Pluto: The IAU Position is Opinion, not Fact

Contrary to what one is likely to read in various publications, textbooks, and online sites, Pluto did NOT lose its planetary status on August 24, 2006.

Today in the US, there is a lot of discussion of just what is “truth,” how one distinguishes between truth, opinion, and outright falsehood, and who gets to decide which version of a particular story is the “real one.”

In this context, it is appropriate to note that the overwhelming majority of publications, journalists, media outlets, textbook companies, and websites (including Wikipedia) got this all wrong 12 years ago when it comes to Pluto specifically and the definition of the term “planet” in general.

What happened on that day in 2006 is that 424 IAU members, most of whom were not planetary scientist but other types of astronomers, in an act that violated their organization’s own bylaws, voted to change the definition of planet. Of those 424, just 333 voted that dwarf planets are not planets at all but some other type of object entirely. 91 voted to class dwarf planets as planets.

Within days, an equal number of planetary scientists signed a formal petition rejecting the IAU decision. This group has maintained their position to this day, as is seen from presentations made in 2017 and 2018 to the Lunar and Planetary Sciences conference about the geophysical planet definition.

By saying that Pluto lost its status as the result of that vote by 424 people, the media gave the IAU a tremendous level of power that is completely unwarranted. They took a vote; so what? Another group, this one composed mostly of those who study planets, issued their own statement within days rejecting the IAU decision.

Why then is a vote by one group given a status of gospel truth while a vote by another, arguably more qualified group, barely gets any mention at all?

Does this remind anyone of the last presidential election, when some candidates were given national media attention for every tweet they wrote, no matter how ridiculous, while others had their increasing popularity completely ignored by the same mass media? If it doesn’t, it should!

If an asteroid had impacted Pluto and lobbed off part of it to the point that Pluto was no longer round—something that actually happened to proto-planets Vesta and Pallas in the belt between Mars and Jupiter—then it would make sense to say Pluto stopped being a planet on a particular day. If it suddenly lost a significant part of itself, Pluto would no longer be spherical, and one could legitimately question whether it would still count as being rounded by its own gravity.

But there was no such asteroid impact.

Frequently, when I and others who oppose the IAU definition explain the geophysical planet definition, according to which a planet is “a sub-stellar mass body that has never undergone nuclear fusion and that has enough gravitation to be round due to hydrostatic equilibrium regardless of its orbital parameters” (credit to planetary scientist Kirby Runyon for this definition), readers respond by stating, Wait! There is a third requirement, that the object has to clear its orbit.

Here is the problem. Who said there is a third requirement? Too many people act as though such a requirement was handed down by God on stone tablets. The truth is, it was not—it is simply the opinion of one group of scientists who prefer a dynamical definition, one that focuses on the influence objects have on other objects, over a geophysical definition, which instead focuses on an object’s intrinsic properties.

There is evidence that this “third requirement” was imposed by those who favor a dynamical definition for the specific purpose of excluding Pluto. This means that those who enacted it first decided the conclusion they wanted, then crafted a definition to fit their desired conclusion.

Commonly used by oil companies who commission studies about the connection between fossil fuel emissions and global climate change, this process looks like science but it is not science. Science does not choose a conclusion first, then fit the data to match that conclusion. Researchers paid by oil companies know where their money is coming from, which is why they arrange the data to fit their facts. This is exactly what took place in Prague in 2006 though the motivation was not money but imposing one particular view on the world.

Furthermore, the IAU definition was set up so that it could never be overturned. New Horizons was already on its way to Pluto when the 2006 vote took place. However, by making orbit clearing the determining factor for planet status, those who voted for this definition assured that no matter what was found at Pluto, no matter how many planetary features and processes it has, none of that would ever matter because Pluto does not clear or gravitationally dominate its orbit.

This is why even now, when the IAU is holding its first General Assembly since all the New Horizons data was returned in late 2016, no effort is being made by IAU insiders to revisit and possibly revise that decision.

And yes, this also applies to other dwarf planets.

Furthermore, to correct another erroneous assumption still circulating online, there are no known dwarf planets or Kuiper Belt Objects larger than Pluto. While Eris was initially thought to be larger, in 2010, a team of astronomers found it to be smaller than expected after measuring it occult a star. This method is regularly used by scientists to accurately find the size of a celestial object.

Even if there were dwarf planets larger than Pluto—and there may be some yet to be discovered—that would not have any impact on Pluto’s status. If such objects exist, they are all dwarf planets, a subcategory under the umbrella of planets—according to the geophysical definition.

An object cannot be defined by the presence of another object. A Mars-sized planet could be found far beyond Pluto, but that would not change what Pluto is. It would simply add another planet to our solar system.

Some people have a notion that our solar system cannot have “too many planets.” There is no scientific basis to this claim. The universe has billions of stars and billions of galaxies. The Milky Way alone likely has billions of planets—more planets than stars. Finding many of one thing does not mean that thing needs to be downgraded because there are “too many.”

Twelve years after this debacle, it is time to set the record straight. The status of Pluto and the question of what constitutes a planet are both matters of ongoing debate. Neither position is more “official” or legitimate than the other. The IAU claim that its definition alone is “truth” is based on nothing more than authoritarianism—an insistence that they and only they get to decide this issue—this in spite of the fact that unlike those who oppose them, the IAU never sent a spacecraft to any planet.

Setting the record straight means getting the word out to textbook publishers, educators, authors, media outlets, planetariums, observatories, and members of the public that the only fair and balanced way to present this issue is as a legitimate debate with two sides. Yes, some people genuinely believe Pluto lost its planet status on August 24, 2006. But that is one view, one opinion, not objective reality, not fact or “truth.” While a controversial debate took place that day, it did not alter what is out there. Those who hold to the geophysical definition—many of whom are the world’s leading planetary scientists—consider Pluto to never have stopped being a planet, and their view deserves as much respect and acknowledgement as that of their opponents.

Let’s get the message out there!