Here’s Why the Pluto Planet Debate Just Won’t Die

A long time ago, a group of astronomers decided that Pluto was not a planet. This was in 2006, before the era of iPhones and Snapchat and Minecraft … and yet scientists still argue over whether Pluto is a planet today. Why the big fuss? Let’s dig in and find out, because the fate of our solar system’s classification is at stake.

No Scientific Consensus

Pluto was disqualified from planethood because it was too small to clear debris nearby its path in space, or at least that’s what the International Astronomical Union decided at a meeting in 2006. But a lot of people don’t agree with that. After all, just look at the hundreds upon hundreds of asteroids threatening Earth; even our much larger planet can’t clear all the little rocks nearby.

new paper in the journal Icarus brings up a fresh argument. Specifically: If we’re talking about how to define a “planet,” we should look at the literature of scientific papers to see what other scientists said.

This is really important because literature is a part of how science works. A scientist makes a discovery, they submit it to a scientific journal, and then other scientists actually review their work before it gets published. Papers can be drastically altered before publication — or even never published at all, if they aren’t valid in the eyes of the reviewing scientists. This “peer review” process is fundamental to science today and has been for hundreds of years.

Here’s the shocker in the new paper: The literature it reviewed about planets — all the way back to 1802, well before the invention of electricity — shows that only one single paper says that a planet needs to clear its orbit. And that methodology was later discredited!

Where Do We Go from Here?

The new paper’s lead investigator is convinced we need to think differently about Pluto, especially after the NASA New Horizons spacecraft showed an incredibly complex world during its quick flyby in 2015. The spacecraft spotted signs of an underground ocean, ancient lakes, and even organic compounds — the building blocks of life — on the surface.

“It [Pluto] is more dynamic and alive than Mars,” said Philip Metzger, a planetary scientist at the University of Central Florida, in a statement. “The only planet that has more complex geology is the Earth.”

New Horizons principal investigator Alan Stern is a long-time advocate for calling Pluto a planet. Back in March, he and postdoctoral researcher Kirby Runyon co-authored an article in Astronomy calling for more flexibility on the definition of planet. Meanwhile, Stern, Runyon, and affiliates have been mounting a media campaign in recent months asking the IAU to reconsider their 2006 definition.

This group says a planet should instead be defined as a round body that never underwent nuclear fusion. “At least 119 peer-reviewed papers in professional, scientific journals implicitly use this definition when they refer to round worlds (including moons) as planets. The publication history for these papers spans decades, hailing from both before and after the 2006 IAU vote,” Runyon and Stern wrote in Astronomy.

It’s hard to say what will happen next, but one thing’s for sure — after a dozen years, this debate is only getting louder and more forceful. In the end, what scientists decide upon could have huge implications. Under this new definition, we wouldn’t have eight planets in the solar system; instead, there would be more than 100. So, Pluto lovers, here’s the rub: If you get Pluto’s planethood back, you also bequeath planethood upon dozens of sibling worlds. Talk about rewriting astronomy textbooks!


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