When scientists decided that the sun was not not turning against the earth but the reverse, it was also a case of 99.999% of the people having no say in this and being told that what they had learned was wrong, based upon esoteric and hard to understand reasons.

And there was also then a movement to adapt science to popular perception.

I thought we had moved beyond that, but sometimes, on wonders.

E pur si mueve.

gee,

there are a lot of things that people learned - in school - or elsewhere that were later found to be wrong or were updated. For example, the earth is now found to be round and is not at the center of the universe afer all.

The IAU are bang on with this approach. I've attended various IAU discussions over the years, and it's always noticeable that the ONLY ones in favour of keeping the 'Pluto is a planet' definition are US astronomers, toeing the line for nationalistic/sentimental reasons.

Both the IAU and the people opposing its deliberations are wasting their time. This is not an important issue.

I disagree...science is not a democracy. Although the current IAU definition of a planet is poorly worded, the planetary scientists had their shot and muffed it and now they're crying foul. But they apparently have no better suggestions, at least none I've heard.

This post is worthless, and I declare myself a frequent follower of this site. What point are you trying to make with the 99%, and people having to rethink what they learnt? Just because *I* think mountains are formed by giants pushing the ground above them because I find plate tectonics esoteric, I don't expect a fully qualified team of proffessionals will care when I "tell them what I think". Atoms should be by definition indivisible, but mad physicists with hard to understand quantum mechanics upset me saying they in fact were very divisable, and not even in smaller bits, but in waves! Bunch of nuts...

Questioning your established ideas is a healthy habit, and people who have trouble with that reasoning they "liked Pluto being a planet" because they were taught so should try shuffling their minds a bit.

You can argue for or against Pluto being a planet, but with scientific facts, not just because it's "right". The only thing that profits from that is esoterism.

I am campaigning to reclassify Earth as a "minor planet", along with all of the other debris in the inner solar system. As Asimov noted, it is clear to any objective observer that our solar system only has four significant planets.

Despite protests to the IAU planet definition by more planetary scientists than voted in Prague, despite polls by CNN, BBC, The Planetary Society, and others that show the public does not like the IAU planet definition, and despite the fact that one of the largest Facebook associations regards Pluto's Planethood, IAU President Cesarsky speaks publicly with the press, saying her position is that nobody much is unhappy with the IAU's planet definition. It would seem that what NASAWatch is doing is simply helping to make Dr. Cesarsky aware of the error of her position. I would think she would very much appreicate such input from both scientists and the interested public.

-Alan Stern

I don't much care what we call it, but the change and what it takes to explain that change allowed me to teach a 13 year old more about space and planet formation in 10 minutes than she probably ever learned in school. IMHO, its a good thing for kids to have to understand "esoteric, and hard to understand reasons"...

I would like to know the source for Dr. Cesarsky's claim that those astronomers trying to get Pluto reclassified as a planet constitute only a "very small part of the astronomy community." Supporters of the 2006 IAU demotion, made by all of four percent of its members, have been spreading this statement all over the Internet for the last week, but they do not back it up with any data at all! This is nothing more than a deliberate political attempt to discredit the legitimate movement among astronomers to see Pluto's planet status reinstated; it is clearly a "spin" in line with the first principle of propaganda, namely, "a lie repeated a thousand times becomes the truth."

The Pluto situation is in no way similar to the other scientific developments cited, such as the fact that the earth revolves around the sun and not the other way around. In the case of Pluto, the decision was not based on any new information discovered about Pluto. There is no data proving that what we learned in school was wrong. What there is is knowledge of additional bodies like Pluto that show our solar system to be far more diverse than originally thought. It's not that Pluto is the lone object that doesn't fit in; it's just that Pluto is the first of a new subtype of planet, the ice dwarfs, of which there turn out to be many. These are still different from most KBOs in that they have achieved hydrostatic equilibrium, meaning they have geological processes that make them far more akin to planets than to asteroids.

The argument that classifying round KBOs as planets will lead to there being "too many planets" is not in any way scientific. If our solar system has 200 planets, then that is what it has. It was not designed for our convenience.

The claim that it is Americans only who want to see Pluto's planethood reinstated is not only untrue; it also reveals an underlying anti-American political bias. It just so happens that more American astronomers are planetary scientists who focus on the composition of individual bodies while more European astronomers are dynamicists, who focus on orbital neighborhoods. This is a scientific division among two groups who look at the same facts but interpret them differently because they are concentrating on different aspects of those facts.

In conclusion, I think Dr. Cesarsky and other supporter of Pluto's demotion will be the ones surprised when New Horizons flies by Pluto and Dawn flies by Ceres in 2015 and both yield findings showing these objects to be a subclass of planets.

I appeciate Dr. Cesarsky's frustration that the planet debate has not subsided since the partisan brouhaha in Prague two years ago where the central question on both sides was the status of Pluto and not science.

How we organize and categorize things is useful as part of the process of science. Different groups of people will find utility in categorizing things in different ways. The IAU definition for planet is useful for dynamicists. The geophysical definition for planet is more useful for planetary scientists (of whom only a minority are astronomers any more).

The problem arises when an organization like the IAU feels it can define a broadly used term for everyone - particularly when it seeks to make its usage narrower than most people are used to, and cannot acknowledge alternative scientific perspectives beyond that of a small group of astronomers. One unfortunate consequence is that the IAU has become a symbol of orthodoxy that does not brook dissent, which is not good for astronomy or science.

Since there has also been an ugly thread of nationalist and anti-US sentiments expressed in these debates, I would like to say a few words in favor of the smallest geophysical planet - Ceres. Ceres was discovered by an Italian, Giuseppe Piazzi, in 1801 and hailed as a great discovery at the time because it filled the one gap in the Titius-Bode's law sequence of planetary orbits. It's planetary status was controversial for decades because in the telescopes of the day it was nothing more than a moving point of light and all other known planets were observed to be disks.

William Herschel, who had discovered the last planet, Uranus, in fact coined the term 'asteroid' (star-like) to describe Ceres in something of a put-down. The acceptance of Ceres as a planet wained as more and more asteroids were discovered. In the following centuries we learned that Ceres contained about half the mass of the asteroid belt and that it was carbonaceous and, curiously, also covered with clay. An intermittent or asymmetric atmosphere was detected in 1990.

In 2005, Hubble resolved Ceres and found it to be a round object with no discernable topography. Analysis of its shape and motion revealed it to be differentiated with a rocky core and ice-rich mantle. Thermal modeling indicates that Ceres may be warm enough beneath its surface to support a liquid ocean today - this raises the question of whether there could be life. The US, in collaboration with Germany and Italy, successfully launched the Dawn mission in 2007. It will arrive at Ceres in 2015 (after it first visiting Vesta in 2011).

We expect to see geology, detect its atmosphere if it exists, and see if there is evidence for communication between the subsurface ocean and the surface (e.g., expanses of evaporite deposits, an atmosphere may also provide evidence). Because there is concern about possible life, at the end of the mission, Dawn is supposed to move into a quarantine orbit in order to avoid possible contamination of Ceres. It may be little and not dynamically important, but Ceres has physical properties that are far more analogous to other planets like Mars than the inert, irregular asteroids. From a geophysical perspective it makes sense to categorize Ceres as a planet.

I am one Planetary Scientist who is not "trying to get Pluto reclassified as a planet." because I have never accepted, or taken seriously, the IAU's lame attempt at reclassification. There is a huge amount of skepticism about this within the Planetary Science community.

Speaking also as a Museum educator, I can say that in this case the children have better instincts than the "professionals" who made this attempt at a definition.

This regrettable chapter is, at least, useful for teaching lessons about why definitions are difficult, how our knowledge of planets is changing, and especially how even organizations of professional astronomers can be wrong and very silly.

Any definition of "planet" which does not, and cannot, work for extrasolar planets is fatally flawed. By this silly definition, you cannot say whether or not something is a planet unless you know the population of small bodies in its neighborhood, something we may never know for most newly discovered planet candidates. So where does that leave them? In quasi-planetary limbo?

The recent silly attempt at defining "Plutoids" is no better. Outside of Neptune's orbit? Where does this leave extrasolar small bodies? (depends on the existence and location of the extrasolar Neptoids?)

I question the IAU's authority to define these terms. The historical role of naming objects is completely different from defining these categories. Having botched the job so badly, one wonders if they can, or should, play this role.

The controversy over reclassifying Pluto has been one I've tried to thoughtfully consider. However the proponents for this change have convinced me they are wrong and their motivation suspect.

As Dr. Grinspoon pointed out, at a time when we are approaching 300 detected exoplanets, the IAU is devising a classification system that will be useless beyond the Sol star system. How does the orbit of Neptune (used to differentiate Plutoids) translate to GL 436 where a hot "Neptune" orbits its red dwarf star in 2.6 days? Do we have to devise a temperature/orbital definition for "Neptunes"? Is there a designated Neptune for every star system?

