Earthsky Interviews
Private: Who else wants Pluto to be a planet again?
01-24-2007 - Space
Alan is a professional astronomer, long-time space advocate and old friend of mine. He also happens to be Principal Investigator for NASA’s “New Horizons”:http://pluto.jhuapl.edu/ space mission to Pluto, which launched a year ago and which is scheduled to encounter Pluto and its moon Charon in the year 2015.
Alan called Earth & Sky yesterday with an update on what some American and European astronomers are currently thinking about last summer’s decision of the International Astronomical Union (IAU) to rewrite its “definition”:http://en.wikipedia.org/wiki/2006_redefinition_of_planet of a planet, thereby stripping Pluto of full-fledged planet status.
According to Alan, the IAU has now formed a committee to clean up its new definition. That’s when he said the word “indefensible.” In fact, many astronomers believe the new planet definition – voted into official existence last summer by only a few hundred astronomers – needs work.
Alan and I talked about some impending events, centered on Pluto and its lost planet status.
One is a special session at this year’s European Geosciences Union “conference”:http://meetings.copernicus.org/egu2007/index.html in Vienna in mid-April. The special session will be called “What is a Planet?” Session organizer Karsten Seiferlin explained to Alan in a note: “Last year’s decision to take away the status of being a ‘planet’ from Pluto … left both the public audience and some planetary scientists surprised and in some cases embarrassed.”
And if the Europeans are embarrassed, American astronomers are likely to have even more conflicting emotions about Pluto. Alan confided that some American astronomers are unhappy that the IAU “kicked out,” as he said, the only planet discovered by an American astronomer, “Clyde Tombaugh”:http://www.icstars.com/HTML/icstars/graphics/clyde.htm.
Meanwhile, in Clyde’s hometown of “Streater, Illinois”:http://www.streatorchamber.com/, the Streator Tourism Council has announced plans to bring a “Planet Pluto Expo”:http://mywebtimes.com/ottnews/archives/ottawa/display.php?id=290464 to the city on May 18 and 19, 2007.
What is happening here? Wasn’t Pluto demoted from planet status forever?
In science, very little is forever. According to Alan and other astronomers with whom we’ve spoken recently, some astronomers are simply choosing to ignore the IAU’s decision. Alan and his colleagues in astronomy are hoping to organize a meeting of their own, so that professional astronomers can come together to discuss Pluto’s status.
To understand the arguments better, download this great (pdf) “factsheet”:http://www.boydsmillspress.com/pluto.pdf about Pluto from astronomer Ken Croswell.
Astronomers like Alan and Ken are trying to keep the public aware that Pluto’s status as a planet is still an open debate.
And speaking of the public reaction, Alan mentioned that he’s heard from parents whose children think that Pluto has just … disappeared. He seemed sad about that.
Pluto hasn’t disappeared, of course. It’s still out there on the edge of the solar system, with a moon nearly as large as Pluto itself, with its two smaller moons, and with mysterious surface features only now being glimpsed by our telescopes.
A lot of us are sad that the planet that was so popular among children is no longer a planet.
Meanwhile, Alan has more reason than most of us to feel unwilling to accept the IAU’s decision. He worked toward a space mission to planet Pluto for much of his adult life. When the New Horizons mission launched on January 19, 2006, it was headed toward a full-fledged planet in our solar system. Now, according to the new “definition,”:http://en.wikipedia.org/wiki/2006_redefinition_of_planet the spacecraft is headed toward a “dwarf planet.”
Maybe there’s nothing to these labels, after all. All just sound and fury, signifying nothing? These bodies in space have pursued their majestic orbits for billions of years. When you think about it in that light, our little notion of whether Pluto should be called a planet or not doesn’t seem very important.
But astronomy is a peculiar kind of science. Although it has a few practical applications, its main use is to give us a larger context for our existence on this Earth. Astronomy is for people. It’s not for a few hundred professional astronomers, voting in a room somewhere. What is their work in astronomy for, after all, if not to expand _our_ horizons?
