Existing theories of the development of the solar system say that the Sun and the planets formed from an ‘accretion disk’ of gas and dust. Many meteorites – fragments of ancient comets – have been found to contain nanodiamonds, which suggests that the tiny carbon particles were abundant in this disk.

But when Bradley’s group analysed a selection of carbon-rich interplanetary dust particles using transmission electron microscopy, they were surprised to find very little evidence of nanodiamonds. According to the team, the simplest explanation for this observation is that nanodiamonds were not present before the solar system formed. But this raises the question of how the nanodiamonds formed in the chemically ‘oxidizing’ environment of the early solar system, when the nanodiamonds found previously in meteorites appeared to have formed in a ‘reducing’ environment.

Alternatively, the researchers suggest that the nanodiamonds could predate the Sun, but that they may have been more abundant in the inner solar system than they were in its outer reaches. This would mean, however, that our understanding of transport processes in the early solar system was incomplete.

“Any one of these explanations has profound implications concerning our understanding of the early solar system,” write Bradley and colleagues.