Saturn Ring Mystery Possibly Solved After Four Centuries of Uncertainty

The rings formed 100 million to 200 million years ago when one of Saturn’s moons, thrown off course by another moon, veered too close to the planet and was ripped to pieces by its gravitational forces, the study suggests. Rubble from the hypothetical moon, which the researchers dubbed Chrysalis, continued to orbit Saturn and over time flattened into the disk of particles seen today, according to the findings, which were published Thursday in the journal Science.

The rings are just 30 feet thick in places but span a diameter of about 170,000 miles. Jupiter, Uranus and Neptune also have rings, but they are smaller, darker and fainter.

The new research represents “a completely new scenario that may solve the age of the rings for good,” said Dr.

Maryame El Moutamid,

a Cornell University astronomer who wrote an editorial accompanying the study but wasn’t involved in it.

In addition to explaining how and when the rings might have formed, the missing-moon scenario offers an explanation for Saturn’s puzzling tilt. Like Earth but unlike Jupiter—the planet in our solar system it most resembles—Saturn rotates at a significant angle with respect to the plane in which it orbits the sun.

“It ties together two puzzles that had previously been treated as separate,” Dr.

Francis Nimmo,

a professor of planetary science at the University of California, Santa Cruz, and a co-author of the study, said of the scenario. “It turns out that you can explain both in a single story.”

For many years, scientists believed Saturn’s rings formed more than 4 billion years ago, when the young planet’s strong gravitational field captured passing comets and asteroids and slowly flattened them into rings. But observations by three National Aeronautics and Space Administration missions—Voyager 1 and 2 in the 1980s and Cassini between 2004 and 2017—called that idea into question. The observations revealed that the rings’ mass and composition were much younger than previously believed.

“If they’re not old, they must be a result of a breakup of a body, like a large comet or a moon later on,” Dr.

Jack Wisdom,

professor of planetary science at the Massachusetts Institute of Technology and lead author of the new research, said of the rings. But figuring out just what sort of object—and exactly how it broke up to form the rings—proved to be a difficult puzzle.

The solution came when the research team’s analysis of Cassini data upended long-held ideas about the complex relationships between Saturn, its moons and the nearby planet Neptune. Scientists had thought that Saturn and Neptune were in resonance, meaning they were gravitationally interacting. But the analysis showed that had been true but no longer was, suggesting that a small moon must have disrupted the resonance when gravitational forces exerted by another moon, Titan, sent it spiraling toward Saturn.

Not everyone is convinced that the question of Saturn’s rings and its tilt has been put to rest.

Jack Lissauer, a planetary scientist at NASA’s Ames Research Center in California’s Silicon Valley, said the new scenario “has a lot of things going for it” but that it is hard to verify the complex series of celestial events it is based upon. Dr. Lissauer wasn’t involved in the research.

The science community needs to spend time reviewing the new research, said Matthew Tiscareno, a planetary-rings researcher at the SETI Institute in Mountain View, Calif., who wasn’t involved in the study. But Dr. Tiscareno called the missing-moon scenario “an exciting new idea that likely gets us closer to solving several intertwined mysteries at Saturn.”

Write to Aylin Woodward at Aylin.Woodward@wsj.com