Annals by Owen Gingerich

Planetary perils in Prague Downloaded from http://direct.mit.edu/daed/article-pdf/136/1/137/1829233/daed.2007.136.1.137.pdf by guest on 30 September 2021

For astronomers, Prague is a singular- years has increased by orders of magni- ly nostalgic city. It was here, in February tude, an immense expansion factor that of 1600, that the young, starry-eyed Jo- only the observations of Tycho Brahe hannes Kepler met the imperious, eccen- rival in their comparative impact. Tele- tric Tycho Brahe. Brahe brought a price- scopes in mountain observatories, plus less trove of precise observations of the spacecraft above the atmosphere com- planets and stars, the likes of which the bined with modern electronics, have world had never seen–singlehandedly reaped a bounteous harvest of exciting he raised the astronomical data bank a new results. hundredfold. As Kepler would later re- But as seen from the Prague press of- flect, Brahe had the building materials ½ce of the International Astronomical for cosmology, but he lacked an archi- Union (iau), the world was ½xated on tect. Kepler became that architect. It was two far more mundane questions: would a conjunction fated to alter the course of little Pluto, in the frigid realm beyond astronomy. Neptune, still be considered a planet; Last August, Prague was teeming with and if so, would some of his icy play- astronomers, roughly two thousand of mates in that remote zone also be ush- them. They came to evaluate an astro- ered into the exclusive planetary club? nomical data bank that in the past few The iau had gotten itself into this feeding frenzy by a procedural question Owen Gingerich, a Fellow of the American Acad- of nomenclature that only indirectly in- emy since 1976, is Professor Emeritus of Astrono- volved Pluto’s status. For this is one my and History of Science at Harvard University of the things international unions do. and a senior astronomer emeritus at the Smith- They follow in Adam’s footsteps by cre- sonian Astrophysical Observatory. Among his nu- dentialing names. The International merous publications are “The Great Copernicus Union of Biological Sciences, for exam- Chase and Other Adventures in Astronomical ple, oversees a committee that establish- History” (1992), “The Eye of Heaven: Ptolemy, es the rules for the naming of birds and Copernicus, Kepler” (1993), and “The Book No- shells and other animals. And the Inter- body Read” (2004). Most recently, he published national Astronomical Union, in its very “God’s Universe” (2006). ½rst General Assembly, in Rome in 1922, established the list of eighty-eight con- © 2007 by the American Academy of Arts stellations accepted today. Committees & Sciences

Dædalus Winter 2007 137 Annals by under its aegis assign names to comets, would not have been so fraught, except Owen Gingerich to minor planets, to planetary satellites, that in 1999 my colleague Brian Mars- and to features on these moons or plane- den, who was directing the iau’s Mi- tary bodies. nor Planet Center, had reached a nice But as for planets, the iau has never round number in tabulating the aster- had an opportunity to name one. In 1930 oids, 10,000, and he suggested it might the Lowell Observatory announced the be reserved for Pluto. Unexpectedly, discovery of a planetary body beyond his rational, practical suggestion creat- Neptune, and they were delighted when ed a ½restorm, and the iau of½cers were

