Planets and Dynamical Dominance: History and Prospects

Since the time of Copernicus, the number of objects called "planets” has changed along with our understanding of the architecture of the solar system. Former "planets” include the Sun, Moon, the first 15 known asteroids, and Pluto.

The IAU definition of a planet as a heliocentric body that "has cleared the neighborhood around its orbit” is problematic. Clearance is never complete because the asteroid and comet reservoirs are leaky, and resonant planet-crossing orbits can be stable. A more accurate criterion for planetary status is "dynamical dominance". An object is dynamically dominant if it sweeps up or scatters other objects from its orbital zone in a time much less than the age of the system (theoretical criterion), and/or if its mass is much greater than the total mass of all other objects in its orbital zone (observational criterion), where "orbital zone” can also be simply quantified.

The eight planets in the solar system dynamically dominate all other heliocentric objects by orders of magnitude. Whether a comparable gap exists in other planetary systems remains to be seen.

Numerical experiments suggest that our solar system is dynamically full, with almost no room left to insert another planet in a stable orbit between the existing ones. Dynamical studies of other planetary systems suggest that this may be a general rule. The "packed planetary system” hypothesis of Raymond and Barnes recently led to the successful prediction of a planet (HD 74156d), the first since the discovery of Neptune in 1846. It suggests that other systems may also contain a distinct set of planets that dynamically dominate swarms of smaller objects.