Astronomers call it “diamond rain”, and no one’s ever seen it – until now.

According to calculations, more than 8000 kilometres beneath the service of the icy giant planets Uranus and Neptune the pressure is so great that carbon atoms are squeezed so tightly together that they form diamonds, which then sink further through ice slush towards the solid core.

Now, scientists using the SLAC National Accelerator Laboratory at Stanford University in California, have succeeded in creating diamond rain under controlled conditions.

A team led by Dominik Kraus from the Helmholtz Zentrum Dresden-Rossendorf research centre in Germany subjected plastic to shockwaves by exposing it to the intense energy produced by SLAC’s X-ray free-electron laser, known as the Linac Coherent Light Source (LCLS).

The experiment caused almost all the carbon atoms in the plastic to combine into diamond-like structures a few nanometers wide.

The Matter in Extreme Conditions instrument at SLAC gives scientists the tools to investigate the extremely hot, dense matter at the centers of stars and giant planets. SLAC National Accelerator Laboratory

“Previously, researchers could only assume that the diamonds had formed,” says Kraus. “When I saw the results of this latest experiment, it was one of the best moments of my scientific career.”

Astronomers think that the forces at work deep in the frozen mantles of Uranus and Neptune are likely so powerful that each of the diamonds formed could weigh millions of carats. It is also possible that the solid cores of both planets are coated with a thick diamond outer layer.

But while the planets have had billions of years over which to create their diamond cargo, the experiment conducted by Kraus’s team lasted just a couple of quadrillionths of a second – long enough, nevertheless, for the process to be observed, measured and recorded.

It is thought that diamond rain inside planets might function as an energy source, generating heat as the gems travel towards the core.

“We can’t go inside the planets and look at them, so these laboratory experiments complement satellite and telescope observations,” says Kraus.

And while the SLAC experiment – written up in the journal Nature Astronomy – was conducted to provide insight into distant planetary processes, it may well produce commercial benefits here on Earth.

Nanodiamonds similar to the type created by Kraus and his team have a wide range of uses in medicine and industry. Currently they are created by means of controlled explosions. Using a laser instead offers a potentially more efficient and much safer method of manufacture.