Our largest planet cannot "gravitationally clear" the trojans asteroids from its L4 and L5 points. How large or numerous can trojan bodies be before denying their obital companion planethood? Are large binary planets sharing an orbit beyond the realm of possibility? (another hint from Pluto and Charon)

By the way, is it inconceivable that we may find star systems with planets that are not all on the same orbital plane? Is it conceivable that we won't?

At what oblateness does a rapidly spinning body violate sphericity and lose planetary status? How about newly forming planets like the 14 Jupiter mass protoplanet around HL Tau? At what point does it officially become a planet? Are substantial planetary rings counted as demerits against evolving to planetary status?

The kind of indifference to dissent, bordering on arrogance, exhibited by some officials at the IAU and change proponents leaves a distinctly unscientific impression. Disagreement seems to be demonized as either emotional populism or nationalism. [See above posts] I've noticed a level of vitriol (elsewhere) by proponents that has surprised me. This legitimizes scrutiny of the accusers' motivations.

And as Laurel stated, if our star system ends up having 200 planets (wanderers) so be it. Changing the classification system to suit some aesthetic is unscientific and hypocritical. Perhaps the IAU has been successful at devising naming schemes for moons and craters, (their schemes for asteroids and comets are questionable) but devising a planetary classification system is obviously beyond them. Maybe it's time to renounce this priesthood.

Plutoids? Don't they make a cream for that?

This is all like arguing over the point when pebbles become rocks and rocks become boulders. There's a nearly continuous size spectrum of objects in the solar system and beyond, starting from sub micron dust grains, to clumps of grains, to rock chunks, to asteroids, to bigger asteroids, to hydrostatically-deformed, nearly spheroidal small objects, to Earth-like bodies, to gas giants, to bigger gas giants, to small stars, to large stars, to very, very large stars. The whole argument about what is a planet is a waste of time. It has no impact on science; it's a pointless and meaningless distraction. Pluto is still Pluto, and you can call it whatever you want. In a century or so, we will probably look back and realize how silly classifying bodies as planets, planetoids, asteroids, or hemerroids (Thanks, Alan. Got a chuckle out of that one) really was. Why don't we argue about crystal spheres while were at it? The notion of planets is a relic of a time when we only knew about the big round stuff you could see through a telescope--the classical planets. That era is over, and the notion of a planet is obsolete. Let's get over it and move from the 16th century to the 21st century. We know better.
We should be teaching that there are lots of things orbiting the sun (and many other stars, including stars that orbit stars). Some are really small. Some are really big. The big ones tend to be nearly spheroidal. Some of these we used to call planets. The biggest object orbiting the sun is Jupiter, then Saturn, then...Pluto, then....and so on, and so on, until you get tired of naming things.

This whole thing resembles nothing so much as the lumper/splitter division in linguistics. Linguists have so much trouble agreeing on the definition of the difference between a language and a dialect because the boundary between the two is fuzzy and partly dependent on politics and nationalism (for example, the Scandinavian "languages" meet the definition of dialects, whereas the Albanian "dialects," Gheg and Tosk, meet the definition for languages). It's the same here. And it's not just "what's a planet," because the upper boundary of planet and the lower boundary of star overlap. This leads people of the splitter frame of mind to come up with more and small categories looking for greater precision. Lumpers want fewer catgeories at the expense of less precision. The time is going to come when, no matter what we decide, our categories will have to be thrown out. Gas giant vs. terrestrial planet? I wonder what the newly-discovered "super-earths" look like, and where the boundary will fall? Do we need official categories like subneptunoid?

Keith:

A perspective on planet definitions:

Remember that we would not have most of this trouble if the IAU had accepted the recommendation of its own advisory panel that a "dwarf planet" is a kind of planet, just like a "giant planet". In this case Pluto and other large KBOs would be planets still, just dwarf planets (like dwarf stars or dwarf galaxies).
Unfortunately, a popular revolt among the astronomers at the IAU General Assembly in Prague (very few of them planetary scientists) led to a vote that a dwarf planet was not a planet. That is what started us down the road that has now led to the unfortunate term plutoid, which further reduces that credibility of the IAU.

I think your organization needs to take a step back from your computer screens and understand that people the world over are waiting for the New Horizons spacecraft to take the first pictures up close and personal of Pluto, the ninth planet. All humans have been taught that this is the ninth planet. I am not sure if you missed it, but all the Earth's publics pay every one of your salaries. I think you may want to give some consideration to their thoughts and feelings, however 'unscientific' you feel they may be. Pluto may be an icy rock to you, but I believe the world has a different opinion.

There are lots of things we learn in school that turn out to be wrong.
Isn't it possible that Pluto was just misclassified when was discovered,
based on a lack of important information that we now have? This is the
nature of science; things change based on additional data. Honestly, I am
not sure what all the fuss is about, and I doubt 99.999% of humanity really
cares all that much.

On a different note, I read your blog regularly and want to thank you for
the service you are doing for the planetary science community.

Cheers,
mitch schulte

Nasa Watch,

The conservative reaction to the demotion Pluto at the latest AAAS convention rang familiar to me.

I recall that a very similar episode took place in the paleontological world in 1975 when the beloved Brontosaurus was officially 'voted' out of existence. As the result of a paper published by John McIntosh and David Bermanbase, it was documented how the genus Brontosaurus had been fabricated in scientific haste by Othniel Marsh, the Professor of Paleontology at Yale University. In 1905, while preparing a dinosaur exibit at the Peabody Museum, Marsh allowed the first “complete” skeleton of the reptile to be created from a patchwork of several unidentified sauropod skeletons. The skull was built by pure guesswork.

For the greater part of the twentieth century, paleontologists turned a blind eye to this scientific sloppiness because the Brontosaurus was so well entrenched in dinosaur lore, and beloved by the public. When the mistake was finally publicly exposed in 1975, the genus known as Brontosaurus was renamed Apatasurus (and given its proper head).

Of course, there was an initial outcry from the public at having been robbed of their prehistoric mascot. Thankfully, sensibility prevailed and the story of this scientific ‘error’ has come to be embraced by nations of elementary school children.

Even today, however, many conservative traditionalists still live in the ‘Age of the Brontosaurus’, and refuse to give in to the new system. Many fictional picture books and Hollywood movies continue to ignore the Apatasaurus.

The similarity of the Brontosaurus issue to the recent Pluto debate is obvious. Let’s accept the new way of looking at things, and move on.

Alex,

The situation with the brontosaurus is not comparable. Marsh did not have complete data, which is shown by the fact that he put the wrong head on his model. The real correction was not so much in the "renaming" but in using the data to create a more accurate reconstuction of this animal. Today, both "brontosaurus" and "apatosaurus" are used interchangeably to mean the latter version with the correct skull. So what if there are two names?

In the case of Pluto, we have not discovered any new information about Pluto itself to warrant a "new way of looking at things." What we have discovered is that there are far more planets out there and far more types of planets than anyone imagined. That doesn't mean we should change how we see Pluto. What it does mean is we clearly need to expand our conception of what is a planet.

New data on Pluto will be available to us from New Horizons, at which point a reconsideration or reclassification may be warranted. Until then, there is no reason for "sensibility" to mean acceptance of what is essentially nothing more than an alternative interpretation of the known facts.

No, we're not moving on. Your comparison with the Brontosaurus is fallacious. PLUTO EXISTS.

One constant refrain heard from the anti-Plutonians was "It will be impossible for schoolkids to learn the planets of the solar system if there are 200 of them".

If we applied the same logic to the periodic table of elements, we could reduce the number from about 100 to 4:

Earth, air, fire and water.

The new IAU planetary definition might well render an earth sized object in the Kuiper belt as a "plutoid", as we would have a hard time proving that such a slow moving object could "clear its orbital path". This is, frankly, preposterous.

I will not go so far as defend the IAU classification scheme, but I will note that it is an attempt to group objects in the solar system...it appears to me that many of the commentators here object to placing Pluto outside the definition 'planet'. I would like to hear alternative planet definitions (as I requested above) that do better (and I'm sure there are, I just don't see people posting them). What is a good definition of 'planet', based in some scientific context?

The stories of Brontosaurus as described above are inaccurate and incomplete. Even the true story doesn't hold much wisdom regarding Pluto, given that it didn't involve a small group of 'experts' trying to impose their opinions regarding classification of fossils onto the greater scientific community or the public.

In 1879 Marsh named "Brontosaurus" (minus its head) because he originally didn't recognize that what he had found (a reasonably extensive post-cranial set of remains) was a genus that he had already named two years earlier (Apatosaurus) based on less material. Even by 1903, Elmer Riggs had determined that Marsh's Brontosaurus specimen was so similar to the earlier find that he proposed renaming it Apatosaurus, per the well-established rules of zoological nomenclature giving priority to the first naming. This technical assignment has held ever since.