That’s why, to me, the IAU’s decision last summer to give us a solar system of eight planets instead of nine seems … wrong. It seems contrary to what astronomy is supposed to do.
What do you think? Who else wants Pluto to be a planet again?
I agree with Laurel Kornfeld: Pluto and Eris are the Sun’s ninth and tenth planets.
It’s not history but science that argues that Ceres is not a planet but Pluto is. Only one year elapsed from the discovery of Ceres to the discovery of the next asteroid. That’s history, but it reflects a scientific truth: lots of other objects like Ceres orbit the Sun between the orbits of Mars and Jupiter. Thus, it’s no surprise that little time elapsed from the discovery of the first asteroid to the discovery of the second.
In contrast, sixty-two years elapsed from the discovery of Pluto, in 1930, to the discovery of the next Edgeworth-Kuiper belt member, in 1992—even though Tombaugh kept looking for new planets until 1943. That’s history, and it also reflects a scientific truth: Pluto is a really BIG member of the Edgeworth-Kuiper belt. It took astronomers seventy-five years after its discovery to find the only known member of the Edgeworth-Kuiper belt that is larger than Pluto, Eris.
Drawing the line at Pluto’s diameter is arbitrary, but it is no more arbitrary than defining “round” or defining “clearing the zone.”
Furthermore, science is full of arbitrary definitions. As proof, look up the scientific definition of a meter.
we think pluto shood steel be a planet !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
we think pluto shood steel be a planet !!!!!
The IAU definition has rapidly taken hold all over the world and will stay, only the hastily drafted wording might be improved eventually. But the essence of the decision is highly unlikely to change: There are eight bodies in our solar system that dominate their ‘neighborhood’ and two belts full of minor objects, none of which being in a dominant role.
Pluto was just “lucky” that its apparent brightness made this one easier to find than its neighbors (though I’ve since found out that the next-brightest one was only a factor of 2.5 dimmer in the 1930’s – Tombaugh might have missed 2005 FY9 just barely). The IAU definition has brought order into the solar system as never before, and those still bickering should take a deep breath and get a life …
Daniel Fischer
science writer
Germany
Utter nonsense. Nearly as many astronomers signed the petition protesting the IAU vote as took part in the vote!
No, the IAU has not brought order into the solar system. The truth is exactly the opposite—it has created chaos with the ridiculous statement that a dwarf planet is not a planet at all (linguistically impossible) as well as a double standard since Neptune, Jupiter and even Earth can also be viewed as not clearing their orbits (Neptune of Pluto; Jupiter and Earth of asteroid fields). In fact, “clearing its orbit” was never an issue regarding planet definition until the IAU’s conference. I strongly believe this decision will not stand. It’s easy to tell people to “get a life” when they challenge your position. Things would be much more convenient for you if those of us who dissent with the IAU decision just “move on.” Well, that’s not likely to happen. Dr. Alan Stern, the author of the petition protesting the IAU decision, plans to hold a conference of 1,000 astronomers later this year to address this issue, and it almost certainly will resurface at the IAU General Assembly in 2009, where you can expect a lot of members who never previously showed up for these conferences. This debate is far from over.
Laurel Kornfeld: Exactly! Well said!
I totally agree with both Magenta and Laurel. The IAU decision was stupid because taken, without proper consultation and preparation, on a subject that has wide implications – zillions of schoolchildren and their teachers, not to mention parents, in 192 countries registered in the UN system.
The very simple solution would have been to classify Eris at the 10th planet… but why be simple when you can show your “erudition” by being complicated?
The people who are quibbling about “round” and “clearing the zone” ought to read Michael Brown’s views on the topic before drawing conclusions about the current definition’s criteria being arbitrary.
Thanks to Bruce McClure, here is the relevant information:
“Michael E. Brown, Professor of Planetary Astronomy (who, along with Chad Trujillo and David Rabinowitz, discovered Eris, the largest dwarf planet), gives his thoughts on the great planetary debate at http://www.gps.caltech.edu/~mbrown/eightplanets/”;
Why is it that people continuously bring up “teaching to kids” as a justification for changing a definition in this context?
