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No peace over Pluto

An artist's conception

shows NASA's New

Horizons probe during its

2015 encounter with Pluto.

The latest round in the planethood debate may well provoke planetary scientists into a revolt against the international body that usually has the last word on astronomical terminology, according to the top scientist for NASA’s mission to Pluto.

This week's announcement from the International Astronomical Union that Pluto and other dwarf planets on the solar system's edge would be known henceforth as "plutoids" has been seen by some as a sign of respect for what was once considered the smallest of the solar system's nine planets.

That's not how Alan Stern, principal investigator for NASA's New Horizons mission to Pluto, sees it. In fact, he wonders whether this will be the last straw for those who think IAU officials badly bungled their definition of a planet almost two years ago.

"They're almost needling the planetary community to go their own way," Stern told me today.

Stern isn't alone - and in fact, there's a wide range of opinions on the planethood question, ranging from outrage to acceptance of the IAU's definition. The main point of contention is the idea that a planet must have "cleared the neighborhood around its orbit" - a definition that Stern maintains could exclude worlds exactly like Earth.

In today's telephone interview, the former NASA associate administrator discussed Pluto, planethood and what planetary scientists might do about those questions. Here's an edited transcript of the Q&A:

Cosmic Log: Let's start with the plutoids: Some people have said that this is at last an opportunity for Pluto to get some respect. Does this end the controversy?

Stern: No, because the controversy isn't about names. The issue is a crucial one to planetary scientists: whether we understand which objects are planets, or not. It's not about respect. It's not about Pluto.

Alan Stern is principal

investigator for the New

Horizons mission to Pluto

and the Kuiper Belt.

There is no equivalent issue in the rest of astronomy. Imagine if stellar astronomers couldn't agree as to what is a star, or galactic astronomers couldn't agree as to what is a galaxy. What if geologists or biologists couldn't make the simplest classifications, like animals vs. plants? It would be unacceptable. As a result of the 2006 IAU meeting, right now we have an unworkable, embarrassing and wrong definition of what a planet is.

It's very easy to demonstrate that. Any definition of a planet would be laughed out of the house unless Earth is a planet. Anytime you take a picture of an object, and the picture is of Earth, that has to be a planet. We live on a planet.

In fact, the IAU definition doesn't come close to allowing Earth to be a planet.

Q: Because Earth hasn't cleared the near-Earth objects out of its orbit?

A: Well, there's a technicality that they didn't write it well. You're right, because of the near-Earth objects, the earth is technically disqualified. But even if you could forgive that ... and clear up the language, the issue is that as you go farther and farther away from the sun, the equations that describe the mass required to "clear a zone" show that the objects have to get more and more massive.

So Mercury qualifies in Mercury's orbit, but Mercury would not qualify in Earth's orbit. Earth might qualify in its current orbit, but if we put the earth where Pluto is - in other words, if Pluto were the mass of the earth - it still wouldn't qualify. In fact, in the Oort Cloud, which is part of our solar system, none of the planets, even Jupiter, would qualify. Which is a really ridiculous way of defining things. It depends on where it is, not what it is.

Q: It's a case of defining a planet not by the thing itself, but by everything that's around it?

A: Exactly. I would like to see a definition that's really simple. I like to use the "Star Trek" Enterprise test. The Starship Enterprise shows up at a given body, they turn on the cameras on the bridge and they see it. Captain Kirk and Spock could look at it and they could say, "That's a star, that's a planet, that's a comet." They could tell the difference. They don't need a Ph.D.

In the case of the IAU, when Kirk asks, "Is it a planet?" Spock would have to say, "I don't know, Captain. We have to make a complete census of the solar system, feed that into a computer, and do numerical integrations to determine which objects have cleared their zone."

Q: So the issue of the nomenclature, whether it's a dwarf planet, or a plutino or a plutoid - as far as you're concerned, that's not the real point.

A: It's really about us "planetary scientists" having a basic understanding of the object after which our field is named. The reason we're having this discussion is because for a long time, we only knew of a few planets, and life was simple. Then, in the 1990s, there was this explosion in the variety of new kinds of bodies orbiting other stars and out in the solar system, because our technology got more sophisticated. We could suddenly see what we couldn't see before.

