Discovery of 12 satellites of Saturn exhibiting orbital clustering

The giant planets in the Solar System each have two groups of satellites. T he regular satellites move along nearly circular orbits in the planet's orb ital plane, revolving about it in the same sense as the planet spins. In co ntrast, the so-called irregular satellites are generally smaller in size an d are characterized by large orbits with significant eccentricity, inclinat ion or both. The differences in their characteristics suggest that the regu lar and irregular satellites formed by different mechanisms: the regular sa tellites are believed to have formed in an accretion disk around the planet , like a miniature Solar System, whereas the irregulars are generally thoug ht to be captured planetesimals(1). Here we report the discovery of 12 irre gular satellites of Saturn, along with the determinations of their orbits. These orbits, along with the orbits of irregular satellites of Jupiter and Uranus, fall into groups on the basis of their orbital inclinations. We int erpret this result as indicating that most of the irregular moons are colli sional remnants of larger satellites that were fragmented after capture, ra ther than being captured independently.