The 1975 issue involved Marsh's original decision to plop a Camarasaurus head onto the "brontosaur" skeleton display because the original specimen had none and the skeleton was superficially similar to Camarasaurus. This error (repeated in many museums) was corrected during the 1970s when the Apatosaurs were classified properly as diplodocids (front legs shorter than the back legs, more elongated skulls with pencil-like teeth among other features) and not brachiosaurids (front legs equal or longer than rear, shorter skulls with more blade-like teeth-including Camarasaurus).

Nonetheless, many paleontologists (bowing in some sense to their acknowledgment of popular sentiment) will refer (at least in public forums) to the sauropod dinosaurs (the long-necks that include both Apatosaurus & Camarasaurus) as "brontosaurs" while still ensuring that their technical papers refer to Apatosaurus as they should.

Thus, unless a few members of the IAU actually try to RENAME Pluto instead of reclassify what it is (almost as silly an idea), I don't see how the story of Brontosaurus is pertinent.

I just don't see why the emotional attachment to the word "planet." Scot Rafkin's post is the most clear thinking here. This is all so much butterfly collecting and sentiment. As I pointed out on the other thread on this, for atmospheric/climate science Titan is much more of a "planet" than Mercury. But it makes no sense to go on a campaign to have Titan reclassified as a planet. Just like Ceres. And just like the big KBO's. Or call them all planets. Just add modifiers, I guess. That way we can make the term planet meaningless by universal application - or actually just meaning "round" - so we have giant planets, terrestrial planets, asteroid planets, outer solar system dwarf planets. In any case, it's just categorization and wording. It's only useful insofar as an organizational idea is conveyed. Maybe something is a planet for one field and not for others? So I guess I'm just as happy with dwaft outer planet as plutiod (fewer words in the latter) - but again, I don't see the emotive drive here.

Mark asks for a good definition of a planet -- which needs to work for exoplanets as well. Aside from the obvious requirement that a planet orbits a star, the simplest criterion is mass, as a proxy for hydrostatic equilibrium. Then within the class of planets, we can have many (and changing) subsets: giant planets, terrestrial planets, dwarf planets, super-Earths, ice dwarfs, hot Jupiters, etc. And these flexible subclasses are a matter of usage and don't need the IAU to define them.

As a scientist and science educator, I have to agree with David Grinspoon and David Morrison (among others). It is ridiculous to have any definition for a planet or other orbiting body that does not, in some way, include the fact that we now have discovered over 300 exoplanets. It is unfortunate that the IAU did not look at an early NASA report where the term Eschew Obfuscation appears---avoid ambiguity.

A good classification system is one which helps communicate ideas. As a middle school teacher recently told me, “Words are auditory symbols by which we identify commonalities between objects, events, or experiences. When we see a repeated pattern, we give it a word. This allows us to communicate readily. A descriptive paragraph can be reduced to one word. This ability, this drive to create language, is part of our humanity.” This has not been the case with the creation of classes of objects called dwarf planets and, more recently, “plutoids.” In an attempt to define the objects in our Solar System, the IAU has caused a lot of confusion, even among scientist and science educators. When I go into a classroom and a student asks me if Pluto is a planet or how big does an object have to be in order to be called a planet or a dwarf planet, I have to admit to them that I cannot give them an answer because I do not know the answer myself. I do not know the details of the math that goes into determining the relationship between size and distance that is required to determine what is a planet and what is not.

I guess that I could have lived with the idea of Pluto, as well as Ceres and Eris, being dwarf planets (small planets just as our Sun is a dwarf star). We already talk about terrestrial planets and gas-giant planets (which are not IAU terms). These terms go a long way to helping us, and our students, understand the properties of these planets: what they look like and something about their surfaces and/or atmospheres.

However, as David Morrison has pointed out, the IAU did not want to have dwarf planets to be small planets. That just would not do. So, with good intensions, they came up with the term “plutoids.” The road to Hell is paved with good intensions. Actually, a very old saying. They have replaced the term dwarf planet with the term plutoid. Oh, sorry, poor Ceres is now the lone dwarf planet. I have a deep fondness for Ceres! Also, since we tend to know very little about the actual nature of these distant objects (are they round?), they have now created a “system” for naming the larger bodies that just might be round. If they are brighter than a certain magnitude, they will be named by two IAU committees, not just the Committee on Small Body Nomenclature. If, in the future, we find out that an object is not round, and thus not a plutoid, then, to quote Anne Robinson of Weakest Link fame: “goodbye.”

As an educator, how am I to explain all of this to my students? I still am not sure I understand all of the implications with respect to the creation of the term plutoids. I assume that Ceres is now the only dwarf planet. However, what happens if there is a Pluto-sized body out there that has an albedo less than 10% or 15%? It would not be bright enough to be initially classified as a plutoid, but is clearly large enough to be round. Would it be reclassified? Would it have to be renamed?

The IAU has created a monster, not a good classification system that can be used to help us understand the nature of the objects beyond Pluto, and clearly not something that helps us communicate our understanding to our students and the public in general.

Now that we have a new classification system, how do we take advantage of this in our classrooms and with the public? To quote an associate of mine, Bill Schmitt, a science educator, “The big issue is not if categories are right or wrong, but rather if they are useful for understanding something. This idea seemed, to me, to be lost in understanding the recent recategorizing of planets. For instance the issue in classrooms should not have been about if Pluto should be a planet or not, but rather about how a new system can be helpful to understanding the science of the Universe.”

To me, the new system is far from helpful in our understanding of our Solar System not to mention the objects being discovered around other stars. As an example, the classification system suggested by David Morrison would be a much better system.

Why is it an obvious requirement that planetary bodies are classed by orbital characteristics? This is not true for stellar objects. Why not include Titan in the list of planets?

Well, Galileo thought the galilean satellites were planets! - naturally enough, since they were the first bodies, other than comets, to be discovered in historical time.

I think we should just call any interesting object in the Solar System a "world" and leave it at that.

Definitions should be useful and as simple as they can be (but not simpler). The comment about pebbles, rocks, and boulders hits a good point. Believe it or not, geologists have agreed upon specific size ranges for silt, sand, pebbles, cobbles, and boulders. These precise limits are useful to facilitate accurate description in professional journals and reports, but in no way prevent anybody else from using the words in meaningful and more-or-less accurate ways...

A planet, as commonly understood, should be a major body orbiting a star (with the caveat that planets can be ejected to become "free floaters"). I like the term dwarf planet, actually, in the sense that it clearly means "small planet," and is easily comprehended by anybody. It allows a somewhat finer parsing of the solar system zoo, in that before you were either a planet or a "bit" (asteroid or comet). Now we have something in between. Is there an obvious dividing line between dwarf planet and planet? No. But like terrestrial planet and gas giant, it is useful, and for the moment there is a mass gap between Mercury and Eris (though Alan Stern is right and this gap won't last)...

The only real problem is that "the IAU" decided that dwarf planets were not planets, and in a transparent move to prevent the planet in dwarf planet ever getting the upper hand, came up with the ghastly term plutoid (which, at the very least, does not fly trippingly off the tongue). The funny thing is that I am a member of the IAU, but I have never once been asked my opinion on this matter (which is why I have "the IAU" in quotes). All of this nonsense could easily have been avoided if an open and respectful scientific process had been allowed to follow its natural course.

As several people have already noted, such as Scot Rafkin on 6/16 at 11:53pm, ("This is ... like arguing over the point when pebbles become rocks and rocks become boulders") the attempt to use "scientific" dividing lines to delineate categories in such a gray area is at best an exercise in futility and at worst, ridiculous.

Even CalTech's Mike Brown (discoverer of 2003UB313 aka Eris), while an initial advocate of Pluto's "demotion," made a valid point on behalf of the cultural desire toward Pluto's retention of planet status in a web article located at:

http://www.gps.caltech.edu/~mbrown/planetlila/

The next three paragraphs are quoted from his website. Note especially the analogy he draws in the 2nd paragraph using the word "continent" (much like Scot Rafkin's analogy of pebbles/rocks/boulders):

"In my view scientists should not be trying to legislate an entirely new definition of the word 'planet.' They should be trying to determine what it means. To the vast majority of society, 'planet' means those large objects we call Mercury through Pluto. We are then left with two cultural choices. (1) Draw the line at Pluto and say there are no more planets; or (2) Draw the line at Pluto and say only things bigger are planets. Both would be culturally acceptable, but to me only the second makes sense for what I think we mean when we say the word planet. In addition, the second continues to allow the possibility that exploration will find a few more planets, which is a much more exciting prospect than that suggested by the first possibility. We don't think the number of planets found by the current generation of researchers will be large. Maybe one or two more. But we think that letting future generations still have a shot at planet-finding is nice.

