Nanometre-sized spinning diamonds found around stars are the source of an unusual radio-frequency emission, reports a paper published online this week in Nature Astronomy.

The anomalous microwave emission (AME) was an unexpected discovery during observations of the cosmic microwave background in the late 20th and early 21st centuries. AME, observed in the 10-60 GHz frequency range, is seen on a large scale as foreground contamination.

Initially, the emission was thought to be generated by rapidly spinning dust grains made of polycyclic aromatic hydrocarbons. However, recent positional cross-correlations of AME and concentrations of these dust grains in our Galaxy came back negative, bringing this theory into doubt.

Jane Greaves and colleagues observed the protoplanetary disks of 14 hot, young star systems using three different radio telescopes, being careful to isolate emission from the disks themselves, and not any incidental material. The authors saw AME emanating from three systems. These are the only systems known to host hydrogenated nanodiamonds, according to previous near-infrared studies. The authors calculate the probability of these two associations being a coincidence to be 0.003%.

Nanodiamonds are common components of meteorites found on Earth and relics of the planet-forming process that occurred around our Sun, a much dimmer star than those observed by the authors. Since stars like our Sun are common, nanodiamonds are also likely to be common around stars, but we can only see them, and their associated AME, when the stars shine brightly.