The element makes up much of these gas giants and research suggests that hydrogen near their centres is a liquid metal. But it’s not clear what happens between there and the wispy clouds at the surface.

Experiments involving squeezing hydrogen in a diamond vice and shooting it with lasers can recreate the conditions within the planets, but the small atoms of hydrogen can easily escape.

Now Stewart McWilliams at the University of Edinburgh, UK, and his colleagues have used a laser pulse lasting just a few microseconds to heat compressed hydrogen to 3000 kelvin. “We do the experiment really fast, so the hydrogen doesn’t have time to get away,” he says.

During this brief window, the team saw the hydrogen enter a phase that doesn’t reflect or absorb light, which they call “dark hydrogen”.

“It becomes essentially like coal, it’s just black,” says McWilliams. The discovery of this intermediate state between the gas and metallic phase of hydrogen suggests that the gas giants have a black layer inside, which could explain the puzzle of how they cooled after formation.

“The temperature of these planets today is one of the big mysteries in planetary science,” he says. “Understanding this coal-like layer inside these planets is very important for understanding their history.”

We should hopefully learn more soon, as NASA’s Juno probe is due to arrive at Jupiter next week, where, among other things, it will study the planet’s magnetic and gravitational fields, giving researchers a better idea of what’s inside.

“We hope that Juno will be very sensitive to the details of the interior structure, and we’ll be able to maybe make better models of these planets by combining the satellite data with these experiments,” says McWilliams.

Source: New Scientist