The concept of atoms, electrons, orbitals, spin numbers, etc. are more complicated than the current definition of a planet and yet we manage to teach them to our kids.
Are we forgetting Einstein’s “Make everything as simple as possible, but not simpler.” here?
Are far as picking criteria for classification, we ought to stick with classifiers that are characteristics of the domain of interest. Size (approximated by roundness), dominance (approximated by ‘clearing the zone’), etc. are concepts within the domain of astronomy.
Pluto as a planet was simply an accident of our record. There is nothing special about the size of Pluto within the domain of interest. It is as unremarkable as the size of Varuna or Quaoar, or 2005 FY9. As such, it does not make a good classifier.
On the other hand, picking the unremarkable value of the size of Pluto as a classifier is detrimental to educating kids in the scientific process. The only way to justify this is along the lines of “Because I/we said so…” This is Science by fiat and really harms the process of enquiry.
I respect the discoveries Mike Brown and his colleagues have made. However, the fact is, he’s gone back and forth on this issue. Before the discovery of Eris, he said Pluto was not a planet. Then, after the discovery of Eris, he said it was a planet. Now, after the unfortunate IAU vote, he says it is not a planet. So, with all due apologies to a certain unsuccessful presidential candidate, he was for Pluto before he was against it.
Setting Pluto’s diameter as the defining property of a planet is indeed arbitrary. But so is setting a certain wind speed for the defining property of a hurricane. Furthermore, roundness and “clearing the zone” require two more arbitrary lines to be drawn. How round is round? If the equatorial axis is 5 percent greater than the polar diameter, is the object still round? What if it’s 10 percent? 20 percent? Where do you draw the line? Wherever you draw the line, it’s arbitrary.
Likewise, where do you draw the line concerning “clearing the zone”? If an object has cleared out 1 percent, 10 percent, 50 percent, 90 percent, 99 percent of its mass? Wherever you draw the line, it’s arbitrary.
There is a very good reason to draw the line at Pluto’s diameter: Pluto is a very large object in the Edgeworth-Kuiper belt. It is the tenth largest object that orbits the Sun. Varuna, Quaoar, 2005 FY9, Sedna, etc., are all smaller, with diameters typically two-thirds that of Pluto. Drawing the line at Pluto thus maintains a fairly high threshold for any aspiring planet to cross. Thus, this definition preserves the current connotation that a planet is an important object. (In all fairness, so does the IAU proposal.) However, conferring planethood onto all sorts of sub-Pluto objects—which has been derided as the “no iceball left behind” policy —dilutes the significance of what “planet” means. That’s why so many astronomers were irate over the proposal by the Gingerich committee. That proposal was so extreme that it provoked an extreme backlash: the demotion of Pluto and Eris. Had the Gingerich committee instead come up with a moderate, sensible proposal—drawing the line at Pluto—it likely would have been accepted.
Magenta makes many excellent points. There are many scientific classifications that are actually arbitrary. Furthermore, the requirement of an object clearing its orbit sets up a ridiculous situation in which objects are defined by where they are rather than by what they are. Objects further from the sun are far less likely to “clear their orbits” unless they are gas giants (even those may not meet this qualification—Neptune does not clear its orbit of Pluto). So an Earth-sized object under this definition would be considered a planet in Earth’s current orbit but not a planet if it orbited beyond Neptune. That definition is very arbitrary and chaotic. Also, it is noteworthy that David Rabinowitz, co-discoverer of Eris along with Mike Brown and Chad Trujillo, disagrees with Brown’s position on Pluto. Rabinowitz is one of several hundred planetary scientists who signed Stern’s petition against the IAU definition. So even the discoverers of Eris themselves are not in agreement on this.
Mike Brown’s emphasis on concepts is important, not whether he waffled on what a planet is. Besides, he acknowledges his bias in the article.
