Blue Collar Scientist

Some in the astronomy and space community are slightly up in arms about National Geographic’s recent announcement of a winner in a contest to come up with a mnemonic phrase to help remember the names of the planets. The phrase that won the contest, proposed by a ten year old, is:

My very exciting magic carpet just sailed under nine palace elephants.

As has been noted, this would cover the eight bodies defined by the International Astronomical Union as planets, as well as three “dwarf planets.” The Bad Astronomer writes:

Um, NatGeo? I hate to break it to you, but our solar system, officially, has eight planets. Pluto was kicked out years ago. If you want to be a Luddite and still accept Pluto as a planet, that’s fine, but really, Ceres and Eris too?

Nope.

He’s formally right, of course. So where does this confusion come from?

It is a result of the IAU’s adoption of the term “dwarf planet” to describe those bodies that are massive enough for their gravity to overcome rigid-body forces and go into hydrostatic equilibrium (i.e., they are round), but is not massive enough to clear their orbits of debris, and which are not satellites.

My most popular astronomy talk, by far, is the one where I talk about whether Pluto is or is not a planet. That talk got a major revision when the IAU kicked Pluto out of the club, and the change only enhanced the popularity of the talk. Every time I give this talk, I explain the difference between a “planet” and a “dwarf planet,” and invariably, every time during Q&A someone says something like this:

Planet is a noun, and dwarf in this case is an adjective, so the dwarf planets are still planets, just little ones, right?

Umm, no. As far as the IAU is concerned, the terms are “planet” and “dwarfplanet,” two separate and distinct concepts. But it doesn’t stop normal, intelligent, common-sense individuals from thinking otherwise every time I give the talk and explain the difference.

Now, you might think, hey, that just means that your talk is crap, and you aren’t explaining the difference very well. Well, I thought of that, and I’ve made adjustments. I’ve run it by focus groups. I’ve consulted with greater experts than I - both experts in astronomy, and in public speaking. The problem isn’t going away.

More telling, I think, are the number of reporters who asked me exactly the same question in the months following the change. I do a fair amount of media consulting and background sourcing, and I probably talked to over 100 reporters who came to this same conclusion. These are intelligent people who read the press releases, all of which specifically said Pluto was not a planet but a dwarf planet, who nevertheless didn’t realize they were distinct concepts.

The confusion persists to this day. I still get questions from teachers about the definitions, and many of them draw the same conclusion about dwarf planets still being planets, just little itty bitty ones. The general public, when they think of us at all, views the astronomical community as being slightly ridiculous on this issue. Many view these definitions as meaningless hair-splitting, with laypeople having pointing out to me that the only distinction between a Planet and a Dwarf Planet is where it orbits. Their logic is that something as small as Pluto, if it were orbiting in a part of the solar system that had been cleared by other planets - say, between Jupiter and Saturn, where lots of stable solar orbits but not a lot of debris exists - would be considered a planet just by happenstance. Others point out that the definitions don’t really have anything to do with genuinely interesting things about the planets (and dwarf planets), such as how they formed or how they were modified by other processes in the 4.5 billion years since their formation. Others point out that in a hypothetical solar system, it would be possible that a “non-planet” today could have its orbit cleared in a billion years, thus achieving the status of planet merely “by failing to disappear in a flash of light and a puff of smoke,” as one person memorably explained to me.

All of these are reasonable observations for laypeople to make. More perceptive critics have pointed out to me that the definitions are so unclear that IAU felt obligated to define the list of planets and a separate list of dwarf planets, by name, in a footnote to the definitions, presumably so there would be no confusion. (That sure didn’t work out very well.) Many have pointed out that the IAU definition specifically excludes any exoplanet from being called a planet, a condition many find unreasonable. Even more perceptive ones have pointed out that real science doesn’t get voted on, so if these definitions aren’t real science, why pay any attention?

There are good answers to all these questions. Real science does include systematic collections of knowledge, and one way of systematizing knowledge is to categorize it - in this case as planet or not-planet, and to do that you have to define your terms - so to this extent the IAU decision is, sort of, scientific. We call planets orbiting other stars exoplanets, so the formal definition isn’t really burdensome. And so on. Ultimately, though, my answer is that the IAU has not done a good job of settling the definition of planet in an understandable way, and for this reason the term “planet” is more of a cultural, than a scientific, one.

Which brings us back to National Geographic. Were they wrong to call Ceres, Pluto, and Eris “planets” for the purposes of their contest? The phrase is going to be used in an upcoming book, called 11 Planets: A New View of the Solar System. It is being written by David Aguilar, who is no lightweight. He is director of public affairs for the Harvard-Smithsonian Center for Astrophysics, the former director of Fiske Planetarium (in Bad Astronomer’s neck of the woods, I’ll point out), etc. This is not, in other words, just some guy who is writing a book. This is just some very knowledgeable guy who knows everything there is to know about the issue, who is writing a book that will in all likelihood be highly respected and widely read, and who probably has one or several good reasons to call it “Eleven Planets.” Just complaining that it doesn’t follow the rules isn’t going to wash - if this book doesn’t follow the rules, the author is at least in a good position to explain why the rules suck.

What’s going to happen, here? Well, in another year, IAU will have its General Assembly down in Rio. And I’d be surprised if someone hasn’t come up with some better ideas about how to classify planets in the last three years. The only question is whether their ideas will be considered at the Assembly, or not. What I know for sure is, regardless of what IAU does, astronomers should expect to see popular accounts of the solar system fail to conform to the neat little boxes we thought we had drawn around things, and however well we have trained the press, we can still expect to see these distinctions fail to penetrate the awareness of the thoughtful public.