An unprecedented surge in planetary moon discoveries continued this week as astronomers reported the discovery of four more moons orbiting Saturn, bringing the total number of moons found around the planet since October to ten.

The ringed-planet Saturn as seen by the Hubble Space Telescope. Photo: NASA/University of Arizona

The discovery of the four moons, temporarily designated S/2000 S 7 through S/2000 S 10, was announced Thursday in a pair of International Astronomical Union (IAU) Circulars released by the Central Bureau for Astronomical Telegrams, the clearinghouse for various astronomical discoveries.

The moons were found by an international team of astronomers led by Brett Gladman of France's Observatoire de la Cote d'Azur and JJ Kavelaars of McMaster University in Canada. Gladman and Kavelaars first spotted the moons September 23 and 24 using the 3.6-meter Canada-France-Hawaii Telescope atop Mauna Kea, Hawaii. Their existence was confirmed with observations in November by Kavelaars and others using one of the four 8-meter telescopes of the European Southern Observatory's Very Large Telescope in Chile, a 2.2-meter telescope also operated by ESO in Chile, and the 5-meter telescope at Palomar Observatory in California.

While the limited number of observations of the moons prevents astronomers from learning much about the moons or even computing accurate orbits, the moons do appear to be somewhat similar to the six other moons Gladman, Kavelaars, and others found orbiting Saturn in October and November, which are thought to be small bodies orbiting far from Saturn. Such moons are often called "irregular" since their orbits suggest they were not formed at the same time as the planet but rather captured at a later date by the planet's gravity. The moons may once have been members of the Centaurs, a class of small icy bodies that orbit the Sun between Saturn and Uranus.

The newly-discovered moons do appear to be fainter, and thus smaller, than those discovered earlier this year. The four moons discovered in late October, S/2000 S 1 through S/2000 S 4, have magnitudes between 20 and 22, corresponding to diameters between 10 and 50 kilometers based on estimates for their albedo, or reflectivity. Two additional moons located in November, S/2000 S 5 and S/2000 S 6, were slightly dimmer. The four newest moons, though, are between magnitudes 23 and 24, suggesting that they may each be only a few kilometers across.

The four moons bring to 28 the total number of moons orbiting Saturn, cementing its position as the planet with the most known natural satellites. Uranus, which once briefly led Saturn in total number of moons, is a distant second with 21. Jupiter follows with 18 while Neptune has eight. The surge in discoveries -- ten moons found around Saturn in just six weeks -- is virtually unprecedented in the history of astronomy. The only comparable period is late 1985 and early 1986, when scientists discovered ten moons of Uranus by analyzing images returned by the Voyager 2 spacecraft as it sped past the planet.

In addition to the ten moons found around Saturn, astronomers have also discovered six moons orbiting Uranus since 1997, five by astronomers, including Gladman, using Earth-based telescopes and one by a reanalysis of Voyager 2 images. Two moons have also been found around Jupiter since last year, including one linked to a 1975 discovery that was never confirmed. Astronomers credit this new surge in discoveries to dedicated searches using large telescopes and improved instruments that allow them to scan large areas around planets to look for the faint signatures of these moons.

All the moons recently discovered will require several months of additional observations to pin down their orbits and remove any slim possibility that some of these objects may be Centaurs or other bodies not orbiting Saturn that were coincidentally near the planet at the time they were discovered. The IAU circular that announced the discovery of S/2000 S 10 also commented that observations were particularly needed of one of the moons found in October, S/2000 S 3, while recent observations of S/2000 S 4 had significantly revised its original ephemeris, a list of predicted positions over time.