Planethood: A Debate

Two leading scientific experts consider what to call Pluto in the wake of a controversial IAU finding.

Artist’s conception of several solar system bodies shown to scale: Earth, Pluto and its moon Charon (far left), Eris (top), Eris, and Earth’s moon. Which one doesn’t fit in? NASA

Who’d have thought such a tiny planet would stir up such a big controversy? Everyone seems to have an opinion on the recent vote by the International Astronomical Union to change the official definition of  "planet" and thereby demote Pluto to "dwarf planet" status.

Among the scientists who disagree with the new definition is Alan Stern, a planetary scientist at the Southwest Research Institute in Boulder, Colorado and the Principal Investigator for NASA’s New Horizons spacecraft mission to Pluto. Michael Brown is a planetary scientist at the California Institute of Technology and the co-discoverer of Eris, a dwarf planet in the outer solar system. The recognition that Eris is slightly larger than Pluto is partly what prompted the search for a new definition of planethood. Brown leans toward classifying planets based on their dynamics—how they interact with other objects in the solar system—which is the scheme that ultimately prevailed at IAU. The scientists spoke with Air & Space Senior Editor Tony Reichhardt in August 2006, shortly after the IAU vote.

A&S: First of all, is it really necessary to have a new definition for a planet, either for scientists or for the public?

Brown: I actually think it was the wrong thing to do, and we don’t need to force a scientific definition for the word "planet." Most scientists would agree that there are at least two rational classification systems you could use in the solar system, depending on if you’re more interested in the dynamics of planetary systems or if you’re more interested in the individual bodies and how they behave. They’re both perfectly good classification systems. And both sides are somewhat crazy trying to force their view of what’s important into the word "planet."

Stern: I agree that given the way this turned out, I wish it had been left alone. I think it’s really damaged the way astronomy is viewed by the public.

But to step back, over the last 15 years we’ve had new discoveries that have pushed the edges of what it means to be a planet in the eyes of the public—hot Jupiters, pulsar planets, and Plutos galore. So I don’t think it was irrational that the drumbeat got louder to define what is a planet. I think sooner or later the field had to face up to it. I have no doubt that in 50 years or 100 years, this will be all sorted out and there will be a good consensus. But over 50 days or 100 days, or maybe even a few years, I think it’s likely to remain in very great flux.

A&S: Do you think there was a bias among the people who voted at the IAU meeting?

Stern: I think the group was pretty heavily biased, although I don’t mean it was a conscious bias. For one thing, planetary scientists are a relatively minor part of the IAU’s membership. I watched the final deliberations [from the Prague meeting] on the web, and recognized very, very few people. That’s also been my experience in attending IAU meetings. Unless it’s a planetary session, they’re not planetary experts. Also, it was a 12-day meeting, and a lot of people had gone home by the time of the final vote. From what I could tell, it was not a representative sample, at least of planetary scientists.

Brown: That’s a statement that’s been made a lot in the press, so I was curious, and did my own very small sampling. I took what you can consider or not consider a representative sample—faculty members in planetary science at Caltech—and asked them what their definition would have been. And the portion who chose the dynamical side was about the same as the proportion in Prague who chose the dynamical side. Most of them were much more comfortable with the idea that dynamics are an important part of the classification. These were people who are geologists and atmospheric scientists and everything in between. So my feeling is that you could poll any subset of planetary scientists that’s randomly selected, and I’m pretty sure the vote would have gone the same way.

A&S: Let’s parse the definition of "planet" passed by the IAU. They said a planet has to be in orbit around the sun, which automatically bothered some people who wanted a definition that includes planets around other stars.

Brown: I think you definitely need to include them. I wasn’t in Prague, but it appears to me that the alternative definition [the one finally approved, as opposed to the draft proposal] wasn’t given the time to mature that it should have had. I don’t think you’ll find anyone on any side of the issue defending the way the IAU went about doing this. I think they basically blew it. So we’re left with oddities in the definition, like the fact that it doesn’t say anything about extrasolar planets.

Stern: I agree with Mike that it’s got to be repaired. This is a great limitation. Usually in astronomy we try to generalize definitions rather than do something very specific like this. There’s a cynical point of view I’ve heard, which is that the restriction to orbiting the sun was imposed basically to gain votes—because if one generalized to all extrasolar systems, the current three-part definition would not have been able to pass.

A&S: Let’s go on to the second criterion. Are you both okay with "roundness" as part of the definition of a planet?

Stern: I certainly am. A lot of people have been writing about this as sort of the hallmark of planethood. You know an object is large if its gravity dominates its material properties [to force its shape to be round, unlike an irregularly shaped asteroid]. Even an asteroid tens of miles across acts like a rock, not like a planet. I think that’s a much better hallmark or standard than, for example, whether it has an atmosphere, or whether it can hold satellites.

Brown: I have no objection to the roundness part. As the definition stands, it’s irrelevant, because all of the objects large enough to fulfill the third IAU criterion are large enough to be round. So there’s no reason to include it. But there’s no objection to including it either.

A&S: Although "roundness" seems to me a fairly vague term. There’s at least one large object in the Kuiper Belt that’s fairly elliptical.

