18 Years Later, Proposed Change to IAU Planet Definition is Not the Answer

 

It isn’t, as the Celestron website one-sidedly states on its page about observing Pluto, a “fun holiday to celebrate.” What today is is the anniversary of a terrible day for science and knowledge, a day in which a very political vote was taken, which has done tremendous harm to science education and communication over the last 18 years.

“It” is the 18^th anniversary of the highly controversial vote by four percent of the IAU on a very flawed planet definition that excluded Pluto and all small planets that do not “clear their orbits,’ or, in other words, gravitationally dominate their orbits.

That definition is problematic because it puts primacy on a celestial object’s location rather than on its intrinsic properties. This runs contrary to the use of taxonomy and definitions in science. It was also a violation of the IAU’s own bylaws, which require a resolution to be put to the appropriate IAU committee before being put to the floor of a General Assembly. This was not done with the resolution adopted by the IAU in 2006.

The passage of 18 years without this travesty being corrected is a disservice to science and to all who are interested in it. During this time, New Horizons revealed Pluto to the world as a geologically active world with some complex processes seen elsewhere in the solar system only on Earth and Mars, yet the IAU and supporters of its definition have completely ignored this new information and doubled down on the “orbit clearing” requirement.

A baby born when the IAU vote took place is now old enough to vote. This means an entire generation has grown up with confusion and misinformation, as the media continue to present the IAU view as gospel truth rather than what it really is—one side in an ongoing debate.

And this year, scientist Jean Luc Margot, who was once a student of Mike Brown, the astronomer who unprofessionally brags that he “killed” Pluto, has proposed to remedy only part of the IAU definition to include exoplanets.

By stating that planets must orbit the Sun rather than a star, the IAU definition precludes any exoplanets from being classed as planets.

The problem with Margot’s proposal, which was presented at this year’s IAU General Assembly but not put to a vote, is that it goes out of its way to preserve orbit clearing, or as more accurately stated, gravitational dominance, as a criterion for planethood.

The proposal establishes a minimum mass for planet status that includes Mercury but does not include Pluto or Ceres.

The paper uses circular reasoning to justify this. It states, “Dynamical dominance looms large in planetary taxonomy because both Ceres and Pluto lost their status as planets once they were found to belong to a belt of small bodies.”

This is circular reasoning because it essentially says, Ceres and Pluto lost their status as planets not because of any change they underwent but because the IAU created a requirement of dynamical dominance that excluded them.

Contrary to media reports and this article, Pluto and Ceres never lost their status as planets. Instead, what happened is that the IAU stopped considering them planets. That doesn’t mean they stopped being planets or that we have to stop viewing them as planets. This is an important distinction.

Pluto and Ceres are far more than objects in belts. They are significantly different from the overwhelming majority of the objects in those belts because they are rounded by their own gravity, a state known as hydrostatic equilibrium. This is crucial because it is the threshold when active geology begins, making these objects significantly different than tiny, shapeless asteroids and comets, which are shaped only by their chemical bonds.

Classifying these objects merely as inhabitants of belts completely ignores their unique characteristics, shared with the terrestrial planets, and blurs the distinction between them and the majority of objects in these belts.

And again, it amounts to putting an object’s location above its intrinsic properties, which is a major problem.

It also precludes any rogue planets, which do not orbit any star but float freely in space, from being considered planets. Many of these objects have been found over the last 18 years. If they aren’t planets, then what are they?

If rogue planets were planets when they orbited their parent stars but stopped being planets when they were ejected from those orbits, then we again have the fallacy that an object’s location is put above its intrinsic properties. The same object can be a planet in one location but not a planet in another location.

The same is true for binary planet systems like Pluto-Charon, as neither object in such a binary “clears its orbit.” As for gravitational dominance in a binary system, that would require one of the companions to be significantly more massive than the other one, which is not the case for Pluto-Charon.

Adherents of the geophysical definition, which does not require orbit clearing or dynamical dominance for planethood, do not discount orbital dynamics or think they aren’t important. There is no reason why, in accordance with a study published in 2000, we cannot distinguish dynamically dominant planets from non-dynamically dominant planets via the use of subcategories.

With the geophysical definition, some planets are gravitationally dominant while some are not. This and its importance are acknowledged by placing these two types of planets in different subcategories. Dwarf planets are simply a subclass of planets that are not gravitationally dominant. This does not mean they aren’t planets at all, just that they are a different subclass of planets.

In fact, the IAU itself had a resolution in 2006 establishing dwarf planets as a subclass of planets. This resolution was voted down in a vote of 186-183, in a group with a membership of 10,000.

So dynamical dominance, to which Margot gives such priority, has this status due to 186 people out of 10,000, a difference of just three votes among the 424 IAU members present at that time.

This belies Margot’s claim that “A dispassionate analysis of the features of solar system bodies yielded a distinct group of eight bodies that have been historically referred to as planets.”

It isn’t a dispassionate analysis that yields this. It is a vote of 186-183 on a particular day in 2006, nine years before the New Horizons Pluto flyby.

Margot’s paper doesn’t miss the chance to push the fallacy that opposition to making gravitational dominance a criterion for planethood is based on emotion. In a highly patronizing and demeaning statement, it says, “Readers who are chagrined that smaller bodies are not recognized as planets should take comfort in the fact that these bodies are no less worthy of exploration.”

Readers and others who oppose keeping dynamical dominance as a criterion for planethood are not “chagrined.” We dissent and do so because we reject this premise and prefer the geophysical definition, which, while not discounting an object’s location, does not put primacy on that as a requirement for planethood.

In other words, our objection is scientific, not emotional.

Margot’s proposal was not put to a vote at this year’s IAU General Assembly but that could be done at its next one in 2027.

At the same time, most planetary scientists believe the IAU is the wrong venue for planet definition. Most IAU members are not planetary scientists but other types of astronomers. It makes no sense that those who study galaxies, black holes, cosmology, etc., should decide what a planet is. That decision should be made by a different organization, a group composed of planetary scientists, and should be done not in a vote but through evolution over time as more knowledge about these worlds becomes available.

Eighteen years is a long time for a bad definition to stick around, but at the same time, the passage of time does not make that definition any more “official” or “legitimate.” Those of us who oppose the IAU definition and recognize its harm to planetary science and public education will continue to fight it and advocate for a better definition. We will never give up, and we will never accept what we know is wrong. And in the long term, I strongly believe, we will prevail.