W hat we understand to be ‘planetary’ largely relates to our experiences of the ultimate planet, Earth. Across the solar system we study the chemistry and dynamics of atmospheres, non-impact surface structures that reveal something about processes underneath the surface, and magnetic fields that reveal processes even deeper beneath the surface. We study erosion due to winds and the flow of liquids. We study geysers and volcanism. On some worlds we search for signs of past and even present life.
The International Astronomical Union (IAU) took a decidedly parochial view in defining the term ‘planet’ at a meeting of its General Assembly in Prague Aug. 14-25 .
Rather than contemplate the intrinsic properties that give rise to the planetary processes we study in the solar system, the IAU decided upon a definition that has nothing to do with those processes at all. A planet orbits the sun and “has cleared the neighborhood of its orbit.” The application of this definition by its proponents results in a scaling back of the pantheon of planets to eight objects, rejecting Pluto and other similar bodies.
At first one might think that this definition defines a size or mass at which planetary processes might arise, but this is not the case. For an object to be a planet, it must be more and more massive as its distance from the sun increases, in order to have either accreted or gravitationally scattered other bodies in its orbit over the age of the solar system.
The definition has many problems. With the term ‘cleared’ itself undefined, it has raised the question of whether Jupiter, with clouds of asteroids straddling its orbit, or Earth, being occasionally impacted by asteroids, could be strictly considered planets — something that will be the predictable bane of high-school teachers who try to teach what was on the mind of the IAU at the time. More seriously, an Earth-sized body discovered in the Kuiper belt would not be considered a planet, because only a much more massive object would have cleared its orbit.
To define a term that is in such broad use, like ‘planet ,’ one needs to consider how it is already being applied, primarily by what is considered to be ‘planetary .’
When the Cassini Huygens probe entered the atmosphere of Saturn’s moon, Titan, and returned images of a nearly craterless surface with what appeared to be rivers and lakes, and features that looked like they were formed by familiar tectonic processes, this distant object was described as “Earth-like” by members of the science team studying it.
Titan is clearly a planetary body. Were it in orbit about the sun and not Saturn, no one would have any problem in designating Titan as a planet, even if at Saturn’s distance it could not have possibly cleared its orbit over the age of the solar system.
The IAU definition is a spectacular pedogical and scientific failure. By taking a narrow dynamical view in defining ‘planet ,’ it has ignored large areas of scientific endeavors. A petition signed by more than 300 planetary scientists and astronomers rejects the IAU definition, stating they will not use it and that a better definition is needed. Signers include world-class experts in many areas of planetary science and scientists engaged in solar system exploration from the Mariner missions to today.
The public perception of astronomy and science in general has been harmed by confusion and controversy surrounding the IAU decision. Under its own rules, the IAU cannot reconsider its action until 2009. Fortunately, scientists and educators are neither bound by IAU decisions, nor constrained by its rules. The Internet gives the science community the means of having an open international discussion of planetary characteristics and a definition of ‘planet’ and ‘planetary bodies’ that more broadly captures those objects that share these characteristics. This will be a wonderful opportunity, lost by the IAU, to explain to the public what we have learned about our own solar system, the more than 200 planets around other stars, and what we expect may be discovered in the future.
Mark Sykes is director of the Planetary Science Institute in Tucson,