Not long ago, the solar system seemed to be a simple, orderly place. Anyone taking a trip to the local planetarium would have heard that the planets have 60 or so moons orbiting them on neat, nearly circular paths. That picture has now become a lot messier. Over the past 6 years, astronomers have discovered a passel of new moons around Jupiter, Saturn, Uranus, and Neptune, more than doubling the known number.

Compared with Earth's moon and most of the other satellites that planetary scientists have studied throughout the solar system, these recently discovered bodies appear downright unruly. They swoop in and out of the plane in which the planets orbit the sun and have highly elongated, rather than circular, paths. Some even orbit in the direction opposite to the rotation of their host planets. Because of these strange features, these objects are known as irregular satellites.

Most irregulars are tiny--less than 100 kilometers in diameter, or smaller than one-thirtieth the size of Earth's moon--and can barely be detected from Earth. Many reside so far from their planetary chaperones that gravity barely holds them in place.

Besides revealing our solar system to be far more cluttered than astronomers had suspected, these piffling objects are providing new clues about what conditions were like during the system's infancy. In particular, these moons may reveal details about a critical, last step in the formation of the outer planets.

For studying planet formation, irregular satellites are "one of the last [unexplored] frontiers," says Matthew J. Holman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

SATELLITE REVOLUTION Until recently, discoveries of irregular satellites were nearly as irregular as are the satellites themselves. For more than a century after researchers spotted the first small irregular satellite, Neptune's Triton in 1846, only a handful had been found. Then, in 1997, a windfall began.

At observatories around the world, exquisitely sensitive solid-state light detectors, known as charge-coupled devices (CCD), had superseded photographic film, enabling astronomers to record objects hundreds of times fainter than ever before. Moreover, using large--format cameras consisting of millions of CCD pixels, researchers could search for the faint objects over large patches of sky.

Those were just the right tools for finding irregular moons, Philip D. Nicholson of Cornell University realized in September 1997. Nicholson and Brett Gladman, now at the University of British Columbia in Vancouver, were traveling to the medium-size Hale Telescope at Mount Palomar in California to search for objects in the Kuiper belt, the reservoir of comets that lies beyond the orbit of Neptune. But while on the airplane en route to Mount Palomar, Nicholson calculated that Uranus would be in the same field of view. He and Gladman decided that during their two nights at the Hale telescope, they and their colleagues would devote any spare time to a search for outlying moons around that planet.

The team succeeded. Gladman and his collaborators discovered the first two irregulars known to orbit Uranus.

A flurry of discoveries followed. Since Gladman's finding, his team and another have spotted 70 more of the irregular satellites. Astronomers announced the latest find, an irregular satellite of Uranus, in an Oct. 9 circular of the International Astronomical Union.

The number of moons added to the roster since 1997 is "stunning," says Joseph A. Burns of Cornell.

Finding irregulars isn't just a matter of scouring the sky with a sensitive detector. In a single snapshot of the heavens, a tiny moon can look just like a galaxy billions of light--years distant. But there's one distinguishing feature: The motions of moons are discernible as each inches across the sky in synchrony with the planet it orbits. In contrast, distant galaxies appear to remain still.

Astronomers use two strategies to identify distant moons. One method, recently employed by Holman, J.J. Kavelaars of the National Research Council of Canada in Victoria, and Gladman uses a medium--size telescope to take a dozen or so precisely timed images of the same patch of sky during a single night.

In each image, they then shift all objects back to the position they would have had, if they truly were moons, at the time when the astronomers took their first exposure of the night. Finally, they combine the images.

In this so-called shift-and-add technique, the objects that are satellites end up in exactly the same position in each superimposed image, producing a bright, easy-to-spot point of light.

The other strategy, adopted by David C. Jewitt and Scott S. Sheppard of the University of Hawaii in Honolulu, uses a large telescope capable of finding extremely faint bodies in individual images. The astronomers take three sequential images of a patch of sky. A computer scans the trio for any object that has changed position from one image to the next. Starting with that information on the object's location, the team can then track the candidate satellite with a smaller telescope to discern its orbit and motion.