I think that the Planet Definition Committee has worked hard and thoughtfully and deserves our gratitude, at the very least for clasifying the issues involved. Unless I hear some very strong counter - arguments in the next ten days, I shall vote for the resolution. I would however, like to make two brief comments.
First, many words have different meanings in scientific and general use. For all physical scientists the words "force" and "energy" have quite district and well-defined meanings. In general discourse they stand for much vaguer concepts and can even be used as synonyms. We should not get too upset, whatever we decide about the status of Pluto, if the general public continues to use the term "planet" in a sense different from our definitions. Probably even many astronomers, myself included, will not change a liketime habit of talking about "minor planets" because of a resolution passed by the GA.
My second comment is to point out a little irony. Our presently favoured theories of the origin of the solar system developed from an idea published by Immanuel Kant over 200 years ago. Among professional philosophers Kant is much more famous for his insistence that we impose our categories on Nature and cannot know the ding an sich - the thing itself. If the definitions are important to the specialists, by all means make them, but do not fall into the error of believing we are talking about reality. I like to think that during the next few days, somewhere in the Elysian Fields, the spirit of Immanuel Kant will be enjoying a hearty chuckle at the seriousness with which we are taking ourselves!
Recent discoveries of objects in the outer reaches of our Solar System have forced scientists to reconsider what it means to be a planet. The International Astronomical Union (IAU) has proposed a new definition of a planet as a celestial body whose gravity is strong enough for it to be nearly round in shape, and which is in orbit around a star but is itself neither a star nor a satellite of a planet. According to this definition, the nine traditional planets in our Solar System would be joined by Ceres (the largest of the asteroids), by Charon (Pluto's largest moon), and by 2003 UB313 (the provisional name for a recently discovered object larger and more distant from the Sun than Pluto). Pluto and Charon would be regarded as a double planet, rather than as a planet and satellite, because their center of gravity lies outside of Pluto itself (the only such case known in our Solar System). There is a candidate list of additional objects that may be large enough to qualify as planets, subject to confirmation by the IAU.
The IAU resolution also recognizes Pluto as the prototype of a new class of planetary objects to be known as "plutons." In contrast to the classical planets, plutons typically have quite non-circular orbits and take more than 200 years to orbit the Sun. With increasingly sensitive and broad searches of the outer solar system well underway, it is quite likely that additional Pluto-like planets will be discovered.
The Division for Planetary Sciences (DPS) of the American Astronomical Society is the world's largest international professional society of planetary scientists. The DPS Committee, elected by our membership, strongly supports the IAU resolution. It was proposed after two years of careful review by an international panel of expert planetary scientists, followed by a broadly representative international group of historians, writers, and scientists. The new definition is clear and compact, it is firmly based on the physical properties of celestial objects themselves, and it is applicable to planets found around other stars. It opens the possibility for many new Pluto-like planets to be discovered in our Solar System.
The proposed definition will be brought to the IAU General Assembly for a vote on August 24, 2006. As representatives of an international community of planetary scientists, we urge that the resolution be approved.
After long consideration, I have come to several conclusions with regard to the new proposed definition of planets that I would like to share with the IAU community. They are outlined below.
The following proposal was presented at the Division III meeting on Friday 18, with an attendance of over 100 IAU members. After an hour of discussion, there was a show of hands to know the opinion of Div. III members. Approximately 20 members voted for the proposal endorsed by the EC, 50 members for this new proposal and 10 abstentions.
There has been a long discussion about what a planet is. This problem appears at both ends: for the very massive bodies and for the smaller ones. At the large end, the limit seems to be clearer; it is now widely accepted that planets must not generate any energy from nuclear fusion, while brown dwarfs generate some nuclear energy from the fusion of deuterium. More problematic is the small end. We think that the definition should be kept as simple as possible and based on physical and cosmogonic reasons.
