Ask a science-smart ten-year-old how many planets there are in the solar system and the answer "nine" will come without a pause. Many youngsters will just as quickly rattle off the order of the planets in distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

Unless some other planet were to be found farther out from the Sun, the subject might seem to be closed. Certainly we are not going to find any new substantial body near the Sun, because if any existed it would have been observed long ago. Since the discovery of Uranus by William Herschel in 1781, the heavens have been scoured - I was going to say "daily," though "nightly" makes far more sense. Johann Galle found Neptune in 1846, but he didn't get much credit because the existence and position of the planet had been mathematically predicted the previous year by John Couch Adams and Urbain LeVerrier, working independently. Pluto came last of all, found by Clyde Tombaugh in 1930 after a painstaking search that would have driven a less patient man crazy.

There has been talk ever since of a possible "Planet X," some massive new world orbiting far, far out. None has been discovered. Thus the proposal, made two years ago, that the number of planets should be changed, may seem odd indeed. Why change, when nothing new has been found?

The answer is, a number of astronomers want the number of planets to be reduced to eight. Pluto, they argue, is not really a planet at all. The natural counter to such a suggestion is to point out that Pluto undeniably orbits the Sun, even though a single revolution takes 248 years and we have so far observed only part of one. If Pluto isn't a planet, then what is it?

At this point, matters become a little more complicated. Millions of bodies orbit the Sun. Many of the largest ones, other than the planets already noted, have names. Most are located in a region of the solar system between Mars and Jupiter known as the Asteroid Belt. This is an unfortunate name, because "asteroid" means "star-like." Planetoids would be a much better term than asteroids, but the latter term is too well established to hope for a change. The first asteroid to be discovered, in 1801, is also the biggest. Ceres has a diameter of 974 kilometers, followed by Pallas (538), Vesta (526), and Juno (268). Ceres is almost half the size of Pluto. If Pluto is a planet, why not Ceres?

Part of the reason is that for a long time, Ceres and the other asteroids were thought to be fragments of a large planet that had disintegrated for reasons unknown. Today, astronomers argue that there never was such a world. Jupiter's gravitational field is so strong, the potential planet between it and Mars was never able to form.

The other reason for naming Pluto as a planet, while denying that honor to Ceres or any other asteroid, was that Pluto originally was thought to be far larger than it is. Percival Lowell, the man who originated the search for Pluto (but died 14 years before its discovery) was hoping to find a world the size of Uranus or Neptune, fifty thousand kilometers across. In fact, Pluto's estimated size shrank steadily from the time of its discovery, from more than ten thousand kilometers across to less than a quarter of that. One reason the size was overestimated was the presence of Pluto's own moon, Charon, not discovered until 1978 but almost half the size of Pluto itself. You might say, wait a moment. Doesn't the fact that Pluto has a moon of its own automatically make it a planet? It does not, because Mercury and Venus have no moons, while some asteroids do (for example, Ida has a moon, Dactyl).

The real reason for Pluto's proposed demotion is not its small size. In astronomy as in real estate, it's location, location, location. Pluto is in the wrong place. Yes, it can be looked on as at the outer edge of the region of planets, although for 20 years of its 248-year orbit it is actually closer to the Sun than Neptune (this was true from 1979 to 1999). However, Pluto can also be regarded as defining the inner edge of something known as the Kuiper Belt, or the Edgeworth-Kuiper Belt. This is a region of planetoids and large comets, whose existence was first suggested by the Irish astronomer K.E. Edgeworth in 1943. The Edgeworth-Kuiper Belt may extend to anywhere from two to twenty times the distance of Pluto from the Sun. Detection of objects out there is tricky, because at such distances the available light is so feeble.

An undiscovered planet at least the size of Mars could be in the more remote regions of the Edgeworth-Kuiper Belt. It would be faint and slow moving, and easily mistaken for a dim star. An object 400 kilometers across, admittedly far short of Mars or even Pluto but a substantial body nonetheless, was discovered in the Edgeworth-Kuiper Belt in February, 2000. There are believed to be hundreds or thousands of other "trans-Neptunian objects" out there.

Here, then, is the issue: which is it better to be? The first, most famous, closest, and perhaps biggest of the class of trans- Neptunian objects, or the smallest, farthest (usually), and coldest of the class of planets, a mere midget of a world compared with all the others?

This question has divided professionals in the field of planetary science into two camps. The first group feels that it represents the traditionalists and the issue was settled long ago. Pluto was a planet, is a planet, and will remain a planet. Demotion is unthinkable. Their opponents, oddly enough, also invoke tradition in proposing Pluto's change of status. They point out that in 1801, when Ceres was discovered, it was considered to be a genuine planet. Only when, in the next few years, Pallas, Juno, and Vesta were discovered did Ceres suffer its own demotion from planet to asteroid.

I grew up with Pluto as a planet, and I would be reluctant to see it downgraded to a mere object. However, I want to point out one human and admirable element in all the recent discussions. The tiny size and far-out position of Pluto were known for many years. The suggestion that Pluto is too small and insignificant to be a true planet remained unvoiced until 1999. I believe that the delay was in deference to its discoverer, Clyde Tombaugh, who died in 1997 in his ninety-first year.

Maybe that was a mistake. Tombaugh was a down-to-earth and practical man. It's a shame that he's not around for us to ask his opinion.

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