Here on Earth, 80 years of advertising has told us that diamonds are special, but our shiny stones have nothing on the weird and wonderful forms seen out in space. Diamonds in meteorites tell the tale of long-lost worlds, it might actually rain diamonds on planets like Uranus and Neptune, and there's even the chance of huge, planet-sized diamonds floating around out there. Now, astronomers have found that mysterious microwave signals around stars could be caused by swirling clouds of diamond dust.

For years, faint light sources known as anomalous microwave emissions (AME) have been detected in protoplanetary disks – the clouds of dust and gas that surround young stars and maybe eventually coalesce into planets. Rapidly-spinning nanoparticles have long been blamed for these signals, but exactly what type of particle remained a mystery.

"Though we know that some type of particle is responsible for this microwave light, its precise source has been a puzzle since it was first detected nearly 20 years ago," says Jane Greaves, lead author on a study describing the research.

To find out what was behind them, astronomers studied 14 young stars across the Milky Way using the Green Bank Telescope (GBT) in West Virginia and the Australia Telescope Compact Array (ATCA) in New South Wales. Of those 14 stars, three of them showed clear signs of AME radiation: V892 Tau, HD 97048 and MWC 297.

Interestingly, all three of those also showed a specific infrared signature that wasn't found around any of the other 11. That infrared fingerprint lines up with hydrogenated nanodiamonds – tiny crystalline carbon nanoparticles with hydrogen-bearing molecules on their surface.

That find also allowed the team to rule out what was previously at the top of the suspect list: a kind of organic molecule known as polycyclic aromatic hydrocarbons (PAHs). These are common throughout interstellar space and also emit infrared light. But the astronomers observed clear PAH signatures around stars with no AME light, indicating there was no connection.

"In a Sherlock Holmes-like method of eliminating all other causes, we can confidently say the best candidate capable of producing this microwave glow is the presence of nanodiamonds around these newly formed stars," says Greaves.

Models have found that nanodiamonds would be common in protoplanetary disks, and the astronomers estimate that they make up about one to two percent of the total carbon available there.

The research was published in the journal Nature Astronomy.