"Astronomers tend to dislike this solution as it is clearly non-scientific. The best analogy I can come up with, though, is with the definition of the word 'continent.' The word [sounds] like it should have some scientific definition, but clearly there is no way to construct a definition that somehow gets the 7 things we call continents to be singled out. Why is Europe called a separate continent? Only because of culture. You will never hear geologists engaged in a debate about the meaning of the word 'continent' though. When geologists talk about the earth and its land masses they define precisely what they are talking about; they say 'continental crust' or 'continental drift' or 'continental plates' but almost never 'continent.' Astronomers need to learn something from the geologists here and realize that there are a few things -- like continents and planets -- to which people have large emotional attachments, and they should not try to quash that attachment.

"Thus, we declare that the new object, with a size larger than Pluto, is indeed a planet. A cultural planet, a historical planet. I will not argue that it is a scientific planet, because there is no good scientific definition which fits our solar system and our culture, and I have decided to let culture win this one. We scientists will continue our debates, but I hope we are generally ignored."

[End excerpt]

As Mike mentions, Pluto--as a planet--is part of our culture. Why not just accept that and move on? And if the scientific community still insists on having a scientific consensus to such an unscientific bit of nomenclature, then why not let the FULL membership of the IAU (~10,000 members, last I heard) decide, rather than glean their "scientific conclusion" from a tiny minority as was done in 2006? Otherwise, how scientific is that?

Pluto has been defined by our culture, and our culture has deemed it a planet. Onward and upward, everyone.

I believe this ongoing debate regarding Pluto's status may contain lessons for all of us that go far beyond the nuances of astronomical nomenclature. Mr. (Dr.?) Carter has done a good job of describing the core issue: a clash between scientific terminology and practice on the one side and the outlook of the "regular" folks across our society on the other. I agree with his conclusion: there is no need for the two "cultures" to be in conflict.

But perhaps a more valuable question is hiding in the cracks here: Why has Pluto, a frozen object a billion miles away that no one has seen with their own eyes, made such a splash across the general population compared to the ongoing debate over retiring the shuttle and NASA's troubled implementation of the VSE, a program that is supposed to be laying the foundation for more people to travel to the Moon, Mars, and beyond?

O Is it purely a matter of media whim, the reporters/producers deciding to cover (and hype)one story more than the other because the notion of scientists redefining what we all learned in school holds irony, or humor, or "gee-whiz look at how silly scientists can be"?

O Is there something about Pluto, really, that schoolchildren find so much more endearing compared to dreaming that they themselves might potentially fly in space?

O What about this debate is so worthy, or so interesting, or so emotional that it merits such thorough coverage and inspires such passionate discourse?

I don't know the answers, but I believe that investigating how and why this story of Pluto's status struck a resonant chord with the greater public may reveal lessons applicable to re-engaging that same greater public with the wider (and certainly more bountiful) pursuit of solar system exploration—and even, perhaps, to re-engaging them more intelligently with science across the board.

First off let's consier that it is all made up, it is a result of human beings being meaning making machines. That said, the geophysical definition makes sense. If we go much beyond spherical bodies orbiting stars and we begin to deal with properties of the object. Is Jupiter dissimilar enough from Mercury that we should forgo the term planet altogether and invent rockoids and gasoids or whatever.

My vote is that the IAU got it wrong and should adopt the geophysical now.

Plutoid? Come on! We've always referred to Pluto as a planet, which to me means that we already established the definition of a planet long ago. It's spherical with a regular orbit about a star.

There are two types of round bodies in space: those with nuclear fusion and those without. The fusion-driven ones (and their remnants, such as pulsars) are called stars. The fusionless are planets. Intuitively, we call "planets" all non-shining bodies round in shape. Irregular, gravel-shaped big rocks are asteroids. They are the junkyard of space so they are not dignified with the name "planet".

Pluto is round, it orbits the sun on its own, it has 3 moons, it acts like a planet, it looks like a planet, it is a planet, end of story.

It would seem new ideas are hard to accept even by scientists who should know better. Lets hope our kids have more sense or there is no hope for humanity. Can we please move on?

This bogus decision that was made in 2006 is a crock of crap. The "qualification" that uses "clearing of its orbit path" is just random B.S. I guess that some other planets, like say Jupiter, aren't "really" planets because they "fail" that test. If it quacks like a duck, swims like a duck and has feathers like a duck then by the stars it IS a duck. And besides, what's this "dwarf planet" jive anyway? It's STILL a PLANET anyway! That very P.C.-ish term says that, doesn't it? Pluto is a planet, no ifs, ands, ors or buts - no doubt about it. Orwellian labelling and Jedi mind tricks CANNOT change this fact and never will. Have a nice day.

A planet should be, by definition, an object that orbits a star. I will add, however, that there should be a mass constraint. A true planet would be one with sufficient mass and gravity to insure a round body. Other objects would be asteroids, or comets,, or meteors.

A moon, or satellite, would be defined similarly - a body that orbits a planet.

In any given solar system astronomers who wish to be technical for science purposes could still use adjectives like gas giants, ice giant, terrestroid planets, etc.

I am in general agreement with the geophysical definition.
I.E. A Planet must be shaped by its own gravity into a speroidal shape,must not support fusion, must be in orbit around a star, pulsar or brown dwarf (I would exclude sattelites of planets from being classified as planets, BUT I would include interstellar free floaters within the definition of "Planet"). Since in any system of categorization a certain amount of arbitrariness cannot be avoided, I would also specify a lower limit of 1,000 kilometers for the diameter of a planet.

Pluto is as near spherical as makes no difference, has 3 moons of it's own including one 1/2 it's own size and seems to have cleared it's own orbit reasonably well - at least I don't know of any other objects that have been discovered floating around in the same orbit. I know there may be quite a few objects crossing it's orbit, but then the same can be said for all the "Planets" in our Solar System. The IAU may have made their proclamation, however, when anyone asks me how many planets we have in the Solar System - I always tell them "at least 9" - to me Pluto is still a Planet.

Wow, Jim Jupiter; are ducklings ducks?

I don't remember voting for any members of the IAU. They don't represent me or anyone else except themselves. Who or what gives them the authority to declare what is or isn't a planet? Therefore, my opinion is just as valid as theirs. I declare Pluto to be a planet, and that's that.

Finally someone’s speaking some sense! Ever since the whole planet classification came to a head, I've been wondering how long it would be before someone pointed out that the new classification would require some detailed investigation not directly related to the 'object' itself. Making the classification process a lengthy and rather imperfect process, this never more so poignant than in this day and age of exoplanetary (exoplutoidian) discovery. Given the vast distances and the seemingly impossible sensitivity required to detect even the largest exo solar planets, could we ever really be sure that planet x isn't a plutoid x?

The decision of what constitutes a planet should be in the hands of scientists who understand them best (clearly not the IAU). The geophysical definition makes sense and is relatively simple. Does the IAU have an aversion to simple yet effective definitions? With the recent boon of new Jovian moons should the IAU reclassify what constitutes a moon to keep those numbers artificially low also?

This new definition is silly. I can't believe there's not been a definitive definition before now...and this rings of "another politically correct" compromise. Plute has been a planet for a long time and trying to change it's designation is crazy...especially with the myriad of definitions now being proposed. Leave it as it was.

I agree with the IAU, Pluto is NOT a planet. Their reasoning is, I concede, somewhat overly complex, but I would like to remind people that the asteroids were not known to be, as a whole, "unround" bodies when they were universally dismissed as Minor Planets in the mid-nineteenth century. They were dismissed because of their ridiculously small stature relative to the body of planets (indeed, even moons of planets!)

Pluto's integrity as a planet is unknown (closer to the sun, it might grow a tail and split into different icy rock bodies, in which case it is nothing more than a comet). This is certainly a realistic assessment of the body, though further study is required.

I propose a different, geological definition. Besides not having, and never having had, nuclear fusion at its core, a major planet should also have a minimum of half the mass of Mercury. For edification on this see this lising of solar system bodies by mass ...
http://en.wikipedia.org/wiki/List_of_solar_system_objects_by_mass

In the mean time, it would be much better if S&T would stop its propoganda that Pluto IS a planet with such inflammatory titles like "IAU Snobbery". There was a time when Copernican views were no doubt dismissed as "Copernican snobbery" and the use of such degrading terms only muddies the scientific discussion. E.g., "Well, there you go again."

The current definitions (used by the IAU) just don’t make sense. Also, definitions are not based on the IAU’s determination of what “they” think definitions should be. Rather, definitions are decided by their usage.

Collectively, if we don’t use these “new” definitions, then they really exist only in the minds of of the IAU.

First off, I am an astronomer who studies exoplanets, and I know not a single other astronomer (in my field or in any other) who actually thinks Pluto is a planet (there are a few who think that the whole debate is ridiculous and irrelevant, but that's another story).

Cultural arguments for why Pluto should remain a planet are, as others have pointed out, nonsense. Once upon a time, people believed the world was flat. There's a not insignificant segment of the american population who believes that evolution is a lie and that creationism (aka "intelligent design", in its most recent rebranding) is the right answer. Fortunately, as someone else pointed out, science is not a democracy, and just because most people don't like the answer doesn't make it false.

