Without leaving my seat, I can visit Venus, Mars and Jupiter. We have sent out a host of planetary probes, and a dozen Web sites offer their spectacular images along with daily marvels acquired by the cameras of the Hubble space telescope. It is easy to imagine that we know everything about the solar system.

We don't, although certainly we are far ahead of the world's first astronomers, peering up into the night sky without benefit of telescopes or much knowledge of science. They did their best to make sense of what they saw, and they sorted objects into four major classes.

There was the sun and the moon. There were stars, thousands of bright points of light that twinkled and always seemed to be in the same relative positions. There were five planets, shining with a steadier light, moving among the stars but always appearing and disappearing on a regular basis. And there were comets, strange objects with fuzzy heads and long gauzy tails that apparently came from nowhere and usually were taken as a warning of dire events down on earth.

The seventeenth century invention of the telescope and the acceptance of the sun as the center of the solar system helped astronomers but left many mysteries. The earth went around the sun, so the number of planets was increased to six. However, the stars still were no more than points of light, and comets were unpredictable mysteries although probably a part of the solar system.

Then, in 1781, came a big surprise. William Herschel, a musician turned astronomer, discovered another planet, Uranus. This surprised scientists and dismayed people who cast horoscopes for a living. However, the solar system was now judged to be complete. The philosopher Hegel in fact proved, on strictly logical grounds, that there could be no more than the seven planets that were then known. The discovery of Ceres in 1801 could hardly have pleased him, even though it was small, more of a planetoid than a planet.

Even so, one could argue that the whole solar system was finally known. Ceres, plus other planetoids in similar orbits, filled up a gap in the orderly arrangement of planetary distances. No one needed another planet, except for one strange fact: Uranus proved to be a poor timekeeper. Using Newton's laws of motion and gravitation, the effects of the sun and planets on each other could be calculated exactly, and their positions predicted. Uranus, annoyingly, was not quite where it ought to be.

Well, Newton's law of gravity might not be exactly right, though it had done amazingly well everywhere else in nature. Or - the thought occurred independently and close to simultaneously to a young Frenchman and a young Englishman - was it possible that some unknown planet was out there, disturbing the motion of Uranus and causing it to show up a few seconds early or late at its predicted position?

John Couch Adams and Urbain Leverrier solved the highly complex problem of estimating where such a planet might be from the tiny discrepancies in the orbit of Uranus. Adams sent his solution to the Astronomer Royal, who did nothing with it whatsoever. Leverrier was luckier. His solution, which he completed a few months after Adams, was used in 1846 to discover the planet Neptune. This started a squabble over priority that in scientific circles seemed like a continuation of the Napoleonic wars. Today Adams and Leverrier are given equal credit.

Now the solar system, with eight planets and lots of planetoids, was finally complete.

Or was it? You may be able to guess what came next. After a few decades of observation, Neptune didn't seem to be quite where it ought to be according to calculation. By this time it no longer seemed preposterous that you might, with no more than a pencil and paper, be able predict the existence and position of a whole new world. Percival Lowell decided that another planet was out there, and he organized a search for it using calculations based on Neptune's irregularities of orbit. Lowell had an unfortunate habit of observing things that weren't there, such as networks of canals on Mars and a non-existent moon of Saturn. This time he was luckier, though he did not live to see the results. In 1930, 14 years after Lowell's death, Clyde Tombaugh of the Lowell Observatory discovered the planet Pluto.

That should have ended the story, at least until we had the chance to observe enough of Pluto's 248-year orbit around the sun to see if it keeps good time. However, there was a snag. While Neptune was a gas-giant planet, many times the mass of earth, Pluto was a midget, smaller than the moon. There was no way it could possibly produce gravitational effects big enough to notice. Percival Lowell, one of the original Boston Lowells, would have turned up his patrician nose at the idea that puny Pluto was his Planet X.

It was calculation time again, but now with computer power beyond the dreams of earlier generations. Suppose you took all the minute discrepancies in position for all the planets, and tried to calculate the mass of a single unknown planet and where it would be.

Those calculations were completed more than twenty years ago. The new planet was predicted to be as big as Saturn, far from the sun, and in an orbit that took it way out of the plane in which all the other planets move. I doubt if the results persuaded even those who presented them, and any search for Planet X must have been half-hearted.

So is the search over? Not quite. Unknown worlds can perturb more than the motion of the planets. They also perturb the comets, those largely unpredictable celestial visitors that often sweep in toward the sun from much farther out than any known planet. A comet of unusual size, more than 400 kilometers across, was discovered in February, 2000. It forms one of the "trans-Neptunian objects," of which hundreds are known. Odder than this comet's size, however, is its path in space, which at its closest point to the sun is well beyond the orbit of Neptune and at its farthest is 13 times that distance.

The logical way to explain such an orbit is through gravitational perturbations. An unknown planet the size of Mars could do the trick. And such a planet is likely to have gone unobserved if it is, say, four times as far from the sun as Neptune. It would be faint and slow moving, and if seen at all it would easily be taken for a dim star.

The solar system still contains mysteries. Planet X may be out there after all. And Percival Lowell may one day grumble from beyond the grave, "I told you so."

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