A Planet By Any Other Name
What is a planet? This seems to be an embarrassing question for a planetary scientist to be asking, but since the International Astronomical Union (IAU) passed a resolution in summer 2006 defining a planet, it has become a topic of increasing discussion and some controversy. At the 40th annual meeting of the Division for Planetary Sciences (DPS) of the American Astronomical Society being held this week at Cornell University in Ithaca, New York, there was even a special session devoted to the topic.
Earlier in the week I attended a talk by Dr. Alan Stern, Principal Investigator of the New Horizons mission to Pluto, at the University of Central Florida where he was visiting our research group. Stern made an eloquent case for ditching the IAU’s planet definition in favor of one that is more inclusive. Yesterday, at the DPS meeting here in Ithaca, I heard an equally eloquent argument by Stern’s colleague Dr. Hal Levision of the Southwest Research Institute defending the IAU definition.
The IAU definition says, in short, that a planet is an object that is large enough to be round and that orbits a star and which has “cleared” its orbit. Large enough to be round means, for all practical purposes, about 500 km in diameter. Stern would leave it at that. That is, he favors a definition of planet based on the intrinsic properties of the object independent of where it is in the universe and what effect it has had on other objects. With this definition, the geophysical planet definition, the Moon is a planet, as are Pluto, the large moons of Jupiter, many objects in the Kuiper Belt as well as Ceres, the largest asteroid. The geophysical planet definition makes no requirement on a planet orbiting a star instead of orbiting another planet or indeed drifting through interplanetary space. It simply must not be a star, which is defined as an object that now or in its past has undergone some form of nuclear fusion in its interior, and must be large enough to be in hydrostatic equilibrium (that is, roughly round due to the force of gravity overcoming its internal strength). With this definition, there are hundreds, perhaps thousands of planets in our solar system, depending on the unknown population of large objects in the Kuiper Belt.
Perhaps the main problem with the geophysical planet definition, as pointed out by Levison at DPS, is that it does not provide a clean break between planets and non-planets. In any planetary system, including our own, there will be objects just smaller than the minimum size for hydrostatic equilibrium that are neighbors of “planets”. So, there could be a spherical object - a planet - in the Kuiper Belt, and another object virtually identical in every way and in nearly the same orbit, that does not meet the “round” criterion and so would not be a planet. This seems arbitrary.
The dynamical definition, the one adopted by the IAU, requires that the object gravitationally dominate its particular region of the solar system. The awkwardness of this definition is that if one could move Earth sufficiently far from the Sun, it would no longer have the gravitational influence necessary to clear its new much larger orbital zone and would not be a planet. Conversely, Pluto would be a planet under this definition if it were much closer to the Sun. The advantage to this definition is that if one looks at the distribution of objects in the solar system, taking into account their sizes and their orbits, there is a very clear break between “planets” and small solar system bodies and “dwarf planets” (the latter being objects that meet the round and star-orbiting criteria of the IAU, but not the orbit-clearing one).
To avoid the problem posed by this dynamical evolution definition (that, for example, Earth would not be a planet if further from the Sun, or in fact, if it orbited Jupiter), Levison proposes modifying the definition so that instead of having one based on clearing an orbit, it is based on not being part of a continuous size distribution of objects in its region of the solar system. Thus, Jupiter in the plot above is not the only object in its orbit (so it has not technically “cleared” its orbit), but there is no object near its size. With this size distribution definition, “planet” refers to objects that are distinctly larger than any other objects in their orbital zone. It also leads to a distinct group of 8 planets in our solar system.
Over the next several years, this question will probably be resolved by the evolution of common public and professional usage rather than new IAU definitions. In the meantime, though, it has provided some interesting discussion on the apparently basic topic of what it means to be a planet.