Ask the Experts

Experts weigh in on potential new definition of ‘planet’

Courtesy of NASA

Pluto may become a planet again if a new definition of 'planet' is instated.

A team of NASA scientists is proposing a new definition for the word “planet.” If the International Astronomical Union approves the proposal, Pluto may regain its status as a planet, along with several other celestial bodies.

The current official definition of a planet, as defined by the IAU, is a celestial body that is in orbit around the sun, has enough mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape and has no debris or other objects within its path of orbit.

Alan Stern, the leader of the team that’s initiating the new proposal and the principal investigator of NASA’s New Horizons mission to Pluto and the Kuiper belt, wants to reinstate Pluto’s planet status. He expressed his anger on Pluto being demoted to a dwarf planet in an interview with Business Insider in 2015.

Part of the reason for this proposed definition change could be political, said Gianfranco Vidali, professor of physics at Syracuse University’s College of Arts and Sciences.

“Not in the sense we understand but in the sense that (Alan Stern) is a planetary scientist and he wants to promote planetary science,” Vidali said.

With the new definition, Vidali said under Stern’s interpretation, planets no longer have to orbit the sun and they don’t need a clear path of orbit. The only requirements for a celestial body to be considered a planet would be that it not have experienced thermonuclear fusion like that of a star, and that it be nearly spherical.

Not only would this reclassify Pluto as a planet, but it would also allow many more celestial bodies, such as moons and satellites, to qualify as planets, Vidali said.

“Now, schoolchildren will have to memorize 100 and something planets,” Vidali said.

Experts agree, though, that the distinction between planets and dwarf planets is justified.

Vidali said the classification between planets and dwarf planets developed when astronomer Michael Brown discovered Eris, another celestial body in the solar system, that was more massive than Pluto. Instead of declaring an additional planet, astronomers decided to further narrow the definition of a planet.

There are multiple differences between planets and dwarf planets. Generally, dwarf planets are smaller than Mercury, according to the IAU website. Also, unlike planets, their orbits are allowed to pass through zones that contain other objects, such as the asteroid belt. Part of Pluto’s orbit passes through an area known as the trans-Neptunian region, which contains many other celestial bodies similar in size to Pluto, per the IAU website.

Anthony Gonzalez, professor in the department of astronomy at the University of Florida, said he believes the distinction can be justified because of the differences in physical size between planets and dwarf planets.

Neither expert fully supported the definition proposed by Stern’s team, but they both suggested potential changes that they would make to the current definition of “planet.”

Gonzalez said even though he thinks the current definition works well for our solar system, he would prefer more precision in the clause that requires a planet to have cleared its path of other objects. There is a difference between small objects that fly by in Earth’s orbit and the path of a planet that goes through the Kuiper belt or asteroid belt, he said.

Vidali said he would take out part of the IAU’s definition that says a planet has to orbit “the sun” and would replace it with the requirement that a planet has to orbit a “parent star.”

He added that while this would make the definition more encompassing, it would still exclude satellites.

“It’s possible to revisit this definition. After all, it’s just a definition,” Vidali said.


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