It is a natural part of human nature to want to divide objects into discrete classes, whether such objects be people, biological species, types of rocks, whatever. Nature, however, appears to adopt the opposite stance, in that it almost seems to delight in filling the world with an entire spectrum of "in-between" objects that defy our attempts at classification. We often come to the point where our classification schemes are based upon standards that are arbitrary and outmoded, and there are times when we are forced to admit that such schemes are essentially obsolete.

This process certainly takes place throughout the world of astronomy. One such example concerns the objects we call "planets;" we have been raised with the idea that there are the nine planets in our solar system -- Mercury through Pluto -- and that any other planets we might find, either in our solar system or around other stars, should be similar to these objects. Characteristics of "planets" would thus include traits such as orbiting (in near-circular orbits) around the sun (or the parent star), being "large" but not "too large" (i.e., not being significantly larger than Jupiter), being made of either rocky materials like our solar system's inner planets or of gas like the gas giant planets of our solar system, and so on.

Right away, though, this definition of "planet" is challenged by long-known objects in our solar system. There are two moons -- Ganymede around Jupiter and Titan around Saturn -- that are larger than Mercury, but because they don't orbit directly around the sun they aren't considered "planets." The discovery in 1978 of a moon orbiting around Pluto has shown us that that "planet" is far smaller than we had previously believed; not only is Pluto smaller than the above two moons, but also smaller than five others (the Earth's moon, Triton around Neptune, and the other three Galilean moons around Jupiter -- Io, Europa, and Callisto).

The discovery within the past several years of over one hundred "planets" orbiting around other stars has also severely challenged our ideas of what a "planet" should be. We have found "planets" many times larger than Jupiter, and we have found "planets" traveling in distinctly elongated orbits. Perhaps most surprising are the so-called "hot Jupiters" -- Jupiter-sized objects orbiting extremely close to their parent stars, where the surface environments are so hot that even iron would be molten.

Another challenge to our entire idea of "planets" has come with the discovery of objects in the so-called "Kuiper Belt" beyond Neptune's orbit. The first such object was found just over a decade ago, and as of today over 650 have been found. A handful of these objects are quite large -- perhaps one-fourth to one-third the size of Pluto -- and this fact has spawned discussion of whether Pluto should be considered a true "planet," or just an unusually large Kuiper Belt object.

The discovery of the largest Kuiper Belt object found so far was announced just last week. This object, dubbed 2002 LM60, was discovered by CalTech astronomers Michael Brown and Chad Trujillo on photographs taken June 4 in the course of the Near-Earth Asteroid Tracking (NEAT) program which utilizes, among other instruments, the 48-inch Schmidt telescope located at Palomar Observatory in California. Images of 2002 LM60 were subsequently located on numerous old photographs taken during earlier years, some as far back as 1956; in one ironic twist, this list includes photographs taken in 1983 (also with the 48-inch Palomar Schmidt telescope) for the express purpose of searching for planets beyond Pluto, but the images of 2002 LM60 were evidently missed at the time. Brown and Trujillo also observed 2002 LM60 with the Hubble Space Telescope during July and August, which has helped pinned down its size and approximate surface characteristics.

2002 LM60, as it turns out, is approximately 800 miles in diameter -- a little over one-half the size of Pluto -- and, like Pluto, appears to have a surface at least partially covered with frost. It travels around the sun in a near-circular orbit some 300 million miles beyond Pluto's orbit, and takes 284 years to journey around the sun once. While this is not "official" yet, Brown and Trujillo have proposed that 2002 LM60 be named Quaoar, after the creation force in the mythologies of the Tongva people who inhabited southern California before the arrival of European settlers.

Quaoar is presently located in the constellation Ophiuchus, a bit to the northeast of the "head" of Scorpius, and is now very low in the southwestern sky after dusk. After being hidden on the opposite side of the sun for the next few months, it will be well placed for viewing next spring and summer. While it is too faint and distant to be visible in backyard telescopes, larger instruments equipped with modern electronic cameras should be able to record it.

It is rather likely that other Kuiper Belt objects the size of Quaoar will be discovered during the years to come, and it is actually well within the realm of possibility that Kuiper Belt objects the size of Pluto, or even larger, may someday be found. Such discoveries will, of course, continue to fuel the discussion as to whether or not Pluto should be considered a "planet." In this author's opinion, however, such discussions are essentially pointless; it is rather clear in the face of all the recent discoveries that the term "planet" as we have heretofore understood it is obsolete. This author instead prefers a term like "world" which can encompass such diverse objects as Earth, Jupiter, Pluto, Quaoar, Ceres (the largest main-belt asteroid), Titan, Europa, the "hot Jupiters," and all the many other exotic objects that are sure to be found during the coming years and decades. Such a term indicates that each of these objects is an individual and unique place, and worthy of study in its own right.