Pluto, 10 Years After It Was Demoted

The summer of 2006 was quite an eventful one. It was a simpler time when we were quoting Borat’s “very nice!” drawl, figuring out how to use headbutts to win soccer games at the World Cup — and when we, mere humans, decided that a little piece of rock at the outskirts of our solar system wasn’t really ours.

Cue crestfallen faces. Pluto — 4.67 billion miles away, often forgotten, randomly floating nebulousness that inspired a goofy Disney dog’s name — ceased to be a part of our lives in 2006. For decades, schoolchildren learned about the nine planets orbiting the sun, that Pluto was the smallest and furthest away, a chunk of rock that sort of got pulled into our solar system party and never really left even though it was on the edge, metaphorically and figuratively. In an instant, a consortium of scientists elected to boot Pluto out and relabel it as a “dwarf planet — a planetary body that continues to orbit the sun, but just isn’t quite big enough to warrant the formal term of planet.”

What happened? Basically, the discovery of other celestial bodies orbiting the sun precipitated the need among the astrophysics community to refine the definition of what we defined as a planet. Eris, for instance, was found in 2005 by Caltech astronomer Mike Brown. Eris is 27 percent more massive than Pluto, so initially NASA took to describing it as the tenth planet of the solar system.

But then scientists faced a conundrum: We’d undoubtedly be discovering more and more planets in the futureT many of which would probably outsize Pluto. the International Astronomical Union decided it was time to give “planet a proper definition.”

So on August 24, 2006, the IAU issued a resolution on what defined a planet:

• It had to orbit the sun.

• It had to be big enough that its own self-gravity rounded its body out.

• It had to clear the neighborhood around its orbit.

Pluto, unfortunately, failed the third condition. Its mass is a blink-and-you’ll-miss-it 0.07 times that of the mass of other objects sharing the same orbit. To contrast, Earth’s mass is 1.7 million times the total mass of other objects sharing its orbit.

Many hailed the decision. Brown was quite possibly the biggest proponent of the new definition. The man’s Twitter handle is @plutokiller, for chrissake.

That’s not to say the scientific community was ra-ra’ing a major victory here. The vote on the resolution was quite narrow, and plenty of other experts did — and still do — question the wisdom of the IAU’s redefinition. The biggest name on the side of opposition is probably Alan Stern, the principal investigator for NASA’s New Horizons mission — which has yielded a treasure trove of new data regarding little Pluto.

“It’s been a disaster for science and for the IAU,” Stern told Inverse. “No scientist I know thinks the definition is a good one. And since the flyby of Pluto, it’s pretty obvious its a planet.

There’s more to being defined “planet” than simply scientific ones, critics argue. Owen Gingerich, an astronomer at the Harvard-Smithsonian Center for Astrophysics, says he believes it was “a mistake for the IAU to try to define ‘planet.’ That is a culturally defined word, which has changed over the centuries as definitions have slowly evolved.”

He acknowledges that is reasonable for scientific unions to define subclasses, and that Pluto is not what we would consider a “classical planet; it is simply too small.” Nevertheless, Gingerich says the whole debate was a very opportune moment to teach both the public and burgeoning astronomers about exactly what a planet is and what should be considered a planet. “The IAU missed a significant teaching moment,” he says.

The consequences of the IAU’s missteps in public outreach seem to still linger. Philip Metzger, a planetary scientist formerly at NASA and now based at the University of Central Florida, is opposed to the new definition, and says “when you explain to the public why it was a bad decision, they are mostly very quick and very happy to agree with you and they seem relieved to hear that a large number of planetary scientists agree with them that it is a planet.”

Metzger explains that within the planetary science community, researchers have a mixed range of responses to the new definition, and this is primarily a consequence of what their expertise is. “Astronomers who mainly do observational work, for example, seem happy with the new definition,” he says. “In my opinion, though, the planetary geoscientists should have been the ones to define this term since the physical essence of a planet is at the heart of planetary geoscience, not at the heart of observational astronomy.” While observational astronomers are most concerned with the orbital clearing criteria, planetary geoscientists care less about a planet’s interactions than with its surroundings, and more about the planet’s composition and mass by itself.

“Based on what it actually is, Pluto is clearly in the same category as other planets,” Metzger argues. “Why does it matter that it isn’t able to clear its gigantic orbit so far from the sun? That fact has no effect upon the evolution and development of Pluto. It does not clear its orbit precisely because it never interacts with those other bodies that are so far away from it. If they never interact with each other, then how for Pete’s sake can that be important in describing the nature of Pluto? It can’t!”

Like Gingerich, Metzger also alludes to other layers surrounding the redefinition that are more cultural than scientific — and the effects extend well beyond Pluto. “It was clearly an attempt to maintain the status quo with an old, outdated view of nature that there are only a few planets and they reign in their orbits like gods, that it is an orderly solar system, and that it is safe, he says. “It was a shock when we started to realize that there are hundreds of planetary bodies beyond Neptune and that they give us a completely new picture of the solar system.”

According to Metzger, there are between 150 and 500 planet-sized bodies out beyond Neptune — but because they are labeled “dwarfs,” the excitement behind their discovery is essentially nonexistent. The idea of a messy, constantly evolving solar system doesn’t resonate with anyone not directly involved with this research. “This is truly tragic,” he says. “The harm that this has done to science in our generation is huge and has not been fully recognized, yet.”

Whether or not you are #TeamPluto or not, one thing is for certain: the IAU, and the scientific community at large, will someday need to revisit the debate behind the definition of a planet. The end of Pluto’s reign as the little planet that could did little to provide clarity to the current picture of the solar system. Scientific progress often does raise more questions than it answers — but then again, that’s precisely the point.