The rise and fall of Planet X: Neptune and Pluto were supposed to "fix" the weird orbit of Uranus. Now, it seems, the orbit wasn't "broke."

In any well-designed, well-conducted survey, you don't stop just because you've discovered something. By completing the survey, you might discover much more. So for the next thirteen years Tombaugh searched more than 30,000 square degrees of sky (out of a total of 41,253 square degrees). He found no more planets with a brightness equal to or greater than that of little Pluto. But his time wasn't wasted. The survey revealed hundreds of asteroids, six new star clusters, and a comet.

But was Pluto the Planet X of everybody's suspicions? Nope. Over the decades, as the measurements of Pluto's mass became more and more accurate, astronomers learned how little the place really is. Turned out it's far too small to account for the residuals of Uranus and Neptune. So Planet X still had to be lurking, undiscovered, in the outer limits of the solar system.

That, at least, was the prevailing belief until May 1993, when E. Myles Standish Jr. of the Jet Propulsion Laboratory in Pasadena, California, published a paper in the Astronomical Journal titled "Planet X: No Dynamical Evidence in the Optical Observations." Standish used the updated mass estimates of Jupiter, Saturn, Uranus, and Neptune that had become available from the Voyager flybys; in the case of Neptune, the mass difference amounted to nearly 0.5 percent--quite large by today's standards.

Assuming that the masses derived from the Voyager missions were accurate (a wise move), and discounting a single set of suspicious measurements made at the U.S. Naval Observatory between 1895 and 1905 (another wise move), Standish recalculated all the orbital parameters. The result? The large systematic trends in the residuals of Uranus and Neptune disappeared, and the remaining small residuals were consistent with the observational uncertainties of the modern data. In plain English: the apparent anomalies in the orbits of Uranus and Neptune could be completely explained within the framework of the presently known solar system. In even plainer English: Planet X was dead. But was it buried?

Several years ago, shortly after Clyde Tombaugh died at the age of ninety, Pluto's planethood was thrown into question. Seven moons in the solar system are bigger. More than half its volume is ice, as is the case for comets. For a twenty-year stretch of Pluto's 248-year journey around the Sun, its elongated orbit takes it closer to the Sun than Neptune gets. And Pluto's moon, Charon, is massive enough to cause the center of gravity of the Pluto-Charon system to lie outside Pluto itself. Each of these distinctions has no counterpart among the other planets. Yet the Rose Center for Earth and Space in New York City got into big trouble with the national press and with third graders for being the first major public institution to demote Pluto in its exhibits on the solar system. Not only was Pluto not Planet X; now poor Pluto wasn't even a planet.

In a further insult to Pluto's ego, Caltech astrophysicists recently discovered Quaoar, an icy world in the outer solar system that (like Charon) checks in at about half the diameter of Pluto. It's made of the same stuff, but Quaoar orbits in a near-perfect circle, something Pluto can only dream about.