Thursday, March 12, 2009

"Pluto a planet" poll

Should Pluto be considered a planet?

Yes....9 [90%]
No....1 [10%]

Just a guess, but I would bet that those voted "Yes" are lay persons and not bona fide astronomers. Such controversy. Simply put...if the astronomers wish to redefine the characteristics of a planet and thus change Pluto's status--that's fine, but for the populace, Pluto can remain a planet.

"Naming Pluto"--a film


Pluto...a reader's contribution

Pluto not fitting definition

"The Pluto Files" by Neil deGrasse Tyson

Tyson, Weintraub, Tombaugh & Pluto

Tyson's interview with "Time" on Pluto and planets

What's in a name?--Pluto


Laurel Kornfeld said...

But it's not clear that astronomers wish to redefine the characteristics of a planet or change Pluto's status. The astronomy community is still very divided about this. Also, consider the possibility that those who voted are either well informed lay persons who have spent time researching this topic and/or amateur astronomers, who can be as knowledgeable as professionals.

Mercury said...


That's certainly true about the layperson, but those people are not professionals whose academic pursuit and income is based on the field of astronomy and who are part of a community of professionals that define the parameters of their field scientifically and by a committee of experts. Those definitions are part of tools by which they do their business and I bet when they go home they refer to Pluto as being a planet...closet Plutonians? Both perspectives can reside comfortably side by side. It is like I wrote before..."aspirin" is "aspirin" to the layperson, but the chemist, pharmacist, and doctor know it as "acetylsalicylic acid".

Can you tell me the current definition of a "planet" and specifically, from that definition, that exempts Pluto? Isn't the idea of redefinition to reclassify Pluto based on size and orbital behavior and to sort out other bodies in the region? A stable classification would be advantageous to the astronomer despite the public status of Pluto in common lore.

Laurel Kornfeld said...

I was actually referring to professional astronomers, many of whom continue to reject the IAU planet definition that excluded Pluto, as can be seen from this petition: . Notably, one of the signatories is Dr. David Rabinowitz, co-discoverer of Eris.

On Tuesday night March 10, the American Museum of Natural History hosted the Isaac Asimov Memorial Debate, which this year was about Pluto and the definition of planet. All the panelists were professional astronomers; half reject the IAU decision outright, and even the dynamicists believe it is problematic. There seemed to be a consensus that it might be too early in the state of planetary science to be establishing a concrete definition. Panelists also agreed that what constitutes a planet is a spectrum ranging from small round objects to ones just below the threshold to begin nuclear fusion and become stars.

I'm not sure I understand your question, "Can you tell me the current definition of a 'planet' and specifically, from that definition, that exempts Pluto?" Are you looking for an explanation of the IAU definition or an alternative definition put forth by scientists such as Dr. Alan Stern?

Objects can be sorted based on size and orbital behavior within the broader parameter of the term "planet," with planet indicating any non-self-luminous spherical object orbiting a star. As Dr. Mark Sykes notes, "that's why God invented subcategories." I have no problem with using the term dwarf planet, which was actually coined by Dr. Alan Stern himself. The problem is in saying that dwarf planets are not planets at all. How would it be any less of a stable classification to establish dwarf planets as a subcategory of planets that are planets because they are spherical and orbit the sun but of the dwarf subcategory because they do not gravitationally dominate their orbits?

Mercury said...


I hope you didn't throw a shoe at the 2009 Isaac Asimov Memorial Debate moderator Neil deGrasse Tyson. So half of the panelists [Gibor Basri, David Jewitt, Jack Lissauer, Sara Seager, Alan Stern, Mark Sykes] rejected "...the IAU decision outright" that correct? Of the 9,570 plus ISU individual members four were represented on the panel [Basri, Lissauer, Stern, and Sykes] that correct? Do you know what three rejected the definition? I am just curious. That " might be too early in the state of planetary science to be establishing a concrete definition." may be correct but one has to start somewhere. I think that's the way science functions...start with a definition and as new data accumulates, changes may be instituted. As a case in point is a current debate considering the sun's core. The traditional model suggests a gaseous hydrogen core. But that has been challenged, and somewhat persuasively, by Dr. Oliver Manuel who believes that the sun's core is iron. Changes in science models are always changing and being challenged. Remember the challenge made of the Ptolemy cosmology by a heliocentric cosmology?

