The catalog of moons orbiting the outer planets has grown dramatically since the late 1990s. Large arrays of sensitive, electronic detectors have allowed astronomers for the first time to rapidly survey large areas of the sky, yielding a bonanza of tiny moons orbiting Jupiter, Saturn, and Uranus.
Although recent findings have raised the retinue of Jupiter’s known moons to 28 (SN: 1/13/01, p. 24), Saturn continues to lead the pack. A team led by Brett Gladman of the Observatoire de la Cote d’Azur in Nice, France, had previously found six small moons of Saturn (SN: 12/9/00, p. 376; 11/4/00, p. 298), bringing the planet’s count to 24. As of February, the team had found six more Saturn moons, upping the count to 30.
The 12 new moons are classified as irregulars, meaning that their orbits are highly inclined relative to Saturn’s equator and that they move in elliptical, rather than circular, paths. The moons are tiny, less then 50 kilometers wide, and are outliers, orbiting Saturn at distances of 10 million to 20 million km.
Gladman and his colleagues found all 12 new moons using telescopes in Hawaii and Chile. “This has been an exciting couple of months, although extremely exhausting, as we chased all these moons, reduced a titanic amount of . . . data, and correlated all the information to make sense of it,” says Gladman.
Several of the newly found moons orbit opposite to the direction in which Saturn rotates, he notes. Likewise, the irregular moons of Jupiter and Uranus move opposite to the direction of rotation of their planets and their other moons. These retrograde orbits have led astronomers to speculate that the irregular moons were captured by the fully formed planets from the vast reservoir of icy debris in the young solar system.
If that theory is correct, Neptune should also harbor a group of irregular moons. None has been found yet, but this could be because the planet’s great distance makes such tiny bodies more difficult to discern from Earth.
“We hope to learn something about the timing and process of outer planet [formation] by unraveling the capture histories of these moons,” says Philip D. Nicholson of Cornell University.