Astronomers now know that planets around other stars are plentiful. But they do not fully understand how they form and there are many aspects of the formation of comets, planets and other rocky bodies that remain a mystery. However, new observations exploiting the power of Atacama Large Millimeter/submillimeter Array (ALMA) are now answering one of the biggest questions: how do tiny grains of dust in the disc around a young star grow bigger and bigger — to eventually become rubble, and even boulders well beyond a metre in size?
Computer models suggest that dust grains grow when they collide and stick together. However, when these bigger grains collide again at high speed they are often smashed to pieces and sent back to square one. Even when this does not happen, the models show that the larger grains would quickly move inwards because of friction between the dust and gas and fall onto their parent star, leaving no chance that they could grow even further.
Somehow the dust needs a safe haven where the particles can continue growing until they are big enough to survive on their own. Such “dust traps” have been proposed, but there was no observational proof of their existence up to now.
Nienke van der Marel, a PhD student at Leiden Observatory in the Netherlands, and lead author of the article, was using ALMA along with her co-workers, to study the disc in a system called Oph-IRS 48. They found that the star was circled by a ring of gas with a central hole that was probably created by an unseen planet or companion star. Earlier observations using ESO’s Very Large Telescope had already shown that the small dust particles also formed a similar ring structure. But the new ALMA view of where the larger millimetre-sized dust particles were found was very different!
“At first the shape of the dust in the image came as a complete surprise to us,” says van der Marel. “Instead of the ring we had expected to see, we found a very clear cashew-nut shape! We had to convince ourselves that this feature was real, but the strong signal and sharpness of the ALMA observations left no doubt about the structure. Then we realised what we had found.”
What had been discovered was a region where bigger dust grains were trapped and could grow much larger by colliding and sticking together. This was a dust trap — just what the theorists were looking for.
