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Pluto's Demotion: My Initial Response

A short time ago, the IAU passed resolutions 5A and 6A, defining what is and isn't a planet. For those who wish to read them, here they are, reposted from the IAU's website:


IAU Resolution: Definition of a Planet in the Solar System

Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation 'planets'. The word 'planet' originally described 'wanderers' that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.


The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:

(1) A planet1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2, (c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite.

(3) All other objects3 orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

2An IAU process will be established to assign borderline objects into either dwarf planet and other categories.

3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

IAU Resolution: Pluto


The IAU further resolves:

Pluto is a dwarf planet by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.


So what does this mean for Pluto?

Well, had Resolution 5B passed, we would have had the overall classification of a "planet" that would have had two sub-classifications: "classical planet" and "dwarf planet." By those classifications, we could have used the sentence "Pluto is a planet" with complete scientific accuracy.

But the IAU rejected Resolution 5B. What this means for our solar system is that there are now two separate categories: "planet" and "dwarf planet." Pluto is now a dwarf planet; and it would be technically incorrect to refer to it as a "planet" without the preceding adjective of "dwarf."

I have to say that I am disappointed with this news. I grew up with a nine-planet solar system, and as I was growing up I marveled at the Voyager spacecraft missions that brought us new knowledge and those spectacular pictures of the planets. I was disappointed that we didn't know what Pluto looked like, and I went on record for many years as hoping for a mission to Pluto. I was delighted when the New Horizons spacecraft launched back in January, as we would finally get to see pictures of the planet Pluto.

But by relegating Pluto to "dwarf planet" status, the IAU has changed the emotional impact that such a mission can have.

I believe the IAU's vote could conceivably stifle the imagination of those of us who still wonder at the glories of our solar system, and who reach for the stars. And I hope that in 2009, when the IAU meets again in Rio de Janeiro, Brazil, they will reconsider the vote that they have taken today in Prague.


I haven't commented on many of these posts, but I am in the other camp on this matter fairly strongly. It has always seemed to me as if people had an intuitive understanding of what a planet is that did not really allow Pluto to be counted. While scientific knowledge may have taken a while to nail down its definitions, Pluto has always felt like the odd man out to me. I admire its pluck and its fervor, but I always felt like definitions of planet that included Pluto were created based on the requirement "let's come up with a definition of a planet that will include Pluto."

The resolution passed today by the IAU matches up with what I have always felt a planet to be, and I am pleased by it. I wish Pluto could have qualified, because I do find it disconcerting and potentially confusing to have a new set of planets, but I don't think it ever really did.

I say, let's let New Horizons be the start of something new. Let's say that planets aren't the only things in our solar system worthy of exploration. There are whole categories within the system that we've been pushing aside as uninteresting; why not see if we can fire the imagination about them as something new and exciting?
You know, I heard one of the committee members discuss this at a symposium recently (and the same member, on the General Technics mailing list, asked for comments on this topic, where it was extensively debated).

I expect that a series of dwarf planets will be named; if they're not, it's pure nostalgia. To paraphrase that committee member, "we don't know how many planets there are, but the number is either eight or greater than nine."

I have to respectfully disagree with the belief that this decision could stifle the imagination of anyone who wonders at the glory of our solar system.

Words are words (and it's hard as heck for me to say that, because I'm a writer, editor, and grammar geek), and names are names. Sure, there's power in a name, but as we discover finer and finer details about the outer reaches of our solar system, a whole new slew of questions pop up. What if Pluto is a captured comet, which in turn snagged a satellite of its own?

That sure gets my brain firing. How do comets deccelerate and sink into more planet-like orbits? Why they do it? How come we haven't seen more of this phenomenon? And if I'm totaly wrong, what is the deal with Pluto, anyway?

It is the end of an era--I will absolutely not dispute that. But I think the power the universe around us has to make us dream and wonder and question goes beyond nomenclature--and if that's not entirely accurate now, then I'd like to do what I can to make it that way.

So, I guess I can sum it up like this. I'm disappointed, but I'm doing my darnedest not to let it get me down. :)
Thanks for covering this here, Michael. This is the kind of detailed information that's really interesting, but probably not something I'd end up digging up on my own.

Oh really. Eight planets is not critical mass for wonder? Granted, nearly everyone working at NASA today, the majority of scientists and engineers, and most science fiction writers were born after 1930. And it wasn't until the kids of the thirties were graduating high school and college that we first sent anything into orbit. But redefining a category of celestial object stifles imagination? That's goofy to me.

So a spacecraft is visiting a really big Trans-Neptunian Object. That's cool! In the past five years, haven't we had spacecraft fly closely by asteroids, land gently, deliberately smack hard into them? Haven't we flown a spacecraft smack through a comet's tail and bring bits back? Haven't we had spacecraft land on other planets' moons? What is not awesome about all that? Go, Small Solar System Bodies!

ugh. That's too long, but I refuse to call them SSSBs.

Besides, anything that makes people think about hydrostatic equilibrium and barycenters (not covered in the definition, I admit) and suchlike is fine by me. My gripe about it is that the definition is specific to this solar system; look anywhere else and we get to gripe about naming things again.
My very excellent mother just served us...

nuts. :-(

December 2016

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