A cutaway of gas giant Jupiter, which is thought to comprise a rocky core surrounded by metallic hydrogen and a layer of molecular hydrogen. Between the hydrogen layers, new research suggests, lies a layer of ‘dark’ hydrogen. HENNING DALHOFF / SCIENCE PHOTO LIBRARY / Getty Images

Physicists have uncovered a new state of hydrogen dubbed “dark hydrogen”, which is neither a metal nor a gas, and suggest it is lurking in gas giant planets.

Stewart McWilliams from the University of Edinburgh and colleagues from China and the US squeezed pure hydrogen in the same conditions as the interior of massive planets and found an intermediate state between a gas and a metal.

This transitionary state does not reflect or transmit visible light, but does pump out heat.

“This observation would explain how heat can easily escape from gas giant planets like Saturn,” co-author Alexander Goncharov says.

Despite being the simplest element in terms of structure, with one electron, one proton and, in the case of the slightly heavier deuterium, one neutron, and being the most abundant element in the universe, there’s plenty scientists don’t know about hydrogen.

How it acts in the extreme heat and pressures found deep within gas giants is a mystery. The Galileo probe that dropped into Jupiter’s atmosphere in 1995 lasted less than an hour before it was destroyed just 150 kilometres into its suicidal descent.

Planetary scientists have calculated Jupiter’s core to be around 44 million times the pressure of sea level on Earth. And with those pressures come high temperatures – around 24,000 ºC, hotter than the surface of the sun.

At these conditions, hydrogen – a gas on Earth – is compressed so much it becomes a liquid that can conduct electricity. This “metallic hydrogen”, it’s thought, makes up the lion’s share of Jupiter’s interior, and is surrounded by “molecular hydrogen”, which is more like the gas with which we’re familiar.

Unsurprisingly, exploring metallic hydrogen on the comparatively cool, less pressurised Earth is difficult. Physicists have been able to recreate Jupiter’s crushing conditions in the lab, but only for a few millionths of a second at a time.

One technique uses a laser-heated diamond anvil cell. This creates high pressures by trapping a sample of hydrogen between the flat faces of two diamonds. A laser pulse heats the sample at the same time.

Using such a device, McWilliams and colleagues observed pure hydrogen up to 1.5 million times normal Earth pressure (150 gigapascals) and 5,500 ºC, and measured the wavelengths it absorbed and emitted as the pressure and temperature rose.

At around 2,400 ºC and 141 gigapascals, they noticed the hydrogen became “semi-metallic” with properties of molecular and metallic hydrogen.

It didn’t reflect or transmit visible light, like molecular hydrogen, but it emitted heat, like metallic hydrogen. And it could conduct electricity, but nowhere nearly as well as metallic hydrogen.

The researchers believe Jupiter hides a layer of dark hydrogen between its metallic and molecular layers. This dark hydrogen may even affect how the gas giant’s churning metallic hydrogen produces a magnetic field.

Prevailing models of hydrogen behaviour in the high-pressure and high-temperature conditions of gas giants, they write, “thus require a significant assessment”.

The work was published in Physical Review Letters.

Related reading: Juno’s epic journey to probe Jupiter’s secrets

Edit: hydrogen atoms do not contain a neutron but deuterium does. This has been amended.