The undulating rings of Saturn indicate an enormous and fluffy nucleus hidden within.

Saturn rings are not only a beautiful adornment - scientists can use this feature to understand what is happening in the deepest part of the planet.

Using the famous rings as a seismograph, the scientists studied the processes inside the planet and determined that its nucleus needs to be 'fuzzy.'

Instead of a solid sphere like Earth's, the core of Saturn appears to consist of a 'soup' of rocks, ice, and metallic fluids that slosh around and affect the planet's gravity.

The new survey used data from NASA's Cassini mission, which orbited Saturn and its moons for 13 years between 2004 and 2017. In 2013, data from the mission revealed for the first time that Saturn's innermost ring, the D-ring, ripples, and swirls in ways that cannot be entirely explained by the gravitational influences of the planet's moons. The new study looked at these movements in the rings of Saturn in more detail to obtain an overview of the processes in its interior.

"We used Saturn's rings like a giant seismograph to measure oscillations inside the planet," Jim Fuller, assistant professor of theoretical astrophysics at Caltech and one of the authors of the paper said in a statement.

"This is the first time we have been able to seismically probe the structure of a giant gas planet, and the results have been rather surprising."

Not only does the planet's nucleus appear lazy, but it also appears to extend over 60% of the planet's diameter, making it much larger than previously estimated.

The analysis showed that Saturn's heart could be approximately 55 times more massive than the whole planet Earth. Of the total mass of the nucleus, 17 terrestrial masses are made up of ice and rock, the rest being a liquid based on hydrogen and helium, suggests the study.

The lead author of the study, Christopher Malkovich, a postdoctoral scholar research associate in planetary science who works in Fuller's group, explained that the motions in the core cause Saturn's surface to constantly ripple. These surface waves cause minute changes to the planet's gravity that then affect the rings.

"Saturn is still trembling, but that's subtle,' said Malkovich in his statement. "The surface of the planet moves about one meter [3 feet] every one to two hours like a gently undulating lake. Like a seismograph, the rings pick up gravitational disturbances, and the particles in the ring begin to move."

According to scientists, the nature of these ring waves suggests that the nucleus, despite its subsidence, is composed of stable layers of different densities. Heavier materials occur in the center of the planet and do not mix with lighter materials closer to the surface.

"For the planet's gravitational field to be oscillating with these particular frequencies, the interior must be stable, and that's only possible if the fraction of ice and rock gradually increases as you go in toward the planet's center," Fuller said.

Malkovich compared the material in the core to sludge, adding that the layered but liquid nature of the core is akin to the salinity of Earth's oceans, which increases with depth.

"The planet's hydrogen and gaseous helium are gradually mixing with more and more ice and rocks as you move towards the center of the planet," said Mr. Mankovich.

The findings might challenge some of the established models of the formation of gas giants, planets with no hard surface, which are composed mainly of hydrogen and helium, the study suggests. These models assume that the rock nuclei of these planets initially formed and subsequently attracted large gas envelopes. If the nuclei of the planets are, however, fuzzy as the study points out, the planets might instead incorporate gas earlier in the process.

In fact, recent findings from NASA's Juno mission suggest that another of the solar system's gas giants, Jupiter, could also have an equally fuzzy nucleus.

"Christopher [Mankovich] and Jim [Fuller] were able to show that one particular ring feature provided strong evidence that Saturn's core is extremely diffuse," said Matt Hedman, a planetary scientist at the University of Idaho, who was part of the team that first discovered that the motions in Saturn's rings can't be fully explained by the gravity of its moons.

"I am excited to think about what all the other ring features generated by Saturn might be able to tell us about that planet," added Hedman, who did not collaborate on the new paper.