more asteroids plus Neptune.) Soon dozens more were added to the asteroid total. By the end of the 19th century hundreds of asteroids had been detected, and they were frequently referred to as “minor planets.” Metzger and colleagues scoured the astronomical literature since 1800 and found that astronomers consistently referred to asteroids as planets. Only Herschel, and in no case anyone after him, complained that shared orbits should disqualify asteroids from planetary status.

In 1951, Metzger and colleagues note, the authoritative Science News-Letter (now Science News, of course) proclaimed that “there are thousands of known planets circling our sun,” designating the big ones as “the chief planets” (including Pluto, discovered in 1930). Other astronomy writers held a similar view. In 1959, the prolific science popularizer Isaac Asimov noted that some preferred to call asteroids “planetoids” (because they are not, in fact, starlike). “Even planetoid isn’t quite a fair name,” Asimov wrote. “The planetoids do not merely have the form of planets; they are planets. To emphasize their small size, though, they are frequently called minor planets and that is perhaps the best name of all.”

But in the 1960s, Metzger and colleagues note, use of “minor planets” diminished as better observations revealed geophysical distinctions between the smaller asteroids and the major planets. Small asteroids had irregular shapes rather than the approximate roundness of big planets. And spectroscopic analyses of asteroid composition plus new ideas about how asteroids formed suggested that most of them were not really very much like planets after all.

“This history indicates that it was geophysical characteristics, not sharing of orbits, that led to the shift in terminology in which asteroids were no longer called planets,” Metzger and collaborators conclude.

Metzger, Stern and coauthors do not say anything about Pluto in their paper. But the implication is obvious: Denying planet status to Pluto is an arbitrary determination (by the IAU) based on a definition without justification in the astronomical literature. It was concocted and voted on at a big meeting, not derived from actual scientific usage. Planetary status should be determined, as it was with asteroids, by the progression of astronomical science, not by voting on an arbitrary definition.

“Voting on key taxonomical terminology and the relationships between taxa is anathema in science,” Metzger and colleagues write. It’s “contrary to the traditions evolved over many centuries to reduce social, political and personal cognitive biases in science. It injects unhelpful dynamics and social pressures into science and impinges on individual scientists’ taxonomical freedom…. We recommend that, regarding planetary taxonomy, central bodies such as the IAU do not resort to voting to create the illusion of scientific consensus.”

But don’t expect the issue about Pluto’s status to be resolved anytime soon. It’s not a case where the question is complicated but the answer is simple. Nevertheless, by reading through the historical literature on planets and asteroids, Metzger and colleagues have improved the prospects for a better-informed debate. After all, “The more that you read, the more things you will know. The more that you learn, the more places you’ll go.”

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