Jupiter’s core isn’t what we thought

It had been thought that a colossal collision with an early planet containing half of Jupiter's core material could have mixed up the central region of the gas giant, enough to explain its interior today.

But a new study published in Monthly Notices of the Royal Astronomical Society suggests its make-up is actually down to how the growing planet absorbed heavy and light materials as it formed and evolved.

Unlike what scientists once expected, the core of the largest planet in our solar system doesn't have a sharp boundary but instead gradually blends into the surrounding layers of mostly hydrogen - a structure known as a dilute core.

How this dilute core formed has been a key question among scientists and astronomers ever since NASA's Juno spacecraft first revealed its existence.

Using cutting-edge supercomputer simulations of planetary impacts, with a new method to improve the simulation's treatment of mixing between materials, researchers from Durham University, in collaboration with scientists from NASA, SETI, and CENSSS, University of Oslo, tested whether a massive collision could have created Jupiter's dilute core.

The simulations were run on the DiRAC COSMA supercomputer hosted at Durham University using the state-of-the-art SWIFT open-source software.

The study found that a stable dilute core structure was not produced in any of the simulations conducted, even in those involving impacts under extreme conditions.

Instead, the simulations demonstrate that the dense rock and ice core material displaced by an impact would quickly re-settle, leaving a distinct boundary with the outer layers of hydrogen and helium, rather than forming a smooth transition zone between the two regions.

Reflecting on the findings, lead author of the study Dr Thomas Sandnes, of Durham University, said: "It's fascinating to explore how a giant planet like Jupiter would respond to one of the most violent events a growing planet can experience.

"We see in our simulations that this kind of impact literally shakes the planet to its core - just not in the right way to explain the interior of Jupiter that we see today."

Jupiter isn't the only planet with a dilute core, as scientists have recently found evidence that Saturn has one too.

Dr Luis Teodoro, of the University of Oslo, said: "The fact that Saturn also has a dilute core strengthens the idea that these structures are not the result of rare, extremely high-energy impacts but instead form gradually during the long process of planetary growth and evolution."

The findings of this study could also help inform scientists' understanding and interpretation of the many Jupiter- and Saturn-sized exoplanets that have been observed around distant stars. If dilute cores aren't made by rare and extreme impacts, then perhaps most or all of these planets have comparably complex interiors.

Co-author of the study Dr Jacob Kegerreis said: "Giant impacts are a key part of many planets' histories, but they can't explain everything!

"This project also accelerated another step in our development of new ways to simulate these cataclysmic events in ever greater detail, helping us to continue narrowing down how the amazing diversity of worlds we see in the Solar System and beyond came to be."