On the surface of giant gaseous planets, hydrogen is a gas. But between this gaseous layer and the liquid metal hydrogen in the planet’s core lies a layer of dark hydrogen, says a team of physicists led by Dr. Alexander Goncharov from Carnegie Institution of Washington and University of Edinburgh scientist Dr. Stewart McWilliams.

Using a laser-heated diamond anvil cell to create the conditions likely to be found in the interiors of gas giants, Dr. McWilliams, Dr. Goncharov and their colleagues probed the physics of hydrogen under a range of pressures from 10,000 to 1.5 million times normal atmospheric pressure and up to 10,000 degrees Fahrenheit (5,538 degrees Celsius).

The scientists discovered a previously unknown intermediate state between gas and metal, which they’re calling ‘dark hydrogen.’

According to the team, dark hydrogen does not reflect or transmit visible light, but does transmit infrared radiation, or heat.

“This observation would explain how heat can easily escape from gas giant planets like Saturn,” Dr. Goncharov said.

The physicists also found that dark hydrogen is somewhat metallic, meaning it can conduct an electric current, albeit poorly.

This means that it could play a role in the process by which churning metallic hydrogen in planetary cores of gas giants produces a magnetic field around these bodies, in the same way that the motion of liquid iron in Earth’s core created and sustains our own magnetic field.

“This dark hydrogen layer was unexpected and inconsistent with what modeling research had led us to believe about the change from hydrogen gas to metallic hydrogen inside of celestial objects,” Dr. Goncharov explained.

The results appear online in the June 24 issue of the journal Physical Review Letters.

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R. Stewart McWilliams et al. 2016. Optical Properties of Fluid Hydrogen at the Transition to a Conducting State. Phys. Rev. Lett. 116, 255501; doi: 10.1103/PhysRevLett.116.255501