CHARLOTTESVILLE

--How many planets are there in our solar system? This seemingly simple question has plagued astrono-mers for decades but by the mid-1990s had grown into a glaring deficiency that could no longer be overlooked. The problem astron-omers faced was that in science one desires clear and well-defined terminology, and such a definition of a planet was lacking.

The definition crafted by the International Astronomical Union in 2006 essentially classified Pluto as one of the larger members of a belt of icy objects that lies more distant from the Sun than Neptune. Known as the Kuiper Belt, this group is analogous to the belt of rocky asteroids located between the orbits of Mars and Jupiter. Both collections are thought to be remnants, leftover bodies that did not get incorporated into the larger planets, and understanding them is key to understanding the solar system's formation.

The original meaning of the word "planet" was simply a point of light in the sky that moved relative to the fixed stars. Over the centuries we have learned more about their nature, and included the Earth as one of them. The question of Pluto's status was forced when, in the 1990s, astronomers began to discover other objects beyond Neptune, many similar to Pluto in size and appearance. One of these, now called Eris, was especially problematic because it is larger than Pluto and was dubbed the 10th planet when it was discovered in 2005. It raised the concern of how to deal with future discoveries of similar objects. Could we end up with dozens or even hundreds of planets?

In order to classify anything one needs not only to know the object's properties but also the criteria for the classification. Is the tomato a fruit or vegetable? To answer, we need to know what constitutes a fruit and a vegetable as well as the relevant properties of the tomato. Neither category is better or worse than the other. Such is the same case with planets. "Is Pluto a planet?" is only half the question, the rest being: "What is a planet?"