They may be tiny, but nanodiamonds shaped like pyramids may have solved a big mystery that’s puzzled astronomers since 1996. That’s when researchers noticed something weird when analysing data from the COBE satellite, which was launched in 1989 to capture cosmic background radiation left over from the big bang.
COBE saw the expected background radiation, but it also captured emissions of microwaves whose source couldn’t be explained. With wavelengths of a centimetre or so, and frequencies of 10 to 60 gigahertz, the “anomalous microwave emission” has puzzled astronomers ever since.
So far, the favoured explanation has been that the radiation comes from tiny specks of carbon-rich grains made from polycyclic aromatic hydrocarbons, or PAHs. These are abundant in protoplanetary discs, the clouds and dust surrounding young stars from which planets eventually form.
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They’re claimed to form dipoles—with north and south poles like a magnet–which can produce microwaves by spinning millions of times a second through collisions with each other.
But the stars where PAHs have been identified don’t always yield the anomalous microwave emissions too, casting doubts about whether PAHs are the true source of the weird microwaves.
Diamond in the rough
Now, astronomers have chanced upon a different explanation after scanning the protoplanetary discs of 14 bright young stars just a million years old or less, plus those of 30 older stars like our own sun. Using radio telescopes in Australia and the US, they discovered that just three of the young stars and none of the older ones were sources of the odd emissions.
They also detected infrared emissions from these three stars alone, with signatures corresponding to nanodiamonds, tiny bits of diamond less than a nanometre across. Puzzled by this, the researchers scoured previously published data on 80 bright young stars and found to their astonishment that nanodiamonds had been positively identified before in only three of them – exactly the same three highlighted by their own study.
“It turned out they were the only three where other teams had found the nanodiamonds,” says Jane Greaves at Cardiff University in Wales. She and her colleagues calculate that the odds of this being a coincidence are vanishingly small. “No one had discussed or thought about nanodiamonds being the source before,” says Greaves.
Spinning pyramids
Already well-known because they’ve long been found in meteorites that have landed on Earth, nanodiamonds can also generate microwaves because they form dipoles too. Greaves says the observed infrared signals tally with pyramid-shaped nanodiamonds that would be able to do this.
“They’re little pyramids of carbon atoms with hydrogen atoms poking out the top, and so are dipoles,” she says. “When they spin millions of times per second, they would produce the microwaves.”
“It’s probable that these nanodiamonds are the sources of the anomalous microwave emission,” says Ashraf Ali at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “This is in clear contrast with previous proposals that PAHs could emit electric-dipole radiation,” he says.
But not everyone’s convinced. “My money’s still on the PAHs and related things, particularly nitrogen-containing PAHs which have dipole moments,” says Louis Allamandola at the NASA Ames Research Center in Moffett Field, California.
Nature Astronomy
