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Scientists debate planet definition, status of Pluto

Pluto was reclassified as a dwarf planet when the International Astronomical Union adopted a dynamical definition of a planet in 2006. Image Credit: NASA / JHUAPL / SwRI

Pluto was reclassified as a dwarf planet when the International Astronomical Union adopted a dynamical definition of a planet in 2006. Image Credit: NASA / JHUAPL / SwRI

Scientists Ron Ekers and Alan Stern debated the planet definition and the status of Pluto in an event sponsored by the Philosophical Society of Washington.

Following the debate, which was livestreamed April 29, 2019, audience members and those watching online voted on their definition preference: either the International Astronomical Union (IAU) dynamical definition or an alternative geophysical definition. The latter won by a vote of 130-30.

Two central points dominated the debate—the question of a dynamical versus a geophysical definition and that of who gets to make the decision on the definition used.

Ekers of Curtin University in Perth, Australia, specializes in radio astronomy.  He was president of the IAU when the controversial vote that adopted the dynamical planet definition was conducted in 2006.

Stern is a planetary scientist at the Southwest Research Institute in Boulder, Colorado, and principal investigator of NASA’s New Horizons mission to Pluto and the Kuiper Belt.

Ekers discussed the history and role of the IAU, which is celebrating its centennial this year, noting the organization was founded after World War I to standardize categorization of celestial objects so astronomers could better communicate with one another. This process is not science but an issue of naming objects, he said.

When Eris was discovered in 2005, it was erroneously thought to be larger than Pluto. The IAU’s 2006 vote was conducted to determine which organization committee should be responsible for naming it—the committee that oversees the naming of planets or the one that oversees the naming of small solar system objects.

The definition adopted by the IAU that year has three components. It requires a planet to orbit the Sun; be rounded by its own gravity, a state known as hydrostatic equilibrium; and clear the neighborhood of its orbit.

That third criterion, which has been at the center of the controversy for over 12 years, means an object must gravitationally dominate its orbit, sweeping out smaller objects in its path. Because it focuses on the effect a celestial object has on other objects, the IAU definition is a dynamical one.

In contrast, Stern and many like-minded planetary scientists reject the requirement of gravitational dominance and favor a geophysical definition that centers on an object’s intrinsic properties. According to this definition, a planet is a substellar object that has never undergone nuclear fusion and is rounded by its own gravity, regardless of its orbital parameters.

Stern argued the IAU definition was adopted by the wrong group of astronomers, as most of those who voted were not planetary scientists but astronomers with other specialties. He discussed what he views as the weaknesses of the IAU definition, one of which is the further a planet is from its parent star, the larger an orbit it has to “clear.”

“Clearing the orbit was made up by the dynamicist planetary community, as a simple description where the giant planets throw things out completely,” Ekers said. “These objects which Alan showed you (dwarf planets in the Kuiper Belt) are in resonant orbits with the big planets. They have no free will; they have no independence, and that’s the real intention of clearing the orbit.”

A 2015 theoretical paper published by scientist Jean Luc Margot of UCLA shows the IAU definition is sound, he added. Stern responded by illustrating a graph from the Margot paper showing how, according to the IAU definition, identical objects are classified differently solely based on their locations.

Because the IAU definition requires an object to orbit the Sun rather than a star, the question of exoplanets has been left unresolved. Ekers emphasized the IAU definition was intended only for our solar system and that exoplanets would be addressed in the future when more is known about them. Stern said there should be a single definition for both our solar system and others.

Ekers said he favors distinguishing dwarf planets from full planets, adding that Kuiper Belt dwarf planets have much more in common with their smaller, Kuiper Belt Object (KBO) counterparts than with the eight gravitationally dominant planets. Stern contested this, noting the 2015 New Horizons flyby revealed Pluto to have the same complex geology as the larger planets.

“This is a world with an atmosphere made of the same stuff that we’re breathing, almost certainly a liquid water ocean on the inside, with mountain ranges and tectonics, with complicated activity that’s going on inside it today and on its surface, with glaciers and avalanches and all of the hallmarks of planetary processes that we see,” Stern said of Pluto.

Video courtesy of PSW



Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

Reader Comments

Pluto is a planet. Probably better suited as a planet than others. Has many moons. Even has a moon tidally locked to it. Has the potentual in the future to become a bigger planet if several massive anchored down cables are joined together between pluto and charon. Then they could, extremely slowly be cinched together to form plutocharon.

Nicely done, Laurel. Your article really gets to the crux of the matter. I saw the debate, too. But a lot of the things you mentioned went over my head. I do not come from a scientific background. My article of the debate was more of a gonzo journalism piece with blatant bias for the great icy orb.

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