What is a planet?

In August of this year, 2,500 astronomers gathered in Prague, Czech Republic, for the 26th triennial general assembly of the International Astronomical Union (IAU). They then proceeded to make fools of themselves by failing to agree a sensible definition of what a planet is. A small committee led by astronomer-historian Owen Gingerich arrived at the meeting to propose the following definition of a planet:

A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

This definition was supplemented with the following definition of a satellite:

For two or more objects comprising a multiple-object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycenter [center of mass] resides outside the primary. Secondary objects not satisfying these criteria are 'satellites'.

If accepted, this joint definition would have raised the number of planets in our solar system from 9 to 12, by adding: the largest asteroid Ceres; Charon, previously treated as merely Pluto's largest moon, but considered to form a double-planet system with Pluto under the supplementary definition above; and 2003 UB313, an icy body more than twice as far from the Sun as Pluto.

However, the proposed definition was rejected by the majority of the participants at the assembly, and, after much belligerent argument, in its place came a definition based upon the orbital dynamics of a planet. The assembly agreed to define a planet to be an object which is neither a star nor a satellite, and which has 'cleared out the neighbourhood of its orbit'. Under this definition, there are only eight planets in our solar system, and Pluto is cast out from the club. One of the advocates of the orbital dynamics approach, Steven Soter, attempts to justify this definition in the January 2007 issue of Scientific American:

www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=93385350-E7F2-99DF-3FD6272BB4959038

Soter rejects the definition proposed by the IAU committee, arguing that "asteroids and KBOs [Kuiper Belt Objects] span an almost continuous spectrum of sizes and shapes. How are we to quantify the degree of roundness that distinguishes a planet? Does gravity dominate such a body if its shape deviates from a spheroid by 10 percent or by 1 percent? Nature provides no unoccupied gap between round and nonround shapes, so any boundary would be an arbitrary choice."

The fact that there is a continuum of object types, and one has to draw a dividing line at some, perhaps semi-arbitrary point, is a poor reason to reject a proposed definition of what philosophers call a 'natural kind'. Such continua, and such arbitrary points of division are endemic to the definition of natural kinds other than elementary particles, and Soter faces exactly the same problem with his own preferred definition: "The IAU may need to amend the definition to specify what degree of clearing qualifies a body as a planet. I have suggested setting the cutoff at a µ value of 100. That is, a body in our solar system is a planet if it accounts for more than 99 percent of the mass in its orbital zone. But the exact value of this cutoff is not critical. Any value between about 10 and 1,000 would have the same effect." Perhaps, however, the choice of objects included or excluded as planets is more sensitive to the choice of a cut-off in the case of the sphericity condition, and Soter might therefore be able to justify his definition on this basis.

Soter ends his article by making the excellent point that, "to be useful, a scientific definition should be derived from, and draw attention to, the structure of the natural world. We can revise our definitions when necessary to reflect the better understanding that arises from new discoveries." Definitions of natural kinds in science are malleable concepts, which change as our understanding changes, unlike the stipulative definitions found in pure mathematics. The debate over the definition of a planet in astronomy is an excellent case study of this.