August 16, 2006, Prague

By Ron Ekers, President of the IAU

The IAU has been the arbiter of planetary and satellite nomenclature since its inception in 1919. The various IAU Working Groups normally handle this process, and their decisions primarily affect the professional astronomers. But from time to time the IAU takes decisions and makes recommendations on issues concerning astronomical matters affecting other sciences or the public. Such decisions and recommendations are not enforceable by any national or international law; rather they establish conventions that are meant to help our understanding of astronomical objects and processes. Hence, IAU recommendations should rest on well-established scientific facts and have a broad consensus in the community concerned.

The boundary between (major) planet and minor planet has never been defined and the recent discovery of other "Trans-Neptunian Objects" (TNOs), including some larger than Pluto, triggered the IAU to form a Working Group on "Definition of a Planet" from its Division III members. While there was general agreement on all the scientific issues related to Solar System dynamics and physical properties of planets, the IAU Division III Working Group could not agree on aspects that were related to social and cultural issues, such as the status of Pluto. In order to include these broader aspects, the IAU Executive Committee (EC) formed a new committee whose membership had backgrounds in history, science publishing, writing and education as well as in planetary science.

Terms of Reference
The Planet Definition Committee of the IAU Executive Committee was charged with:

1. discussing the broader social implications of any new definition of a planet and recommending a course of action that balances the scientific facts with the need for social acceptance of any change;
   
   
2. addressing the status of Pluto, and of the newly discovered TNOs in the light of recommendation (i);
   
   
3. considering whether the current naming procedures for planets and minor planets have exacerbated the problem of defining a planet and recommending whether revisions are needed; and
   
   
4. attempting to frame these recommendations as a resolution, or resolutions, that could be put before the Prague GA in August 2006 for possible adoption.

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The Path to Defining Planets

By Owen Gingerich, Harvard-Smithsonian Center for Astrophysics/IAU EC "Planet Definition" Committee chair

Celestial nomenclature has long been fraught with controversy. Galileo proposed to name the large satellites of Jupiter the "Medicean planets"; William Herschel named his new planet after the English monarch, George III; Hevelius honoured the defender of Vienna with "Scutum Sobieski"; and Bode named a northern constellation after the comet hunter Charles Messier. None of these appellations have stood the test of time except for the fragment "Scutum".

At its inaugural meeting in 1922, the IAU standardized the constellation names and abbreviations. More recently IAU Committees or Working Groups have certified the names of asteroids, satellites, and planetary and satellite features. Until now, however, the IAU has never named a planet, and it has been unclear whether there are potential planets to be named.

How, in fact, should the word "planet" be defined?

This was the controversial question facing the committee established by the IAU Executive Committee with the charge to recommend a definition for an IAU resolution. The seven members represented a spectrum of opinion and expertise. We all knew that modern scientific advances have taught us that the Solar System is a far more complicated place than William Herschel and his contemporaries ever imagined, not only containing an assortment of planets, asteroids, and comets, but rocks, gravel, dust, and ions. We met in Paris for a vigorous discussion of both the scientific and the cultural/historical issues, and on the second morning several members admitted that they had not slept well, worrying that we would not be able to reach a consensus. But by the end of a long day, the miracle had happened: we had reached a unanimous agreement.

On the scientific side, we wanted to avoid arbitrary cut-offs simply based on distances, periods, magnitudes, or neighbouring objects. One physical criterion seemed pre-eminent: was the object shaped by hydrostatic equilibrium, that is, was it basically a round object? This criterion became the basis of our proposed definition. Objects with mass above 5 x 1020 kg and diameter greater than 800 km would normally be considered to be in hydrostatic equilibrium, but borderline cases would have to be established by observation. Even among these round Solar System objects there is a distinct difference between the major planets, whose orbits lie near the ecliptic plane, and those smaller objects with more eccentric, tilted orbits. Had astronomers realized in 1930 that Pluto was smaller than our Moon and with a mass well under 1% that of the Earth, perhaps some special designation would have been devised for it. On the cultural/historical side, combined with contemporary science, our committee felt that the time was ripe to recognize Pluto as the prototype of a different sort of planet. Consequently, we propose to distinguish between the eight classical planets discovered before 1900, and a new class of Trans-Neptunian Objects, for which we recommend the name "plutons."

