Welcome back Pluto? Study urges scientists to rethink the definition of a ‘planet’

ORLANDO, Fla. — For decades, Pluto was the ninth planet from the sun. Then, scientists suddenly decided in 2006 that Pluto was a planet no more. The International Astronomical Union’s decision is still controversial to this day. Now, a new study is arguing that the IAU’s current definition of what constitutes a planet is based more in folklore than in science, and in serious need of a revamp.

Study authors from the University of Central Florida argue that the IAU’s definition should remove the requirement stating a planet must clear its own orbit. Instead, the new definition should emphasize that a planet is or has been geologically active. For a planet to “clear its own orbit,” it must be the largest gravitational force in its orbit, and not share or cross its orbit with other bodies in space.

In Pluto’s case, however, Neptune’s gravity just happens to influence its neighboring former-planet. Moreover, Pluto shares its orbit with frozen gases and objects in the Kuiper belt. Thus, the IAU’s 2006 change in the definition of a planet led to Pluto losing its famous status.

So, why change it back now?

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Pluto as seen from the New Horizons spacecraft in 2015 at a distance of 476,000 miles. (Photo by NASA on Unsplash)

After all, it’s been about 16 years since this all happened. Why stick up for poor old Pluto after all this time? According to lead study author Philip Metzger, a planetary scientist at UCF’s Florida Space Institute, it’s a problem that goes far beyond just our solar system. New and ever-expanding technology, such as the brand-new James Webb telescope, is facilitating the discovery of more and more new planets beyond our solar system.

“There’s an explosion in the number of exoplanets that we’ve discovered over the last 10 years, and that’s only going to increase as we put better telescopes in space,” says Metzger in a university release.

“So, we will have more motive to do good taxonomy, and we need to fix this now before we get too far in this revolution with exoplanets” he continues. “We want to be doing excellent science because this great influx of data is making it vastly more important to define our new discoveries correctly.”

Researchers performed an exhaustive review of the last 400 years of literature on planets. The world’s first historical definition of a planet comes via Galileo in the 1600s, who argued a planet is a geologically active body in space. Since the 17th century, the scientific community has largely accepted Galileo’s definition up until the 20th century. Between 1910 and 1950, researchers noted a “pronounced decline in the number of scientific papers written about planetary science.”

“We’ve shown through bibliometrics that there was a period of neglect when astronomers were not paying as much attention to planets,” Metzger explains. “And it was during that period of neglect that the transmission of the pragmatic taxonomy that had come down from Galileo got interrupted.”

Undermining Galileo’s work

The emergence of almanacs over the prior two centuries also played a role in all of this as well, researchers say. Almanacs are annual books often containing weather predictions based on astrological factors including planetary position. Such predictions require an orderly and limited number of planets.

“We found that there were enough almanacs being sold in England and in the United States that every household could get one copy every year,” Metzger says.

Even though the popularity of almanacs faded in the 20th century, their impact remains relevant.

“This was a key period in history, when the public accepted that the Earth orbits the Sun instead of the other way around, and they combined this great scientific insight with a definition of planets that came from astrology,” Metzger comments, adding that during this period, astrological views including the notion that moons and satellites are not planets, “crept into scientific literature.”

“This might seem like a small change, but it undermined the central idea about planets that had been passed down from Galileo,” Metzger explains. “Planets were no longer defined by virtue of being complex, with active geology and the potential for life and civilization. Instead, they were defined by virtue of being simple, following certain idealized paths around the Sun.”

The Space Race launches a new debate on planets

This continued until the 1960s. At that point, the Space Race reignited both interest and research into planets and objects in the solar system. During this time, some scientists tried to go back to using Galileo’s broader definition, while others stuck with the more astrological interpretation that there are actually a small number of planets in the solar system.

Study authors argue that these ideals resurfaced in 2006 when the IAU changed the formal definition of a planet. To validate their beliefs, Metzger believes the group proposed an extra planet requirement (clearing its own orbit).

“So, some scientists tried to develop a method to mathematically justify a small number of planets, which was the criterion that a planet has to clear its own orbit,” Metzger claims. “And this was really developed post facto to keep an orderly, small number of planets.”

The study argues that while orbit clearing may be an indicator of a planet’s current trajectory, it does not provide any true insight into the inherent nature of that object. Furthermore, additional research finds scientists had never viewed orbit clearing as a requirement for planet status in the past.

“It’s a current description of the status of things, but if, for instance, a star passes by and disrupts our solar system, then planets are not going have their orbits cleared anymore. It’s like defining ‘mammals,’” Metzger notes. “They are mammals whether they live on the land or in the sea. It’s not about their location. It’s about the intrinsic characteristics that make them what they are.”

Bringing back the ninth planet

In conclusion, Metzger recommends that scientists reinstate Pluto as the ninth planet. He would also like all scientists and science textbooks to start using the geophysical definition of planets.

“When Galileo proposed that planets revolve around the Sun, and reconceptualized Earth as a planet, it got him jailed under house arrest for the rest of his life,” the researcher says. “When scientists adopted his position, he was vindicated, in a sense, let out of jail. But then around the early 1900s, we put him back in jail again when we went with this folk concept of an orderly number of planets. So, in a sense, we rejailed Galileo. So, what we’re trying to do, in a sense, is get Galileo out of jail again, so that his deep insight will be crystal clear.”

“For the term planet, myself and most planetary scientists consider round icy moons to be planets,” adds study co-author Charlene E. Detelich, a geologist and researcher with the Johns Hopkins University Applied Physics Laboratory. “They all have active geologic processes that are driven by a variety of internal processes, as does any world with enough mass to reach hydrostatic equilibrium. As a geologist, it is immensely more useful to divide planets by their intrinsic characteristics than by their orbital dynamics.”

“I’ve always been bothered by the argument to preserve the eight-planet solar system model for the sake of easy memorization for school children,” she concludes. “Imagine how much more perspective they’d have if they had a full understanding of the diversity of the universe and our place in it? We are not one of eight planets, we are one of upwards of 200.”

The study in published in the journal Icarus.

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