In my blog on June 18, planetary scientist Alan Stern commented on the recent reclassification of Pluto and other outer solar system bodies as “plutoids” instead of planets.

We’ve been talking again, this time in response to a conference, "Great Planet Debate: Science as a Process," at the Johns Hopkins Applied Physics Laboratory in Baltimore August 12. It was a chance for scientists to discuss the issues underlying what is being called “the great planet debate,” or simply GPD.

The debate is far from resolved, as this e-mail exchange I had with Alan Stern demonstrates. Stern participated in the GPD conference. Here’s what he had to say.

Dan Pendick: What were the big issues you discussed at the meeting, what did people say, and what do you think about it?

Alan Stern: The only big issue is what science chooses to call a planet. That cannot be decided by voting because science doesn't work that way. Instead, science works by consensus, i.e., scientists making their mind up as to what works best and what description of nature best fits the data.

There are basically two camps at present. One camp wants to classify what is and is not a planet based on its attributes. The other camp wants to look at a solar system as a whole and decide what the dominant objects are and call those planets.

I like the first approach, i.e., the attribute based approach. I think of it this way: When a spaceship (let's say, the starship Enterprise) pulls into orbit around a body, the science officer (in this case Spock) has to tell the captain (here, Kirk) what kind of object they are orbiting. In an attribute-based planet definition, the science officer makes some measurements of the body they are orbiting (e.g., its size, shape, and other physical properties and then based on what he has measured), [and] he tells the captain they are either orbiting a planet, or some other kind of object.

In contrast, with the second approach, the science officer has to take a complete census of the entire solar system they are in and decide about the nature of object they are orbiting based on what *other* objects orbit the host star, and where each object orbits. And in fact, if you like dynamical dominance — whether or not a planet has cleared its orbit of debris — as a criterion, then the science officer also has to integrate a lot of orbits for a long time to decide if the object they are orbiting dominates, and therefore is or is not a planet.

I think that context-based approach is flawed, and for several reasons. First, it's making something hard out of something easy. Second, what may be a dominant object in one solar system may not be dominating in another solar system; e.g., Saturn or Mars is a planet in our solar system, but they might not be in others because they might not be dominant objects. Such an outcome seems silly to me.

In fact, the IAU's 2006 planet definition (which is widely recognized as problematic) takes the context-based problems to a whole new level of ridiculosity (Is that a word?). Why? Because the "clearing its zone" criteria means that Mercury is a planet in the orbit it is in, but it would not be a planet if we moved it just a tad farther out in our own solar system! And Pluto *would* be an IAU planet if you moved it far enough in in our solar system! This is beyond silly.

Worse, even Earth-sized objects in our outer solar system will not be classified as planets by the IAU’s definition. Now tell me, how do you explain *that* to kids and the public? "Oh, I know it's 8,000 miles across, has 7 continents, a blue sky, oceans, rivers, clouds, a moon, and cities on the surface, but it's not a planet." That just doesn't pass the basic smell test, as they say.

Now you might agree with me about this, but not all my scientific colleagues do. Some feel strongly that the context-based way of doing this is superior. Others don’t, but what we learned at the GPD meeting was that the community is deeply split on which way to go and there is no consensus. Even on the narrower question of whether Pluto is a planet, some scientists say yes, some scientists say no, and there is no clear consensus just yet.

I do think the situation will clear up over time, but planetary science is adjusting to new norms as we discover all kinds of exotica — dwarf planets, hot Jupiters, super-Earths, pulsar planets, etc. As long as the data keep turning up wild new revelations, I think this debate will continue, and I think it’s premature for texts, planetaria, and schools to teach anything other than [that while] the debate is ongoing.

DP: I understand the context -vs.- attribute split. And I understand the value of classifying by the object's intrinsic qualities — it's the simplest way. What would be the advantage of using context? In exchange for all the complexity and confusion, what do you gain besides containing the number of "major" planets to eight in our solar system? There must be something more to it than making sure school kids can memorize them.

AS: Honestly, I don't see what you get from the context thing except a small number of planets and a sieve for which objects control the clearing, and even that is based on a non-trivial equation with all kinds of factors in it that boil down to saying the situation of what is and is not a planet in every solar system across the galaxy will be different.

Now to me, controlling the number of planets to be small is irrelevant and actually detrimental: We don't find a definition for rivers that keep the number small, nor mountains, nor species, nor stars, nor galaxies. Who cares? You know, using rivers as an example, teachers just teach kids the 7 great rivers of the Earth and the ones by their town, and every kids knows there are too many to count and they can look up any ones they want after the few they are taught.

To my mind, that is how we should treat planets — both in our solar system and in others. I wonder what your readers at Astronomy think?

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For related articles about the Great Planet Debate, visit Astronomy.com's special package, How we lost Pluto.

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