There is an apocryptful story that Abraham Lincoln once posed the following problem to a friend: if you counted a horse's tail as a leg, how many legs would he have. Lincoln's answer was: still four, because calling a tail a leg doesn't make it a leg. This story comes to mind when I think of the fuss over whether to call Pluto a planet. Whatever we decide to call it won't change the nature of the object one bit. Shakespeare put it more briefly: a rose by any other name would smell as sweet.

There is a very good reason why astronomers have been concerned with the nature of Pluto that is rarely pointed out. In order to understand what that reason is, we need to consider the structure of our solar system, and the nature of our Sun's smaller companions.

Astronomers speak of an inner and an outer solar system. The sun and the planets as far out as Mars are in the inner solar system. Jupiter, and further objects form the outer system, with the asteroid belt forming a kind of transition region between the two systems. The inner one is much smaller than the outer one. Jupiter's distance from the Sun is nearly 3.5 times that of Mars, while Neptune is nearly 20 times further than Mars. If we were to represent the inner solar system by a golf ball, Neptune's orbit would correspond, roughly, to a basketball. The outer system is so much farther from the Sun than the inner one, it should not be at all surprising that objects in the two regions are very different.

The four planets, Mercury, Venus, Earth, and Mars of the inner solar system are called terrestrial planets. The Earth is the largest terrestrial planet, though Venus is only a little smaller. These planets are composed of metallic cores with rocky outer shells, or mantles. There are also four planets of the outer system, Jupiter, Saturn, Uranus, and Neptune. These outer, or Jovian planets, are composed primarily of hydrogen and helium gases under various stages of compression. The smallest Jovian planet, Neptune is a little smaller than Uranus, though it is somewhat more massive; it is some 17 times the mass of the Earth. Jupiter is more than 300 times the Earth's mass.

We can see that there are two families of planets, quite different in both their composition and the location of their orbits. These facts were well known by the time of the discovery of Pluto in 1930. Initially, and for more than a decade, it was thought that Pluto was both comparable in size and mass to the terrestrial planets.

It was entirely reasonable, in the mid-twentieth century, to consider Pluto to be the ninth planet of the solar system. Just why another terrestrial planet would be found beyond the Jovian family was a puzzle. Modern measurements make it clear that Pluto's mass is nearly 500 times less than the Earth's and its radius somewhat smaller than our Moon's. The mass and radius enable a good estimate of the density, about twice that of water, and roughly half the density of Mars. Thus Pluto's composition cannot be metal or rock like that of the terrestrial planets, nor is it primarily gaseous like the Jovian planets. Pluto must be largely ice, with only a small admixture of rock and metal.

The outer solar system is replete with objects similar in composition to Pluto. Of the four satellites of Jupiter found by Galileo, all four are more massive than Pluto. Two are larger in size, and two slightly smaller. The densities of the two outer Jovian satellites are about double that of water, like Pluto. Like Pluto, they are primarily made of water ice, with additional frozen methane and ammonia, along with small amounts of rock and metal.

Pluto was once thought to be an escaped satellite of Neptune, but this is now considered less likely. Indeed, Pluto has a satellite of its own, named Charon. In the last few decades of the twentieth century, a variety of objects have been found which orbit the Sun beyond Pluto. Some of these objects are comets, predicted to lie in a zone now designated the Kuiper Belt, after the Dutch-American astronomer Gerhard Kuiper. Comets are typically several powers of ten less massive than Pluto, though they are icy in nature. However, modern observations revealed more and more Kuiper-Belt Objects (KBO's), and eventually one was found that was demonstrably larger than Pluto itself.

With the discovery of the KBO's, it became logical for astronomers to think of Pluto as more closely related to them than to either the terrestrial or Jovian planets. Some astronomers unwisely began to say that Pluto was "not" a planet, and the rest has been a history of confusion and misunderstanding. Some of this misunderstanding is purely semantic.

The vast majority of astronomers know that the revision of Pluto's status has come along with a more complete knowledge of its size, structure, and composition. But in the media, and among amateurs, this rethinking has been construed as a demotion or even insult, and sides appear to have been taken. Inevitably, some appear to have taken an anthromorphic view, and feel embarassed that Pluto has been insulted.

In August of 2006, the International Astronomical Union (IAU) adopted a revised definition of the term "planet." According to this definition, an planet must (1) orbit the sun, (2) be sufficiently massive to be spherical in shape, and (3) be massive enough to have cleared its orbital region of minor orbiting bodies. Pluto fails to fulfill the latter requirement. Unfortunately, it is possible to argue that other legitimate Jovian planets fail on this same basis. Jupiter itself, is shadowed by a famous grouping of asteroids known as the Trojans, which follow virtually the same orbital path.

Most likely, astronomers will continue to call Pluto a planet--or not--as fits their context. Members of the laity may do so too, in the full knowledge that their usage will not change the nature of Pluto a whit.