Three astronomers in Prague for the great planet debate of the summer of 2006

Every three years the world’s astronomers, including those from UNSW, gather for the General Assembly of the International Astronomical Union (IAU). In 2006 it was held in Prague, the city where Brahe and Kepler fought while laying the foundations of modern science in the Renaissance period in Europe. Normally the General Assembly is a placid affair, but this year there was something different – astronomers had to decide what a planet was! At least, that was what the world’s press thought we had gathered for. To the average astronomer planets are incidental bodies, and not many had given serious thought to what they actually are. But in the last three years we’d run into a problem. An object larger than Pluto had been found in the Kuiper Belt, a realm of icy bodies in the outer Solar System that are residue from its birth 4.6 billion years ago which hadn’t assembled into a planet. This clearly wasn’t a planet in the sense that we ‘understood’, but what did this mean for Pluto? To most astronomers Pluto has never actually been a planet, it was only that popular culture had decreed it so. But now something had to be done!

The IAU, knowing the intense interest the issue would generate, decided to set up a committee of venerable experts to consider the issue in private, and to report back on its considerations at the next General Assembly. Being scientists, the IAU then assumed that the astronomers would all see the rationale and accept the recommendation. But that’s when it all fell apart. Astronomers turned out not to be rational beings after all, but had just as many prejudices and misconceptions as the general populace, and certainly were not going to accept the recommendations of any committee until they had had their say! The result was complete chaos, and what started as a set of clearly defined scientific criteria that defined what a planet was (basically something large enough for self-gravity to make it spherical, but small enough not to have fusion, that is in orbit around a star), ended up as anarchy as resolutions were proposed from the floor, with amendments made on the run. The original proposal would have ended up creating several new planets, but the end result was to have kicked Pluto out, relegating it the role of a ‘minor planet’. But a very dubious clause that was accepted and involved ‘clearing out its orbit’ has left astronomy in the ambiguous situation where Jupiter itself might not actually be a planet anymore! The IAU are going to have to come back to this one, and see if they can get the world’s astronomers to behave like rational scientists…

Michael Burton