Defining what it means to be a planet is difficult, and I comment the IAU for doing their best at what is clearly an almost impossible (and thankless task). No, you didn't vote for members of the IAU -- prominent and respected scientists who know what they're talking about did. One of the interesting things about the 8 planets in our solar system is that they are all unique -- no two are alike in more but the coarsest possible measure. Pluto, on the other hand, is far from unique. It looks *exactly* like several other objects within our system, and there are probably more plutoids out there that we haven't found yet. Alas, "uniqueness" is also a really poor way of defining what it means to be a planet, but I think that the contrast between the planets and the plutoids is really quite clear.

None of this changes the fact that Pluto may be an interesting place; it merely places it in a context where it makes sense.

Here's mine:


Anything that orbits a star that hasn't undergone stellar ignition is a planet. Planets are classified by mass only. Each class begins at the midpoint between classes (i.e. centiplanet class begins at .005 planets and goes up to .05 planets):




1 milli-planet 0.001 Pluto,Ceres (.0021) is in the milliplanet class

1 centi-planet 0.01 Mercury at .055 is in the centiplanet class

1 deci-planet 0.1 Mars at .107 is in the deciplanet class

1 planet 1.0 Earth (1), Venus (.6) are in the planet class

1 deka-planet 10 Neptune (17.147) is in the dekaplanet class

1 hecto-planet 100 Saturn & Jupiter are in the hectoplanet class

1 kilo-planet 1000 Upsilon Andromedae d (1,248) is in the kiloplanet class



The brown dwarf limit is 4,131 planets, or 4.1 kiloplanets. Most comets and asteroids are microplanets.



See, that wasn't hard.

If we defined stars using the criteria "SR" suggests (hmm, why isn't SR using his/her name?) we would likely have to get rid of M dwarfs because they arent "really" like the hotter stars and there are too many of them anyway. The dwarf planets are a completely new class of objects in planetary science that meet every reasonable criteria of planethood: they have atmospheres, geology, cores, and satellites, and they are large enough to reach hydrostatic equilibrium-- so they become rounded by self gravity. This last aspect is key because this is *the* hallmark criteria for when objects graduate from "rocks" to planets. At this size scale the physics of the body is no longer controlled by mechanical properties, but by gravity. Objects beginning about 1/4th Pluto's diameter lie on the planetary side of this boundary. Yes, there are a lot of them in our solar system, and very likely, more of this type of planet in the galaxy than any other. Perhaps you feel badly that terrestrial and jovian planets have been displaced, that's ok, but feeling badly is not a scientific criteria relevant to achieving a good classification system. Science is about adapting to new data, and in the past 20 years planetary science has learned that dwarf planets are the norm, not the exception. The IAU will eventually catch up. As hundreds of planetary scientists like me have said, if and until they do, then I will simply not use the IAUs definition and will encourage others to do the same because it improperly associates dwarf planets with rocks rather than with planets.

Obviously the only way to resolve this debate is to go back to basics. "Planet" is derived from the Greek word for "wanderer" and referred to those "starlike objects" that wandered across the sky and were visible to the naked eye.

So (per standard scientific naming protocol), only those objects ORIGINALLY classified as "planets" are permitted to be called PLANETS: Mercury, Venus, Mars, Jupiter, and Saturn. Nothing else qualifies by the original, legally proper definition, so we'll need to assign proper, scientifically proper terms to everything else:

- Let's just call Earth "EARTH". There is only one, after all.
- Uranus & Neptune can be URANOIDS; that would be gas giants discovered by telescope orbiting the Sun between the orbits of Uranus & Neptune.
- Ceres is a CEROID, but all the other rocks in independent solar-centered orbits (that don't have tails) can stay ASTEROIDS.
- Pluto, Eris, and Sedna shall be PLUTOIDS...or maybe KUIPEROIDS since so many folks are upset about "plutoid."
- If something ever sprouts a tail, it may be called a COMET...but only while it has a tail. Otherwise it goes back to being an asteroid or kuiperoid...or OORTOID.
- If it's in an orbit around something that orbits the Sun, it's to be designated a moon. But we must be careful not to confuse this term with "the Moon", which is Earth's Moon. [Perhaps we should rename "the Moon" Artemis or Selene or Diana just to keep things clear?]
- Those things orbiting other stars can be EXOIDS.

THERE! That should clear everything up.

I am pretty sure Pluto is a banana. Not literally, of course, but metaphorically speaking. Ask a botanist, horticuralist and a chef and you'll variously learn a banana is a fruit, a berry and a spice. Throw it open to the public and you will mostly hear passionate arguments in favor of whatever the poster first or most authoritatively learned a banana is.

Language can be a sloppy and imprecise tool and English is worse than many others. We've spent a couple thousand years the word planet as if it had a lot of meaning when in fact it only really meant "wanderer" to distinguish planets from other dots of light that were more orderly. It's too late to get a knew definition everyone will agree on. Classification systems are useful tools, but they are invented and almost always rely on judgement calls somewhere along the way. The nature of Pluto does not change based on how we classify it, but many of us sure feel passionately about how it gets classified.

Personally, I've never liked Pluto as a planet and was thrilled with the IAU decision. My beliefs, though, are based on personal taste (Pluto never measured up to the other eight for me) and historical issues with discovery, classification and naming. But I hold that up as at least as valid as "it's what I learned in school".

BTW, here's a link for more on the banana debate: http://wiki.answers.com/Q/Are_bananas_berries_herbs_or_fruit

All this discussion is based on a "faulty" definition. Adding to the current 9 planets is fine with me- if the definition fits. Reducing the number of planets is fine with too- if the definition fits. BUT the current IAU definition should also exclude Earth, Jupiter, Neptune and Mars. But, only Pluto got the ax. If we go by the IAUs definition we should only have 4 planets. Again, that’s O.K.- we’ll just call Earth a “Humanoid”…..a place where humans dwell. LET'S TAKE A VOTE! All in favor- say "aye".

Hello Paddy, I saw your posted comment and to answer your question: OH YEAH! You betcha. You got it. Keep cool.

Actually this is a perfect time for the public to pay attention to planetary science. The discovery of exoplanets has opened a door to what can only become a huge field of research and discovery long into the future.

Geophysicists and Planetary Scientists should be the ones to define planethood since they'll choose the defacto standard through usage anyway.

At this early stage a broad and flexible definition of planets would make sense. David Morrison makes an excellent case, and Alan Stern seems to parallel David's thinking.

The broadest definition would be non-stellar bodies orbiting a protostar, star or stellar remnant (white dwarf, neutron star, black hole?). Add "natural" bodies, or man-made stellar satellites could become planets. Finally, require some minimum mass or dust grains could become planets.

As someone else mentioned, mass would be the best (and most detectable) property for classifying size. We will not be able to measure radius or how spherical small distant bodies are. Per Sara Seager et. al. planetary radius is degenerate (material makeup controls radius not mass). Perhaps Geophysicists can determine one minimum mass value that makes sense for any material a planet can be made from (iron, carbon, silicon, water, all the way down to H/He).

Problem cases: determining if a barycenter lies outside a body's surface for distant binary planets if we can't determine planetary radii (binary planets vs. planet w/moon). Also "planemo" vs. planet depends upon detecting nuclear fusion within the smallest of stars.

Someone a few posts up took a swipe at the "Pluto as Planet" supporters by comparing them to the willfully blind people who reject evolution. This is unfair, the Pluto enthusiasts are, if they care about this issue, also science enthusiasts. The anti-evolution crowd either hates or badly misunderstands science and has no desire to understand it, therefore this charge is unfair, shame on him or her that made it, that person owes us all an apology. Just for the record, this is NOT a swipe at religious people, many religions accept evolution.
Oh, and by the way PLUTO IS A @#&*$%# PLANET!!!!!

Whatever happens to the definition of "planet", I think there should atleast be a word for all substellar objects large enough to be in hydrostatic equilibrium, we could call them "worlds". A planet could be defined as a star orbiting world (where stars include stellar remnants and brown dwarfs). For a world orbiting a substellar object - lets use the word "moon" (i.e. Tethys), and use moonlet for sub-worlds that orbit substellar objects (i.e. Phobos). A world that is not in orbit can be called a "rogue".

A "major planet" could be a planet that has a large gravitational influence on the local structure of its star system, this should satisfy the dynamicists. A world that orbits with a planet that is within 1 order of magnitude of mass - could be a "companion planet" instead of a moon.

Hmmmmm. And won't we all feel stupid when some form of life is finally found.... and it is on the 'thing formerly known as a planet', Pluto.

The planet category is too diverse to be useful no matter what the definition. There should be more narrow categories, e.g. terrestrials, gas giants, plutoids, etc, and the word planet could be left undefined and used by scientists and others any way they please. It sure is a useful word for those extra-solar orbiting bodies that no one knows enough about to categorize more narrowly.

IAU is in denial!!!!