Brown: That’s my favorite one in the whole solar system. It’s 2003 EL61. It’s bigger than Pluto if you look at it at the right moment in the right dimension. "Roundness," though, is only the public face of the definition. The scientific face is "hydrostatic equilibrium" [where compression due to gravity balances the outward pressure].

Stern: Yes, when properly worded, this criterion is pretty straightforward. It says that the object is large enough to become rounded by gravity. It doesn’t mean that it is round. You don’t actually have to see the object if you have a measure of its mass or radius or both.

Brown: I think 2003 EL61 is the case you’d use to explain this. Because everyone would point out, "Gee, this one’s not round." It’s hydrostatic equilibrium that’s important, not the actual shape. Even the Earth is not perfectly round.

A&S: Let’s go on to the third requirement, the one that caused much of the debate—this notion of "clearing the neighborhood around its orbit." People immediately pointed out that Jupiter, for one, might not qualify [since it has asteroids in its orbital path]. Maybe the IAU meant something more specific by this, too?

Stern: I just know what they wrote, and what they wrote is subject to exactly that kind of criticism. If Johnny’s teacher says, "The IAU says that to be a planet an object must have cleared its neighborhood," then Johnny could say, "What about the Near Earth Asteroids?" It just becomes unteachable.

My bigger criticism with this dynamical part of the definition is that it creates a situation—untenable, in my view—that a given object can be a planet in some circumstances and not in others. For example, the Earth, by this definition, would count as a planet at its current distance from the Sun. But if you moved the Earth out into the distant reaches of the solar system and discovered it there, it would not be a planet [because it wouldn’t have cleared its orbit]. And the same is true if you put Jupiter in the Oort cloud. It would not be a planet by this definition. You can play endless games with these architectures, and it’s nothing but confusing. To my philosophy, that’s disturbing. An object, it seems to me, either is a planet or it’s not. The paleontologist doesn’t care where he finds the bones of a dinosaur. It’s still a dinosaur.

Brown: All Alan has said is that he’s not interested in the dynamics of the planetary system when it comes to classifying planets. That’s a perfectly acceptable philosophical point. But it’s an aesthetic point, not a scientific point. It’s just not the way he prefers to classify things in the solar system. These examples—Earth further away, Jupiter further away—I would agree you wouldn’t classify those as planets by this definition. But that actually makes it very interesting. Why would you not classify a Jupiter in the Oort cloud as a planet? Because it behaves very differently. It has had a very different history than these other eight planets.

I hate to keep using the word "planet" when my preference is to not even use the stupid word. But these eight largest objects we are now calling planets have had very different histories than anything like a Jupiter in the Oort cloud or an Earth out at 300 times its current distance from the Sun. Dynamically, the difference between these things is like night and day.

I disagree that it’s difficult to teach. I have been having a lot of discussions about this, and I find [the IAU definition] not only easy but delightful to teach. I feel like I finally get to teach people the structure of the solar system—how planets form, why there are belts, why there are individual solitary bodies. I love the question, "What about the Near Earth Asteroids?" You can talk about dynamically unstable orbits. What about Trojan asteroids? You can talk about how Jupiter has captured the Trojans at a Lagrangian point. The quibbles are where the science gets interesting. Any definition that is so clear that there’s no room for discussion, is, I don’t think, a very interesting definition.

Stern: It’s not true that I’m unhappy with dynamics as a criterion for planet classification. I co-wrote an article after the 2000 IAU meeting with Hal Levison, who’s a dynamicist, where we coined the terms uber-planets and unter-planets in a jocular way, because we didn’t want to use pejorative terms like "major" and "minor." I do think from a dynamical standpoint, there are planets that really matter in the architecture of the solar system, and those that don’t. They’re both planets. Just as you can have wet and dry planets, or life-bearing and non-life-bearing planets, you can have dynamically important planets and dynamically unimportant planets.

I would make a term for "planetary bodies." And anything that’s large enough to be in hydrostatic equilibrium would qualify. Then, if it were orbiting a star, I would call it a planet. If it were orbiting another planet, I would call it a satellite that’s a planetary body. And if it’s ejected from the solar system— which we expect has happened although we haven’t detected objects like that—I would call it a "free floater" or "rogue planet." But they’re all planets.

With stars, we don’t classify whether something’s a star based on whether it’s in a cluster or it dominates a region of the galaxy. It’s a star because of its intrinsic properties. In astronomy objects are typically defined by what they are, not where they are or what they’re near.

Brown: I’d be much happier with Alan’s physical definition than the one the IAU initially proposed. I thought their initial proposal was philosophically flawed in that you had things that were planets only if they were in orbit around a star. But if you put it in orbit around another planet, it suddenly becomes a satellite. If you really go with the physical criteria, you don’t care where it’s located.

A&S: What’s the right number of planets? The draft IAU definition would have opened the door to dozens of planets, or even a couple hundred, in our solar system. Would that bother you? Is it important to have a number that can fit on ten fingers, or in an easy rhyme?

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.

The eight survivors: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. IAU/Martin Kornmesser
A true-color view of Pluto, with the best resolution obtained to date. Eliot Young (SwRI) et al., NASA
IAU members may have cast their votes, but the argument is far from over. IAU/ Lars Holm Nielsen