There is a wide consensus that planets formed by the accretion of small bodies – the planetesimals. The accretion process led to the formation of embryo planets that, as they grew in size and acquired more powerful gravitational fields, went to a process of runaway accretion in which the size of a few of them detached from the rest of the bodies of their neighboring zones. Given the powerful gravitational fields of these massive bodies - that we can call at this stage protoplanets - they were able to clean the population that had close encounters with them. The bodies interacting with the protoplanets were finally incorporated to the planets or scattered to other regions.
From a cosmogonic point of view, it therefore makes more sense to consider a planet as an object that acquired a mass large enough to clean a zone around its orbit. According to this definition, only eight planets, Mercury (perhaps marginally), Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune fulfill this condition. It is obvious that, at least for our solar system, this cosmogonic definition implicitly carries the condition of objects with a roundish shape determined by self-gravity.
From our definition, Pluto, Ceres and other large Trans-Neptunian objects in quasi-hydrostatic equilibrium  should be not considered as planets, since they never were the dominant bodies in their accretion zones. It is suggested that Pluto be kept unnumbered by historical reasons.
Is may be possible that in the near future cases of objects not foreseen at present could appear beyond our solar system, as for instance free-floating planets, stray planets, or double planets. We think that we should not advance definitions at this point for these exotic cases and leave their discussion when if they became a part of the observed world.
 From our present knowledge of the Solar System, we know that objects as small as Mimas (D~400km) are roundish. If this were the lower limit for an icy body to be in hydrostatic equilibrium, then we would already have several tens of bodies fulfilling this requirement.
(1) A planet is a celestial body that (a) is by far the largest object in its local population , (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape , (c) does not produce energy by any nuclear fusion mechanism .
(2) According to point (1) the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane are the only planets of our Solar System. All the other objects in orbit around the Sun are smaller than Mercury. We recognize that there are objects that fulfill the criteria (b) and (c) but not criterion (a). Those objects are defined as "dwarf" planets. Ceres as well as Pluto and several other large Trans-Neptunian objects belong to this category. In contrast to the planets, these objects typically have highly inclined orbits and/or large eccentricities.
(3) All the other natural objects orbiting the Sun that do not fulfill any of the previous criteria shall be referred to collectively as "Small Solar System Bodies".
 The local population is the collection of objects that cross or close approach the orbit of the body in consideration.
 This generally applies to objects with sizes above several hundreds km, depending on the material strength.
 This criterion allows the distinction between gas giant planets and brown dwarfs or stars.
 This class currently includes most of the Solar System asteroids, near-Earth objects (NEOs), Mars-, Jupiter- and Neptune-Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs), and comets.
List of adherents to the above proposal:
|Julio A. Fern?ndez||Uruguay|
|Mario Di Martino||Italy|
|David Vokrouhlicky||Czech Republic|
|J. E. Arlot||France|
An unpublished paper on the history of the planet definition discussion, based on a talk in a conference in Munich, Germany, 2003, and written in July 2003. Hence the most recent research results (e.g. "Xena") are not yet included.
gropp.pdf [PDF 86 kB]
The Division III discussion on the draft IAU Resolution 5, intended scientifically to define the word "planet", was at times heated, amusing and intellectually informative. A large majority of those present sought to abandon the draft resolution in favour of an alternative proposition still to be precisely defined, but which included a phrase to the effect that a true "planet" should be by far the most massive body in its part of the solar system. An overwhelming majority were in favour of continuing the traditional numbering and naming conventions for newly discovered "planetary" objects, i.e. among the lists of minor planets, as for example Ceres and Sedna. The stage is set for an interesting week. Will beer and sandwiches in the smoke-filled rooms of the Resolutions Committee be enough to sustain the principal participants in their efforts to square this awkward circle of planetary nomenclature?
Stripping the argument to its essence, the key issue is whether the word "planet" can sensibly be defined entirely in terms of a small solar system body's intrinsic properties, as in the word "star" (a stellar object undergoing nuclear burning), or whether extrinsic factors play an important role in finessing an object's precise status. In fact, as in distinguishing the Moon from Charon, the inclusion of extrinsic factors has been accepted. Secondary factors are how to draft unambiguously a definition that builds on the history of planetary nomenclature in the solar system, and is broad enough to encompass the new solar system discoveries and the wide range of new planetary types observed around other stars and stellar remnants (and possibly around brown dwarfs as well), and be inclusive of the further planetary objects that will eventually be found orbiting the Galaxy independently, perhaps as orphans from a parent star, floating freely in interstellar space.