The concepts are size and dominance. The “roundness” is simply a symptom of size. For instance, 2003 EL1 is egg-shaped, but there is no doubt that hydrostatic forces are a dominant influence on its shape. For hydrostatic forces to come into play, an object must be big enough. In fact, another astronomer (Basri) proposed that this be the lower limit for a planet a few years ago, which would be about 435 miles (or is it Km?).
As Magenta points out, it is reasonable to desire some distinguishing characteristic between the heavyweights and the also-rans. This is where dominance comes into play. I will refer the reader to my earlier posts (#s 29, 31) for some of my thoughts on dominance.
Magenta’s threshold criteria applies equally well in picking Mercury, or 2005 FY9, or Varuna as the cut-off.
I agree that the IAUs “clearing the neighborhood” is not very well worded. All it underscores is that we need a better way to express the concept of dominance in operational terms.
I am perfectly fine with a definition that allows a non-planet to be bigger than a planet. Ganymede (of the Jovian system) and Titan (Saturn’s moon) are both bigger than the planet Mercury and I see no serious proposal to classify them planets. Heck, Titan even has a permanent significant atmosphere and Ganymede actually has its own magnetic field.
As far as arbitrary classifications go, in science whenever a better classification scheme comes along it tends to supplant one that has arbitrary formulations.
In the particular example of the Tropical Depression-Tropical Storm-Hurricane continuum, I haven’t heard of a proposal that is less arbitrary, which is funny because the Meteorologists themselves think that even a Tropical storm is significant enough to name.
In the context of a planet, the proposal jettisoning Pluto was less arbitrary than the previous status-quo.
Neptune so strongly influences the orbits of Pluto, Eris and their brethren that they look like its lost children. Similarly, planetary theorists credit Jupiter’s influence for preventing the formation of a planet where the asteroid belt currently lies.
I have no objection if someone can come up with a reasonable proposal that will allow us to designate Titan and Ganymede as planets, or include Pluto et al., in which case the classifier’s core concepts will have to change from the current basis of size and dominance.
However, insofar as dominance is still a core concept (which it obviously is, otherwise we would be talking of Titan and Ganymede too) it is logical to demote Pluto, Eris, et al.
The problem is, I object to both roundness and “clearing the zone” as defining characteristics of planets. Both lead to absurdities.
Historically, Uranus, Neptune, and Pluto were considered planets because they were a) large and b) orbited the Sun. These are the ONLY two criteria that should be used to define a planet of the Sun.
Here are three examples of the absurdities that result when employing the “clearing the zone” concept. The first of these has already been discussed by Laurel Kornfeld:
1. Imagine a planet with Mercury’s mass, density, and diameter—in other words, a perfect twin of Mercury, which everyone agrees is a planet—orbiting the Sun in the Edgeworth-Kuiper belt, or in some more distant belt of objects. Under the IAU proposal, the real Mercury would be considered a planet, while its twin would NOT. That is absurd.
2. Imagine, as some science fiction writers have, that there is another Earth orbiting the Sun on the other side of the Sun. This anti-Earth has exactly the same mass, diameter, and density as the real Earth, and it goes around the Sun once a year, just as the real Earth does. Under the IAU proposal, neither Earth nor this anti-Earth would be considered planets—because neither has “cleared its zone” of the other! Absurd!
3. Imagine that in 2010 a planet with the same mass, diameter, and density as Earth is discovered 200 AU—200 Earth-Sun distances—from the Sun. Is this new Earth a planet? Of course, you say. But wait. Until astronomers determine whether or not this object has “cleared its zone,” this object cannot be classified as a planet or a nonplanet. Instead, it will linger in limbo, probably for decades, because detecting smaller bodies at such an enormous distance from the Sun will require new technology.
In contrast, under the common-sense planet definition (planet of the Sun = Sun-orbiting body that’s Pluto-sized or bigger), in each of these three scenarios, the hypothetical object would quickly be determined to be a planet, once its diameter is measured and found to be larger than Pluto’s.
The roundness concept leads to similar absurdities.
Magenta the current classification uses size and dominance as its basis.
