The Space Reviewin association with SpaceNews

IAU GA vote
Astronomers at the IAU meeting last month cast their votes on a resolution defining the term “planet”. (credit: IAU/Lars Holm Nielsen)

Inside the planet definition process

It just didn’t feel right, especially on this huge poster that Public Affairs of the International Astronomical Union had put up in an interview room on August 16: It showed all the newly defined planets of the solar system, all twelve of them as just proposed by its very own Planet Definition Committee. There were the four rocky worlds, including the third rock from the Sun we inhabit; there were the four gas giants; and there were another four tiny spots, one in between Mars and Jupiter and a pair plus another single one beyond Neptune. This was not what we had learned, step by step, about the structure of our solar system since the 19th century: there are these eight planets—all right, “classical” if you wish—but in addition there are two important belts of countless smaller objects spanning all sizes from some 3,000 kilometers down to meter-sized bodies. It just didn’t seem a good idea to single out one in the inner “main” belt of asteroids, Ceres, and three in the outer “Edgeworth-Kuiper” belt, the Pluto/Charon pair and 2003 UB313, still without a proper name (but the sticky nickname “Xena”). And to make them something very special, real planets, while leaving their kin literally in the dust, with another 12 on an uncertain candidate list for possible later upgrade to planet status.

These were the gut feelings of this writer, covering the 26th General Assembly (“GA”) of the International Astronomical Union (IAU) in Prague in the Czech Republic this August. This was also the immediate reaction from many participants, as one could fathom easily: it seemed highly uncertain from the beginning whether they would accept this bold proposal. These triannual gatherings are unique in that they bring together astronomers from some 70 countries and of all research fields in one place, to attend large and small conferences on numerous topics, from the solar system to cosmology, all running in parallel. Not all GA participants are members of the IAU, an honor bestowed only to working astronomers of some standing, and not all—almost 10,000 now—IAU members can make it to the GAs. Key decisions on administrative matters are made by national representatives, and one such decision made early at the 2006 GA was to permit individual IAU members to vote on scientific matters; this right had been abandoned during an earlier GA. With the membership in power again—though only those present in person at the meeting on voting day—the stage was set for the one decision that the whole world was apparently bracing for: the adoption of an exact definition of the word “planet”, the first one ever.

It just didn’t seem a good idea to single out one in the inner “main” belt of asteroids, Ceres, and three in the outer “Edgeworth-Kuiper” belt, the Pluto/Charon pair and 2003 UB313.

Normally resolutions to be voted on are formulated and made public many months ahead of a GA, and there were four others on the table following the protocol. However, with the fifth the IAU Executive Council (EC) wanted to run things differently: the planet resolution was written only weeks before the assembly, and then held secret until its third day, a mere eight days before the vote! We could have done things differently and put the draft onto the web, then-president Ron Ekers told The Space Review, but the EC was afraid of a heated pre-GA debate and the flood of e-mails that would certainly be triggered, thus the secrecy. While journalists were informed of the proposal one day ahead and under strict embargo (which didn’t stop the Czech media from disclosing it right away), the GA participants and most of the rest of the world saw the document only in the morning of August 16 when it appeared in the daily conference newspaper, together with numerous explanatory documents. The Planet Definition Committee—four planetologists, two other astronomers (including one historian of astronomy), and a writer—had, after surprisingly short deliberations this summer, “unanimously” settled on the radical idea that all bodies massive enough to force themselves into an approximately spherical shape should be called planets, regardless of their role and place in the solar system.

Only those accompanying vastly larger planets would be denied that honor (e.g. our own Moon), but everything else in hydrostatic equilibrium would join the club, regardless of its actual size, its surroundings, or its role in the solar system’s evolution. This viewpoint to the Planet Definition Committee looked physically motivated, with gravity—being strong enough to overwhelm rigid body forces—at its core and (an important consideration) simple. Still it required four paragraphs and four footnotes to write it down (“like written by lawyers”, one critic would soon quip), and the consequences would be dramatic. Not only would Pluto remain a planet—which was a coincidence and never a consideration, the Committee chair claimed in a posting on the H-ASTRO mailing list on August 16—and be joined by the bigger 2003 UB313. Pluto’s satellite Charon would also become a planet (because the barycenter of the pair resides in the open space between their bodies) and the same would be happening to the asteroid Ceres, which was once considered a planet but had lost that designation over a hundred years ago. Plus, since it’s awfully hard to determine whether hydrostatic forces have reshaped a distant, small object, there was a candidate list of another 12 bodies in both the main asteroid and the Edgeworth-Kuiper belt which another committee might at one point upgrade to planet status as well, based on future observations.

