What is a ‘planet’? Historical study revisits the debate over Pluto and other dwarfs

Pluto and Charon
A composite image from NASA’s New Horizons spacecraft shows enhanced-color views of Pluto at lower right and Charon, its largest moon, at upper left. (NASA / JHUAPL / SwRI Photo)

Twelve years after the International Astronomical Union voted in a definition of planethood that reclassified Pluto, the debate goes on.

A newly published study uses the historical record to take aim at the definition’s most controversial clause: the idea that a planet in the solar system has to “clear the neighborhood of its orbit,” so that no other worlds are at a similar orbital distance.

That requirement was employed in 2006 to exclude Pluto, whose orbit periodically crosses paths with the orbit of Neptune’s orbit. More to the point, other, smaller worlds have been detected in the same region of the solar system frequented by Pluto.

Complicating the debate is the fact that a steadily increasing number of icy celestial objects are being found in the Kuiper Belt, a broad ring of sparsely distributed material beyond Neptune. One object discovered in 2005, known as Eris, was found to be more massive than Pluto. That discovery is what sparked the IAU’s effort to settle the matter, and ultimately led to the orbit-clearing clause.

The clause not only leaves out Pluto and Eris, but also Ceres in the main asteroid belt and other worlds discovered beyond Pluto. Those worlds, which are still massive enough to crush themselves into a round shape, were designated as “dwarf planets.”

In the new study, published in the journal Icarus, four planetary scientists argue that the orbit-clearing clause doesn’t have much historical precedent and isn’t commonly used in current research. Their survey of the scientific literature found only one publication that used the orbit-clearing requirement — back in 1802, just after Ceres’ discovery — and that requirement was based on assumptions that have since been disproven.

“The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be a defined on the basis of a concept that nobody uses in their research,” lead author Philip Metzger, a planetary scientist at the University of Central Florida, said in a news release. “We now have a list of well over 100 recent examples of planetary scientists using the word planet in a way that violates the IAU definition, but they are doing it because it’s functionally useful.”

Metzger and his colleagues favor a definition based on a geophysical standard: Is the celestial object large enough that its gravity allows it to be spherical in shape? That criterion has historically been used to distinguish irregular asteroids from larger celestial objects. And it’s already part of the definition set down by the IAU for planets as well as its separate category of dwarf planets.

“That’s not just an arbitrary definition,” Metzger said. “It turns out this is an important milestone in the evolution of a planetary body, because apparently when it happens, it initiates active geology in the body.”

As an example, Metzger pointed to the observations made by NASA’s New Horizons probe as it flew past Pluto in 2015. Those observations served as evidence that Pluto has towering mountains of water ice, an underground ocean, evidence of ancient lakes and a multilayer atmosphere with clouds.

“It’s more dynamic and alive than Mars,” Metzger said. “The only planet that has more complex geology is the Earth.”


Science journalist Alan Boyle is the author of "The Case for Pluto: How a Little Planet Made A Big Difference."

NASA’s Dawn spacecraft, which is nearing the end of its scientific mission at Ceres, has turned up similar evidence of subsurface water that helps drive an active geology on that dwarf planet.

Metzger and the paper’s other authors have long been critical of the IAU’s definition, and are part of the New Horizons science team. That led Caltech astronomer Mike Brown, who discovered Eris and wrote a book titled “How I Killed Pluto and Why It Had It Coming,” to dismiss the Icarus paper.

“I fixed that headline about Pluto you may have seen. ‘I know you weren’t convinced by our first 78 arguments, but what about this one?’, said one of the Exact Same Dudes,” Brown tweeted from his @Plutokiller account.

Brown and fellow Caltech astronomer Konstantin Batygin are leaders in the search for a potentially massive planet far beyond Pluto and Neptune, which they call Planet Nine. They suspect the planet exists based on gravitational perturbations affecting other objects that have been observed on the solar system’s edge. Direct observations of a candidate for Planet Nine have not yet been reported, however.

Batygin suggested that, scientifically speaking, labels such as “planet” and “dwarf planet” weren’t worth obsessing over.

“Reclassifying Pluto as a planet is akin to calling an island a continent — kinda silly,” he tweeted. “But in the end, let’s all remember that astrophysical bodies are characterized by mass, radius, orbit, etc. These quantities are what’s important, not what we call them.”

In addition to Metzger, the authors of “The Reclassification of Asteroids From Planets to Non-Planets” include Mark Sykes, Alan Stern and Kirby Runyon. Stern, a planetary scientist at the Southwest Research Institute, is the principal investigator for the New Horizons mission.

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