About a year ago, NASA's Cassini spacecraft, which has been orbiting Saturn since 2004, took a series of images of the planet's outermost rings, documenting an unusual protuberance along the outer ring rim. Now, astronomers report that the disturbance may be the result of a newly forming Saturnian moon.

Saturn's outermost rings are the planet's largest and brightest, but one of the disturbances documented in the April 15, 2013 photograph is an "arc" about 750 miles long, 6 miles wide and about 20 percent brighter than its surroundings.

NASA astronomers report that this arc, along with other observed protuberances in the usually smooth ring edge, may be caused by the gravitational effects of a nearby object, possibly a new moon.

Writing in the journal Icarus, the astronomers report that the object is not expected to grow any larger and could even be falling apart, but either way, it provides a rare opportunity to understand more about how Saturn's icy moons may have formed in the planet's rings long ago.

"We have not seen anything like this before," lead study author Carl Murray, of Queen Mary University of London, said in a statement. "We may be looking at the act of birth, where this object is just leaving the rings and heading off to be a moon in its own right."

The object, which has been informally named "Peggy." is too small to be seen in images so far, NASA reported.

Cassini will move closer to the edge of Saturn's outer ring in late 2016, where it will be able to study the object in more detail and perhaps even image it.

Theories into Saturn moon formation suggest that the planet's ring system was once much larger than it is today and that the moons were birthed from the rings. The planet's largest moons are also the farthest away from the planet, and it's possible that the moon-making process has ended with Peggy, the rings now too depleted to make more moons.

"The theory holds that Saturn long ago had a much more massive ring system capable of giving birth to larger moons," Murray said. "As the moons formed near the edge, they depleted the rings and evolved, so the ones that formed earliest are the largest and the farthest out."

Cassini Project Scientist Linda Spilker, of NASA's Jet Propulsion Laboratory in Pasadena, Calif., said astronomers are trying to get as much information out of Peggy as they can.

"Witnessing the possible birth of a tiny moon is an exciting, unexpected event," she said.