Debate over Pluto's Planet Status Still Contested

Millions around the world went through school with the understanding that Pluto qualifies as a planet. Then, in 2006, that changed. A team of researchers from the University of Central Florida has a problem with that change, and put out a recent study detailing why they think Pluto deserves to be a full-sized planet.

The key reasons for Pluto losing its planet status are invalid, researchers note. 

UCF planetary scientists led by Philip Metzger from the Florida Space Institute said the standards for classifying planets isn't supported in research.

The debate is back and @DrPhiltill's new research is suggesting that Pluto is indeed a planet ?

Thoughts? We want to hear them ? https://t.co/7NbatGo8iN

— UCF (@UCF) September 6, 2018

Standards not supported

The International Astronomical Union is responsible for determining and qualifying full-sized planets. Full-sized planets must meet three criteria: 1) it has to be in orbit around the Sun, 2) it has to have sufficient mass to assume hydrostatic equilibrium (or have a really round shape), 3) it has to have cleared the neighborhood around its orbit. 

The IAU contended in 2006 that Pluto only meets two out of the three qualifications. They said it has yet to "clear its neighborhood."

Metzger said the third part of the definition (and Pluto's hinderance) isn't good enough. He noted moons like Saturn's Titan and Jupiter's Europa have been called planets by hundreds of years of planetary scientists. 

"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.”

I have so much more to say about this: the vital role of taxonomy in science, how voting on taxanomy is anathema since it undermines the process, how Galileo argued "planet" must now be a geophysical, not dynamical, taxon because this usefully advances the relevant science... https://t.co/TbmzAjVwIy

— Dr. Phil Metzger (@DrPhiltill) September 6, 2018

“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 continued, talking about 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.”

“We showed that this is a false historical claim,” Runyon says. “It is therefore fallacious to apply the same reasoning to Pluto.”

History of Pluto and title changes

In its relatively short history of being discovered, Pluto remains one of the most popular celestial bodies around. Part of that stems from the debate surrounding whether or not to classify Pluto as a 'real planet' in the first place.

The small body was originally discovered in February 1920 at the Lowell Observatory in Flagstaff, Arizona by astronomer Clyde Tombaugh. Tombaugh had assistance in the discovery from William Pickering. 

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Originally called Planet X, Pluto was a massive discovery both as a new planet and for the Lowell Observatory's latest camera and photography systems.

In 2006, the International Astronomical Union downgraded Pluto's status to that of a 'dwarf planet' seemingly out of the blue. This left the remaining planets divided into two groups: terrestrial planets (Mercury, Venus, Earth and Mars); gas giants (Jupiter, Saturn, and Uranus).

The mysterious definition of 'planet'

Metzger has one suggestion to the IAU: change their definition. Metzger recommended changing the definition to include the planets intrinsic properties rather than the status of a planet's orbit.

“Dynamics are not constant, they are constantly changing,” Metzger says. “So, they are not the fundamental description of a body, they are just the occupation of a body at a current era.”

Metzger explained that if it's large enough to get a spherical shape due to gravity, it deserves consideration as a full-sized planet. 

“And that’s not just an arbitrary definition,” Metzger says. “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.”