Everyone agrees that the Sun has eight principal planets. The question that has exercised most people has been the status of Pluto. Years ago, when Ceres was the first (and only) asteroid in the solar system, it was assigned planetary status. Soon after, following discovery of the second, third and fourth minor planets, it became clear that Ceres was not alone. In redefining Ceres as a minor planet (the word "asteroid" was deliberately derogatory) nineteenth-century astronomers implicitly recognized what modern cosmogonists have known for years: an object's influence depends on its size and the company it keeps.
The same story has been repeated with Pluto. Soon after it was found, in 1930, Armin O. Leuschner, the first graduate student of Lick Observatory and a founding member of the Astronomical Society of the Pacific, was able to write: "What shall we conclude ... if the future shows that Pluto, as is the case with comets, has no mass sufficiently appreciable to affect other bodies in the solar system? It may then have to pass into the class of objects known as minor planets, the largest of which, Ceres, has a mass considerably less than one one-thousandth that of the Earth. But we are not quite so sure any more whether we should call an object a comet or a minor planet, even if the object shows all the characteristics of the former and none of the latter. ... I think the implications I have in mind are clear..."
While Pluto was a singleton, it could reasonably be regarded as a planet. But now, when we know there are at least 100,000 icy objects in the near trans-Neptunian zone with diameters larger than 100 kilometres, and at least a million similar-sized objects slightly farther out (not even considering the Oort cloud), the position is far less clear-cut. In the face of this evidence, to count Pluto as a planet and to give a similar status to the other large icy objects orbiting in the same part of the solar system, seems perverse.
The draft IAU resolution introduced the word "pluton" to describe the largest of these objects, which are presumably very little different from the large satellites of the four major planets. However, the similarity of this word to that for "Pluto" in some other languages is confusing. "Planetoid", or "dwarf planet" are alternatives, though to find an appropriate name misses the point. Whatever they are called, and whatever mass/diameter range is finally assigned to the term "dwarf planet", where do we logically draw the line between these objects and minor planets (and perhaps even comets)? Meanwhile, there is the practical difficulty of introducing a new category of planet when so many so-called dwarf planets (Ceres, Sedna etc.) have already been numbered and named within the framework of the existing catalogue of minor planets.
The criteria of size relative to its neighbours, and of orbital stability and environment ("the company one keeps") are clearly key factors in determining whether a new solar system object should be counted as a planet or a minor planet. To paraphrase Leuschner, the implications with respect to Pluto should be clear.
Putting the planets of the Solar System according to their increasing(or decreasing) eccentricities, the following distribution is obtained.
e2 e8 e3 e7 e5 e6 e4 e1 e9
(e1=Mercury eccentricity e2=Venus eccentricity ......... e9=Pluto eccentricity)
We notice then, in this succession, the eccentricities of the planets are distributed around Jupiter eccentricity in four couples. From this representation some equations can be drawned:
_______________ \/ (AU1+AU9) / AU5 = e7 / e3 ( AU1=Mercury AU9=Pluto AU5=Jupiter ) _________ \/ e7 / e3 = e4 / e6 _________ \/ e4 / e6 = e8 / e2
The three equations can be made more compact in this way:
___________________ \ / __________________ \ / / __________________ \/ \/ \/ ( AU1 + AU9 ) / AU5 = e8 / e2 or ( ( ( e8 / e2 )2 )2 )2 = ( AU1 + AU9 ) / AU5
The equations put together, with differences of some millesimals, all nine planets of the Solar System. In this equation Pluto is necessary.