Laurel, the bottom line here is a struggle for a workable [for astronomers] and by committee definition...what massive bodies fit into an agreed definition. And I kind of get the feeling that if the classification is too broad there will be too many planets and that maybe astronomers won't accept. It may be agreed that a body's orbital preference be dropped as a criteria or establish a cut off figure regarding mass. That's what the IAU is supposed to discuss and adopt. They are the ones to make definitions for astronomers. The populace can still cherish Pluto as a planet. I do but I don't get upset over astronomers reclassifying Pluto to suit their scientific needs.

Laurel Kornfeld said...

No shoes were thrown at the debate, which was actually very congenial and friendly. All the speakers including Tyson presented a good mix of humor and serious discussion. David Jewitt had to cancel at the last minute, and Steven Soter took his place. Stern and Sykes strongly opposed the IAU decision, and Basri pretty much agreed with them although his concern is with the upper end of planets--brown dwarfs and sub-brown dwarfs. Basri and Seager study exoplanets, and neither was happy with a planet definition that addresses only our solar system, noting we might find very different arrangements in other systems. Soter and Lissauer, both dynamicists, emphasized the distinction between the eight gravitationally dominant planets and everything else in the solar system but were critical of the IAU wording, specifically the choice of the term "clearing the neighborhood of its orbit." They also were critical of the process by which the decision was reached. It seemed like a lot of the panelists agreed with Stern that voting is not a good way to decide things in science altogether, that ideas should rise or fall over time in an organic process.

I don't know if Soter and/or Seager are IAU members.

Even Tyson seemed reconciled to the notion that this debate is not over and will likely continue for quite a while. One of the panelists noted that fifty years from now, at a debate on this same topic, people might look back in amazement about how little we knew back in 2009 and how decisions were made with a very limited knowledge base.

Tyson expressed your concern about a planet classification being too broad, but Stern and Sykes pointed out that use of multiple subcategories could easily alleviate that situation by using adjectives to distinguish the different types of planets. There didn't seem to be concern about having too many planets, just about having a definition that is too broad to tell us anything specific about them.

I still take issue with your treating the IAU vote as a decision by astronomers as though it were uncontested and a done deal when that is not the case. Sykes and Stern clearly took issue with the concept that the IAU are the ones to make definitions for astronomers, emphasizing that many professional astronomers have rejected the IAU decision. Stern got laughs when he referred to the IAU as the Irrelevant Astronomical Union. I think it is crucial to recognize that the IAU decision is not representative of the entire community of professional astronomers, but only of one part of it. Sykes mentioned other, larger groups of planetary scientists such as the American Geophysical Union, that are conducting their own discussions on the definition of a planet with the presumption that the issue is far from settled.

Mercury said...


As I understand it, the IAU draft resolution has recognized eight traditional planets [Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune] and three "plutons" [Pluto, Charon, and UB313]. Apparently, the IAU is not the ultimate arbiter for as Robert Williams [Vice President of the IAU] wrote: "Statements of scientific importance are expressed by the IAU in resolutions of the General Assembly. Although resolutions are non-binding they do represent the consensus scientific judgment of the members, and are arrived at by a process that involves member input and debate." The 2006 resolution [B5] gives this the definition of a planet: "A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit." And that "Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of Trans-Neptunian Objects." I draw from this that Pluto has been definitely demoted by the IAU's definition and that, and more importantly, the IAU's resolution is not binding.

I am beginning to lose the focus of your concern. Would you state succinctly what it is you desire. Do you want Pluto's status reinstated [thus initiating a new definition of what a planet is]? And why is that important? I have yet to read your thoughts regarding what I have suggested in that Pluto may remain in popular lore while a more scientific status of Pluto is quite legitimate for professional astronomers.