Those small planets in the trans-Neptunian world would furthermore be called “plutons”, to distinguish them from the larger planets from Mercury to Neptune. All plutons, as well as Ceres, would also be called—though informally—dwarf planets. Finally, the term “minor planet” (used for a long time as an alternative to asteroids) was to go: everything not a planet would become a “small solar system body”.

Except for a rushed statement from the Division of Planetary Sciences of the American Astronomical Society and an editorial in Nature, this author is not aware of any substantial support for the proposal from the Planet Definition Committee. Instead, it was practically torn apart at the General Assembly. This was supposed to be the simple, clear definition of “planet” that the world was waiting for? After all, the whole definition effort was mainly driven by a huge desire in the public at large and much less by astronomers themselves: other than for some formal purposes (e.g. which authority would catalog and name new discoveries), it didn’t matter much whether Pluto, UB313, and other objects were called planets or something else.

With the 2005 discovery of 2003 UB313 a decision could no longer be deferred: would the new ones (but down to which size?) be accepted as planets too, or would they—and Pluto with them—rather be listed as minor planets?

Such a view, however, would overlook the cultural significance of the term in mankind’s understanding of its place in the Universe. From earliest times, planets were something special, bright lights moving among the fixed stars often associated with important deities. With the Copernican revolution and the invention of the telescope, and later astrophotography, the meaning of planets changed several times, with the Moon and the Sun losing the title, the Earth gaining it, and Uranus, the first few asteroids, and Neptune being added to the pantheon, the asteroids being kicked out again when they became too many, and finally Pluto being added in 1930. The latter would soon be seen as—literally—an oddball, with a diameter tiny compared even to Mercury and a very out-of-plane and out-of-round orbit. Then, from 1992 on, numerous other bodies were discovered in similar orbits, some of at least half its size, and—as one and a half centuries earlier with the asteroids—there was talk of reclassifying Pluto as something other than a “real” planet. Proposals to that effect, like assigning the minor planet number 10,000 to Pluto, were shot down by fierce resistance, especially in the US, and the uneasy status quo survived into the 21st century.

Then came the summer of 2005 with the discovery of three neighbors of Pluto that were of the same size or, in the case of 2003 UB313, even larger. Now a decision could no longer be deferred: would the new ones (but down to which size?) be accepted as planets too, or would they—and Pluto with them—rather be listed as minor planets? At this time a group of 19 planetary researchers was already pondering a planet definition for the IAU Division III, which deals with the solar system, but they had deadlocked and eventually delivered three different possibilities to the IAU: make everything larger than 2,000 kilometers across a planet, demand hydrostatic equilibrium, or insist that there are no other bodies of comparable size in the same orbital neighborhood. With the GA approaching in 2006, the only opportunity in three years to put the issue to vote for the world’s leading astronomers, the IAU EC decided to give it another try and set up the Planet Definition Committee. Perhaps one day scholars can access their notes and find out how their deliberations could reach a conclusion so quickly, but according to their official story they converged on the hydrostatic equilibrium as the key criterion practically overnight. The EC slightly streamlined the resolution text the Committee had delivered and endorsed it (as did all Division presidents), and now it was up to the community to decide.

An alternative definition

After lots of heated hallway discussions and statements made to the press, the first formal opportunity for debate came on August 18, as part of the business meeting of Division III. The meeting room was packed, and soon the arguments flew. Contrary to some reports the atmosphere was not overly aggressive, and hardly any astronomers were misbehaving. Instead, the debate was a striking exercise in sound and deep scientific debate, even under time pressure: it immediately became clear that a majority of the planetary astronomers present saw things quite differently from the Planetary Definition Committee. The solar system was a system to them, and it mattered a lot where a body was located and what it had done there. (There are eight fully formed planets and hundreds of thousands of “fascinating byproducts of the formation of these eight planets”, in the words of exoplanet researcher David Charbonneau. One Definition Committee member had already conceded to The Space Review that dominance in one region of the solar system would have been a good concept, too.) The argument that a star is a star regardless of where it resides, so the same should hold true for a planet, didn’t fit. It was also highly unpopular that there would suddenly be several more planets (including former asteroids and moons!), with the list open-ended and in all likelihood growing to several dozen planets in the future.

