The story starts over 200 years ago, in the back garden of a house in Bath, when William Herschel discovered a new planet—the first in modern times. In an act of spectacular grovelling, Herschel tried to call the planet Georgium sidus (George's star). The name didn't stick—the planet was named Uranus—but George III took the point; he granted Herschel a pension that allowed him to give up teaching music to young ladies and take up astronomy full time.
Herschel hadn't stumbled on Uranus by chance. He was one of the first astronomers to realise the importance of systematic surveys, and over several thousand cold nights, he surveyed the whole sky five times. During the second of these surveys, he discovered Uranus, which he knew must be a planet because it slowly moved across the sky from night to night, unlike stars, which stay fixed in position.
The discovery of Uranus soon led astronomers to another planet. They realised that Uranus was not following the path predicted by Newton's law of gravity, and in 1846 the French mathematician Urbain Leverrier showed that the discrepancy could be explained if Uranus's orbit was being perturbed by the gravitational field of a planet further from the sun. A German astronomer, JG Galle, discovered a planet—later named Neptune—at the predicted position, which led to another outbreak of human nature. It turned out that a young British mathematician, John Couch Adams, had done the same calculation before Leverrier, but none of the senior British astronomers had taken him seriously. This produced a predictable Anglo-French spat over which scientist should share the credit for the discovery.
Pluto was always the planetary misfit. The four planets before Pluto in distance from the sun— Jupiter, Saturn, Uranus and Neptune—are essentially balls of gas and are much larger than the rocky inner planets (Jupiter has a mass 300 times greater than the earth). Pluto, though, is solid and very tiny. Its mass is not only much less than that of the next smallest planet, Mercury, but is also only one sixth of that of the earth's moon.
The discovery in 1992 of an even smaller object began the long process of Pluto's demotion. In the 1980s "charged-couple device" (CCD) cameras became available to astronomers. (Progress in astronomy is often made not as the result of clever ideas, but because of new instruments, years of hard work or luck.) A CCD camera is the same as the digital cameras one buys on the high street—we just got them two decades before everyone else. For an astronomer, the advantage of a CCD camera is that it is much more sensitive than a photographic plate. At the end of the 1980s, Dave Jewitt and Jane Luu at the Institute for Astronomy in Hawaii decided to use CCD cameras to look for small objects beyond the orbit of Neptune. They had a hunch that because there are small objects in the inner solar system—the asteroids—there might also be small objects further out. They went on a fishing expedition.
The method they used was the same as that of Herschel and Tombaugh: look for objects that move relative to the fixed stars. For five years they had no success, but then in 1992 they discovered an object that was moving so slowly it had to lie beyond the orbit of Neptune. The International Astronomical Union gave it the uninspiring name of 1992 QB1 (Jewitt and Luu wanted to call it Smiley—the object had eluded astronomers for so long and Tinker, Tailor, Soldier, Spy was then on television in the US). 1992 QB1, or Smiley, has a diameter of about 200km, roughly one tenth that of Pluto.
Any idea that Smiley was the "tenth planet" was quashed when, a few months later, Jewitt and Luu discovered a second object, and then a third. Over a thousand of these trans-Neptunian objects have now been identified, and because it is only possible to survey a tiny area of sky with CCD cameras, astronomers estimate that there are over 100,000 objects orbiting the sun beyond Neptune. This belt of objects is the counterpart of the belt of asteroids in the inner system, and is called the Kuiper belt, after the American astronomer Gerard Kuiper.
In the time it takes Neptune to orbit the sun three times, Pluto orbits the sun twice, keeping it safe from the gravitational effect of the larger planet. Pluto and Neptune are thus said to be in a 3:2 orbital resonance, and soon after the discovery of the Kuiper belt, astronomers found that some of its objects were in the same orbital resonance with Neptune—objects which they named "plutinos." The discoveries of the Kuiper belt and, especially, plutinos gave rise to a suspicion that Pluto was not really a planet but merely a large Kuiper belt object. For several years this remained just a suspicion, for two reasons: Pluto was much bigger than the other trans-Neptunian objects, and, uniquely, it had a moon, Charon.
But the suspicion began to harden into something more definite in 2004 with the discovery of an object in the Kuiper belt, Sedna, with a diameter of about 1,000km—almost half that of Pluto. Moreover, by now astronomers knew that Pluto was not unique in having a moon. More than ten Kuiper belt objects are now known to have tiny moons. Finally, in July 2005, astronomers at the California Institute of Technology announced that they had discovered an object even bigger than Pluto. When journalists heard about this object, which was initially given the name 2003 UB313 but has now been named Eris, they raced off and wrote stories describing the discovery of "Planet X," the tenth planet.
