Finally, the International Astronomical Union has produced a definition of a planet, and Pluto does not qualify. This demotion of the “ninth planet” has been long overdue, and will make possible a more coherent description of our solar system.
Predictably, many have protested this decision, based on sentimentality or nationalist bias (since its discoverer was an American), as well as criticisms of the ambiguous and arbitrary nature of the definition. None of these criticisms can escape the fact that there is no consistent physical description of a planet that could include Pluto without also including several other existing asteroids, as well as potentially dozens of other trans-Neptunian objects yet to be discovered. Pluto has met the same fate as Ceres did in the nineteenth century. Originally thought to be one of a kind, it has been found to be smaller than initially thought, and but one of many objects in an asteroid belt.
Of course, in one sense it is certainly arbitrary to make a distinction between a planet and an asteroid, since both may have similar composition and orbital behavior. Nonetheless, the distinction that a planet dominates its orbital neighborhood suggests a further stage of development and greater prominence in a solar system. It is no argument to complain that with this new definition we may not know the true status of an object until we have explored the area around it. Many objects are necessarily classified ambiguously or erroneously due to lack of information upon discovery.
When we consider that there are only eight planets, we find many striking similarities among them that are not shared by other objects, such as Pluto. All have metallic cores, and all orbit the sun in the same direction, in the same plane as the sun’s rotation within a few degrees, in ellipses with low eccentricities that all have the same perihelion-aphelion orientation. The fact that the planets have such striking orbital similarities suggests that they formed around the same time as the solar system. We have four terrestrial planets, followed by a rocky asteroid belt, then four gas giants, followed by an icy asteroid belt. The asteroid belts are probably leftovers from planetary formation. Lastly, the comets, with their extremely eccentric orbits, are possibly stray interstellar objects caught by the sun’s gravity. At any event, this model is much more conducive to examining the problem of planetary formation than one that makes no distinction between the elegantly aligned eight planets and the chaotic asteroid objects that failed to attain this harmonious state.