The question immediately arises about the status of Pluto. Although Pluto remains a planet by the proposed definition, it will generally be preferable to call it a pluton to emphasise its role as the prototype for a physically distinct category of planetary bodies.

Specialists will at once recall that there are over a hundred so-called "plutinos," Trans-Neptunian lumps of rock, ice, and snow, each with the period 248 years (thus in a 3/2 resonance with the period of Neptune). These faint objects are in general not plutons. Plutons are at present very rare objects: Pluto, Charon, 2003 UB313, and perhaps several more, and anyone who finds a new pluton should be appropriately celebrated.

Savvy astronomers will notice that our definition also makes Ceres a planet, and if Pallas, Vesta, and Hygeia are found to be in hydrostatic equilibrium, they will also have to be considered planets. Without making a formal definition, we suggest that it might be convenient simply to refer to these small round members of our inner Solar System as "dwarf planets."

Did our committee think of everything, including extra-solar system planets? Definitely not! Science is an active enterprise, constantly bringing new surprises. Undoubtedly some future IAU committee will have to revisit this question and define the upper limit for "planet", probably well before 2106!

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The Process of making a Resolution on the Definition of a Planet

By Robert Williams, Space Telescope Science Institute, Vice President of the IAU

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. As explained in the accompanying articles the question of the definition of a planet is of great interest within the Union and among the public, and Division III and the Executive Committee are attempting to set forth criteria that define planets and provide for a nomenclature for the different Solar System objects.

A Working Group under Division III was established to formulate a recommendation on the definition of a planet that could be put before the Executive Committee. Although that Working Group did not achieve a clear consensus, it did succeed in defining the important criteria and framing the discussion of issues to be considered. The EC studied the Division III Working Group report and decided to form its own advisory group, the Planet Definition Committee, to attempt to resolve the issue in a manner that had a solid scientific basis and which might achieve consensus support among members of the Union. Prof. Gingerich has described the work of the Planet Definition Committee, whose report has been received by the EC and used as a basis for framing the draft resolution that is now being put before the General Assembly. The current draft of the resolution "The Definition of a Planet" that has been approved by the EC and the Resolutions Committee appears with these articles.

The process by which resolutions are considered by the IAU is set forth in the Working Rules. It involves consideration by the Resolutions Committee and the Executive Committee, and discussion by the General Assembly before a vote taken in the second business meeting of the GA. Because of the potential impact of this resolution the EC is undertaking extra measures to assure full discussion of the draft during the General Assembly that will allow for possible revisions to the current version before it is presented to the GA at the closing business meeting. They include a discussion and debate of the resolution by Division III-Planetary Sciences at its scheduled meeting this Friday, 18 August. In addition, the EC is convening an extraordinary plenary session of the General Assembly to take place next Tuesday, 22 August, during the lunch break, which will be devoted entirely to a discussion of the draft resolution, and after which a "sense of the meeting" vote will be taken on the resolution as presented. We are fully aware of the potential difficulty in achieving a consensus on this complex issue, and we wish to provide ample opportunity for input from members in the formulation of the final resolution to be considered next week.

The key events that bear on the substance of the final resolution to be presented at the closing business meeting, and in which all IAU members are encouraged to participate, are (1) the discussion at the meeting of Division III on Friday, 18 August at 11:00 am in Club B, and (2) the Plenary Session on the Definition of a Planet on Tuesday, 22 August at 12:45 pm in Forum Hall. The Closing Session of the GA will be held Thursday 24 August at 14:00 in the Congress Hall and here the final resolution will be presented, discussed, and voted upon.

The EC reiterates our desire to benefit from members' input into this issue by your participation in these events, which are an important part of the IAU's mission to communicate the discoveries of astronomy to the public.