Saturn has a much larger, and less well defined gassy core than was previously assumed, according to scientists studying movements in its ring system.
Data gathered by the NASA Cassini spacecraft was used by astronomers to reveal that Saturn possesses a fuzzy or diffuse core without clearly defined boundaries.
The probe, which operated in orbit around Saturn for more than 13 years from 2004 to 2017, gathered extensive data on the gas giant and its ring system. It allowed a team from the California Institute of Technology in Pasadena to discover that the core is made of a mixture of helium, hydrogen and heavier elements, that it extends out to 60 per cent of the planet and there is no clear boundary line.
They describe this as 'fuzzy' interior, rather than the rocky core previously suspected, with as slush of ice, metallic fluids, rock and gasses.
Previous studies into the core of gas giants involved inspecting the detailed configuration of their gravitational field as seen from an orbiting spacecraft.
For the new study astronomers used seismic measurements from the rings, as they interact with the gravitational field of the planet below.
This allows them to paint a more detailed picture of the alien world's inner most regions and how it changes.
Oscillations in the interior of Saturn cause movement in the planet, revealed through the gravitational field, creates ripples in the ring system.
The problem with simply using the gravity method to determine the interior makeup of a gas giant is that the impact the core has on the gravitational field is relatively minimal, limiting the level of precision it is possible to achieve.
This issue prompted astronomers Christopher Mankovich and Jim Fuller to look at the wider dataset coming from the Cassini spacecraft.
They discovered things were fuzzier than was previously assumed - with former studies predicting a metal core and firm boundary line with the outer envelope.
That outer envelope is the thick layer of hydrogen and helium that we can see when looking at the outside of the planet, and the team found it makes up about 40 per cent of the radius of Saturn, with the core making up the remainder.
The other discovery was that the core and the envelope were made of similar material - hydrogen, helium and heavy elements.
Researchers found that the surface of Saturn moves a metre every two hours, 'like a slowly rippling lake' causing ring particles to 'wiggle around'.
The frequency of the ripples in the rings allowed them to find that the interior of the planet is stable and when first forming, it formed stable layers with heavy materials like rock and ice moving towards the centre and gasses moving to the outer layers.
Determining how Saturn's structure developed is a challenge for standard planetary formation models due to the weak gravity interaction of the core.The new study, and more detailed observations and measurements 'provides important constraints as to its mass accretion history,' the authors explained.
They suggest that in the early years of the solar system Saturn may have had a more abrupt boundary between the core and outer envelope, but that it eroded over time.
There is so much rock and ice within this massive planetary core you could recreate the Earth 17 times, according to the study authors.
In fact this 'fuzzy' core is 55 times more massive than the Earth and is similar in structure to that of the giant planet Jupiter.
'This gradual distribution of heavy elements constrains mixing processes at work in Saturn,' according to the researchers, who say it 'may reflect the planet's primordial structure and accretion history,' which is how it first formed billions of years ago.
The models they used to create their measurements place 'tight constraints' on the mass and size of the heavy-element core of Saturn.
But add that their new findings, that it is more diluted than earlier predictions, require a more nuanced description of the structure than previous models.
'The fuzzy cores are like a sludge,' Mankovich told CNN, adding the 'hydrogen and helium gas in the planet gradually mix with more and more ice and rock as you move toward the planet's centre.'
'It's a bit like parts of Earth's oceans where the saltiness increases as you get to deeper and deeper levels, creating a stable configuration.'
The findings have been published in the journal Nature Astronomy.