In talks I give at schools, I have, since the middle of the 1990s, been telling children that there are eight planets, not the once standard nine. My own professional research is on the origin and evolution of galaxies, but the scientific arguments about the definition of a planet are simple enough for anyone to understand. The advantage for me of not working in planetary astronomy is that I am unbiased—I really don't care how many planets there are. But since the discovery of the Kuiper belt, it has seemed obvious to me that the solar system contains eight planets and two belts of builders' rubble left over from the formation of the solar system 4.5bn years ago. Pluto and Eris in the Kuiper belt, and Ceres, the largest object in the asteroid belt, are in this view just particularly large lumps of rubble; and the story of Ceres, which was the first asteroid to be discovered, in 1801, and which was briefly labelled a planet, has always seemed good evidence to me that Pluto too should be downgraded. The schoolchildren I addressed never seemed very upset by the idea.
The International Astronomical Union (IAU) became involved in this issue when Sedna was discovered. The IAU was set up in 1919 for astronomers to organise meetings and provide names for minor objects in the solar system. This latter task forced it to start considering whether Pluto is a planet. At the time of Sedna's discovery, some astronomers argued that its size meant that it too should be regarded as a planet. The IAU does not have a committee for naming planets, because there had never been the need for one—it is almost 80 years since Pluto was discovered. Sedna is still much smaller than Pluto, and so it was eventually named by the committee on small body nomenclature, the IAU body responsible for naming the builders' rubble. But the IAU executive realised that sooner or later somebody would discover an object as big as Pluto, in which case it might not be appropriate for it to be named by the existing committee. They formed a new committee to consider what we mean by a "planet." The committee, chaired by the British astronomer Iwan Williams, made no firm recommendation but produced three possible definitions for a planet.
1) The cultural definition is that an object is a planet if enough people say it is a planet. Supporters of the cultural definition argue that it is impossible to come up with a precise scientific definition of a planet—like the concept of a continent, it is inherently fuzzy. It would make little sense for geologists to propose a precise definition of a continent that led to Antarctica, for example, being reclassified as an island, because the public would either be upset or, more likely, wouldn't pay any attention and would continue to think of it as a continent. According to this definition, Pluto should remain a planet because people have regarded it as one for nearly 80 years.
2) The structural definition is based on the commonplace observation that planets are round. This observation is not quite as obvious when one realises that small bodies in the solar system have a variety of shapes; an asteroid usually looks more like a potato than a sphere, for instance. The reason for this difference is that if the mass of an object is large enough, the internal gravitational forces in the object—its weight, essentially—will exceed the internal forces between the atoms that keep the object rigid, and the rock will gradually flow so that the object takes up the lowest-energy shape, which happens to be a sphere. Thus one possible definition of a planet is any object with a mass sufficiently high that gravity shapes it into a sphere. For the defenders of Pluto, this was a convenient definition because the critical mass is equivalent to a diameter of about 1,000km, roughly half that of Pluto.
3) The dynamical definition arises from the fact that the orbits of small objects are unstable if they are too close to the orbits of large objects. If a small object has an orbit that takes it close to Neptune, for example, sooner or later Neptune's gravitational field will send it scuttling away, either towards the sun or, more likely, out of the solar system completely. Thus one possible definition is that an object is a planet if there are no other objects with similar orbits—if it dominates its neighbourhood. By this definition, of course, Pluto is not a planet.
There are problems with all these definitions. The cultural definition is useless for deciding the status of a new object. The structural definition is precise but hard to apply in practice because the shape of a planet does not just depend on its mass but also, although to a lesser extent, on its density and internal structure. The problem with the dynamical definition is that it is very unclear how to make more precise the phrase "dominates its neighbourhood."
The discovery of Eris, an object even larger than Pluto, made it necessary to choose one of these definitions. The IAU set up another new committee to draft a resolution on the definition of a planet which could be presented to the general assembly of the IAU, which was due to meet in Prague in August 2006. The new planet definition committee (PDC) was chaired by Owen Gingerich, a historian of astronomy, and included five professional planetary scientists and one non-astronomer, Dava Sobel, a science author (she wrote the bestselling Longitude) who was chosen to represent the views of the public.