an eleven-year-old schoolgirl in Oxford bombarded with protests at the appar- Downloaded from http://direct.mit.edu/daed/article-pdf/136/1/137/1829233/daed.2007.136.1.137.pdf by guest on 30 September 2021 suggested the name Pluto, because its ent threat of Pluto’s demotion. Hence, ½rst two letters were the initials of the they were understandably nervous. Af- observatory’s founder, Percival Lowell. ter a large committee from their Plane- The iau held General Assemblies only tary Systems Sciences Division not only every three or four years and didn’t have failed after many months to ½nd a solid one until 1932, so by then the name was consensus, but also seemingly had not a fait accompli. considered the public-relations aspects In 1930 Pluto was assumed to be at in their debates, the Executive Commit- least as large as Earth, and maybe a few tee decided to appoint a broader-based times larger, for Percival Lowell had ‘Planet De½nition Committee.’ Since I presumably predicted its approximate had credentials in both astrophysics and position by its gravitational perturba- the history of astronomy, I was tapped to tions on the giant planets Neptune and lead the way across the mine½eld. Uranus. In the decades that followed, Many suggestions poured in via the however, observations showed that not Internet about how to de½ne the word only was Pluto much smaller–indeed, planet. Someone suggested it was already smaller than our moon–but also the de½ned by its Greek origins: wanderer. If apparent perturbations stemmed large- it moves against the starry background, ly from the use of an erroneous mass let it be a planet. Considering that orbits for Neptune in the celestial mechanical are already known for approximately computations. Thus, Pluto’s planetary three hundred thousand asteroids, not status was in some jeopardy. In 2000, to mention comets, the suggestion was when the planet walk was constructed straightaway tossed into the obsolete for the new Rose Center and Hayden ideas bin. Others appealed to history: Planetarium in New York, Pluto was freeze the status quo with its nine plan- conspicuous by its absence. ets. But history is a ½ckle guide, for Matters came to a head in 2005 when throughout the ages the number of plan- an icy ball discovered far beyond Nep- ets has varied both with cosmology and tune proved to be as large as or slightly with discovery. larger than Pluto. Was it, or was it not, a Meanwhile, the committee had con- planet? Which committee had the nam- ferred for two days in Paris to craft a ing rights? Meanwhile, it suffered under scienti½c, but culturally sensitive, de½- ub a technical designation, 2003 313, or nition. There were two ways, not com- the nickname Xena from a popular fan- pletely independent, to de½ne a planet tasy television show. scienti½cally: either by what it is, or by As far as the Executive Committee of where it is, that is, by its relationship to the iau was concerned, the situation its neighbors. Planetary scientists and

138 Dædalus Winter 2007 geologists are keen on studying planets trans-Neptunian objects, with a name Planetary as physical bodies. Some of them would such as ‘plutons’ to recognize the his- perils in Prague even cheerfully think of the giant satel- torical role of Pluto. In this way, Pluto lites–Jupiter’s Ganymede and Callisto, would be promoted while being demot- and Saturn’s Titan, objects that rival or ed, which, as some analysts smugly not- exceed Mercury in size–as planetary ed, was worthy of political solutions in- bodies. They are de½nitely in the what side the Beltway. camp. The dynamicists, on the other The frenzy of the iau press room hand, ½nd the dominating relationship in Prague was at ½rst exhilarating, but

of a planet on its neighbors particular- in retrospect I realize it was a prime Downloaded from http://direct.mit.edu/daed/article-pdf/136/1/137/1829233/daed.2007.136.1.137.pdf by guest on 30 September 2021 ly fascinating, and for them, where is of source of strategic error. The chief press prime signi½cance. of½cer was convinced that reporters While these alternative approaches to would want to know two things: Is Plu- de½ning a planet had rami½cations for to a planet? And how many planets are the status of Pluto, its standing was not there? The committee had never count- a major consideration in the negotia- ed because we all knew that the num- tions, and we never inquired where the ber in August would not be the same as seven members of the committee stood the count in December. I tried in vain on that issue. So while Pluto remained to convince him that the press release the elephant in the parlor, the members should say, ‘eight classical planets and a pretty much stayed in the kitchen. For growing number of plutons.’ The press, my committee, de½ning a planet as an which feeds on controversy, easily found object seemed simplest and more open- critics who declared the number of ended, especially considering the cur- twelve planets ridiculously complicated rent discoveries of large numbers of exo- because it included the round asteroid planets, that is, planetary bodies orbit- Ceres as well as Charon, a satellite of ing distant suns. Furthermore, rather Pluto that had snuck in through a foot- than establishing an arbitrary cut-off in note that wasn’t part of our proposed size, we chose the most obvious physi- resolution. cal characteristic as the dividing line. If Our recommendations met with en- a body had enough mass and therefore thusiastic approval from the Division enough gravity to pull itself into a ball, for Planetary Sciences of the American let it be a planet. Naturally, there would Astronomical Society, the largest inter- be an ambiguous boundary zone, but national group of planetary scientists. science and scienti½c taxonomy are full Where our proposed resolution ran in- of such debatable cases. to vehement and raucous opposition We immediately understood that a was from the dynamicists, who believed what de½nition would open the gates they hadn’t been consulted and who felt to a dozen more solar-system planets, stabbed in their psyches because we had and maybe as many as thirty or even not given primacy to their favored where forty, and that these would primarily de½nition. Feeding on the discontent of be dirty iceballs of the Pluto class, and those who felt uneasy about admitting not major planets like Uranus or Nep- too many dwarfs into the club, they mar- tune. I therefore proposed that we shaled support for a hastily worded al- should describe Mercury through Nep- ternative de½nition. tune as ‘classical planets,’ and make Our committee met with the leaders Pluto the prototype of a new class of of the opposition, and I showed them an

