For over 20 years, an unusual excess of microwave radiation emanating from parts of the Milky Way has raised more questions than answers.
Astronomers have long relied on cosmic microwave background—described by astronomer Erik Leitch as the “the afterglow of the Big Bang,” or a faint light suffusing the universe—to unlock the secrets of the skies. But one lingering question remained: why were some parts of the sky brighter than others?
Now, a new study published in Nature Astronomy offers a radical resolution to the mystery of anomalous microwave emissions (AME): spinning clouds of nanodiamonds—diamonds no bigger than the length of a typical bacterium.
According to The Guardian’s Ian Sample, lead author Jane Greaves, an astronomer at Wales’ Cardiff University, was studying dust-ringed stars when she chanced upon unusual levels of AME emitting from several of the young specimens. Greaves visited the Green Bank Observatory in West Virginia to investigate further and ended up with similarly strange results.
“I was looking for emissions from small dust particles” that would offer insight on the process of early planet formation, Greaves tells Popular Science’s Mary Beth Griggs. Those readings should have started faint and gotten steadily brighter, “but this emission got brighter and then got fainter again as you went along in the wavelength—and that's a really difficult thing to make by most astronomical processes.”
Sample reports that Greaves conferred with Anna Scaife, an astronomer at Manchester University, and learned that her colleague had found similar microwaves emanating from stars. After pinpointing three of these stars—V892 Tau, found in the constellation of Taurus in the northern sky; HD 97048, in the southern sky constellation of Chamaeleon; and MWC 297, in the southern sky constellation of Serpens—the astronomers realized they had selected the only stars known to be encircled by hydrogenated nanodiamonds, or minuscule carbon crystals layered with frozen hydrogen.
According to a Cardiff University press release, nanodiamonds are typically found inside rings of dust and gas, or protoplanetary disks, surrounding young stars. The “extremely hot and energized conditions” within these disks are conducive to nanodiamond formation, although Sample explains that the exact creation process is unclear. Potential explanations include hot carbon vapor and emission from exploding stars.
Previously, astronomers had suspected the microwaves were generated by polycyclic aromatic hydrocarbons (PAHs), organic molecules that Scientific American’s Shannon Hall describes as the “cosmic equivalent of soot, albeit produced by aging stars rather than smokestacks.”
When Princeton University’s Bruce Draine conducted a 2016 study testing this hypothesis, however, he found no connection between PAHs and the mysterious glow. Although Draine tells Hall he finds Greaves’ new findings “appealing,” he cautions that the link between nanodiamonds and microwaves could simply be coincidence.
Greaves and her team are more certain of their results, stating that there is a one in 10,000 chance or less that the connection is due to chance.
“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,” Greaves said in a press release.
According to the press release, nanodiamonds are significant for their structure, which allows them to emit electromagnetic radiation as they spin, and extremely small size, which enables them to spin incredibly fast and emit light in the microwave range rather than the meter-wavelength range.
Moving forward, Greaves tells Hall that she and her team will attempt to replicate their findings in colder environments, such as clouds of interstellar gas and dust. Griggs adds that Greaves hopes to explore potential connections between nanodiamonds and interstellar AMEs, with an overall goal of determining whether the results apply beyond three specific protoplanetary disks.
“It’s not often you find yourself putting new words to famous tunes,” co-author Scaife concluded in the press release, “but ‘AME in the Sky with Diamonds’ seems a thoughtful way of summarizing our research.