ONE WILD WORLD — NEWSLETTER

Issue #011 | Useless But Fascinating | April 2026

Somewhere in our solar system right now, on two planets we will probably never visit, diamonds the size of icebergs are forming in the sky and slowly raining down through the atmosphere toward the core. This is not science fiction. This is real, peer-reviewed physics.

BY THE NUMBERS

In the early 1980s, physicist Marvin Ross at Lawrence Livermore National Laboratory proposed something that sounded absurd.

Deep inside Neptune and Uranus, he argued, the conditions are so extreme that methane molecules would be ripped apart. The carbon atoms would separate from the hydrogen. The carbon would compress under unimaginable pressure. And then it would crystallise into diamonds.

Diamonds that would slowly sink, falling through the atmosphere, drifting down toward the planet’s core like glittering snow.

It was a beautiful idea. It was also impossible to verify.

Then, in 2009, Stanford University completed something called the Linac Coherent Light Source — the world’s first X-ray free-electron laser. Combined with high-power optical lasers, it could finally generate the extreme conditions needed to test Ross’s theory.

In 2017, an international team of scientists at the SLAC National Accelerator Laboratory put a thin sheet of plastic into the machine. Plastic is mainly carbon and hydrogen — a reasonable substitute for the hydrocarbon-rich material inside Neptune.

They fired their lasers. The plastic was instantly compressed to pressures of around 150 gigapascals — about 1.5 million times Earth’s atmospheric pressure — and heated to temperatures of approximately 5,000 kelvin.

Then they used X-ray pulses to look inside the plastic, in real time, as it was being crushed.

Dominik Kraus, the lead physicist on the experiment, later said that watching the diamonds form was “one of the best moments of my scientific career.”

The theory was no longer a theory. Diamond rain was real.

The diamonds the SLAC team produced in their lab were tiny — nanometres across. But the laboratory could only sustain those extreme conditions for a fraction of a second.

On Neptune and Uranus, those same conditions exist continuously, for thousands of years.

Researchers calculate that the diamonds inside Neptune and Uranus could grow to be enormous. Some estimates suggest individual diamonds could reach millions of carats in weight — each one larger than any diamond ever found on Earth.

Here’s where it gets even stranger.

Researchers now think that over thousands of years, these diamonds may sink through Neptune’s liquid layers and accumulate around the planet’s core in a thick shell. A layer of diamond, possibly thousands of kilometres deep, wrapping around the centre of the planet.

In a 2024 follow-up experiment, scientists at SLAC found that diamond rain probably forms at lower pressures and temperatures than previously thought. That means it could be even more common than scientists believed — and the diamond layer could be even thicker.

It also means diamond rain may exist on other planets we haven’t yet discovered. There could be billions of planets across the universe right now, slowly accumulating diamond cores. Each one a treasury that nobody will ever reach.

There is no practical use for diamonds you can never reach. Neptune is 4.5 billion kilometres away. The diamond rain forms 10,000 kilometres beneath an atmosphere of crushing pressure, freezing temperatures, and supersonic winds. We will never mine those diamonds. We will never see them with our own eyes.

But the experiment that proved it had a side benefit nobody expected.

The same lab process that simulates diamond rain on Neptune can be used to manufacture nanodiamonds on Earth. These tiny synthetic diamonds have applications in medical imaging, drug delivery, and high-precision electronics. The journey to confirm a useless cosmic curiosity gave humanity a new way to make industrial diamonds.

Right now, on a planet you will never visit, diamonds the size of buildings are forming in the sky and falling through the atmosphere.

They will never be seen. They will never be mined. They will never be set in a ring.

And yet they are real.

SOURCES

Kraus, D. et al. — “Formation of diamonds in laser-compressed hydrocarbons at planetary interior conditions”, Nature Astronomy (2017)

SLAC National Accelerator Laboratory — “Scientists Create ‘Diamond Rain’ That Forms in the Interior of Icy Giant Planets” (2017)

SLAC — “Diamond rain on giant icy planets could be more common than previously thought” (2022)

SLAC — “Diamond rain on icy planets offers clues into magnetic field mysteries” (2024)

American Scientist — “On Neptune, It’s Raining Diamonds”

Lawrence Livermore National Laboratory — diamond rain experiment archive