The Pluto Firestorm Continues

The Issue is NOT Planetary Status

I recently got into a fascinating late-evening computer chat with a someone who is really incensed that Pluto is no longer a planet. He seemed pretty upset about it, and although not a planetary scientist, seemed cognizant of the IAU’s role in the supposed “demotion” of Pluto a few years ago. Since we just passed the anniversary of that silly vote that led to all the commotion, and in light of my conversational partner’s concerns about this distant world, I thought it a good time to talk about Pluto again.

Essentially, there are two issues:  the definition of planet (and where Pluto fits) and the IAU vote. People get upset about the second issue without understanding the scientific implications of the first. And, whether or not the IAU voted the way it did, there was and is still a healthy conversation going on in the planetary science community about just how we define solar system objects, particularly planets.

In less than a year, we’ll know more about Pluto than at any other time in human history. The New Horizons spacecraft will have just completed a successful flyby of Pluto, looked at Charon (its companion), and its moons (and maybe will have found a few more!). It will be exciting, and as Alan Stern (PI of the New Horizons mission put it on a recent NASA press conference), ” A year from now, we’ll write the textbooks on Pluto.”

Pluto is now termed a “dwarf planet”. As I’ve said many times before, in many public talks, here on this blog, in Twitter, on Facebook, that’s NOT a demotion. It’s simply a definition of the TYPE of planet that Pluto is thought to be. It is no more a demotion than “dwarf star” or “dwarf galaxy”, terms that astronomy has used sensibly for many years to describe objects that are stars or galaxies but aren’t the same size or mass as their larger siblings. To be clear, those things are still stars and galaxies, just as Pluto is still a planet. The qualifier “dwarf” simply conveys (in a shorthand way) our understanding of some specific characteristics of the “dwarf”.

Heck, the Sun is now known as a “yellow dwarf”, of stellar type G V. Those indicate something about the Sun’s mass, density, and the light it emits. Dwarf in this (or any sense) is not pejorative. Not in the least. It’s a useful descriptor. And, “dwarf” is not an insult to Pluto. It’s part of this world’s description. So, as it stands now, I don’t see a huge issue about whether Pluto is a planet or not (although there have been tantalizing discussions over the years about how it could be a world-sized comet — leading to suggestions that it could be a planet AND a comet).

Of course, there are some who focus on the opinion that Pluto is not a MAJOR planet (but a minor one).  I find the term “major” and minor” sort of outdated, for reasons I’ll get to in a moment. But, suffice to say, those two terms don’t really tell us much about what it means to be “major” or “minor” in terms of physical characteristics. At least not without a lot of tedious and ad hoc explanations.

I know my opinion is in disagreement with my friend Neil Tyson’s and others. That’s okay. We can disagree all we want until we know more about this distant world.  More data=greater understanding. That’s part of science and I wouldn’t have it any other way.

So, the other part of the Pluto argument, isn’t really about what Pluto is or isn’t. It’s with the IAU’s high-handed tactics in raising a vote at a meeting at which many planetary scientists (the folks who, you know, actually KNOW something about solar system objects) were not present. The definition that the small percentage of IAU members voted on was NOT what had been carefully discussed by a panel specifically convened for the purpose. I was astounded to see that kind of weirdness coming from an organization that was created by astronomers to serve astronomy.

What the IAU left the world with was a mess that didn’t need to be made. There’s no other polite way to put it. Heck, planetary science doesn’t even have the definition of “planet” nailed down yet — and if anybody’s going to come up with a definition, it SHOULD be those folks. Not the IAU in one afternoon after jettisoning the work of a committee that was specifically tasked to come up with a workable definition of “planet”. The planetary science folks are the ones in the field, doing the work, and can see the nuances of terminology. Let it be THEIR call.

Look, the objects in the solar system occupy various continua of size, density, orbital characteristics, composition, and evolutionary progress. In order to understand each type of object’s characteristics and put them along some continuum, scientists classify them. Today, “planet” is a classification — albeit a very broad one. We have terrestrial (rocky) planets. We also have gas giants. Lately, I’ve seen the term “ice giants” thrown around (for Uranus and Neptune). We have comets, classified as long- and short-period. Asteroids have whole families of classifications. And, there are in-between objects, worlds and worldlets that don’t fit wholly into one category or another. Nothing fits perfectly, and I think you could sift through the solar system with finer and finer sieves and make many more classifications, such worlds with atmospheres, worlds with volcanism/cryovolcanism, and so on.

The outer solar system is a place we know so little about that it’s not even funny. Next year, that will change. We’ll know more about the Kuiper Belt, which is the region where Pluto resides. It could well be the defining object of the Kuiper Belt, and scientists will need to come up with whole new classification schemes to describe the worlds they find “out there”. There are likely many, many more dwarf planets out there, and that’s just something we’ll have to get used to. But, they WILL be planets. The modifier “dwarf” will describe their size just as “gas giant” describes Jupiter and “terrestrial” describes Venus and Mars.

Forget “major” planets and “minor” planets. These belong to a quainter, earlier age when our telescopes weren’t able to tell much about distant worlds except “big” and “small”. If we can further refine the descriptor of a world, and we can do that now with our newer instruments, then that’s what we should do. We have terrestrial, gas giant, ice giant, and dwarf planets. We have types of comets and asteroids. Heck, we have asteroids that used to be comets! And we have moons and rings (that maybe used to be moons). All of those qualifiers are a great start toward spreading understanding of the diversity of objects in our solar system. And, don’t forget, the New Horizons mission is taking us to Pluto (figuratively), and the news is bound to be great!

If you want to fuss about something, fuss about the IAU and its role in the making of this mess. While the IAU does some things well, it didn’t handle a vote well, and it certainly could use some redirection into a role that serves astronomy, not dictates it.