Today, July 14, 2023, is the eighth anniversary of New Horizons' successful Pluto flyby in 2015. In celebration of this occasion, I am sharing a guest blog by John Vester, which was initially published in the March/April 2021 issue of Analog magazine.
Planet or Not?
by John J. Vester
In 2015, after 16 years of planning and herculean effort, Alan Stern and his team took us all to Pluto aboard the New Horizons planetary probe. But something else that happened in 2006 (the year of the New Horizons launch) cast a slight shadow over the mission, and has since tasked Stern almost as much as the mission itself did…the loss of Pluto’s status as a planet, and the struggle to regain it.
When The International Astronomical Union (IAU) voted to demote Pluto to “dwarf” status, they embarked, Stern argues, on a course for which their membership was unqualified to make pronouncements. The arguments pro and con are many, rage still, and have crystallized into two opposing camps, all of which is brought into clear focus by the April 28, 2019 debate, hosted by the Philosophical Society of Washington DC.1
The IAU Planetary Definition (IPD) has three main parts. 1) A planet orbits the sun, 2) it is large enough to achieve a spherical shape, and 3) a planet clears its orbit of other objects. Against criticism, the adherents of the IPD mainly defend the vote on procedural grounds (although they admit it probably should not have been planned behind closed doors). Naming and science, they say, are two different things and naming is important. It helps science function by creating a common language. (It should be pointed out, though, that the IAU did not name anything, but merely attempted to change the definition of an existing word.)
Stern maintains that expertise is also important, and promotes his Geophysical Planetary Definition (GPD). He points out that planetary scientists, who were significantly absent from the vote in Prague, and who are more qualified than astronomers to decide on planetary nomenclature, do not use or recognize the IPD. The GPD considers a planet 1) to be a sub-stellar object that never underwent fusion, and 2) has enough mass to assume a spherical shape.
The IPD is more concerned with the dynamics of an object…where it is and what its orbit is and does. The GPD is more about the object itself…what it is regardless of where it is. The IPD confines itself to objects in our solar system, while the GPD applies to exoplanets equally. The IPD’s clearing of orbits requirement has the odd consequence that the same object can be considered a planet at one distance from the sun, but not if farther away. GPD proponents are quick to point out that under the IPD’s clearing of orbits stipulation, even the Earth would not qualify as a planet.
Both definitions set up complex requirements for entry into the planet club. One of the reasons given in support of the IPD is the seemingly absurd notion that we need to limit the number of planets so school children won’t have so many to memorize. Yet there are still 50 states and the periodic table still lists north of a hundred elements.
Both definitions are a little vague about moons, rogue planets and brown dwarfs. Each side snipes at the other, basing some of their arguments on outlier cases such as these. This is no way to establish overarching categories.
There has to be a simple way out of this morass, and maybe there is. Here’s an immodest proposal that does not offer either side a conclusive victory, but does not condemn either to final defeat. All combatants agree that this Pluto/planet dustup is a categorization problem. But more than that, I think it is a language problem.
So my proposal, a linguistic solution, is this: Anything orbiting any star is a planet, and anything orbiting a planet is a moon. That simple.
Stern defends the GPD by pointing out that neutron stars, pulsars, and red giants are all still considered stars. (This points up a conundrum in the IPD—calling something it does not consider to be a planet a “dwarf planet.”) To illustrate his point, Stern shows a picture of a Chihuahua and a Great Dane next to each other. Although quite different in size and appearance, we still call them both dogs. There would be no value (not to mention the tidal wave of push back) if scientists demanded we all start calling these animals, instead, canis lupus familiaris.
Imagine the uproar if the IAU began to tinker with the definition of sun, or star. For many things, the common name in the language is best.
So it is with the word “planet,” which originally was a language thing. It meant “wanderer.” It had nothing to do with size. Back then, the planets were mere pin pricks of light that moved (wandered) relative to the background stars (and also exhibited weird apparent retrograde motions at times). But we know that planets don’t wander. They have circumscribed perambulations…they pace. So “planet” has been decoupled somewhat from its original, literal meaning.
To understand the proposed linguistic rationale being offered here, consider the word “snow”. Since we do not live constantly with snow, the English language (along with many/most others) has only one word to encompass the entirety of this seemingly homogenous phenomenon (though extremely varied in the details of its many manifestations). But expertise does matter. Therefore Eskimos/Inuits, who live their whole lives in an intimate relationship with snow, have, it is said, a great many words, one for each of its important forms, all subsumed under the general category that we label “snow.”
With our quickly expanding knowledge of the bodies in our solar system, planets are no longer mere specks of light wandering the sky, but real worlds. This has resulted, during the last forty years, in a huge increase in the number of scientists who consider themselves planetary scientists, not astronomers, and very few of them are members of the IAU, and this is telling.
But the expertise of scientists, whether astronomers or planetary scientists, is not entirely relevant here. They commandeered the word planet from the language of the common people. It is the language (therefore people, not scientists) that has jurisdiction. The Chihuahua/Great Dane example is absolutely relevant. Great Dane is a subcategory of dog. As for heavenly bodies, we know enough about them now, thanks to all those planetary scientists, that we should be more concerned with subcategories. We already speak of the “rocky planets,” the “gas giants,” and the “ice giants.” We understand all these to belong under the heading of “planet.” Why not bring asteroids, comets, Trojans, KBOs, TNOs, and Oort Cloud objects into the same grouping? They should all take their places as subcategories of planets.
This is certainly simpler than the tortuous redefining attempted by the IAU. Clearing an orbit is irrelevant by this linguistic definition. Objects in uncleared orbits would themselves be different flavors of planets.
“Spherical” is a matter of degree, not kind. Even if it mattered, Ceres and Pallas, once considered planets, are today in limbo as spherical asteroids. They are all planets again by the proposed new scheme, and maybe we’ll call them “rocky dwarfs.” As for Pluto, Eris, Sedna, etc., how about “icy dwarfs?”
Size is also irrelevant by this proposed revision to the definition of the larger category, planet. But this shouldn’t create difficulties in discussing these objects. Asteroids would be planets we call “asteroids” (its subcategory). A rock or even a grain of sand orbiting the sun is a planet that we would call a grain of sand or a “meteor.” Once it burns up in a planetary atmosphere or hits the ground and stops orbiting the sun, it is no longer a planet and we might call it part of a “meteor shower” or a “meteorite.”
Even the outlier cases can be easily dealt with by this suggested protocol. The GPD allows that some moons can be considered “worlds” or “planets.” The only way a moon could be proclaimed a planet would be if the center of mass, around which both objects revolve, falls, not inside the larger body, but in the space that separates them. Then it is a double planet, or a binary.
Rogue planets, called in the GPD “unbound planets” become “unbound bodies.”
A brown dwarf is simply a planet if it never initiated fusion in its core. If it doesn’t orbit a star, it is an unbound body.
There will, of course, be scientific hairsplitting on whether an object belongs in one subcategory or another. For the rest of us, though, it will be much simpler to know that they are all planets, simply by virtue of the fact that they orbit a star. One of the criteria Stern offers for recognizing a good naming solution is that it is simple, logical, and intuitive. What could be easier for the general public to understand and embrace than this language-based solution?
The IAU is probably the best place to do the subcategory hairsplitting (provided the planetary scientists are represented and have their say). For my money, the IAU should pass a resolution adopting this idea, and also abdicating their jurisdiction over the overarching category, planet.
Words matter. So, by this proposal, planet would become the people’s word again. Power to the people!
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