Suddenly a draft for a very different resolution on defining a planet appeared on the projection screen, already carrying the signatures of several astronomers. According to that text a planet “is by far the largest object in its local population”, the latter being defined in a footnote as “the collection of objects that cross or close [sic] approach the orbit of the body in consideration”. The hydrostatic equilibrium demand was also included, but was rather redundant since all the eight known “largest objects” easily fulfill that criterion. A straw poll was held at the end of the debate, and the outcome was clear: the alternative won hands-down over the Committee’s draft, with at least 60, if not 75 percent of the vote in its favor! The original draft, one could say, was dead on that day, and indeed it lost in another straw poll on August 22 when the EC tried one last time to sell it (with slight modifications) to the assembled astronomers, this time in a plenary session. By the end of that day, the Executive had given up on the original idea and started to move in the direction of the alternative proposal. By the morning of August 24 the EC had come up with a totally new text that was basically yielding to the demands of the assembly but included some “special effects”. In order to make it more palatable to the community it was split into four parts, to be voted on separately, one after the other.

Suddenly a draft for a very different resolution on defining a planet appeared on the projection screen, already carrying the signatures of several astronomers. A straw poll was held at the end of the debate, and the outcome was clear: the alternative won hands-down over the Committee’s draft.

In resolution 5a a planet (of the Sun; extrasolar planets will be dealt with at a later time) was now to have reached hydrostatic equilibrium and to have “cleared the neighborhood around its orbit”. A “dwarf planet” was introduced as a body fulfilling the first but not the second criterion. The rest would be classified as “small solar-system bodies”. The details go right into the spelling, with “dwarf planet” seen as a compound noun, best written in quotes, and solar-system written with a “-”. It was obvious and stated even in a footnote that there are eight planets, while “an IAU process” would determine which other objects were small solar-system bodies and which were “dwarf planets”. Resolution 5a passed with an overwhelming majority (about 90%, it was estimated) of the about 420 voting IAU members present in that crucial session in the afternoon of August 24. But there was no majority for resolution 5b, which would have added a “classical” to the planet definition above and made “planet” an umbrella term for both those big planets and the “dwarf planets”. No, the community wished to clearly distinguish between (real) planets and “dwarf planets”, thus creating three distinct categories of solar system inhabitants: eight planets, three dwarf planets (Ceres, Pluto and 2003 UB313; Charon is again seen as a mere satellite of Pluto) and hundreds of thousands of small solar-system bodies, including asteroids/minor planets (this term is “allowed” again), comets, and Kuiperoids.

With the failure of resolution 5b, Pluto ceased to be a planet at 15:34 CEST (9:34 am EDT) that day, but it would at least get a convoluted consolation prize half an hour later. For resolution 6a passed again, in which Pluto “is recognized as the prototype of a new category of trans-Neptunian objects”. This category remains without a name for the time being, however, because resolution 6b, which would have called these bodies “plutonian objects”, failed to pass. Someone at the IAU will now come up with an alternative name for these trans-Neptunian “dwarf planets”—a category that is hardly necessary as all “dwarf planets” except Ceres (it’s unlikely that other main belt asteroids would be promoted to “dwarf planet” status) are trans-Neptunians. This fact was pointed out repeatedly in the final short debate preceding the votes, but perhaps the astronomers assembled in Prague felt a bit guilty of “demoting” Pluto. In any case the category of objects introduced to honor the latter has been all but ignored in news accounts of the vote. Those now pondering a name for it have to be very careful as the Planet Definition Committee already had stumbled badly with its proposal to call them “plutons”: Not only is Pluto actually called Pluton in a lot of languages, the term pluton has also been in use for a long time in geology! The Committee chair had to admit that they had thought it was uncommon because two word processor spell checkers hadn’t known it.

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