'Make Pluto a Planet Again' Say Scientists After Controversial Downgrade

Pluto should be reclassified as a planet, according to research published in the journal Icarus.

In 2006, the International Astronomical Union (IAU) voted to downgrade Pluto to a dwarf planet, sparking a highly controversial debate that has yet to truly subside.

Pluto's status began to be questioned in the early 1990s following the discovery of several similarly sized objects in the Kuiper belt—a ring of bodies that orbit the Sun, extending from the path of Neptune at 30 astronomical units (AU) to a distance of 50 AU from the star.

The discovery of Eris, a dwarf planet that is 27 percent more massive than Pluto, prompted the IAU to formally define the term "planet" in 2006, stating that an object had to meet three conditions to be defined as such.

This definition stated that the object must orbit around the Sun, be massive enough to be rounded by its own gravity and have "cleared" the neighborhood around its orbit—or in other words, be the largest gravitational force in its orbit.

Pluto fails to meet the last condition because it is influenced by Neptune's gravity and also shares its orbit with other objects in the Kuiper belt.

The declassification has been met with strong resistance from many in the astronomical community, with scientists pointing out flaws in the definition.

Now, a team led by Philip Metzger from the University of Central Florida's Space Institute has shown how the IAU's standard for classifying planets is not supported by the research literature.

Metzger and colleagues reviewed studies from the past 200 years on the classification of asteroids, finding only one publication—from 1802—that used the clearing orbit requirement to distinguish planets from asteroids and other celestial bodies.

"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 in a statement.

"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. 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."

Scientists considered asteroids to be planets until the 1950s, when distinctions began to be made based on how both sets of objects formed, according to the review. But it is often claimed—as the IAU did with Pluto in 2006—that clearing orbits is the standard for distinguishing asteroids from planets.

The review of the literature, however, shows that this is factually incorrect. Asteroids were broadly recognized as a subset of planets for 150 years. As more of them were discovered, the definition of planet stretched to include ever-smaller bodies.

For a while, scientists found this situation useful as it enabled them to argue for the leading hypothesis of planet formation, known as "Laplace's nebular hypothesis". But in the 1950s, developments in this field meant it was no longer useful to classify asteroids and planets in the same way.

"At approximately the same time, there was a flood of publications on the geophysical nature of asteroids showing them to be geophysically different than the large planets," the authors wrote in the study.

"This is when the terminology in asteroid publications calling them planets abruptly plunged from a high level of usage, where it had hovered during the period 1801–1957, to a low level that held constant thereafter," they wrote. "This marks the point where the community effectively formed consensus that asteroids should be taxonomically distinct from planets."

The evidence demonstrates that this consensus was formed on the basis of geophysical differences between asteroids and planets, not the sharing of orbits.

"We suggest attempts to build consensus around planetary taxonomy not rely on the non-scientific process of voting, but rather through precedent set in scientific literature and discourse, by which perspectives evolve with additional observations and information, just as they did in the case of asteroids," the authors wrote.

According to Metzger, the definition of a planet should be based on intrinsic geophysical properties, rather than ones that can change, such as the dynamics of a planet's orbit. He argues that classifying a planet should be based on whether it is large enough that its gravity allows it to become 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."

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An image of Pluto. NASA

Pluto, for example, features a multilayered atmosphere, organic compounds, evidence of ancient lakes and multiple moons. In fact, Metzger describes the distant world as the second most complex and interesting planet in the solar system.

"It's more dynamic and alive than Mars," he said. "The only planet that has more complex geology is the Earth."

Planetary scientist, David Grinspoon, who was not involved in the latest study, broadly agrees with the views expressed in the Icarus paper.

"The fact is, planetary scientists and astronomers ignore the IAU definition every time they refer to Pluto as a planet," he told Newsweek. "The IAU definition is widely viewed as flawed and will obviously need to be revamped if it is going to endure and be taken seriously. It really wouldn't be that hard to fix in a way that would make more sense and satisfy most everyone."

"Personally, I don't really care if the IAU fix the broken definition or not, or if it simply continues to be widely ignored," he said. "It doesn't really lead to confusion among scientists. It leads to a little confusion among schoolchildren, but then also becomes a teachable moment about the nature of orbiting bodies."