In 2006, International Astronomical Union, a global group of astronomy experts, tightened its definition of planets and changed Pluto’s status to dwarf-planet. The new definition required the planet to have a clear orbit and be the largest gravitational force in its orbit. Since Neptune’s gravitational force also influences Pluto, and Pluto shares its orbit with various other objects in the Kuiper belt, Pluto was denied the status of a planet. Many researchers have been fighting over that decision ever since.
A new study led by Dr. Philip Metzger, a planetary scientist at the University of Central Florida (UCF), has said that the the new definition which denied planet status to Pluto has no basis or support in the research literature. He studied scientific literature from past 200 years and only found one publication (from 1802) that used the clear-orbit condition to classify a planet.
“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,” Metzger said. “And it would leave out the second-most complex, interesting planet in our solar system.” “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,” he said. “It’s a sloppy definition,” Metzger said of the IAU’s definition. “They didn’t say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit.”
Metzger instead recommends to have a new definition of a planet that says the planet needs to be big enough to allow its gravity to make the planet spherical in shape. “And 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.”
“It’s more dynamic and alive than Mars,” Metzger said. “The only planet that has more complex geology is the Earth.”
The co-author of the study, Kirby Runyon, from Johns Hopkins University in Maryland, also claimed that the 2006 IAU definition was incorrect. “We showed that this is a false historical claim,” Runyon said. “It is therefore fallacious to apply the same reasoning to Pluto.”, he said.
This is not the first time researchers have questioned the IAU’s definition of planet. NASA’s New Horizons team also proposed a newer definition in 2017 which also classified Pluto as a planet. The IAU spokespersons have said that while there’s plan for reclassification of Pluto, these debates are good for the scientific community.
The study was published in the Journal of Icarus.