1. David Morrison

Nasa ames research center

As a participant in the recent International Astronomical Union (IAU) meetings and votes on the definition of a planet, as well as previous debates on the same subject, let me note some of my thoughts on both the process and the result.

First, let's look at the actual new IAU definition of a planet in our Solar System. Note that the upper limit in size for a planet (distinguishing it from a brown dwarf star) had earlier been set by requiring that it must not have sufficient mass to derive energy from fusion reactions at any point in its evolution. To this upper limit we now add,

The IAU… resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:

(1) A planet is 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 neighborhood around its orbit.

(2) A dwarf planet is 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, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite [of a planet].

(3) All other objects orbiting the Sun shall be referred to collectively as Small Solar System Bodies.

The IAU further resolves that Pluto is a dwarf planet by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

This new definition is controversial, and not just among scientists. Such public interest is appropriate, because the definition is not primarily a science issue. No scientific study or mission has been waiting for this decision, and scientists can (and often do) use all sorts of jargon without official approval. The whole issue is of interest primarily because nonscientists, including school textbook writers, want a definition. Now they have one. It remains to be seen whether planetary scientists will adjust their terminology because of the IAU votes.

The process was highly convoluted. I was a member of the IAU committee of 19 elected planetary scientists who debated for months and finally adopted a definition by a vote of 11–8, in which Pluto remained a planet, along with any new objects discovered with a radius greater than 1000 km.

This proposed definition, which would eventually have led to several new trans-Neptunian planets, was not even on the table at the IAU. Instead, several versions were debated that were formulated by a committee (which I was not on) chaired by historian of science Owen Gingerich. Rightly or wrongly, many astronomers considered this process to be lacking in transparency, and they seemed to be unwilling to trust their representatives on either committee. During the IAU meeting, the Gingerich committee reported and alternate suggestions were made, so that the issue was in a constant state of flux. This process ultimately led to a public vote on a series of issues, with no amendments allowed. The vote was taken on the last day of the IAU meeting and involved fewer than 500 astronomers, which is only about 5% of all IAU members.

One of the major rifts evident in discussions at the IAU was between astronomers who study physical properties of objects and those who study orbits (dynamics). The dynamicists dominated at the IAU, and many of them would not accept any definition that was based solely on physical properties such as size. They rejected the analogy with stars, which are classified based on what they are, not where they are located (e.g., in a star cluster).

Some of the dynamicists noted the historical distinction of planets as “wanderers,” as well as the established distinction between moons (satellites) and planets, even though some moons (such as Ganymede and Titan) are larger than the planet Mercury. In contrast, many planetary scientists who study these objects as individual worlds prefer a definition based on the intrinsic properties of the object itself. The final compromise definition contains elements meant to satisfy both camps.

Is Pluto, then, still a planet? Yes and no. The answer is semantic, based on whether dwarf planets are planets, just as dwarf pines are pines and dwarf galaxies are galaxies. I would say that Pluto is a planet, but it is a dwarf planet, and the first example of the class of trans-Neptunian dwarf planets. Less defensible (to me) is the use of the term dwarf planet for Ceres, the largest asteroid. There is some precedent, because Ceres was considered a planet for the first decades after its discovery in 1800, and it fits nicely into Bode's law tables. But I suspect that Ceres will continue to be thought of by most scientists as the largest asteroid, not as a dwarf planet.

There remain many uncertainties in applying the new definitions. The lower limit in size for a dwarf planet is defined by its roundness, something that cannot be measured for trans-Neptunian objects when they are discovered. In practice, astronomers will probably need to adopt brightness as a proxy for the unmeasurable size, just as size is itself a proxy for roundness. (Large objects become round from the pull of their own gravity.) The meaning of “clearing its orbit” is also undefined, but in practice, it must permit both stable populations of companions, such as Jupiter's Trojan asteroids, and transient populations, such as the near-Earth objects (NEOs) that cross the orbit of Earth. The usefulness of the new definitions will depend on the decisions to be made in their implementation.