I am extremely dis-satisfied with the IAU's definition of a planet. I have grown up all my life recognizing Pluto as one of the nine planets of the solar system of the star we know as "Sol". To have the IAU reverse that fundamental educational piece from me is ignorant, at best. I accept, and always will view Pluto as the ninth planet in our solar system. The geophysical definition of a planet is the proper and just definition. The attempt of the IAU to reclassify Pluto has, in my opinion, orphaned the IAU into a substandard scientific body that deserves the same classification that Pluto has received from them. Until the IAU reverses their decision on Pluto, I will not recognize the IAU as any type of legitimate scientific body of any importance or siginicance in the scientific community.

The article by Alan Stern makes a lot of sense.

The IAU definition of "Plutoid" is just as bad as the first definition of ice dwarf planet where a "planet" is not a planet because it is not clearing out its neighborhood.
Give me a break! There are Stars. Stars are hot from Nuclear fusion reactions inside.
One can envision an object which could look like a star, which could be heated to incandescence not by gravitational collapse but by nuclear isotopic decay and possibly even by nuclear fission reactions. If for instance the cloud of nebular material the object condensed out of contained a high percentage of U-235 or Pu-239. It might well have a hydrogen outer envelope, would it's spectrum show it to be heated by fission? If not, would it be classified as a star, or a planet?
If a planet in the Jupiter-plus range is "spun up" to have an exceedingly high rotation rate, such that is extremely oblate, not anywhere near spherical. Is it still a planet?
If a Jupiter size planet has a co-orbital companion in the L-4, or L-5 point, and that object is say, Earth size, or even Neptune sized, would any of them be a planet? (the Big one has not cleared out it's neighborhood, the smaller objects are obviously not Moons. What the heck are they?
Two objects sharing an orbit that co-orbit each other close to each others Roche limit (Such as Dr. Robert Forwards Rocheworld) are they planets? If they have atmospheres, or not?
A small body, only a few kilometers across could potentially pull itself into a spherical shape if it was in fact fluid enough to do so. Think ball of lava, ball of Mercury, ball of Water? Think Bubbles?
We will probably find as we explore the universe objects we cannot even imagine made of all kinds of strange and wonderful combinations of materials and dynamic forces. We need to come up with a definition of "Planet" that includes everything except the too large and the too small.

My definition: if a human being could get a running jump and go into orbit around it, it's too small to be a planet. If nuclear fusion makes it glow in the dark, it's too big to be a planet.
Everything else is a PLANET!

Mnemonic Verses Educate Many, Can Justify Silly Unique Nouns Planets Challenge Exceptional Quotations, Satisfied?

Mercury Venus Earth Mars Ceres Jupiter Saturn Uranus Neptune Pluto Charon Eris Quorar Sedna. "Here Be Planets"

Clear Skies.

All the comments to Dr. Stern's article are very interesting. As for my 2 cents - I agree with Dr. Stern.

O.K. NASA WATCH-

I've taken your advice, and that of Alan Stern, to "get involved".

Check out my new website: dwarfplanetsRplanets2.com

Spread the word and GET INVOLVED!

Pluto is, what it is. It does not matter to it whether we consider it as planet or dwarf planet. From childhood we have been studying that there are nine planets in our solar system. Still we feel the same. The classification criteria which might have been formulated long back, should be revised in the modern times so that Pluto remains in the family.
-Rajesh

Simple is better, except perhaps for certain arrogant dynamicists. The geophysical definition Alan Stern proposes is simple and (what a bonus) it makes sense.
Let the revolt proceed !!!

I disagree with the IAU and essentially agree with Stern. To be short and sweet, while Pluto may be a dwarf planet or a particular kind of dwarf planet called a plutoid, it is essentially a "dwarf PLANET", as opposed to "DWARF planet."

I think Pluto is a Lagrangoid, formed at the 1:1 resonance (Lagrange points L4 and L5) with Neptune. Planets are formed in the Titius-Bode resonance, like Ceres. Since Ceres is small, dwarf planet fits. Dr. Stern is trying to justify the sales pitch he used to get Congress to approve the New Horizons mission; calling Pluto a planet makes it "macho" and that appeals to politicians. Well, thank you, Dr. Stern, for launching the probe, but the whole Kuiper Belt is fundamentally different from the planetary system and such classifications as plutoid, plutino, cubewano, scattered disc object, Centaur, etc. (including misfits 2004 XR190 and Sedna} are very important to understanding. Simplification is for Congressmen.

I'd like to see the definition of a planet contain just a size/mass/diameter contraint and a circular shape plus the fact that it must orbit a star. The IAU's definition with "clearing its orbit of other objects" seems overly complex and arbitrary.

ultimately whether we call an object a planet or not is not important, what is is what can be learned about how these objects played a part in the formation of the solar system.

I think it should have been left as a planet, since it has been for 70 or 80 yrs.,and changed back to that now, I guess no matter what anybody says or thinks my thinking is pluto is and always has been our furthest planet that is what I was taught and still believe this day, beside there are alot of more important issues that need be address, Thanx & have a great day !!!

Perhaps its time to drop such a generic classification term as "planet". There should be multiple classes of planetary objects, say "Class1 Planet", "Class2 Planet", "Class3 Planet", ... up to "Class5 Planet". Protostars have different classifications such as Class 0 Protostar, etc. Planet classes could be based on physical parameters such as Mass, Atmospheres (Nitrogen/Oxygen, Carbon Dioxide, Tenuous, etc.), Composition (Rocky or Gaseous). Since there are so many different types of planetary objects with different compositions and physical parameters, the generic term "planet" is just not adequate to describe them as a single class of celestial object. Any planetary classification scheme MUST include ALL planets (those within our solar system and extrasolar planets). The classification system must apply systematically for any planetary objects, whether they exist within our own solar system or in other solar systems. For example, the class "Super Jupiters" could apply to planetary objects with masses between Brown Dwarfs and Jupiter mass objects. "Hot Jupiters" are Jupiter mass objects that orbit extremely close to their stars (those exoplanets have orbital period on the scale of days with masses similar to that of Jupiter). Small objects such as Pluto would most likely be denoted as "Class5 Planets" having tenuous atmospheres and masses on the order of large Kuiper Belt objects.

This debate has gone way past ridiculous. If it orbits the sun it's a planet. That's been the understanding of the word "planet", or it's equivalent in other languages, for hundreds of years. In todays world we need to replace "sun" with "star". Let the various disciplines define sub-classes that "may be useful" within their field of study, but let's get past the circus.

It looks to me like there are a few "eminent scientists" without enough to do.

Hmmmm... If your two "editor's notes" along with the topic's title is supposed to serve as some form of an argument against the IAU position, then it utterly fails. The reason why is because, with regards to the counter-position, it would also be the case that what I learned in school was wrong. Instead of there being only nine planets, now there are around 200 (most of whose names I don't know). The reason to make this move is seemingly just as esoteric (i.e. the universe wasn't built for our convenience).

Science seeks the truth, the fundamental nature of reality. Our scientific definitions and theories should not be determined to be the best or approximately true ones by the vote of people who really don't know what they are talking about. Unless you think we should also hold a worldwide vote to determine whether the standard model in QM is true or not. Heck, that would save billions of dollars because we wouldn't need to run any of those accelerator experiments anymore. Scientific investigation leads to refinements and even occasionally radical changes in the way we perceive the world. That would seem to entail that the "common" folk will on occasion be inconvenienced with a request to modify their current beliefs about the world around them.

If the people who know what they are talking about believe it is best to have eight major planets and a series of dwarf planets in the taxonomy of our solar system, then so be it. Our emotional and seemingly irrational sentiments to past views not withstanding.

Matthew, "the people who know what they are talking about" have not determined that it is best to have eight planets and a series of dwarf planets in our solar system. A tiny percentage who happened to be in a room in Prague in August 2006 determined this. Planetary scientists, who know planets better than any other types of astronomers, overwhelmingly believe dwarf planets should be classified as a subcategory of planets, which gives us a solar system with far more than eight planets. Their arguments are based not on sentiment but on sound scientific reasoning, namely that an object in hydrostatic equilibrium in orbit around a star is a planet. If we are to give credence to "those who know what they are talking about," we need to include ALL of them, not just a select few.

(I'm Mark from some of the earlier comments) - Laurel, I know a lot of planetary scientists (I am one, too) and I'd like to know where you got your statistics that they 'overwhelmingly believe dwarf planets should be classified as a subcategory of planets'. Many (most?) planetary scientists I know don't feel that way - in fact, I'd wager that most of them don't think it matters one way or the other ultimately (except perhaps some that feel that funding will be affected for Pluto system studies, maybe?), because they are interested in the planetary system, and what Pluto tells us both about itself and its place in the system (e.g. it is unique in some ways, but at the same time is one of the largest and closest members of a numerically plentiful and important subset of the planetary system).

