Astronomers using the Hubble Space Telescope say they've spotted at least two, and possibly four, new moons orbiting Saturn, in addition to the ringed planet's swarm of 18 known natural satellites.
Amanda S. Bosh of the Lowell Observatory in Flagstaff, Ariz., and Andrew S. Rivkin, a graduate student at the University of Arizona, reported this week that Hubble focused on Saturn on May 22, as Earth passed through the same plane as the debris-filled rings.
Rare events such as this one, called Earth ring crossings, provide an edge-on view of the rings and dim their reflected light. That makes it easier for astronomers to glimpse the planet's faint collection of small moons.
When Dr. Bosh and Mr. Rivkin studied 27 images, taken over 11 hours, they found four objects that seem to be orbiting the solar system's second largest planet.
Dr. Bosh estimated that the moons are from seven to 40 miles across.
The satellites orbit Saturn at a distance of between 85,000 to 91,000 miles from the planet's center.
It is possible, she said, that two of the objects may be previously discovered moons, Atlas and Prometheus.
If so, those satellites are not where they were supposed to be, based on calculations of their orbits.
The two astronomers are more certain that the remaining two moons have never been seen before.
If the discoveries are confirmed, the International Astronomical Union will, by tradition, give the new moons names drawn from Greek mythology.
Stephen Maran, spokesman for the American Astronomical Society, said the objects may be too small and dim to be seen by ground-based telescopes.
Research with the orbiting Hubble is directed out of Baltimore's Space Telescope Science Institute. The satellite is run by NASA's Goddard Space Flight Center in Greenbelt.
"As we get better resolution with Hubble, we're seeing smaller things than ever before," Dr. Maran said.
Even before this week's announcement, Saturn's 18 confirmed moons was the most of any planet. (Several other moons of Saturn have been reported, but their existence has not been confirmed).
Jupiter has the next largest group of known moons, Dr. Maran said, with 16. Uranus is third with 15.
A Cornell scientist plans to use Hubble to make images of Saturn's rings during another Earth ring crossing on Aug. 10. That could yield the data needed to better define the orbits of the objects spotted in May, Dr. Bosh said.
"Once we do, we can definitely say these are not previously known moons," she said.
Dr. Maran said that, as telescopes improve and smaller objects become visible, astronomers face a conundrum: When is a moon just a rock?
"Planetary scientists have not yet faced up to that," he said.
In the meantime, Dr. Bosh and Mr. Rivkin are reading about Greek and Roman gods, compiling a list of recommended names for the astronomical union.
"We're not very well versed in our mythology," she said.