A Fan's Playground

The thing about all the ongoing discussion about what constitutes a planet and whether Pluto counts as one is that the term “planet” enforces a strict yes/no binary onto physical properties of objects that exist on a spectrum.

And breaking down the complicated spectrum of reality into categories you can make generalizations about is often good and useful, because there are clearly important differences between, say, Jupiter and 2014 JO25, but then the question always becomes where you draw the lines between categories.

We’ve gone through the argument about what’s a planet before - when Uranus was discovered it was first called a comet because the idea of there only being five planets beyond Earth was so entrenched; Galileo called Jupiter’s moons planets and there was a period where planetary satellites were all referred to as planets; when Ceres was first discovered it was considered a planet, and asteroids in general were classified as planets for decades until the number of identified asteroids began creeping up. Planet is clearly a term with uncertain historical boundaries, and our understanding of space always has room to change as we gather more data. Currently the IAU’s planetary definition which cuts out Pluto still stands, but there’s no absolute consensus amongst scientists about how planets should be defined.

It’s a hard question! There’s merit to the IAU’s definition. But it could also make sense to have a broader of what a planet is, so that you can include things like exoplanets, which do not orbit any star, or Pluto, which has not cleared its orbit, and yet which still have some key features in common with planets of the narrower definition. Though even then there would continue to be arguments about what counts as a planet - for example, I gather it’s not as straightforward as one might want to draw the line between a small brown dwarf star and a large gas giant planet.

So scientifically speaking the exact definition of a planet is not a shut and closed issue. But what about culturally speaking, since planets are also definitely a cultural phenomenon? Culturally, it seems to me at the current time a broad definition doesn’t make sense because with a broader definition, there might be hundreds of planets in our solar system or even up to 10,000 (according to estimates I’ve seen of how many dwarf planets there might turn out to be), and there’s a cultural and historical relevance to the standard list of planets that is not present in far-distant tiny rocks that haven’t even been named yet in the outer reaches of our solar system. No elementary school curriculum is going to mandate learning a list of more than 50 planet names that is ever-growing and reaching eventually into the hundreds or thousands, but it does still seem reasonable to expect knowledge of Mercury, Venus, Earth, Mars, and so forth.

But the only way it makes sense for Pluto to be a planet but Eris and her other dwarf planet sisters to not be planets is to define the term strictly culturally instead of scientifically. And people clearly desperately, deeply want Pluto to be a planet. Not too long ago I again saw multiple excited posts on tumblr about how PLUTO IS A PLANET AGAIN!!! when there’s not actually any new news on the topic officially.

So does it make sense to have the same word mean different things to scientists and laypeople? That would just get confusing. Is there other vocabulary we could use instead to make the difference clear? I have trouble believing it would catch on.

I can’t discount people’s feelings on the subject, because that sentimental attachment is real, and isn’t part of the point of the study of space to be connected with our universe? If people’s love for Pluto helps them feel a part of space I don’t want to shut them down. But the cultural feelings about how many planets our solar system should have, and the scientific data we have about how many planets our solar system could have depending on definition, do not line up anymore. I don’t think there’s any reasonable scientific definition of planet according to our current knowledge which would land our solar system exactly at 9.

But why must people’s love for Pluto be connected to its label? Pluto itself is unchanged, whatever we choose to call it! And it’s not a hierarchy of labels, where being a “planet” is the best kind of solar system object to be, as people seem to think. Pluto continues to be incredible, fascinating, beautiful, alien, and unfathomably distant. It continues to be important scientifically, historically, and culturally. It doesn’t need to be called a planet to be any of those things.

Our labels are just human attempts at categorization. And like any system of categorization it is of necessity imperfect. There’s no one true answer to how best to classify the incredible complexity of reality. And the universe continues on whether we observe it or not, and continues to be wonderful.



(I Am Not A Scientist, only an enthusiastic layperson, so if I am misunderstanding anything or have details wrong, please let me know in a kind and helpful manner rather than as a “gotcha”)