We’ve been through this Pluto thing before

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?