Rewrite the textbooks!

The IAU hasn’t yet ruled, but it looks like our solar system is shortly going to “grow” from 9 to 12 planets.

new IAU solar system model

This blows my mind. I’ve seen societal change due to technology — the rise of email, of PCs and blogs and the wired way we live and work now. But I’ve never seen a chunk of human culture changed by technology, not like this, not that I can recall. We may soon no longer have the nine planets that seemed untouchable, unchangeable icons to someone my age.

There’s a few surprises in the list of new planets: they include Ceres, a large asteroid (discovered in 1801!) that orbits the sun between Mars and Jupiter, and Pluto’s MOON (????), Charon! The third proposed new planet is UB313/”Xena,” the larger-than-Pluto Kuiper belt object that orbits the sun out beyond Pluto.

Rather than rehash everything appearing in the media, I’m just going to link to Phil Plait’s take on all this. Plait is a writer, skeptic, and professional astronomer, and his thoughts on the new solar system family are some of my favorite things he’s written so far. He’s done a fine summation and analysis. So just go here.

Here’s a summary of all the new stuff (this is not a rehash, it’s a summary, people). I summarize, therefore I am:

According to the proposal, Pluto would still be a planet. It would become part of a newly created official group of smaller planets called plutons that includes Charon and UB313 as well. Plutons are objects past the orbit of Neptune that “typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years.” Pluto is the prototypical pluton. [All hail the pluton.]

It looks like we may have 12 planets soon: eight classical planets, one “dwarf planet” (Ceres), and the three plutons (SO FAR). There will be more plutons. There are more out there (including some objects, like Sedna, that have already been discovered and simply await evaluation under the new definition). We just haven’t found them all yet.

Pluto-Charon would become a double-planet system. As the Pluto-Charon center of mass is outside of Pluto’s surface, Charon would no longer considered to be a satellite of Pluto. Charon would become a planet unto itself.

The IAU has at last proposed an official definition of the word planet for use as a scientific term used to classify astronomical objects. Here’s the definition:

(1) A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

(2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a “dwarf planet.”

(3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call “plutons”.

(4) All non-planet objects orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

According to Plait, this definition is flawed. [IMO the most fascinating part of a fascinating post is Plait’s argument that it’s impossible to defiine what a planet is, as the universe can always find a way to make a planetlike body that’s not quite part of that definition. I’d always thought of planethood as something that changed as technology and understanding of the universe changed. He’s made me wonder if the process of “defining” what a planet is isn’t far more difficult and arbitrary than I ever believed.]


Which brings me, finally, to my big point. This is all incredibly silly. We’re not arguing science here. We’re arguing semantics. For years people have tried to make a rigid definition of planet, but it simply won’t work. No matter what parameter you include in the list, I can come up with an example that screws the definition up. I’ve shown that already, and I’m just warming up.

The problem here is simple, really: we’re trying to wrap a scientific definition around a culturally-defined word that has no strict definition. Doing this will only lead to trouble. Why? For one thing, it’s divisive and silly. How does a definition help us at all? And how does it make things less confusing than they already are? Charon is a planet? It’s smaller than our own Moon!

A big step in understanding a new object is being able to categorize it. Is it icy, or rocky? Is the orbit circular, elliptical, far from the Sun, nearby, tilted? This type of information leads to insight on how the object formed, what it’s doing, and how it behaves. This is all important, and so it is a good idea to try to categorize objects. But definitions are like little boxes, containers in which ideas sit. But sometimes they’re more like prison cells. They frame our minds, make us see things too rigidly. Thinking of Pluto as a planet might make us miss some important characteristic because we’re too narrow in our thinking. I’ve seen it happen before, even with me. It’s too easy to be rigid with a definition in your hand.

However, in a sense this doesn’t matter. What’s in a name? Scientists will probably still of Pluto as they always have– an ice ball at the edge of the main solar system. The public will still think of it as a planet, so that won’t change. And, well, there is something cool about this new set of rules. Maybe, just maybe, in a few years we’ll have a solar system with hundreds or even thousands of planets, instead of just the 9 — nuts, I mean 12 — that we have now.

Jennifer’s note: Looks like some of my timelines for getting IAU General Assembly updates were wrong. Oh well. I’ll post as I find out what’s being announced over the course of the assembly, which lasts through August 25.

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