I don't think the IAU definition is all that useful, but I am unconvinced that Pluto is a 'planet' at the same time. Regarding most of the pro-planet comments, there is a major component that seems simply emotionally invested in Pluto being a planet - sorry, but that doesn't cut it (such as 'I was taught it was planet so it's a planet, end of story' is just not acceptable from a science perspective). We should strive for an acceptable definition of planet, or (as was suggested above) drop using it at all.

Is there a definition the 'works' for everybody? Probably not. There are proposals that seek to place mass ranges on what constitutes a planet. At the low end this can only be an arbitrary cutoff, I believe. You can put it higher than Pluto, or lower, but where do you draw the line? At the high end the limit is perhaps better defined (core fusion) but is still muddled by brown dwarfs and what separates them from super-jovian planets. Perhaps on how they form (brown dwarfs forming much as low mass stars in multiple systems, where 'planets' form in an accretion disk) but these are not well understood processes, either, and the definition will be harder to apply to extra-solar systems.

Why don't we just accept the fact that we don't know all the types of bodies that exist, and that the definition of planet will inevitably change in the future? Wait till we really start geting good data on extrasolar planets. It will be a real zoo. Pluto and Ceres are significantly different even from something as small as Mercury and are just not the same kinds of bodies. I have far more of a problem lumping rocky Ceres and icy Pluto into a single category.

As a geologist, I have no problem comparing the Moon and Mercury and calling them both planets in informal terms, but the distinction between planet and satellite is important in other contexts, and needs to be preserved. So what's the problem with creating a new class for small, gravitationally shaped bodies?

I am all for the proposal to ignore the IAU if we so choose. This isn't the first dumb thing they've done - redefining "north" from a universal rotational sense to something based on the ecliptic plane is far worse. I often say that those who can, teach, those who can't, do, and those who are utterly incapable of a productive career in science get on the bodies that define nomenclature.

Perhaps the idea of a "planet" is in itself outdated. There are after all significant differences between the outer gas planets, the inner terrestrial planets and smaller spherical objects like Pluto and Ceres. Ideally these objects should be classified by origin. But whatever the classification is it must be consistent and independent of environment. for example earths status should be the same if it is where it is now, or if it was where Pluto is. I have no objections to keeping Pluto as a planet if all other objects similar to Pluto are also made planets. Instead of arguing over Plutos planethood we should distinguish that difference between dwarf planets and terrestrial planets is similar to the difference between gas giants and terrestrial planets. Just as a side note: I personally think that Mercury is too different from the other terrestrial planets to remain in that taxonomy, and should probably be demoted in like manner as Pluto was.

What I meant was that most planetary scientists believe the dynamical definition adopted by the IAU is insufficient in that it does not take into account what an object is made of and the resulting geological processes that result from its composition.

The statement that those who argue in favor of Pluto retaining its planet status do so out of largely emotional concerns or desire for funding is a "straw man" relied upon heavily by supporters of the IAU definition. Some commenters may argue that Pluto is a planet because they were taught that way in school, but this is not the central reason people like Stern want to keep Pluto in the planet category. That reason is very clearly expressed by Stern in his highlighting of hydrostatic equilibrium as being the best defining measure of whether or not an object should be considered a planet.

Pluto and Eris are planets because they are not simply the most prominent members of the Kuiper Belt but because they have enough self-gravity to pull themselves into a round shape, which distinguishes them in many ways from the majority of tiny, shapeless KBOs (the same is true for Ceres regarding the asteorid belt).

Advocates of the IAU definition frequently use arguments that are also unscientific and could be classed as emotional, namely that we cannot have too many planets, as this would make it impossible for children to memorize them or that those who oppose removing Pluto's planet status are simply resistant to change. The fact is, change has to be based on genuine new knowledge, on the emergence of new facts that fundamentally alter what we previously knew about something--such as Galileo's confirmation that the sun is the center of the solar system. In this case, there is no new information, just what is clearly a political division among astronomers. Many members of the public see through this and cannot see making a change when that change is based on nothing more than the subjective opinion of one group of astronomers with a specific interpretation (i.e., dynamicists).

Maybe we need to take some advice from biology - living organisms are divided up into kingdoms, phylla, classes, etc - maybe we should sort out compact astronomical bodies in a similar way. Consider the animal kingdom, most people have no problem calling cats animals, but they may have a problem calling frogs, fish, and ants animals - but all of these are in the animal kingdom. Some folks usually equate animal with mammal, but if that were the case "animals" would need to be a class instead of a kingdom. The question is, should planets be like a kingdom, or like a phyllum or class - I think it would be best to consider it more like a kingdom, since we commonly use the phrase "stars and planets", and stars seem to be like a kingdom. We could consider as the kingdoms of compact anstronomical bodies: stars, planets, asteroids, and debree. Planets could have many phylla which includes gas giants, dwarf planets, terrestrial planets, moons, etc.

Every round rock or ball of ice is not a planet. OK class, can u name the 200 planets, or is it 842 planets?

Just because most people believe in something, doesn't make it right. I still believe the world is flat, but some stupid scientist decided that the world was round. Do you know how upsetting that is. Next, scientists will start saying we're descended from monkeys.

Well, the snow has stopped, I'm going out to make some planets in my yard.

Again, I don't have to support the IAU definition of planet to believe that Pluto is not a planet (and I stated that I don't think their definition is useful). I am simply unconvinced one way or the other as of yet. As for 'no new knowledge' I think that the discovery over the past ~15 years of the large and growing number of KBO's is in fact 'new', and places Pluto in a different context. Whether that moves it from planet to not planet, I don't know, but I can certainly see the argument against planethood (and for it, as well). I don't have a problem if we come up with a definition of planet that allows many more planets in our solar system than the current low number - what I want is a definition for this. Mass limits, the related (but distinct) hydrostatic equillibrium, etc, are attempts but we probably will never have a satisfactory 'definition of planet'. On a related topic, I would like to understand how Dr. Stern would classify satellites, especially those that might have been captured (e.g. Triton) or with massive atmospheres (Titan)? Are they planets? Was Triton a planet and now it isn't?

I have been studying astronomy since the early 1950s. I now teach it at a four year university. On the internet are
several articles concerning Quaroar ( possible spelling error here ) and Sedna. Plutoids? Dwarf planets?
I do note that the major magazines on astronomy call
extra Solar planetary systems "Solar systems". So wrong. There is only one Solar system in the entire universe, it is our one and only Solar system named after our star, Sol.
Those other systems are Stellar systems which would be
named after the respective star that they orbit. Get it, editors of those magazines????
My students are so confused now, they do not know if they are coming or going. Now those "people" muddy the waters even more.
By the way, people with SOME scientific knowledge dispute the existent of intelligent design ( God ). When I speak to people with GREAT scientific knowledge ( surgeons and people who really study the universe ) they "admit" that the body or univers is so vast and complex that the body or universe did not simply "poof" into existence.
I line up toy animals and show how cats, dogs, quail, turkeys, fish, goats and finaly gorillas morphed into man.
I show a little horse and explain how it stretched its neck to get to that fruit and changed into a giraffe in 367.75640384329 million years. ( number made up ). They of course laugh knowing that true evolution from nothing is a big joke...those are true intelectual students.
In closing, be humble, think rational and consider that something out there is bigger than us.
Jim Cook,
Professor.

Mark Gurwell (hi Mark!) asks how I classify very large, planet-sized satellites.

Well, bodies like Titan, Triton, the 4 Galileans, etc. are, geophysically, planets in their own right. There is no question that these things should ALSO be called satellites or moons, because doing so tells you something important about their location. To my mind, the planetary descriptor is the more important one, since it informs you what type of body we are discussing, and the satellite descriptor is important but secondary, as it relates only to the locational circumstance where we find the body.

So, just as we have stars that orbit stars and galaxies that orbit galaxies, planets can orbit planets. I see no reason why planetary bodies should be different from other astronomical bodies in this regard.

Although I think this view is a minority among scientists at present, my intuition is that when extra-solar planet searches reveal two Earth or two Jupiter orbiting one another, or an Earth circling a Jupiter, the logic here will become more mainstream. There is no contradiction in saying, for example, "Titan is a planet and a satellite," since the first descriptor ("planet") tells us what kind of physical objects it is, and the second ("satellite") tells us something about its location. A good analogy would be to say I am a human being and a Virginian, both obviously so, and the first describing what kind of object I am, the second describing my location.

By the way, I also have no issues calling planets in interstellar space (e.g., those ejected from solar systems during the clearing stage) planets, since that is what they physically are....just like "a man without a country" is still a man despite his locational predicament.

The root principle here is that descriptors like planet, star, comet, asteroid, black hole, ...whatever..., should be descriptors of the *type of body*, rather than where it is or what is near it. ...Were we to neglect that in biology and classify based on what an object is near, as the IAU currently likes to do with planets, then a cowboy
would "become" a cow when he herds his cattle.

-Alan

Hi Alan, if we are to describe all objects which are in approximate hydrostatic equilibrium as 'planets', but masses below which instigate fusion, then I can agree it makes for a reasonable definition, though with some blurry edge (e.g. the mass boundary will be blurry and depend slightly on composition. Thanks for responding!

