Saturn's status as commander of the largest retinue of satellites in the solar system has been restored. Today, an international team of astronomers announced the discovery of four new moons orbiting the ringed planet, bringing the total to 22--one more than Uranus's 21. The discoveries should help researchers understand not just how the new moons were formed but also how the giant planets themselves came to be.
At the meeting of the Division for Planetary Sciences in Pasadena, California, astronomer Brett Gladman of the Observatory of Nice and seven colleagues reported that state-of-the-art light detectors revealed four new bodies 10 to 50 kilometers in diameter that "almost certainly" are orbiting Saturn. Although their orbits have not been determined yet, the new moons are probably "irregular" satellites. Whereas most major satellites form from dust and gas orbiting a planet, irregulars are outsiders captured by a planet into distant, inclined, and sometimes highly elongated orbits.
Two groups of four irregular moons orbit Jupiter in opposite directions. Astronomers take that arrangement as a sign that two large bodies approached a still-growing Jupiter, broke into pieces, and went into orbit. With four more examples to study, astronomers may be able to choose between two theories of how precursors of irregular satellites were captured. One model involves close encounters or even collisions with existing moons; the second points to drag from the last wisps of gas accreting to a new planet.