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The eight survivors: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. (IAU/Martin Kornmesser)

Pluto's Planethood: What Now?

Two leading scientific experts debate whether eight is enough.

Brown: The ability to memorize the planets is not important. So if the number is 50 and kids couldn’t memorize them, it’s actually not the important criterion here. I don’t understand the complaint that this is unteachable. To any level of sophistication, you can explain the difference between—Alan has described it perfectly himself—the large, important bodies in the solar system and the ones that aren’t important. The problem is the way it happened at the IAU meeting. The majority wanted a dynamical definition, but were not given the time to come up with a precisely worded one. Most of the people in the dynamical camp really did not want the word "dwarf planet," but that was forced through by the pro-Pluto camp. So you’re left with this ridiculous baggage of dwarf planets not being planets.

Stern: I’ve heard a lot of my colleagues say "we just can’t have a lot of planets." I’m not of that opinion. I think nature gives us whatever it gives us. There used to be a countable number of stars. Now there’s an uncountable number of stars. We don’t limit the number of stars to make it easier on ourselves or fit that old notion of Arabic astronomy. If it’s really 50 or 500, to many of us it doesn’t matter. But there are clearly people on both sides, even in the scientific community.

Brown: This is why this whole debate is not about science. There is no correct definition of the word "planet." If you think the physics of the body is important, you have hundreds. If you think the dynamics are important, you have eight. And it’s unlikely that there will be any compromise. It’s a religious schism.

A&S: So what’s the next step forward? Unfortunately—and this may have been the fault of the press—the public was told astronomers were going to decide once and for all on a single definition. And that didn’t work.

Brown: Given the circus-like nature of what happened, my recommendation is that we drop the word "planet." Why don’t we use new scientific words to describe these concepts instead of forcing the word "planet" to mean things it was never intended to mean. The public is dying to know how many planets there are. Is Pluto a planet? Is Eris a planet? I think the best answer from the IAU in Prague would have been to say the word planet is just not a scientific word, and we have these other words to describe the solar system. But we are not going to mess with this one. It’s a cultural word, like the word "continent." And culture can figure out what it wants to do. If culture wants Pluto to stay a planet, great. If it wants Pluto not to be a planet, fantastic. Deciding which of several equally rational classification systems gets to own the word "planet" has absolutely nothing to do with science and everything to do with culture.

Stern: I disagree that it’s just about culture, I think it’s actually about science. And I think it’s naive that we will get rid of the term "planet." It’s a useful construct in space exploration and astronomy. We’re all reductionists. There are certain broad classes of objects in the universe, and I think planet is a perfectly legitimate class that has a lot of diversity in it. We just to have learn to adapt to the fact that there are many classes of planets that we didn’t recognize before because we didn’t have the data to see it.

A&S: Do you think we’ll see a repeat of this at the next IAU meeting in 2009?

Brown: Who knows? Maybe we’ll do it forever.

Stern: I’m perfectly happy with an outcome where it takes years or decades to shake things out between these two major philosophical approaches. But what I’m adamantly opposed to is the perception in the public that because the IAU has spoken, and produced what even Mike says is a flawed definition, that it’s over and we’re all just going to accept that as dogma, and all the textbooks will change. I’m much happier with saying, "We haven’t decided. It requires more observation, and science has to progress for another X years."

Brown: But if, as I believe, the debate is not about science, then no amount of observation is going to change people’s minds. The current scientific culture has voted in favor of a dynamical definition for the word "planet." No advances in science will ever change the results of a vote like this, just a change in scientific culture.

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