New moons for Saturn

Move over Uranus, the ringed planet is now king.

When the Cassini spacecraft arrives at Saturn, it will have more to look at than scientists had expected. Astronomers last month reported the discovery of four new moons orbiting Saturn. If confirmed, the findings raise the ringed planet’s moon count to 22, making it the solar system’s record holder. Uranus is the runner-up, with 21 known moons.

All four of the new moons are outliers, orbiting at least 15 million kilometers from Saturn’s surface. Brett Gladman of the Observatoire de la C´te d’Azur in Nice, France, and his colleagues spotted two of the moons on Aug. 7 using the European Southern Observatory’s 2.2-meter telescope in La Silla, Chile. Gladman and J.J. Kavelaars of McMaster University in Hamilton, Ontario, then recorded the same two objects, as well as two other moons, in late September at the Canada-France-Hawaii Telescope on Mauna Kea in Hawaii.

An estimated 10 to 50 km across, the objects show up in images taken an hour apart as faint dots of light moving around Saturn. The moons have looping orbits, and astronomers propose that they were captured by Saturn long after the planet formed.

In contrast, the inner moons of Saturn, which have nearly circular orbits, probably coalesced from the disk of dust and gas that surrounded the planet during its formation. Gladman’s team describes its findings in Oct. 25 circulars of the International Astronomical Union.