Posted on: Friday, May 6, 2005

Newly found Saturn moons may shed light on mystery

By Kevin Dayton
Advertiser Big Island Bureau

HILO, Hawai'i — Astronomers using telescopes on Mauna Kea have found another dozen moons circling Saturn, bringing the total number of known moons orbiting the planet to 46.

Co-discoverer David Jewitt of the University of Hawai'i said the "irregular" moons may help solve the mystery of how the largest planets in the solar system managed to trap passing objects and hold them as satellites. Since such captures would be impossible under the conditions in the solar system today, study of the new moons could provide hints about what conditions were like at the time the satellites were trapped, which happened at about the same time as the planets formed.

Jewitt, a UH physics and astronomy professor, said the new finds were made Dec. 12 using a wide-field camera on the nearly 27-foot-diameter Subaru telescope on Mauna Kea. They were confirmed in recent months with observations by the Subaru and the Gemini North and the W.M. Keck telescopes on Mauna Kea.

The newly discovered moons range in size from about 2 to 4 miles across.

"Regular" satellites move in almost circular orbits close to the equatorial planes of their planets, which suggests they were formed through a fusing of dust or other debris trapped along the plane early in the history of the planet. The orbits of "irregular" satellites tend to be larger and move at odd angles from the equatorial plane. They also may orbit opposite the planet's spin.

To astronomers, those differences suggest the irregular satellites were originally orbiting the sun and somehow became trapped around Saturn and other planets.

It would be impossible for Saturn to capture such objects in its gravitational field today because in the vacuum of space there is no force strong enough to slow the objects from their vast orbit around the sun and allow them to drop into much smaller orbits around a planet, Jewitt said.

Scientists theorize that perhaps the planets once had vast atmospheres, and those gases created a drag force that slowed passing asteroids enough that they could be captured. Another possibility is that the gravitational pull of the planets abruptly increased in the late stages of the planets' formations, which allowed them to trap nearby objects.

Jewitt said that under a third theory called the "3-body capture," a collision of two objects near a planet could reduce the kinetic energy of one of the objects enough so that it could become trapped in an orbit around the planet.

Reach Kevin Dayton at kdayton@honoluluadvertiser.com or (808) 935-3916.