THE TENTH PLANET.

There are nine planets in our solar system. Or there are eight. Or there are ten, or maybe even twenty-three. No one knows. A planet is an arbitrary thing, a notion. In a matter of weeks, however, the International Astronomical Union, meeting in Prague, is likely to issue a definitive description. For many years, the planets have been the nine largest objects in orbit around the sun. The case for nine regards Pluto as the ninth planet, as it has been since it was discovered, in 1930, by Clyde Tombaugh, who grew up on a farm in Kansas and was twenty-four when he found Pluto, after painstakingly observing the sky for months by means of photographs he took through a telescope in Arizona. The case for eight eliminates Pluto. It says that Pluto, the smallest planet and the most remote, is more properly regarded as a transneptunian object--that is, a body of rock and ice resident in the region past Neptune. The transneptunian region is also called the Kuiper Belt, after Gerard Kuiper, who predicted its existence in 1951; it was discovered in 1992. Pluto is the second-largest transneptunian, or Kuiper Belt, object. The case for ten planets accepts Pluto and considers the largest Kuiper Belt object, currently called 2003 UB313, as the tenth planet, since it is larger than Pluto. The argument for twenty-three planets regards a planet as any big, round object in our solar system. Very large bodies can only be round--the force of their gravity insists on it. If you were to disassemble one somehow--if you could take a hammer to the moon, for example, and shatter it like a piece of pottery--it would become round again. Slightly smaller than Pluto is the size at which an object might deviate from round, so Pluto and perhaps fourteen other objects qualify.

2003 UB313 was discovered on January 5, 2005, by an astronomer named Mike Brown, a professor at Caltech. It is the largest object discovered in the solar system since Neptune's moon Triton, in 1846. Beginning in the late eighteen-hundreds, astronomers spent a good deal of time searching for a new planet, which they called Planet X. Eventually, they decided that all the planets had been found, and the search was abandoned. Partly in their honor, Brown calls his object Xena. Brown, however, is a serious person, and does not intend to suggest Xena as a name. He wishes he could call it Proserpina, after Pluto's wife. Pluto was the lord of the underworld, and Proserpina lived a third of the year in the underworld with him and the rest on earth. When she was underground, the earth was in winter. Xena spends part of its orbit near Pluto's, and the rest at some remove. Unfortunately, there is already a minor planet called Proserpina, and the I.A.U.'s rules won't allow the name to be used twice.

Brown found Xena in a region of the sky where astronomers hadn't expected there to be a planet. Other than Pluto, the planets pursue their orbits on a plane that is called the ecliptic, which is very roughly level with the equator. Pluto's orbit is at seventeen degrees to the ecliptic; Xena's is at forty-four. Excluding the Milky Way, which is so bright that it is impossible to take clear photographs of territory near and within it, Brown intends to examine a hundred per cent of the sky in the Northern Hemisphere. He has so far seen two-thirds of it. He suspects that he might find in the Southern Hemisphere other objects the size of Xena. He has already found the seven largest objects in the solar system that aren't currently thought of as planets--he has also found the ninth largest--all of which reside in the Kuiper Belt. If the I.A.U.'s definition allows twenty-three planets, Brown will have found eight of them, more than anyone else ever has. He will, in other words, have discovered nearly half of all the planets.

Brown began teaching at Caltech in 1997, when he was thirty-one years old. On a stormy night, he was at the university's Palomar Observatory, waiting for the weather to clear so that he could use a telescope he had reserved, when he decided to take a walk. Palomar is on a mountain, and at the time had four telescopes. (It now has six.) He came to one that he had not visited before, the Samuel Oschin Telescope, which is relatively small. Unlike the newer telescopes designed to look at tiny areas of the sky, this one took in large areas. If you were to hold your hand at arm's length, with your palm facing the sky, it would cover approximately the area that the telescope encloses. The big ones cover roughly the amount of sky framed by the head of a pin held at arm's length. No one was using the Oschin telescope, and Brown realized that its range would allow him to look at whole regions of the sky and find objects that he could point the big telescopes at. "It isn't that I had thought to look for planets," he says. "It's that if you don't have this telescope you don't know that it can be done."

In the middle of July, 1998, Brown began looking at the sky above Palomar. The telescope recorded what it saw on photographic plates that were fourteen inches square. The plates were exposed for twenty minutes, and then developed like film. The only way to know whether an object in a frame is a star or a planet is to observe it moving. "You look at one picture, there's a star," Brown says. "You look at two, there's still a star, but it's moved. It doesn't take college training to know that." Brown took photographs of each section for three nights in a row. "If you have enough plates and enough time to cover a particular swath," he says, "presumably you're eventually going to enter the swath where any new planets might be." He did this for three years, then he ran out of plates. (Kodak, the manufacturer, had stopped making them.) He had covered about five per cent of the sky, a not insubstantial area. On July 31, 2000, he wrote in his logbook, "Found nothing. But how deep did I go?"

As Brown was preparing to write up his results, which would take a year, he heard from some astronomers at Yale who had a new digital camera that could capture large tracts of sky. They needed a telescope to try it out on and wondered if they could share the Oschin telescope with him. With a digital camera, Brown could photograph a patch of sky three times in one night, instead of once every three nights. The digital format absorbs the image more quickly, can photograph objects that are fainter, and requires no developing. Also, the image could be installed immediately on Brown's computer.

In 2000, Brown made a bet with a friend that someone--he hoped it would be him--would find a new planet before December 31, ...