On 16th August 2006, the IAU issued a press release containing a draft of the resolution that would be placed before the general assembly a week later. The PDC had effectively adopted the structural definition. According to its resolution, the solar system had 12 planets: the standard eight plus Pluto, Eris, Ceres and even Charon, Pluto's moon. Charon qualified as a planet for the somewhat technical reason that the centre of mass of the Pluto-Charon system, the point around which both bodies orbit, lies outside the body of Pluto, and so the PDC claimed Pluto and Charon were a double-planet system rather than a planet and moon (opponents pointed out that because our moon is gradually moving away from the earth, if this definition were adopted, at some point the moon would acquire planet status).
During the week before the vote, astronomers packed the bars in Prague, arguing about the resolution. There was also vigorous debate in cyberspace, as astronomers around the world posted articles on websites for and against the resolution. When it came to the vote on 24th August, the IAU assembly rejected the PDC resolution by a large majority. It adopted an alternative definition, also by a large majority, in which for an object to count as a planet it must satisfy three criteria: it must orbit the sun; it must be large enough that gravity moulds it into a sphere; and it must dominate its orbital neighbourhood. Although this looks like a compromise, the critical criterion is the third one, which is the dynamical definition. Because it didn't satisfy this criterion, Pluto was demoted from planet status. As a sop to the defenders of Pluto, objects that satisfy the first two criteria but not the third were given the new title "dwarf planet," but this did not change the basic result: the solar system had lost a planet.
Why did the rebellion occur? I was one of the 8,000 members of the IAU who was not at the general assembly (about 400 members voted, making the participation rates in British elections look quite healthy). From a distance, there are two readings one could give of the events in Prague.
First, in framing a definition that kept Pluto a planet, the PDC was expressing fear of the public. The public tends to think of science—or at least scientists think the public thinks like this—as a collection of facts, and the committee may have thought the shock to the public would be too great if one of the most important list of facts turned out to be wrong. In an article he wrote after the events of August, Gingerich quoted a colleague: "Don't demote Pluto. Little children love Pluto. They'll be heartbroken if you tell them Pluto isn't a planet."
Second, the PDC didn't want to demote Pluto because it was the only planet discovered by an American. The planetary science community is dominated by Americans—and so in this reading the refusal of the PDC to demote Pluto was based on national chauvinism. However, my sources on the PDC have convinced me that this reading is wrong. The Americans on the committee were in a minority, scientists are by nature citizens of the world, and although one American member of the committee had a reputation as a "Pluto defender," my sources were happy that all members approached the question with open minds. (There were vested interests outside the committee. One of the most vigorous defenders of Pluto was the leader of the Nasa mission to Pluto and Charon, who now has to explain to the Nasa administrators why they are paying $700m to send a spacecraft to two pieces of builders' rubble.)
My sources were also confident that the committee had not been egregiously scared of public reaction. One of them revealed that the lay member on the committee, Dava Sobel, who might have been expected to be a Pluto defender, had argued that the public would not mind if Pluto was demoted, as long as the reasoning was clearly explained.
It seems to me that part of the reason the PDC got it wrong was that it worried too much about cultural sensitivity and forgot why scientists classify things. Physical scientists are not experts in classification systems, which is understandable because we don't use them much. For biologists, on the other hand, classification is crucial. A system which put cows and horned toads in the same class because both have horns would not be very useful, because we know that cows and horned toads have very different underlying structures, metabolisms and evolutionary histories. In the solar system, the underlying reality is that there are eight big objects and two belts of smaller objects. This configuration must have arisen when the solar system was formed, and thus Pluto is more likely to be similar, in early history and composition, to the other objects in the Kuiper belt than to the large objects in the solar system. The classification system proposed by the PDC did not reflect this reality, lumping several objects in the belts with the eight big objects. The committee was attracted to the structural definition of a planet, which seemed most precise and logical; but a biologist would have chosen the definition which best suited the underlying reality. Most astronomers realised this, which is why they rejected the PDC proposal.
This is not a serious indictment of the committee. They just made a mistake because of inexperience in classification systems, lack of time, and the pressure of making decisions they knew everyone was going to pick over. In their concern for cultural sensitivity, however, they missed an opportunity to show the public the true meaning of science. The biggest misconception about science is that it is just a collection of facts. The truth is that science is a powerful method of finding out about the world that is easy to understand and available to anyone. Lists of facts are simply provisional conclusions about the world, which may be turn out to be wrong. The fundamental law for scientists—often hard to live up to in practice—is always to be prepared to admit to mistakes. The PDC missed an opportunity to demonstrate this on the largest public stage. Tombaugh's discovery of the ninth planet was a provisional conclusion which, 76 years later, turned out to be wrong.
