The Issue is NOT Just Planetary Status
I recently got into a fascinating late-evening computer chat with 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.
I think it’s very important to emphasize that science is not dictated by an “authority” or “authoritative body.” No one voted on the “truth” of relativity or gravity. Anyone who says “Pluto is no longer a planet” is giving the IAU and its decision power and legitimacy that neither should have. Those of us who adhere to a geophysical planet definition have never stopped viewing Pluto as a planet.
Your understanding of dwarf planets as a subtype of planets makes sense and is what Dr. Alan Stern originally intended when he coined the term in 1991. However, the problem is that the IAU vote specifically says that dwarf planets are not planets at all! That to me is problematic and confusing.
I agree with replacing the terms “major planet” and “minor planet” because they do not reflect the complexity of what is out there. The term “minor planet” has been historically used as a synonym for asteroids and comets, or bodies now referred to as “Small Solar System Bodies.” Pluto does not fit this category because it is not a comet or asteroid.
Pluto should not be described as a comet. It is 70 percent rock whereas comets are largely made of ice. Comets do not have geology and weather. Pluto may even maintain an atmosphere throughout its 248-year orbit. Any planet brought close enough to its parent star would grow a tail. Mercury has a tail, as do many giant exoplanets.
I agree that planetary scientists should be the ones writing the textbooks about planets. But we’re still left with the fact that the IAU made a mess they won’t clean up, one that unfortunately determines the content of textbooks and educational materials. The IAU needs to fix the mess it made and publicly state that in the future, the data alone will drive such classifications.
How about using a schematic like the Herszprung-Russell Diagram, used to classify stars, for planets? We could have a spectrum of planet types and subtypes based on the reality of what is out there, encompassing objects like rogue planets, that do not orbit any star. Some have suggested using the Star Trek classification system, which is not a bad idea.
At the end of “Cosmos,” Tyson admitted he could be wrong about anything and urged us to question everything, so he likely won’t mind your disagreeing with him! 🙂
Hi Laurel, I was wondering when you’d reply. 😉
I am not ready to release the idea of Pluto as a giant comet yet, simply because we aren’t there yet. We just have to widen the definition of comet. Not saying that it is, just that we don’t know enough about it yet. Likely that’s a very generous interpretation of “comet” though. 😉 Did you look at the ICQ link I had in the story with a discussion about that very thing? It is always a good exercise in critical thinking to consider all the possibilities.
As for a classification chart, I’ve carried a mental one around for years that has all the solar system bodies on a continuum, largest to smallest, icy to rocky, and so on. I suspect we’d have more than one diagram. 😉
The IAU issue is something that the membership should force on its “leaders”. There are indeed members who are NOT happy with the IAU’s rather high-handed treatment of such matters, as well as the whole nomenclature thing. THAT is a HUGE mess. And the IAU can only blame itself for years of inaction in that regard. The IAU has a role in astronomy, it just needs to learn what it is.
By the way, I studied planetary science and astronomy for some years at the University of Colorado. My specific area was comets, with an emphasis on plasma tail formation and evolution, but we did, of course have to know about the whole comet, too.
Many of the issues of planethood, makeup of comets, Pluto, discovery of KBOs, etc. were subjects of intense discussion in my classes and in our offices at the U. I’m not ignorant of basic planetary science. Also, Alan Stern was a contemporary during my time there. It’s been great to renew our friendship after each of us has pursued our careers here and there. We did spend some time talking about dwarf planets during that time (early ’90s), as well as cryovolcanism (which was a ‘sort of’ new topic then).
I would never assume you are ignorant of planetary science! I do, however, question the backgrounds of some of the 424 IAU members who voted on this because many never studied planets at all. At times, I will see writings by them justifying the decision with extremely vague statements that do not incorporate the detail you do in your blog entries.
I also regularly read and hear comments stating that “The discussion is over; ‘they’ voted and decided Pluto isn’t a planet anymore.” That viewpoint is problematic for many reasons.
Somehow, teachers, textbook writers, the media, etc. need to get the message that the IAU vote is not the deciding factor here but one voice among many. Hopefully, this will happen when Stern and others write new textbooks based on the data from New Horizons.
I will check out the discussion about comets in the link, which should be really interesting. My response is specifically to Tyson’s reasoning in his book, which is that Pluto is really a large comet because if brought near the Sun, it would grow a tail. He did not go into any more detail than that.
Well, Neil could be right in that if Pluto were brought near the Sun, it would grow a tail, but that’s not the only defining characteristic of a comet. One could make the same conjecture about any of the icy outer solar system bodies. However, if you follow this idea that the object is rounded by its own gravity, then Pluto could also be a planet (dwarf, by size). So, like humans, I suspect that SS objects don’t all fall neatly into any one category, but could exhibit characteristics of different objects.
This whole hoo-hah is why I write often about this subject, and devoted an entire section in my latest book (p. 76, Astronomy 101) to Pluto. I describe it in rather general terms, and really didn’t have time to go into the semantical discussion I did here. We have been talking about a new edition of the book for late 2015, and I would guess that the Pluto section will be greatly changed by the findings of the NH mission.
Many of us do the best we can to get the word out on this subject — it’s more nuanced than the press likes to go into (except for those of us who go into it anyway), and probably more so for teachers who actually have less chance to teach astronomy and planetary subjects than ever before.