It has been apparent for at least a couple of decades that Pluto is much different from the eight “classical” planets. In particular, its mass is much smaller than that of other planets and also much smaller than had been estimated when it was discovered in 1930. Further, Pluto's orbit crosses the orbit of Neptune. If it were found today, Pluto would probably not be classified as a major planet. In addition, we now recognize Pluto as the prototype of a new category of trans-Neptunian objects of great interest to scientists. There are thus good arguments for distinguishing Pluto from the eight larger planets. Calling Pluto a dwarf planet is a reasonable choice of nomenclature, and it also makes sense to extend this same terminology to similar objects found in the outer Solar System, such as Eris.

We now have a reasonable definition of a planet, including the new category of dwarf planet. Why, then, does the public find this decision so controversial, especially in America? I suggest the following three reasons:

1. Bad connotations of the word dwarf. Although astronomers have a long tradition of using this word (e.g., dwarf star, dwarf galaxy), this is not a common word in general usage. To many people, apparently, dwarf has a negative connotation, which was not intended by the IAU. (There may also be some problems with translating this word into other languages.)

2. Affection for Clyde Tombaugh and the “poor boy makes good” story of his discovery of Pluto. This seems to be primarily an American reaction. Some of my non-U.S. colleagues suggest that much of the support for Pluto could be a form of U.S. nationalism.

3. Concern about teaching the new definitions or about asking students to “unlearn” the nine planets they memorized in grade school. This seems to me to be a minor concern. This is only nomenclature, and classifying it as a dwarf planet in no way diminishes the scientific importance of Pluto. I see this as a teaching opportunity. At the simplest level, it is a story of new discoveries: (1) that Pluto is a lot smaller (less massive) than it was thought to be when it was discovered and classified as a planet, and (2) that we have found other large trans-Neptunian objects (of which Pluto is the prototype) that reveal a fascinating part of the Solar System that was undreamed of when Pluto was discovered. An even more important lesson about the nature of science is that (3) scientists change their ideas when new data become available.

One of the problems of the new IAU definitions is that they exclude extrasolar planets. If the new definitions are to be applied beyond the Solar System, considerable interpretation is required because neither “roundness” nor “clearing its orbit” are properties that can be observed at stellar distances. This is a question to be resolved in the future, and it ensures that this issue of defining planet and dwarf planet will continue to generate controversy among astronomers.

2. Owen Gingerich

Harvard-Smithsonian Center for Astrophysics

I have been silently following the conversation about the definition of a planet with great interest, and I wanted to add a few comments. I am indifferent as to whether Pluto was demoted. Our committee's proposed resolution effectively demoted Pluto from the ranks of “classical planets” by making it another kind of planet. I was, however, unhappy with the resolution as passed, because it was clumsy and not well worded. For example, defining dwarf planet as not a planet is linguistically preposterous.

If the IAU voters had recognized that there can be different kinds of planets and had simply agreed that by their dynamicists' definition, they were defining only “classical planets,” things would have been much better. The wording of the resolution that passed is not clear to the public and to many astronomers, though its scientific intent is a reasonable alternative to the definition proposed by our committee: namely, that a planet needs to be round. Although it is scientific, the new definition is not sensitive to the cultural and historical context, which our committee was explicitly charged with considering. I think that the social context is hugely important for the kids (who are our future astronomers) and for the taxpayers who support our very expensive enterprise.

Ultimately, usage will win the day, and that may be difficult to control. There is a lot of rebellion out there now. Pluto will, I believe, never make it back into the ranks of being a major planet because it is so small and very characteristic of the Kuiper Belt objects, but it may well be considered a planet of a different kind. I hope that the community adopts plutonians for that class of bodies and cereans for the round rocky asteroids, because a few more large asteroids besides Ceres will prove to be round, and hence dwarf planets. Given these two very useful educational categories, we can then drop the confusing term dwarf planets.

At the IAU meeting, there were certainly some vociferous voices not interested in compromise, which made the process particularly unpleasant but interesting. It reminded me of trying to do diplomacy in the Middle East!