I think that the Planet Definition Committee has worked hard and thoughtfully and deserves our gratitude, at the very least for clasifying the issues involved. Unless I hear some very strong counter - arguments in the next ten days, I shall vote for the resolution. I would however, like to make two brief comments.
First, many words have different meanings in scientific and general use. For all physical scientists the words "force" and "energy" have quite district and well-defined meanings. In general discourse they stand for much vaguer concepts and can even be used as synonyms. We should not get too upset, whatever we decide about the status of Pluto, if the general public continues to use the term "planet" in a sense different from our definitions. Probably even many astronomers, myself included, will not change a liketime habit of talking about "minor planets" because of a resolution passed by the GA.
My second comment is to point out a little irony. Our presently favoured theories of the origin of the solar system developed from an idea published by Immanuel Kant over 200 years ago. Among professional philosophers Kant is much more famous for his insistence that we impose our categories on Nature and cannot know the ding an sich - the thing itself. If the definitions are important to the specialists, by all means make them, but do not fall into the error of believing we are talking about reality. I like to think that during the next few days, somewhere in the Elysian Fields, the spirit of Immanuel Kant will be enjoying a hearty chuckle at the seriousness with which we are taking ourselves!
Recent discoveries of objects in the outer reaches of our Solar System have forced scientists to reconsider what it means to be a planet. The International Astronomical Union (IAU) has proposed a new definition of a planet as a celestial body whose gravity is strong enough for it to be nearly round in shape, and which is in orbit around a star but is itself neither a star nor a satellite of a planet. According to this definition, the nine traditional planets in our Solar System would be joined by Ceres (the largest of the asteroids), by Charon (Pluto's largest moon), and by 2003 UB313 (the provisional name for a recently discovered object larger and more distant from the Sun than Pluto). Pluto and Charon would be regarded as a double planet, rather than as a planet and satellite, because their center of gravity lies outside of Pluto itself (the only such case known in our Solar System). There is a candidate list of additional objects that may be large enough to qualify as planets, subject to confirmation by the IAU.
The IAU resolution also recognizes Pluto as the prototype of a new class of planetary objects to be known as "plutons." In contrast to the classical planets, plutons typically have quite non-circular orbits and take more than 200 years to orbit the Sun. With increasingly sensitive and broad searches of the outer solar system well underway, it is quite likely that additional Pluto-like planets will be discovered.
The Division for Planetary Sciences (DPS) of the American Astronomical Society is the world's largest international professional society of planetary scientists. The DPS Committee, elected by our membership, strongly supports the IAU resolution. It was proposed after two years of careful review by an international panel of expert planetary scientists, followed by a broadly representative international group of historians, writers, and scientists. The new definition is clear and compact, it is firmly based on the physical properties of celestial objects themselves, and it is applicable to planets found around other stars. It opens the possibility for many new Pluto-like planets to be discovered in our Solar System.
The proposed definition will be brought to the IAU General Assembly for a vote on August 24, 2006. As representatives of an international community of planetary scientists, we urge that the resolution be approved.
After long consideration, I have come to several conclusions with regard to the new proposed definition of planets that I would like to share with the IAU community. They are outlined below.
There has been a long discussion about what a planet is. This problem appears at both ends: for the very massive bodies and for the smaller ones. At the large end, the limit seems to be clearer; it is now widely accepted that planets must not generate any energy from nuclear fusion, while brown dwarfs generate some nuclear energy from the fusion of deuterium. More problematic is the small end. We think that the definition should be kept as simple as possible and based on physical and cosmogonic reasons.
There is a wide consensus that planets formed by the accretion of small bodies – the planetesimals. The accretion process led to the formation of embryo planets that, as they grew in size and acquired more powerful gravitational fields, went to a process of runaway accretion in which the size of a few of them detached from the rest of the bodies of their neighboring zones. Given the powerful gravitational fields of these massive bodies - that we can call at this stage protoplanets - they were able to clean the population that had close encounters with them. The bodies interacting with the protoplanets were finally incorporated to the planets or scattered to other regions.
From a cosmogonic point of view, it therefore makes more sense to consider a planet as an object that acquired a mass large enough to clean a zone around its orbit. According to this definition, only eight planets, Mercury (perhaps marginally), Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune fulfill this condition. It is obvious that, at least for our solar system, this cosmogonic definition implicitly carries the condition of objects with a roundish shape determined by self-gravity.
From our definition, Pluto, Ceres and other large Trans-Neptunian objects in quasi-hydrostatic equilibrium [0] should be not considered as planets, since they never were the dominant bodies in their accretion zones. It is suggested that Pluto be kept unnumbered by historical reasons.
Is may be possible that in the near future cases of objects not foreseen at present could appear beyond our solar system, as for instance free-floating planets, stray planets, or double planets. We think that we should not advance definitions at this point for these exotic cases and leave their discussion when if they became a part of the observed world.
[0] From our present knowledge of the Solar System, we know that objects as small as Mimas (D~400km) are roundish. If this were the lower limit for an icy body to be in hydrostatic equilibrium, then we would already have several tens of bodies fulfilling this requirement.
(1) A planet is a celestial body that (a) is by far the largest object in its local population [1], (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) does not produce energy by any nuclear fusion mechanism [3].
(2) According to point (1) the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane are the only planets of our Solar System. All the other objects in orbit around the Sun are smaller than Mercury. We recognize that there are objects that fulfill the criteria (b) and (c) but not criterion (a). Those objects are defined as "dwarf" planets. Ceres as well as Pluto and several other large Trans-Neptunian objects belong to this category. In contrast to the planets, these objects typically have highly inclined orbits and/or large eccentricities.
(3) All the other natural objects orbiting the Sun that do not fulfill any of the previous criteria shall be referred to collectively as "Small Solar System Bodies".[4]
[1] The local population is the collection of objects that cross or close approach the orbit of the body in consideration.
[2] This generally applies to objects with sizes above several hundreds km, depending on the material strength.
[3] This criterion allows the distinction between gas giant planets and brown dwarfs or stars.
[4] This class currently includes most of the Solar System asteroids, near-Earth objects (NEOs), Mars-, Jupiter- and Neptune-Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs), and comets.
List of adherents to the above proposal:
| Name | Country |
|---|---|
| Julio A. Fernández | Uruguay |
| Marcello Fulchignoni | France |
| Daniela Lazzaro | Brazil |
| Gonzalo Tancredi | Uruguay |
| Alessandro Morbidelli | France |
| Mario Di Martino | Italy |
| Paolo Paolicchi | Italy |
| Antonella Barucci | France |
| Giovanni Gronchi | Italy |
| David Vokrouhlicky | Czech Republic |
| David Nesvorny | USA |
| Fernando Roig | Brazil |
| Hugo Levato | Argentina |
| Steven Chesley | USA |
| Alsonso Sena | México |
| J. E. Arlot | France |
| I. Shevchenko | Russia |
| Patrick Michel | France |