Gosh, am I glad I was not a member of the International Astronomical Union committee tasked with the thankless job of proposing a definition for “planet.” These seven unfortunate souls were inserted between the proverbial rock and hard place. Given the wide range of objects in our solar system, and the many different opinions within the astronomical community and general public, there was no chance that they could come up with a definition that would satisfy everybody.
So I don’t want to sound overly critical of the proposal, which has since been strongly endorsed by the American Astronomical Society’s Division for Planetary Sciences. The committee’s proposal essentially defines “planets” as nearly round objects that orbit stars. I don’t have a strong opinion as to whether the IAU General Assembly should vote “yes” or “no” to this proposal on August 24th (I bet it will pass by a large margin). The committee deserves credit for coming up with a definition based on physical principles that can be applied to objects inside and outside the solar system. But there are several glaring inconsistencies in the proposed definition that will open up a can of worms if approved.
For example, what exactly is meant by “nearly round?” The committee defines it as an object in “hydrostatic equilibrium” (i.e. its mass is sufficient for gravitational compression to overcome its material strength and force it into a nearly round shape). But where does one draw the line between an object that is in hydrostatic equilibrium and one that is just slightly out of hydrostatic equilibrium? If they haven't done so already, astronomers will find borderline cases, so the decision whether or not to include certain objects as planets will be arbitrary.
Making matters worse, the committee is including Pluto’s largest satellite Charon as a planet, because the system’s center of gravity is located in the space between the two objects. In other words, the committee members are saying that Pluto and Charon form a double-planet system.
That sounds clean cut, but it’s not, because the location of a system’s center of gravity depends both on the objects’ masses and their physical separation. At the moment, the Earth–Moon system’s center of gravity is inside Earth. But tidal interactions cause the Moon to recede from Earth by about 4 centimeters per year. In many billions of years, the center of mass will reside in outer space. Does this mean that future astronomers will suddenly have to change the Moon’s status from satellite to planet? This flies in the face of common sense. Given that Pluto is 7 times more massive than Charon (which means the system’s center of gravity is much closer to Pluto), a lot of folks will think it’s patently obvious that Charon should be considered a satellite rather than a planet.
For those who bemoan this new definition because it includes Pluto, I ask the following: what should we do when astronomers find a body, either in the outer solar system or around another star, that is halfway or two-thirds of the way between Pluto and Mercury in size? Such a discovery is just a matter of time. As I wrote in my essay last Thursday, wherever one draws the line that distinguishes planets from non-planets, it will be arbitrary.
And for astronomers who want to divide planets into various subclasses, like giant planets, terrestrial planets, and ice dwarfs (and the proposed definition recognizes Pluto-like objects as “plutons”), Mother Nature will always create objects that don’t fit cleanly into the categories. We already know of several extrasolar planets that would not fit into any classification scheme based on our solar system, and there are tens of billions of planets in our galaxy alone. These planets are going to display a bewildering variety of sizes, masses, orbits, physical characteristics, and so forth. And even if you count all the known round asteroids and Kuiper Belt objects in the solar system, we still know of many more planets outside our solar system than within it. Any sensible definition of “planet” must take them into account.
So despite all the arguments I have heard over the past few days, my position remains unchanged. The simplest way to define “planet” is to use Pluto as the minimum size of a planet, and state that any body found orbiting a star (or brown dwarf!) the size of Pluto or larger is a planet. And despite the fact that astronomy does not operate in a cultural vacuum, my conclusion is not based on sentiment or history, or the desire to prevent kids from having to memorize the names of dozens of planets. It’s based on the fact that Pluto has many characteristics that we commonly associate with planets: a respectable diameter that’s well above the minimum size to be spherical, an atmosphere, a multiplicity of moons, and probably rings. I freely admit that my definition is arbitrary, but I challenge anyone to come up with a less-arbitrary scheme.
Finally, most of the public’s attention has been focused on the low end of the planetary size regime. But the controversy extends to the upper end as well, and the proposed definition fails to address this problem in a satisfying manner. Basically, this issue boils down to the question of how astronomers should draw the line between planets and brown dwarfs. Currently, objects between about 13 and 75 Jupiter masses are generally considered brown dwarfs, because they briefly fuse deuterium in their cores (anything above 75 Jupiters is a star). But there are many gray areas. Should it matter whether an object orbits a star or another brown dwarf, or how it formed?
For example, Geoff Marcy and Paul Butler’s group found a star that has 7- and 17-Jupiter mass objects that are coplanar, meaning they probably formed in a disk. Should the 17-Jupiter-mass object be considered a planet because of its origin, or since it's above the deuterium-fusion threshold, should it be termed a brown dwarf? What about the dozens of known free-floating objects (not bound to stars) that have less than 13 Jupiter masses? Should we call them planets because of their low mass, do we call them sub-brown dwarfs since they probably formed in stellar-like processes, or do we have to adopt the horrible acronym PMOs or planemos, short for “planetary-mass objects”? What about the 5-Jupiter-mass object that orbits at a very far distance from the 25-Jupiter-mass brown dwarf 2M 1207? That system probably formed like a very-low-mass binary star, but the 5-Jupiter-mass bugger is well below the deuterium threshold. The proposed planet definition either fails to clarify many of these ambiguities, or it leaves us with unpleasant outcomes.
I want to make it clear that I don’t have a strong disagreement with the proposed definition, but it’s an imperfect solution to a complex problem. Don’t be surprised if there are modifications down the road.