Then to be a planet it's not so important the shape, structure or something else but to belong to an equation.
|Numerical values:||AU1(Mercury) = 0.387||AU9(Pluto) = 39.440||AU5(Jupiter) = 5.203|
|Eccentricities:||Uranus ( e7 ) = 0.047...||Earth ( e3 ) = 0.017...|
|Mars ( e4 ) = 0.093...||Saturn (e6 ) = 0.056...|
|Neptune ( e8 ) = 0.009...||Venus ( e2 ) = 0.007...|
The International Astronomical Union resolves that for purposes of assigning naming responsibility to the appropriate IAU committee and for application of the appropriate rules for cartographic coordinates, planetary status shall be assigned to newly discovered Solar System bodies with negative absolute magnitudes.
For reference, absolute magnitudes of various Solar System bodies:
With this criterion, the Solar System currently consists of four rocky planets, four larger gassy planets, and three smaller icy planets.
Despite the furor surrounding the planet definition debate, there are two remarkable areas of agreement that should be empahsized:
The problem comes in the transition between these two classes. It spans an order of magnitude in size range from ~500 km diameter to ~5000 km diameter. In our solar system objects in this range are all in orbits that are shared by a large ensemble of similar objects (i.e. the asteroid belt and the Kuiper belt). Defining objects in this size range to be dwarf planets recognizes the transitional nature of this group of objects. They are inherently more complex than smaller objects, particularly at the upper end of the size range where atmospheres, internal differentiation, and other "planetary" processes are possible. It also retains sufficient ambiguity in the terminology that hardcore partisans on either side of the question will be only partly satisfied. That, I submit, is the sign of a good compromise.
Whether to define the dwarf planet/planet boundary as a simple and somewhat arbitrary diameter or to make a more complex environmental distinction for each object on a case-by-case basis remains to be determined. As we do that, it will be good to keep in mind that the next decade is likely to yield the discovery of transneptunian objects significantly larger than Pluto as well as a continually growing number of extrasolar planets. A successful definition will be able to accomodate these new discoveries without modification.
It can sometimes be instructive to observe how other scientific fields have dealt with definition problems. In geology the question of what is a boulder has a specific answer that will surprise many: Anything larger than 256mm in diameter is considered to be a boulder according to the commonly used Udder-Wentworth scale . However, there are other competing systems and no general agreement between them .
Our geologist and geophysicist friends appear to be living happily with inconsistent taxonomy. I'm off to find some boulders to climb...
When did you first became fascinated by astronomy? Was it as it was for me, when you were five years old? Maybe when you were ten? And what was it that fascinated you about it? Was it that the Earth is just one of nine planets? The idea that there might be life on another world? The idea that those dim little points of light we see at night, the stars, are really just the same as the Sun? Or was it somehow that there is a reality beyond the Earth and the sky that you can talk about, wonder about and understand? For most of us, our fascination for astronomy was forged when we were very young; the result of an adult, an older brother or sister, or a book that lit the fire that drives us even as grown-ups. Making sure that the children of this and future generations will continue to be fascinated by the Universe is a duty we all have as astronomers. This duty is even greater toward those children who will not become astronomers, since they are the ones who support our science with their fascination, their actions as government ministers and as private donors.
I think that this truth has an important bearing on the recommendation before us to remake the definition of a planet. I am against this proposal because if it passes, I can no longer explain to my six year old son, who is passionate about astronomy, what a planet is. What meaning does hydrostatic equilibrium have for him? I can tell him that planets are round, of course, but how do I explain to him that Charon is a planet and the Moon, is, well, a moon? And why are there 12 planets this year and perhaps 15 next year, even though the newest three planets are already known? Will he, and millions of other schoolchildren around the world have to understand what a barycenter is before they can understand what a planet is?
I reject the argument stated in the first sentence of the resolution that it is important that our nomenclature reflect our current understanding. If that were the case, why do we still call flaring stars novae when we have known for centuries that they are not "new" stars. And are all nebulae actually clouds? There may in fact be something more important than precision for our astronomical definitions, and that is clarity. Anyone, even a six year old can understand what a planet is as we currently understand it. When he or she gets older we can provide the necessary nuance to describe the wonderful richness that our recent observations have uncovered about the solar System.