1. IAU’s current definition of a planet makes no mention of “roundness.” The property is “hydrostatic equilibrium.” As I pointed out in previous posts this provides a built-in size cutoff. Repeat – the property is not roundness, it is hydrostatic equilibrium.
In simple terms this means that a body will tend towards a spherical shape if it is large enough (think of it as a spheroid).
2. The Earth-AntiEarth in the stable configuration you describe is impossible to form via natural processes.
3. If an Earth-sized body is found at a later time, whether or not it is a planet depends on the absence of another big object “dominating” its orbit. This is the same logic as to why Titan and Ganymede are considered satellites (moons) and not planets. If Titan or Ganymede were in the presumed empty space by themselves past the Edgeworth-Kuiper belt they would qualify as planets (provided there isn’t another body pushing them around).
The wording of “clearing the neighborhood” is silly, but the concept of dominance is sound. Astronomers need to work on a better formulation for dominance.
4. If you would like to see Pluto back as a planet you would have to propose/support a framework which excludes dominance as a basis for the classification. In such a case, you would have to consider the possibilty that the number of planets could easily top 25 (8 classical planets, 7 moons that are bigger than Pluto, The big EK objects and asteroids(Eris, Pluto, Charon, Sedna, Quaoar, Varuna, Lxion, 2003 EL1, 2005 FY9, Ceres, etc.). Further, the framework would have to alleviate, as you put it, the “no iceball left” issue.
5. A proposal based on an arbitrary or historical size cutoff is likely not going to be acceptable, simply because that is exactly what people are trying to correct.
I don’t accept dominance or hydrostatic equilibrium as criteria for planethood. They have never been criteria for planethood. I see no similarity between Titan, which orbits Saturn, and Pluto, which orbits the Sun. The former is a moon; the latter is a planet.
I, and many others, have already proposed a definition that excludes dominance: a Sun-orbiting object whose mean diameter equals or exceeds Pluto’s.
What is so hard about that?
And how do you know the Earth-anti-Earth scenario is impossible? There are moons of Saturn that share the same orbit around Saturn.
You can read about the orbital relationship between Saturn’s moons – Janus and Epimetheus – at http://www.answers.com/topic/epimetheus-moon. Epimetheus means hindsight in Greek, and in hindsight, it looks like the phrase “clearing the orbital zone” opened a big can of worms!
In reading all of these most interesting comments, I am struck by the fact that we have two groups of well-informed, intelligent people who are talking right past each other, neither quite able to grasp where the other is coming from.
On the one side are the traditionalist scientists, who believe that Pluto (and by extension, Eris) should be labeled as “planets” because that’s the way we’ve always done it, so why change now? After all, doing it this way gives us a nice, tidy list, and it is very easy even for novices to understand. If you’re an enthusiast, you’ll soon learn about asteroids, comets and Kuiper Belt Objects, and you’ll begin to grasp the more subtle nuances of our solar system. But the layman won’t really care about all this.
On the other side are the rigid scientists, who want everything completely quantified, with precise definitions and cut-offs, because that’s how they do their work.
As a computer programmer myself, with a four-year degree in mathematics, I can understand wanting rigorous taxonomic precision, but I also have to call myself a traditionalist in that I doubt I’ll ever stop thinking of Pluto and Eris as planets no matter what the IAU decrees. I have a traditional nine-planet orrery on my filing cabinet, and the only way I’ll ever consider replacing it is if “The Noble Collection” comes out with a ten-planet version.
Unfortunately, the universe itself does not always oblige us with having everything neatly pigeon-holed. The data release program I manage at work tends to crash at unpredictable intervals because some unexpected data configuration comes along that it wasn’t set up to handle gracefully. In astronomy, even “hydrostatic equilibrium” is somewhat fuzzy; I recall seeing a chart, perhaps on Wikipedia, listing the largest asteroids in descending order of size. As you work your way down the list from Ceres, officially recognized as a “dwarf planet”, you come to a point where the mathematically “round” objects are interspersed with ones visibly irregular. Where, then, do you draw the line between “small solar system object” and “dwarf planet”?
