Is it time to restore Pluto’s status as a planet?

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No matter what anyone says, Pluto is a planet, according to Kirby Runyon.

Europa, commonly known as a moon of Jupiter, is, too. So are the Earth’s moon and more than 100 other celestial bodies in our solar system denied this status under the prevailing definition of “planet.”

The International Astronomical Union’s 2006 definition demoted Pluto, dropping the consensus number of planets in our solar system from nine to eight.

Pluto “has everything going on on its surface that you associate with a planet. …There’s nothing non-planet about it.”

That change—a subject of much debate at the time and since—makes no sense, says Runyon, lead author of a short paper that makes the pro-Pluto argument.

Icy, rocky Pluto had been the smallest of the nine planets, its diameter under three-quarters that of the moon and nearly a fifth of Earth. Still, Pluto “has everything going on on its surface that you associate with a planet. …There’s nothing non-planet about it,” says, Runyon, who is finishing his PhD this spring at Johns Hopkins University.

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Runyon, whose Earth and planetary science dissertation focuses on changing landscapes on the moon and Mars, led authors from five institutions in drafting a proposed new definition of “planet” and a justification for that definition. Both will be presented at the Lunar and Planetary Science Conference.

All the authors were scientists on the New Horizons mission to Pluto, which the Johns Hopkins Applied Physics Laboratory operated for NASA. In 2015, the New Horizons spacecraft became the first to fly by Pluto, some 4.67 billion miles from Earth, passing within 8,000 miles and sending back the first close-up images ever made of Pluto.

The researchers argue for a definition of “planet” that focuses on the intrinsic qualities of the body itself, rather than external factors such as its orbit or other objects around it. They define a planet as “a sub-stellar mass body that has never undergone nuclear fusion” and that has enough gravitational heft to maintain a roughly round shape—even if it bulges at the equator because of a three-way squeeze of forces created by its own gravity and the influence of both a star and a nearby larger planet.

This definition differs from the three-element IAU definition in that it makes no reference to the celestial body’s surroundings. That portion of IAU’s 2006 formula—which required that a planet and its satellites move alone through their orbit—excluded Pluto. Otherwise, Pluto fit the IAU definition: It orbits the sun and it is massive enough that the forces of gravity have made it round.

Coauthor Alan Stern, the principal investigator for the New Horizons mission, has argued in the past that the IAU definition also excludes Earth, Mars, Jupiter, and Neptune, which share their orbits with asteroids.

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The proposed new geophysical definition omits stars, black holes, asteroids, and meteorites, but includes much of everything else in our solar system. It would expand the number of planets from eight to approximately 110.

That expansion is part of the appeal of the new definition, Runyon says. He would like to see the public more engaged in solar system exploration. As the very word “planet” seems to carry a “psychological weight,” he figures that more planets could encourage that public interest.

“I want the public to fall in love with planetary exploration as I have,” Runyon says. “It drives home the point of continued exploration.”

The new definition, which willing researchers can adopt without approval from a central governing body, is also more useful to planetary scientists, Runyon says. Most are closely affiliated with geology and other geosciences, thus making the new geophysical definition more useful than the IAU’s astronomical definition.

The Planetary Science Research Discoveries, an educational website founded by scientists at the University of Hawaii at Mānoa, has already adopted the new definition.

Additional authors of the paper are from the Southwest Research Institute, the National Optical Astronomy Observatory, the Lowell Observatory, and George Mason University.

Source: Johns Hopkins University