The ‘Is Pluto a Planet’ Debate May be Heating up Once Again
A new study claims that the criteria used to classify planets is wrong, which means you should prepare for a new round of “Is Pluto a Planet” debates.
The International Astronomical Union (IAU), a group of astronomy experts from around the world, isn’t really the type of organization most would consider controversial. But in 2006, its decision to reclassify Pluto and strip it of the title of “planet” made them headline news – and not necessarily in a good way.
For reasons that are still a matter of debate and anger, the IAU decided to set firm rules to define what constitutes a planet. After heated debate, the consensus – which was far from unanimous – decided that an object needed to match three criteria:
- It has to be in orbit around a star
- It has sufficient mass to have formed into a sphere
- It has “cleared the neighborhood” around its orbit
Of the three rules, the third was the most controversial as it became the justification to reclassify Pluto from being the ninth planet in our solar system to a dwarf planet instead. That would put it alongside other dwarfs like Ceres and Eris, and it forced schools everywhere to redo their science syllabuses. It also outraged people around the world who weren’t thrilled with a group getting together and almost arbitrarily changing something the entire world had accepted as fact for over 50 years.
Since that decision, there have been several attempts to get the IAU to reconsider. In 2015, NASA’s New Horizons spacecraft began sending back close-up images of Pluto, further inflaming the calls for a reclassification. It hasn’t led to much, but a new study may have found a compelling argument.
A paper recently released by the University of Central Florida in Orlando has pointed out that the IAU’s standard, especially the third rule about an object clearing its orbit, is not supported by research literature. The study reviewed scientific literature going back 200 years and found only one publication that had ever used that orbit-clearing classification to name an object as a planet, and it was from 1802. The study then showed that the research in that article was actually based on since-disproven theories anyway.
The study goes on to point out the inconsistency in the IAU’s arguments based on other research. Certain moons, namely Saturn’s Titan and Jupiter’s Europa, have both been referred to as planets since the days of Galileo.
“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,” UCF planetary scientist and the study’s co-author Phillip Metzger wrote. “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.”
Metzger went on to deride the use of the IAU’s definition, calling it a “sloppy.”
When it came to Pluto, the IAU pointed to Neptune’s gravitational influence on the one-time planet, then went on to reference several frozen gasses and objects in the Kuiper Belt that were in Pluto’s orbit. That was the final nail in the coffin of Pluto’s planethood.
“[The IAU] didn’t say what they meant by clearing their orbit,” he continued. “If you take that literally, then there are no planets, because no planet clears its orbit.”
The study went on to note that the gap between what we consider a planet and what is simply a celestial body truly began in the 1950s. It can be traced back to a paper written by famed astronomer Gerard Kuiper, which argued the difference between a planet and an object like an asteroid was down to how it was formed. That has since been disproven as a factor in deciding the difference between the two, but its influence remains.
Given that the standard of clearing an orbit is not used to show the difference between an asteroid and a planet, the rest of its 2006 definition becomes more open to debate. The IAU used this argument, claiming that asteroids can’t clear their orbit, so if a celestial object can’t clear its orbit, it must be closer to an asteroid than a planet.
“We showed that this is a false historical claim. It is therefore fallacious to apply the same reasoning to Pluto,” said co-author Kirby Runyon, with Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.
The study goes on to argue that the definition of a planet should be based on its intrinsic properties, not factors that can change, like its orbit.
“Dynamics are not constant, they are constantly changing,” Metzger said. “So, they are not the fundamental description of a body, they are just the occupation of a body at a current era.”
The study goes on to recommend that the definition of a planet be simplified and can be based on its gravity. If an object is large enough that it has become spherical in shape, it should be classified as a planet.
“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.”
Pluto may be small, but it is far more than an asteroid or even a dwarf planet like Ceres (the second largest dwarf planet, if you count Pluto as the first). Pluto has an underground ocean, a complex atmosphere, multiple moons, and evidence of ancient lakes.
“It’s more dynamic and alive than Mars,” Metzger said. “The only planet that has more complex geology is the Earth.”
The new study alone may not be enough to get the IAU to reconsider its controversial decision – a decision they’ve had to defend for over a decade now with very little sign of wavering – but it could be one more step toward eventually help Pluto regain its title as the ninth planet.