Laurel Kornfeld said...

The 2006 planet definition resolution you quote is actually 5a. Resolution 5b, which failed by 333-91 (again, remember, no absentee voting was allowed) would have established both "classical planets" (the eight you mentioned) and "dwarf planets" under the broader umbrella of planets. Unfortunately, that resolution failed, leaving the IAU with a definition stating that dwarf planets are not planets at all.

Resolution 6a was the one that established Pluto as a prototype of a new category of trans-Neptunian objects. Resolution 6b, which proposed calling these objects "Plutonian objects," failed. An earlier proposal to call them "Plutons" was scrapped after geologists objected because that term is already in use in geology.

What I want to see is the adoption of a new planet definition in place of the one adopted in 2006, specifically one similar to resolution 5b, that distinguishes between gravitationally dominant planets and the smaller dwarf planets but still recognizes both categories as planets. I would also like to see the IAU allow absentee voting so all 10,000 of its members can participate even if they are not in a particular room on a particular day. It would also be useful for a caveat to be inserted noting that any definition of planet may be modified in the future based on new data and discoveries.

I prefer the above scenario to your suggestion that Pluto remain a planet in popular lore but be defined differently by astronomers. I don't believe that demoting Pluto from planet status is a more scientific definition. The issue is important for educational purposes, communicating astronomy with the public, and having a definition as accurate as possible. To that end, the criterion of hydrostatic equilibrium is crucial. Denying any difference between small planets in hydrostatic equilibrium and shapeless asteroids that have not attained this state blurs an important distinction and is therefore a disservice to everyone. I believe very strongly in keeping the term "planet" as broad as possible and then using subcategories to distinguish the many types of objects that fall under this umbrella.

Mercury said...


Clarification of resolutions noted.

"I don't believe that demoting Pluto from planet status is a more scientific definition." Why not? For the professional astronomer is would be a more refined definition and classification.

"The issue is important for educational purposes, communicating astronomy with the public, and having a definition as accurate as possible." I don't think it is that relevant. Students are smart and can understand a popular acceptance and a more scientific definition. Furthermore, this is just one small part of teaching astronomy...there is the whole universe. Surely educational venues wouldn't hinge on Pluto's status.

"I believe very strongly in keeping the term "planet" as broad as possible and then using subcategories to distinguish the many types of objects that fall under this umbrella." I suppose that would work.

Maybe the whole IAU should restructured. Maybe the citizens of Earth should make the decisions for an accurate definition. Nope, that would be foolish.


Laurel Kornfeld said...

Here is an article by Dr. Alan Stern on the scientific rationale for keeping the term planet broad and then distinguishing types of planets through use of subcategories:

Mercury said...


"Planetary scientists must care about planet definition because without knowing what a planet is, a planetary scientist cannot describe what the field’s central objects of study are."


I might ask what your definitions of a "planetary scientist" and an "astronomer" are. It appears to me that a "planetary scientist" "is" an astronomer working under the disciplines of astronomy [physics, chemistry, etc.] including definitions established by the IAU and especially here regarding a working definition of what constitutes a planet.

"Like many of my planetary science colleagues, I consider a planet to simply be any natural object in space that is large enough to be rounded by self-gravity. It therefore behaves as a body whose shape is dominated by self-gravity, rather than by the body’s own mechanical strength, as with rocks and asteroids."

Well, that's fine for him, but he is not the one who makes the definitions codified and he may have many who believe his definition too.

"And finally, because educators have in this classification revolution an utterly inspiring, teachable moment emanating from the debate. And what is that? It is that science adapts to new data—such as the prevalence of dwarf planets—rather than running from it, and finds its way to better paradigms, not by fiat or voting but by reasoned debate that converges to consensus."

Okay, I have no problem with is a dialectic and stacked with models that do change--like the one being considered involving the sun's core.

I am not going to change my position for I feel that a popular stance is justifiable as well as a totally different definition for astronomers that is subject to modification.