Astronomers stick to using magnitudes rather than fluxes, and a crazy lettering scheme for the spectral classes of stars for historical reasons, rather than for any truly rational one. And it is this long, historical tradition that adds richness to our field of study. I believe this even though I sometimes bemoan our use of magnitudes and other arcana when I teach them to my students. There is something to be said for keeping to a simple system that everyone, children and laymen, knows and is comfortable with and is not subject to yearly revision. The nine planets have been with us since 1930, more than seventy-five years, and we should keep them as is, even if we eventually wind up calling them the nine classical planets. We all lose something if the IAU adopts Resolution 5, because we will have made astronomy more distant from all of the people on whom we depend with almost nothing gained, as far as I can see. Three cheers for history and clarity.
The problem of definition in astronomy is twofold. An ever-increasing number of objects need to be named or catalogued, and categories are perceived to be necessary in order to arrive at some sort of understanding of the physical characteristics of the objects in a given class. Before proceeding further, it is useful to distinguish clearly between nomenclature (the naming of objects and features) and terminology (in the case of planet largely to do with classifying objects into types). Issue 3 of Dissertatio cum Nuncio Sidereo (pp. 3-4) makes it clear that no such distinction has been made in the deliberations of the IAU Planet Definition Committee. What seems to be the object of the current discussion is the definition not of the word but the concept of planet. It is the common lexical error of confusing the definiens (the definition) with the definiendum (what is being defined).
While it is true that in the past the nomenclature (naming) of planets and constellations has been fraught with difficulty, it is nevertheless the case that ,once agreement has been reached internationally, the names have tended to stick. Since its inception in 1919, the IAU as the body charged with the task of naming new celestial objects and planetary features has deftly minimized potential social, political and nationalist objections to the names it has chosen, with the result that all now accept its authority in the naming of celestial objects.
Unfortunately, astronomers are not in the enviable position of, say, zoologists, who have elaborated a hierarchical nomenclatural structure of phyla, classes, orders, families, genera and species based on detailed observations of specimens in the laboratory. In astronomy, we do have a sort of hierarchy of objects ordered more or less according to mass, but we possess little concrete knowledge of the boundaries between the different kinds of objects and we lack anything like a full inventory of the Cosmos. On the largest scales, we now conclude that observable (i.e. known) matter amounts to a mere 5% of the total mass of the Universe, which, so it seems, is being driven to accelerate its expansion by a mysterious dark energy. Closer to home, the once so simple Solar System we now find to be peopled, not only by planets, satellites, asteroids and comets, but also by TNOs and the newly posited plutons. And we are by no means certain that the Solar System bestiary is now completely accounted for. Astronomers today resemble the early botanists and zoologists that accompanied the European voyages of discovery, each sky survey and interplanetary probe reaping a fresh harvest of hitherto unknown "species".
The problem of classification is severe in astronomy. We derive the apparent properties of objects that are usually inaccessible directly on the twin bases of imperfect theory and biased observations. We strive to distinguish among categories always keeping in mind that new theories and better observations will most assuredly lead us to shift the boundaries between categories, or even to abandon the entire system of classification altogether as different criteria evolve from new results. So incomplete is our inventory of the Cosmos that our nomenclature cannot safely go beyond the naming of objects: we are far from evolving a hierarchy of objects (if indeed, that is a genuinely physically meaningful thing to do on a universal scale). The discovery of exoplanets has further muddied the waters as far as our understanding of planetary systems is concerned and has introduced the theoretical nightmare of planetary migration. We have, it seems, much knowledge of individual objects but little understanding of underlying physical principles as far as the evolution and taxonomy of planetary systems goes. Each new planetary discovery and survey, far from confirming our preconceptions, has often produced complete surprises and has caused us to modify our earlier ideas quite drastically. To use a zoological metaphor, with the little information we have so far, we are still caught up in the process of identifying species and are nowhere near being in a position to decide on genera or higher categories. And that broadly is the problem that presents itself with the definition of the concept of planet.
