As a participant in the recent International Astronomical Union (IAU) General Assembly at Prague and votes on the definition of a planet, let me share some of my views on both the process and the result. First of all, let us look at the approved IAU definition of a planet in our Solar System.
(1) A planet is a celestial body that (a) is in orbit around the Sun,(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A ¡§dwarf planet¡¨ is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as ¡§Small Solar System Bodies¡¨.
The IAU further resolves that Pluto is a ¡§dwarf planet¡¨ by the above definition and is recognized as the prototype of a new category of Trans-Neptunian Objects.
According to this new definition, our Solar System has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto together with Ceres (an asteroid) and Eris (a Kuiper belt object) are ¡§dwarf planets¡¨. This new definition is controversial, and not just among scientists but also the general public. This is not surprising because Pluto is not a planet anymore and the wording of the definition is not clear to the public and to many astronomers. The primary objection is the use of the phase ¡§cleared the neighbourhood around its orbit¡¨. What does it really mean? If you just read the official definition, then even Jupiter and Earth are not planets because there are asteroids around their orbits and hence they do not clear the orbits. Similarly, Neptune is not a planet because Pluto crosses the orbit of Neptune. The problem here is that the wording is misleading but the concept is clear if you are a planetary scientist. Neptune is almost 10000 times massive than Pluto and totally dominates the dynamics of Pluto and its surrounding region known as Kuiper belt (there are over 800 Kuiper belt objects have been discovered in the belt since 1992). In fact, Pluto orbits in a 3:2 orbital resonance with Neptune meaning that for every three complete orbits that Neptune makes around the Sun, Pluto completes two. Similar argument also applies to other planets.
From my point of view, the definition of 1c is entirely reasonable but clearly the wording might need to be changed. Perhaps a more precise definition would say that a planet ¡§dominates¡¨ its orbital zone. On the other hand, one of the original proposals is to drop the definition of 1c. Therefore, everything that is round and orbits around the Sun is a planet. It is an easily acceptable proposal and is consistent with what we commonly know. With this definition, Pluto as well as Eris, Ceres, and Charon (Pluto¡¦s satellite) are planets and we will have 12 planets with more to be confirmed. There is nothing wrong with this definition. However, we will realize sooner or later that those Pluto-like objects will form a distinct population as their physical properties are different from the eight planets. So someone may introduce qualifiers to distinguish different types of planets. In other words, we still have to remark Pluto-like objects as oddballs among planets. The advantages are that the definition is more straightforward and we can still label Pluto as a planet.
After all, I am indifferent whether Pluto is a planet or not, and I asked myself a fundamental question: Is a precise definition of a planet necessary? Some of my friends are surprised that scientists make decision by voting. I do not feel comfortable about this process because voting means that there is no consensus and politics will play a role in controlling the result. However, science does not work in that way. The scientific motivation behind this issue is to classify objects in the Solar System. There is no right or wrong classification scheme, but only a good or bad classification. Classification helps us to understand the nature of similar objects and there are many different ways to perform classification.In the case of planet definition, it is clear that there is no perfect solution and in fact the IAU working group considered every possibility before drafting the final proposal. Astronomers know exactly what they are referring to and whether Pluto is a planet is not crucial. The important thing is to understand theformation and evolution of the Solar System by studying the physical properties of different objects in the Solar System. In fact, geologists do not come up a unified scientific definition of ¡§continent¡¨. Depending who you are talking to, there are five, six, or seven continents. So why do we bother eight, nine, or fifty planets? The key point is that Pluto is totally different comparing to other eight planets. If Pluto was discovered today, it would unlikely be classified as a major planet.
It is true that Pluto is now a ¡§dwarf planet¡¨ while ¡§dwarf planet¡¨ is not a planet. All media uses ¡§demotion¡¨ or similar negative wording to describe the status of Pluto. Indeed ¡§dwarf planet¡¨ is also a negative description. I feel that it is unfair and I would see that the new IAU definition is to reclassify Pluto. Although Pluto is not a planet, it is the prototype of a new category and it is a very significant object. It is as important as the other eight planets in our understanding of the origin of the Solar System because Pluto and its family members carry some unique information that the eight planets cannot provide.
Apart from the wording ¡§cleared the neighbourhood around its orbit¡¨, it is also confusing that a ¡§dwarf planet¡¨ is not a planet. But in planetary science, we all accept that a ¡§minor planet¡¨ is not a planet too. While in English we can replace minor planet with asteroid, there is no such a replacement in Chinese. The other criticism is that less than 5% of IAU members voted on the definition. I totally support that the IAU should consider electronic voting system and replace the old-fashioned hand-raising. One serious problem is that the process before the voting is a chaos. The planet definition proposal keeps changing during the two-week meeting and we only read the final proposal on the day of voting, not to mention a few minor amendments just before the voting. Another major problem is that the IAU working group is not open enough and there is no formal consultation before the assembly.
The definition of planet is not only a science issue, but also a social issue. Indeed, I think the general public has a stronger feeling comparing to many astronomers. That is why the press follows the development closely and in fact I learned about the early development through newspapers before heading to the assembly. The IAU definitely underestimates the social impact of this voting. After all, the common usage of ¡§planet¡¨ may be different from the scientific definition just like the usage of ¡§continent¡¨. But the most important thing now is to educate people about the new definition and its implications. I think the IAU as well as other astronomical societies should take a major role. Although the current definition is not clear enough, I would expect some changes or even another major debate during the next assembly in Rio de Janeiro in 2009.
When I was heading to Hungary by train after the assembly, two English girls sitting next to me discussed the new definition of planet. While they felt somewhat confusing, I am glad that they now know that there are eight planets in our Solar System.