As for orbital dominance, Magenta points out a certain arbitrariness in determining this, though I’ve read that it wouldn’t much matter to the IAU’s current definition of “planet” if we made the mass ratio cutoff a factor of 10, 100 or 1000.
Ultimately, I don’t think the two camps are ever going to be completely in agreement over the definition of “planet”, because they have two different world views. I’ve resigned myself to watching the astronomical community bitterly wrangle over the matter for years to come, while I’ll have to content myself, for the most part, with sitting on the sidelines. After all, I’m only an enthusiast.
I still think the only workable solution to this conundrum is to agree to disagree: let’s define a cultural/historical “Traditional Planet” category for public consumption, in parallel with the more rigorous scientific definition. But I fear I’m very much a voice in the wilderness. So be it.
Thank you, Deborah Byrd, for giving us this forum for discussion. It has been most illuminating.
Thank YOU, Bob, and thanks to Magenta, Thought Shaman, Laurel and others who have made this discussion so interesting.
I agree that different world views will make the Pluto controversy hard to resolve!
Bob you are quite right in that there are two camps.
However, it is not the case that they are talking across each other. One camp has a typical case of what Stephen Colbert terms “truthiness.” They want to hold on to their positions regardless of what the facts are.
I disagree with your implication that it is the traditionalists that are comfortable with ambiguity. In fact, it is those who are willing to consider Pluto et al., to not be planets, who are attempting to look at the facts of the domain, and are comfortable with a not so precise “formulation” for classification (insofar as the “principles” are sound) who are being open.
It is sad that some people do not seem to understand their own principles for classification.
I have pointed out that hydrostatic equilibrium is simply a formulation for an object being sufficiently big, yet people keep complaining about it. The size criterion is a carry over from the traditional way of thinking.
The other point of dominance is also a carry over from the traditional way of thinking. The reason Titan and Ganymede are not considered planets is because Saturn and Jupiter respectively so dominate their orbits that they have captured the two smaller bodies and forced them into orbit around themselves.
Note that “orbits” is a transitive property. If Y orbits X, and if Z orbits Y, Z also orbits X.
Location obiviously matters regardless of size in traditional classification. If Titan and Ganymede were elsewhere in space they would likely be planets even in the traditional view.
Therefore, size and dominance are the two traditional criteria that form the basis for the classification of a planet.
All I ask is that people apply the classifier to the entire domain consistently.
It is inconsistent to reduce the domain to only those bodies that directly orbit the star(s) and then claim there is no concept of dominance. Especially since the concept of dominance was applied to reduce the domain by eliminating the satellites (moons) in the first place.
As far as the view of Pluto’s size being proposed as the cut-off, it is simply classifying by fiat, and is not good science. As I mentioned in a previous post, in science a better classification tends to supplant an earlier one with arbitrary formulations. Further, if we are using this view then we can equally well pick Mercury, or the much smaller Varuna.
The historical argument for Pluto being the cut-off doesn’t hold either simply because it is the historical “inaccuracy” of Pluto that is point people are trying to correct.
I agree with Bob that it may be possible to have two definitions, one colloquial and one scientific. However, this could be a little strange as we’d have to teach them both to the populace. I can imagine a quiz question on the number of planets being answerable both ways causing some mayhem.
The other approach is to simply designate that a planet is a cultural body of interest (from a scientific perspective, the absence of definition of planet will not affect the process of enquiry). In this case, we can simply vote as to which bodies we consider to be planets periodically, say every 5 years or so. Just imagine the fun people can have gathering votes for Pluto or poor little Varuna
.
Bob, I don’t think one has to be an astronomer or a member of the IAU to present views and thoughts. I thank you for yours and thanks to Magenta, Bruce, and others as well.
Pluto isn’t a planet? Oh NO! Somebody get the wambulance! Next thing you now, the Salt Lake will be re-classified as a dwarf lake! What will I tell me children?! Stupid scientists are ruining my life! And I don’t even have one yet.