In these circumstances, attempts at setting up boundaries among the various planetary species is largely vitiated by our lack of knowledge of those species that remain still to be discovered and that might substantially alter in the fairly near future whatever we decide on today. Quite apart from attempts to build a two-dimentional nomenclatural structure for planetary bodies (such as that devised by the IAU Planet Definition Committee), decisions taken in ignorance of what else might be out there would only serve to trammel our thinking and possibly hinder our appreciation of the true physical nature of Solar System bodies. It is probably therefore wiser to leave astronomers and planetary scientists with the freedom to define their own makeshift categories in order to ensure the proper debate of opposing ideas and the unobstructed flow of creative thought. Different uses of terminology tell us something about the way we do science. Confusion in terminology is normally a useful indicator of lacunae in our knowledge of what we are attempting to classify.
Let the IAU continue to do what it does best (nomenclature). The IAU is perfectly entitled to attempt to lay down the law on matters terminological, but astronomers are also free to vote with their feet by continuing to use terms in the way that suits their individual modes of thought. The best course is to withdraw all proposals for a universal, IAU-approved definition of planet and let current usage, based on active research, be the final arbiter when a sufficient knowledge base for a realistic inventory of Solar System planetary bodies eventually becomes available.
Agreeing a consensus on classification might seem like a good idea, but a consensus view based on structural ignorance will quick reveal serious deficiencies and inconsistencies. What, then, should we do about terminology in general and "planet" in particular? Doing nothing would clearly be unacceptable in the climate of this GA. Instead, I would suggest following a more positive and useful approach based on the lexicographical principle of making a statistical study the corpus of current usage (ADS is the perfect tool for doing this) and listing all current usages of the term "planet" and presenting a full – non-normative – report at the Rio de Janeiro GA. The problem must be tackled in a proper scientific and quantitative manner. This corpus-based approach will be adopted in defining the multiple past and present uses of the term "planet" in the Springer Dictionary of Astronomy, a descriptive work now being prepared based on actual rather than recommended usage of lexical terms in astronomy.
The present planet debate has uncomfortable parallels with the arguments of those medieval theologians who wrangled over how many angels could dance on a pinhead. However, although the underlying tenets of the theologians were questionable, to their eternal credit they were at least trying to be quantitative.
The definition of a planet can be made principally on the basis of its formation and orbit, or principally on the basis of its nature and structure. If the division between planets and smaller bodies is chosen on the basis of its formation and orbit, then the logical division is whether the object gravitationally dominates its orbital region.
However, recent discoveries of planets around other stars have emphasized the importance of dynamical alterations in planets' orbits in the final organization of extrasolar planetary systems. It is now believed that large numbers of (what might be considered) planets are ejected from extrasolar systems, due to interplanetary dynamics or to interstellar dynamics in clusters of stars. Free-floating planets have now been observed in the Orion Nebula. It is likely that some objects may at some times be satellites of other planets, and at other times circle their star by themselves.
Therefore I suggest that the most stable definition of a planet would be a physical definition, without a dynamical constraint. There are several cutoffs that are reasonable. Roundness is certainly one, but would lead to possibly hundreds of planets, especially if satellites of planets may also be planets. A more restrictive choice would be the possession of a substantial atmosphere throughout its orbit, kept in place by the gravitational field of the planet.
The definition of a "substantial" atmosphere allows some room for choice; I would suggest that an atmosphere with a pressure less than ~1% that of Earth's be the dividing line. This would produce the following 8 planets: Venus, Earth, Mars, Jupiter, Saturn, Titan, Uranus, and Neptune. The choice of the possession of an atmosphere would connect well to our cultural and historical understanding of a world; these planets will show weather, complex surface processes, and, in many cases, possibly could possess life.
Summarizing text of Daniel Fischer, what actually happened during the 26th GA IAU and what is the results of GA IAU vote about the planet redefinition, with lots of additional material (links to other reports, other opinions, etc.) can be found here: http://www.astro.uni-bonn.de/~dfischer/mirror/300.html.