Thank you for responding Alan. It is "currently" far easier to describe planets smaller than Ceres using the 18 secondary planets (moon planets) since the primary sub-Ceres dwarf planet candidates are still only known as a few pixels. I do not have a problem with the 2006 IAU definition but I do wonder if their could be a better definition. The question is how inclusive does society want the term planet to be?

I have no problem with the Earth being a double planet since Theia would have been a dwarf planet before it collided with the Earth. I do not think the barycenter should define a double planet since that is an orbital characteristic. Triton was (still is?) a dwarf planet before it was captured by dynamical Neptune. I agree that we should consider what society wants to do when we do find an extrasolar Earth-like-moon orbiting a gas giant planet. What would Titan be like if the Saturn system formed where the Earth is located? Saturn not only has the best set of rings, it also has the most (spherical) secondary planets.

Secondary planets would be "dwarf planets" by definition.

-- Kevin Heider

The geophysical definition of "planet" as it has been discussed here seems very reasonable to me. While there will always be examples of objects that are very close to the boundaries of any definition, the geophysical definition minimizes those cases to a much greater extent than does the current IAU definition.

I am particularly impressed that the geophysical definition addresses situations for which the current IAU definition cannot be easily applied. Objects in planetary systems around other stars and objects in interstellar space are two examples that the geophysical definition addresses nicely.

55 Cancri f could theoretically have an Earth-like-moon, then what would people think of secondary planets? Yes, this image is way too Earthly, but it gives the idea.

==Earthoids== :-)

Planets:

Mars (Failed Earth;simple life?)

Venus (Greenhouse Earth; Did it have life 2 billion years ago?!)

Moons:

Titan (Primitive cold Earth; gas, liquids, and solids)

Europa (Snowball Earth with subsurface liquid ocean? Life?)

Ganymede (Subsurface ocean?)

Io (Most volcanically active body in the solar system)

In my previous post I meant to be more specific:

Secondary planets would be dwarf planets by definition, where dwarf planets would be a subcategory of planets. Planets would be defined by their geophysical properties.

-- Kevin Heider

Dear People,

Any scientific definition of naturally occurring objects should be determined by tables of characteristics using valid sets of population statistics. For “planets” and other sub-stellar objects, relevant tables of characteristics would include such things as the orbital elements, mass, surface temperature, composition, and so forth. In particular, the description should also include “epoch.” Using “mass” alone, one might argue that there are only four planets, Jupiter, Saturn, Uranus and Neptune - under this sort of scheme the notion of a “dwarf planet” would surely include the Earth.

Historically, the problem with the word “planet” is its Hellenic derivation – a definition that originally included both the Sun and the Moon, but not the Earth. Once the Sun and Moon were dropped off the list, a later definition of “planet” then included the Earth, Uranus and Neptune; then Ceres, Pallas, Juno, Vesta, Astraea, Hebe, Iris… Oops; then Pluto; then Chiron was discovered; then Charon; then TNO’s; then extra-solar planets (including the "Hot-Jupiters"), and interstellar rogues(?).

I would like to argue that the Moon is a planet because its orbit is always concave towards the Sun, despite the fact that the barycenter of the Earth-Moon system resides inside the Earth. Such a suggestion would reinstate part of the cultural Hellenic definition of "planet." Another suggestion, perhaps, would be that the Earth-Moon and Pluto-Charon are examples of “binary planetary systems,” a notion based on their “roundness,” "differentiation" and “relative masses.” For the gas giants, the total mass of their satellites is circa 1/10,000 the mass of the primary. Using natural mass divisions alone, Earth-Moon and Pluto-Charon form distinctive solar-orbiting sets.

In physio-chemical terms, the only things of consequence are mass, composition (mineral assemblages, atmosphere, and etc.), whether the body is differentiated (e.g., atmosphere, crust, mantle, core). In this case, whether the body has a solar orbit or is a satellite is not really important. Natural lists might be “rocky crustal bodies” (Mercury, Venus, Earth-Moon, Mars, Vesta (?), Io); “gas-giant bodies" (Jupiter, Saturn); "ice-giant bodies” (Uranus, Neptune); “icy crustal bodies” (Ceres, Europa, Ganymede, Callisto, Saturn’s satellites, Chiron, Pluto-Charon, and etc.) – however the tables of characteristics group themselves statistically. Compositions vary greatly in the oxygen-rich solar system (oxygen > carbon in the solar nebula), and are likely to be quite peculiar in other star systems (e.g., see Sara Seager's wonderful article in Sky and Telescope, January 2008, pp. 22-25).

If “epoch” or “age” are accounted for, then such things as whether a “planet” has cleared out its orbit (essentially impossible for Trojans), whether it has not yet fully formed in a young stellar disc and so forth become irrelevant. All things evolve, and all data are “snapshots in time.” In forty years as a geologist, I have seen classification schemes change for all kinds of geological phenomena that were formerly called something else – that’s natural in the “observational sciences” – ideas evolve as data increases.

For what it's worth, my suggestion for the planetary system would be Mercury, Venus, Earth-Moon (binary), Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto-Charon (binary), Eris, plus whatever else is found. This “might” be further divided into the disc-population bodies (Mercury, Venus, Earth-Moon, Mars, Jupiter, Saturn, Uranus, Neptune), and scattered-disc population bodies (Ceres, Vesta (?), Chiron, Pluto-Charon, Eris, and so forth) - with suitable adjectives or adjectival phrases.

Is Pluto a planet? Absolutely! Pluto-Charon is the first to be discovered scattered disc icy binary planetary system, as the Earth-Moon is the first to be discovered disc-population rocky binary planetary system (in the current epoch). Moreover, there should only be one “moon,” the Moon. All other “moons” should be called “satellites.”

Best Regards,

Hadyn Butler

Charon is a planet because it has a mass that is 11% that of Pluto and orbits far enough from Pluto that the Pluto-Charon barycenter is outside of Pluto? Ganymede is not a planet since Jupiter is relatively too large? If Jupiter was large enough to be a small star, making the solar system a binary star system, Ganymede would be a planet? This is the thinking that I didn't like about the original 2006 IAU proposal.

The Sun-Jupiter barycenter is outside of the Sun and thus the Sun orbits a barycenter just above its surface. Yet, Jupiter is not a star. If Jupiter orbited where Mercury is, the Sun-Jupiter barycenter would be inside the Sun.

Should Sirius B be reclassified as a brown dwarf since it is small compared to Sirius A? Stars are not defined as being a star based on what surrounds them or relative masses.

From 1610 until the discovery of Hyperion in 1848 (238 years), all known satellites were spherical. Even in the 1868 book, "Smith's Illustrated Astronomy", satellites were called secondary planets. Perhaps it is time to officially define a "major satellite" as a satellite that "has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape". I doubt the public cares how many 1km rocks are orbiting the gas giants.

I still prefer to classify planets as either special (the 8 major dynamical bodies), or as a very inclusive group including spherical secondary planets.

-- Kevin Heider

There are many problems with the IUA's definitions.

Both sets of definitions are scientifically incorrect. In the first run, Pluto is not a planet because it doesn't clear its own orbit. Neither does Neptune, Jupiter, Mars, or the Earth. Also, the first run of definitions is ONLY for our Solar System. That's like saying that one plus one equals two, ONLY in our Solar System. Definitions for science should be universal.

To say that a dwarf planet is not a planet is absurd. Dwarf stars are still stars, and dwarf galaxies are still galaxies. So therefore, dwarf planets are still planets.

And lets not go into Plutoids; how confusing and convoluted that makes the whole thing.

As far as the IAU vote. The first run in 2006 was actually illegal, according to the IAU's own set of By-laws. The resolution was NOT posted on the web site for months before, as their laws say. And the final version was only written the night before the vote. And 237 members out of 10,000 does not consstitute a majority, not in this solar system or any other.

The IAU has lost touch with reality. As a Planetarium Director and Professional Astronomer, I have no intention to let a bunch of confused so called scientists change this simple fact: Pluto IS a planet !!!!

I completely agree with Alan Stern that a single object can have a dual identity. As a matter of fact, I defended that thesis regarding Pluto in an article in a philosophy journal, which you can find at the URL I've given. Keep up the good work, Alan! -- Joel

I like what Scott Rafkin said. The spectrum of sizes and functions is wonderfully overlapping! Let Jove be Jove with some trojans that are not swept up. Stars orbiting other stars act like planets. What we said was a 'fixed star' circles the galactic core. We might find moons that are in the habitable water zone; we have moons that are like 'asteroids' while 'asteroids' are no more 'star-like'.
We have extra-solar Joves with twice the mass that are what: little brown stars! Far out! I'm waiting for the BigBangers to realize that the Name Creator presupposes a Creation. Both have always existed --one is unchanging and one continuously changes. Think big or think little: The Universe has a place for you. It started infinitesimally small and could be "infinite in range and deathless in duration".