We’ve been through this Pluto thing before

February 17, 2016

Pluto and other former planets

Mike Taylor sent me a link to this post on Quora about why Pluto should still be called a planet. Here’s the response I sent back after reading that piece, only lightly edited and with some links added:

I am unmoved. If we go back to the classical definition of planets as “wanderers” then there are 7: the sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn. Clearly we have the ability to redefine the term from “naked-eye visible objects that move against the background stars as seen from Earth” to “big round things that orbit the sun”.

As for the idea that Pluto was a planet for 70-odd years before being demoted, so what? Ceres and Vesta were planets for half a century, and Juno and Vesta were planets for more than 40 years. Eventually people decided that it was unworkable to classify everything in the rocky belt between Mars and Jupiter as a planet – asteroids are a different class of objects, and referring to them as planets conflates two very different phenomena. Recently many people – an acting majority – have decided that it is equally unworkable to classify everything in the icy belt beyond Neptune as a planet, for the same reason. Kuiper Belt objects are a fundamentally different kind of thing from either the rocky planets or the gas giants. We’ve been through all of this before. We can have 8 planets, or hundreds.

Finally, I am completely opposed to this essentialist idea that terms have to keep their meaning forever and aren’t available for revision as we learn more. I think it’s a harmful doctrine, not least because it’s strongly at odds with the reality of how people actually use language – in science, and in all other areas of life.

That said, thanks for the link. I haven’t seen the pro-Pluto argument couched quite that way before. I disagree with almost all of it, but it’s still useful to know that people think that way.

To which Mike replied:

I’d agree with pretty much all of that, for what it’s worth. I thought it was interesting to see the strongest case made for Pluto’s planetary status by someone with a stake and some expertise, and then to see how far short of being persuasive that was.

What do you think?



  1. Interesting…..about the only thing in the referred to post that holds logically is that the entire IAU was not in on the vote. I don’t know what their quorum rules are, and I also don’t know if it’s members are just slack on attending, but if the rules allowed the vote, there really shouldn’t be an argument.

    His argument of common name versus scientific name holds less weight. We have always had, and always will have, both. But, this is an apple to oranges comparison. The topic isn’t what Pluto should be named, it is how to classify Pluto. A better comparison would be whether a tomato is a vegetable or a fruit. Not whether it should be called a tomato or a tomahto. It hasn’t been said that a robin or turkey aren’t birds, or a buffalo isn’t a mammal.

    The uproar is in the changing on previously known facts, in my opinion. We learned in school there are 9 planets. Now our kids or grandkids are telling us there are 8. Those kind of changes don’t sit well.

    This is a huge problem with dinosaur names and which is the largest or longest or fastest, etc. The whole book on dinosaurs has changed from when I was a kid. I’ll leave the details to Matt!

    So, in science we are constantly refining, usually based on an increase of data. That, is what happened to Pluto. It wouldn’t surprise me if Neptune and Uranus are eventually questioned. I’d go with Matt’s visibility qualification, though maybe, just maybe, they were visible at some point when skies where clearer.

    My only inclination of reclassifying Pluto as a planet has come with the pictures and analysis of Pluto itself. The problem is, there are more ice giants out there with a similar makeup, and it is difficult to draw the line. And…I don’t think SIZE should be part of the definition for planet……

  2. I agree that Pluto isn’t and should not be a planet. That doesn’t make it less special and less exciting and less interesting. An object doesn’t need to be a planet to be awesome.

    Personally, I don’t like the dwarf planet designation at all. I accept that’s an emotional response not a scientific one. But I also feel that dwarf planet is sort of appeasement to those emotionally tied to Pluto as a planet.

    I think Pluto should be called a KBO, like its siblings, and Ceres et al are all Asteroid Belt objects. That makes like cleaner in my book.

    Of course there are burry bits, when does a KBO or ABO object become of sufficient size to be a planet? Stuff out there never falls into nice neat definable categories, we should celebrate this diversity and not fall out about how we name things. The name and the category is less important than the science. 🙂

  3. David wrote:

    The uproar is in the changing on previously known facts, in my opinion. We learned in school there are 9 planets. Now our kids or grandkids are telling us there are 8. Those kind of changes don’t sit well.

    Yes, quite. Science deals with provisional interpretations that are always open to being revised, not immutable facts. When scientists make those inevitable revisions, I think at least some laypeople interpret that as a sign of confusion or some other weakness, like, “When are these eggheads going to get their story straight?” But being able to change our interpretations is not just a strength, but the central engine that makes science work at all.

    I wrote about this problem over my paleontology blog, too (link).

    limey, I agree with every single word you wrote. Well said all around.

  4. Just wanted to add some thoughts on the actual meaning of the word “planet”. To quote Mike: “[Planets are] naked-eye visible objects that move against the background stars as seen from Earth”.

    The Greek adjective “planetes” (“wandering”, “roaming”) is related to the verb “planaein” (“to cause to wander”). A “planet” is something being “caused to wander” in the sense of “being led astray” or “doing a thing irregularly or with variation”.

    Therefore, I would suggest a slight modification to Mike’s definition: “[Planets are] naked-eye visible objects that move _erratically_ against the background stars as seen from Earth”.

    Let’s put ourselves in the position of Greek stargazers in the olden days: Thousands of stars (no light pollution) moving with the precision of a Swiss clockwork (not a valid metaphor at that time). However, a handful of bright objects move back and forth in the sky, like drunkards, like “planets”.

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