Dædalus Winter 2007 139 Annals by alternative compromise resolution that dent of the iau and a member of the Owen began: Gingerich Planet De½nition Committee, Cather- ine Cesarsky, made an impassioned and The predominant part of the solar sys- statesman-like plea to this effect, but tem is a dynamically linked suite of eight unfortunately brilliant floodlights blind- mutually-interacting planets, Mercury to ed those on the stage, and they did not Neptune. Each of these produces observ- see and recognize her until after the vote able perturbations on at least one of its had been taken. As Kepler wrote to his neighbors. The hundreds of thousands teacher, Michael Maestlin, “Experts can- of lightweight bodies individually have

not live off themselves or on air. There- Downloaded from http://direct.mit.edu/daed/article-pdf/136/1/137/1829233/daed.2007.136.1.137.pdf by guest on 30 September 2021 no observable dynamical effects on the fore, let us act in astronomical affairs in heavyweight planets. We retain this group such a way that we hold on to supporters of classical planets as the essential de½ni- of astronomy and do not starve.” tion of “planets.” Unfortunately, one becomes too soon Alas, they would have none of it, old and too late wise. I realize in retro- claiming that even the asteroid Ceres spect that the iau should never have at- could perturb Earth by a few centime- tempted to de½ne the word planet. It is ters and that eventually our instruments too culturally bound, with elastic de½ni- would be sensitive enough to detect that tions that have evolved throughout the minuscule amount. In the end, some- ages. What the iau could legitimately how, the confusing but de½ning phrase have done in its role of naming things that a planet was a body large enough was to have de½ned some subclasses, “to have cleared its zone” was added such as ‘classical planets,’ leaving the to the resolution and was adopted by planetary door open not only for pluto- the ½nal assembly. Appropriately, they nians and cereans but for the exoplan- voted that Pluto would be considered ets as well. These terms would be emi- the prototype body of an unnamed class nently teachable and would help stu- of dwarf planets, but which by the new dents understand the complexity and de½nition would not be planets. And by richness of the solar system that mod- the narrowest of votes they failed to give ern science is revealing. And astrono- the name ‘plutonians’ to the new class mers could have left Prague without of objects for which Pluto stands as the muddle on their faces. prototype. In the aftermath of Prague, the iau In their zeal for science, the voting committees joined forces to accept an astronomers in Prague seemed to forget appellation proposed by Mike Brown, that for the most part they don’t own the leader of the team that discovered the telescopes, the space probes, and 2003 ub313: it is now Eris, appropriate- the instruments on which they depend ly named after the classical goddess of for their researches. It is the taxpayers discord and strife. who own them. And it was American taxpayers who felt they owned the Hub- ble Space Telescope so much that they made an outcry when nasa of½cials decided to abandon it. It was our pub- lic constituency who forced a change in their plans. It behooves us to pay atten- tion to public relations. The new presi-

140 Dædalus Winter 2007