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."

But once again, all was not orderly in the solar system. Uranus was still behaving badly. Its orbital residuals got smaller, but they didn't go away, even with the gravity of Neptune accounted for. And Neptune's orbit had some residuals of its own. Could yet another planet be waiting to be discovered?

In 1894 Percival Lowell, an independently wealthy American astronomer, built the eponymous Lowell Observatory in Flagstaff, Arizona. Lowell indulged a fanatical fascination with Mars, claiming that intelligent civilizations were in residence there, but he devoted most of the rest of his life to the search for the object he called Planet X ("X" for the algebraic unknown)--the mysterious body in the outer solar system that continued to perturb Uranus and Neptune.

One way to look for a planet is to make two photographs of the same patch of sky through a telescope, several days (or years) apart. But the next step is the rub: nobody wants to pore over images of the sky made up of countless millions of dots, hoping to spot the one that moved between one photo and the next. Fortunately, an ingenious mechanical-optical device known as a "blink comparator" would come to the rescue. This contraption, an early-twentieth-century innovation, exploits the remarkable ability of the eye to detect change or motion amid an otherwise unchanging field. First you place the two photographic images side by side in precise alignment. Next, you flash the two images back and forth in rapid succession. Against the background star field, any speck on the two photographs that brightens, dims, or shifts position from one photograph to the other becomes immediately apparent. In the search for Planet X, the blink comparator minimized many sources of human error, including spurious measurements made by sleepy astronomers in the middle of the night.

At about four in the afternoon on February 18, 1930, a twenty-four-year-old amateur astronomer named Clyde W. Tombaugh discovered Planet X. He had been hired the year before by the Lowell Observatory to continue the arduous search. (Lowell himself had died in 1916.) The young fellow was looking at a pair of photographic plates he had taken on January 23 and 29 of the region around the star Delta Geminorum. Tombaugh became the third and last person ever to discover a planet in our very own solar system. On March 13 the observatory announced the news.

In Tombaugh's day many people associated the name given to the ninth planet with Pluto Water, a widely used laxative bottled on the grounds of the palatial French Lick Springs Hotel in Indiana, about fifty miles south of Bloomington. Other suggestions for names included Artemis, Atlas, Bacchus, Constance, Lowell, Minerva, Zeus, and Zymal. But "Pluto" eventually triumphed because Pluto is, after all, the god of the underworld, the realm of darkness--and what else, if not darkness, prevails four billion miles from the Sun? And because Jupiter and Neptune are Pluto's mythological brothers, the name also maintains a happy family. Finally (and perhaps fortuitously), the first two letters of "Pluto" are the initials of Percival Lowell, who instigated the search in the first place. [See "Pluto's Honor," by Neil deGrasse Tyson, February 1999.]