Thanks to its trademark disc of icy rings, Saturn is one of the most recognisable planets in the Solar System. But exactly how this distinctive feature formed has remained something of a mystery.


Now, a computer modelling study carried out by researchers at MIT suggests that the gas-giant’s rings, and the nearly 27° tilt at which it moves around the Sun, maybe due to an ancient moon careening out of orbit, clipping its host planet and being smashed into hundreds of pieces.

The team made the discovery by using gravitational data collected by NASA’s Cassini spacecraft, which orbited Saturn from 2004 to 2017. After running several computer models designed to emulate the orbits of Saturn and its satellites back through time, they determined that billions of years ago, Saturn had at least one more moon, which they have named Chrysalis.

Through gravitational interactions with Chrysalis, Saturn kept its tilt in sync with Neptune. However, around 160 million years ago, Chrysalis became unstable, ventured too close to Saturn and subsequently pulled apart.

The loss of this additional moon was enough to push Saturn out of sync with Neptune, leaving it with its current tilt. Moreover, some fragments of Chrysalis’s shattered body may have remained in orbit and eventually broken down to form the planet’s signature rings.

The researchers estimate that to achieve this effect Chrysalis would have been around 1,500km across – roughly the same size as Iapetus, Saturn’s third-largest moon.

“Just like a butterfly’s chrysalis, this satellite was long dormant and suddenly became active, and the rings emerged,” said Jack Wisdom, professor of planetary sciences at MIT and lead author of the study.

“It’s a pretty good story, but like any other result, it will have to be examined by others. But it seems that this lost satellite was just a chrysalis, waiting to have its instability.”

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Jason Goodyer
Jason GoodyerCommissioning editor, BBC Science Focus

Jason is the commissioning editor for BBC Science Focus. He holds an MSc in physics and was named Section Editor of the Year by the British Society of Magazine Editors in 2019. He has been reporting on science and technology for more than a decade. During this time, he's walked the tunnels of the Large Hadron Collider, watched Stephen Hawking deliver his Reith Lecture on Black Holes and reported on everything from simulation universes to dancing cockatoos. He looks after the magazine’s and website’s news sections and makes regular appearances on the Instant Genius Podcast.