It's as if we were biologists trapped on a single desert island, and the only kinds of plants and animals that we knew were on that desert island. And then we were taken on a world tour of the flora and fauna of the earth. It would blow our minds. You would have two choices: You could say, 'Anything I didn't recognize from my own island is not living. I'm not counting that, because there would be too many varieties and I can't keep up with them.' Or you could say, 'I'm a scientist, and I have to adapt to new data. Wow, I really underestimated the situation.'

I think that's what's happened. But the IAU's reaction is, 'No, stop, I won't have any more planets. We have to limit the number because I'm more comfortable with a small number.' So you get this arbitrary algorithm that produces ridiculous results.

Q: Some of the discussion has focused on whether the approach to having an IAU that is the arbiter is not the right way to go nowadays. There's even talk about setting up an alternate organization.

A: That's right. Most things are done these days open-source and by consensus. You don't find little committees of 10 people speaking for 10,000, all without some sort of a sanity check. The fact that the IAU would claim that the world's astronomers have somehow met and decided something when it was a small committee of a dozen ... where were the experts in this field? What kind of process is that?

So people are asking, "What do we need these guys for? We'll set up an alternative." The IAU has no special claim. They have no police force or army. They're not the Supreme Court. If they're doing a bad job ...

The fundamental issue is that not many planetary scientists even belong to the IAU. The vast majority of its members work on galaxies, and stars, and black holes and cosmology. The reason most of the IAU doesn't care is because it's not their issue. The people who actually understand the physics, the chemistry, the work on planets aren't in the IAU. It's kind of like having a bunch of French professors deciding issues regarding the German language.

Q: You're going to have a fair number of "German-language" experts, so to speak, gathering in August to discuss the planet controversy. Will this issue come up there?

A: It will certainly come up. It's going to be a lot of fun, because it's going to be a scientific discussion. I don't think any conclusions will be reached, but it's going to move the ball along. That's just how science works. We don't actually come to vote. Except for the IAU, I don't know of anyplace where we vote at the end of a scientific meeting.

Q: Would that meeting be a good model for the kind of process you're talking about?

A: It's a step. It's like a lot of things. We figured out that water was once prevalent on Mars, not by getting together and arbitrarily calling committees to vote on it, but because over time the body of evidence became overwhelming.

Astronomers, and particularly planetary scientists, have to grapple with the much greater degree of diversity. And it's not just the diversity. The original view, until 10 or 15 years ago, was that we had four Earthlike terrestrial planets, four gas giants and the misfit Pluto. But the new view is four terrestrial planets, four gas giants and hundreds of Plutos. It's jarring, because it's the Earthlike planets - which we thought were 40 percent of the total - that are the misfits.

It's like the Copernican revolution: We're displaced from the center of things. A lot of people didn't want to buy it for a long time. We had to get used to that. The church opposed it. Now the IAU opposes this.

The only difference is that the smaller objects are smaller. They're not fundamentally different, in the sense that a chihuahua is still a dog. A dwarf human being has all the same genetics as other humans. From my perspective, that's fine: These are dwarf planets. I coined the term, in 1991. The only contention that planetary scientists have is with excluding dwarf planets from planets, as if dwarf people weren't people, or dwarf stars weren't stars. In fact, the sun is a dwarf star. It's just an adjective describing what kind.

Q: So you think there eventually will be a consensus, which emerges not by taking a vote but by gathering more evidence?

A: I do. Let me give you an example: I think it's now widely expected among experts that we will find objects substantially larger than Pluto in the deep outer solar system, because now we really understand how easy planet formation was and how many kinds of things were thrown into the outer regions by the giant planets.

So just watch: When a Mars-size body of an Earth-sized body is found, it will be widely accepted that there will be a planet that doesn't fit the IAU's definition. At that point, even the last vestiges of the definition's defenders will say, "Wait a minute, we have to rethink this." ... The whole thing will be shown for the farce that it is.

Q: I wanted to make sure to get a progress report for the New Horizons mission. Is there anything new that can be said about that, or is it under deep cover until it gets closer to the next milestone?

A: Well, it's our job to be good stewards of this spacecraft across this long cruise. We just passed Saturn's orbit, and that means there are now only two operating spacecraft that are farther out, and those are the two Voyagers that were launched 30 years ago.

The spacecraft is very healthy. In fact, our team is writing all the software for the Pluto encounter [in 2015].  So we're very busy, and not really in the deep slumber you're thinking of.