The great planet debate

New suggestions for defining a planet would put Pluto and many other objects back on the list.

LAUREL, MD. — Ask planetary scientist Mark Sykes where NASA’s Dawn spacecraft is headed, and he will say it is on its way to the largest asteroid and the smallest planet.

POSSIBLE PLANETS Ceres (top) is thought of as a planet to some and Vesta (bottom) could be too. The International Astronomical Union does not define the two asteroids or even Pluto as a planet, but planetary scientists are pushing to perhaps have the organization change that idea. Vesta: NASA, ESA, L. McFadden, J.Y. Li, M. Mutchler, Z. Levay, P. Thomas, J. Parker, E.F. Young, C.T. Russell, B. Schmidt Ceres: NASA, ESA, J. Parker, P. Thomas, L. McFadden, M. Mutchler, Z. Levay

Dawn launched in September 2007 and is scheduled to rendezvous with the asteroid Vesta in 2011 and then with the dwarf planet Ceres in 2015.

But a dwarf planet is not a planet — at least that is what the International Astronomical Union declared in 2006. Technically, Sykes’ comment is incorrect.

But Ceres is a planet, “my favorite planet,” Sykes said August 14 during the Great Planet Debate Conference held at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. Sykes, who is director of the Planetary Science Institute headquartered in Tucson, Ariz., is one of many scientists calling for a definition of the word “planet” other than the IAU definition.

A planet in the solar system, the IAU says, must: orbit the sun; have enough gravity to make it nearly round; and have gobbled up or sent packing any objects found in its orbit. A dwarf planet, under IAU rules, is not a planet. The IAU says a dwarf planet orbits the sun, is not a satellite, has enough mass to make itself nearly round and has not booted objects from its orbit.

But how can a dwarf of something not be considered one of that thing? Sykes asked.

That sentiment was expressed again and again by many scientists at the conference. “It is grammatically and logically weird that a dwarf planet is not a planet. That rule is unacceptable and violates laws of logic and grammar,” said planetary scientist David Morrison of the NASAAmesResearchCenter in Moffett Field, Calif.

The IAU definition of planet pleases no one, which is ironic because words are to be useful and easy to understand, he said. So during the conference, Morrison called for the withdrawal of the IAU definition, an action he said would be unlikely. He then suggested that the IAU definition be ignored.

And that is what Sykes is doing, he said — at least partially. He is selecting the part of the IAU definition that he finds useful, arguing that a planet is anything that orbits a star, doesn’t fuse elements in its core and has enough internal gravity to be nearly round.

Those criteria would make Ceres a planet. It would remake Pluto one too. There would be at least 13 planets in the solar system with many more, possibly thousands to come, he said. The thousands would lie in the Kuiper Belt, the ring of planet-like chunks of rock and ice in Pluto’s neighborhood.

Not all conference attendees agreed, though. “It is easier to determine if a larger object is dynamically dominant, meaning it dominates the orbit, not necessarily clears it, compared to determining whether a smaller object is round,” said astrophysicist Steven Soter of the American Museum of Natural History in New York City. He did note that he was not advancing the IAU’s definition, but rather was suggesting that using dynamics to define objects is more straightforward than defining a planet based on its gravity establishing its roundness. That in essence means the planet’s internal gravity is strong enough to make the object nearly round.

Labeling planets based on their dynamics around the sun distinguishes the planets as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

But if Earth were orbiting the sun out in the Kuiper Belt, based on a dynamical definition and the mass of Earth, it would not be a planet, Sykes and other scientists pointed out.

“The dynamics perspective misses the point of planet classification,” which is to group like things together, said planetary scientist Alan Stern of NASA’s science mission directorate based in Washington D.C. And, “it ignores the 300-plus planets found outside the solar system,” he added. “A definition based on the physical, the intrinsic properties of a planet does not,” he noted.

Such a definition might seem to add confusion because it would include a planet’s moons as planets too, Stern said. “But we are just going to have to get over that,” he said, because what makes a broad, physical-based definition of a planet useful is that it allows scientists and educators to “put like things together in the same bin,” and then make sub-bins or subcategories of planets such as satellite planets, dwarf planets and extrasolar planets, he explained. Those subcategories could be added to already existing categories, such as terrestrial planets, gas planets, rocky planets, inner planets and outer planets.

But making moons and others objects planets is a “radical step” away from the definition of planets as the public knows it, Morrison said. And since “planet” is a cultural term, it is dangerous to change the term to that extent, he argued.

Stern countered by saying that his concept of a definition — one “based on the physical, the intrinsic properties of a planet” — is how he defines a planet. It also pushes the bounds of what a planet is. When, or if, there is ever a consensus, he thinks the definition of planet should fall between his “radical” definition and the more restrictive, dynamics-based IAU definition.

At any rate, when Dawn gets to Vesta and then Ceres, and NASA’s New Horizon mission gets to Pluto and other Kuiper Belt objects in the 2010s, the information gathered is going to be important, whether or not the objects are planets, said planetary scientist Hal Levison of the Southwest Research Institute in Boulder, Colo.

photo of Ashley Yeager

Ashley Yeager is the associate news editor at Science News. She has worked at The Scientist, the Simons Foundation, Duke University and the W.M. Keck Observatory, and was the web producer for Science News from 2013 to 2015. She has a bachelor’s degree in journalism from the University of Tennessee, Knoxville, and a master’s degree in science writing from MIT.

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