Solubility in metallic hydrogen and the mystery of Jupiter's core

In two related studies, scientists uncover surprising insights into the chemistry of giant planets.

What is metallic hydrogen?

Metallic hydrogen is believed to be the most abundant condensed material in the universe, existing deep inside giant planets such as Jupiter and Saturn.

Because metallic hydrogen can only form under immense pressures, it has been almost impossible to study in the laboratory. As a result, much of what scientists know about this material comes from theoretical models.

A new theoretical study has revealed that chemistry in the extreme environments of metallic hydrogen could be far more complex than previously thought.

Scientists used quantum mechanics to investigate how carbon, nitrogen and oxygen behave in metallic hydrogen.

Their simulations revealed that these elements can react with each other and with hydrogen to form stable, covalently bonded molecules — including CH₆, C₂H₈, C₃H₁₀, OH₃, NH₄ and CH₄OH. These compounds are known as hypermolecules, due to their unusually high hydrogen content.

Remarkably, the findings suggest that organic molecules can survive in the extreme pressures and temperatures of planets such as Jupiter and Saturn — conditions that are very different from those found on Earth.

Juno space probe

If organic molecules can survive in the extreme pressures of these planets, one might assume we could find them there.

Planets are expected to have a structure where the heaviest elements fall to the centre, forming a heavy core: for Earth this is mainly molten iron. Giant planets are mainly formed of hydrogen, and as a result of the extreme pressures deep in their interior, the hydrogen becomes first liquid, then metallic.  

Cast your mind back to 2016, when NASA’s Juno space probe arrived on Jupiter to undertake a mission to explore the planet. It was predicted that Jupiter’s core might consist of giant diamonds, rocks, ice or heavy metals. What they found was more surprising - there is no core.

In studies led by the Edinburgh team, results show that almost all elements will dissolve in metallic hydrogen, and so any heavy elements on a planet such as Jupiter are largely dissolved within the hydrogen layer.

Crucially, the study also indicates that carbon, oxygen and nitrogen may dissolve readily in metallic hydrogen. This raises new questions about the internal structure of giant planets and whether they can contain distinct rocky cores, as often assumed.

By providing the first detailed picture of possible chemical interactions in metallic hydrogen, these research studies open a new window on planetary science and high-pressure chemistry.