| Astronomers in the Netherlands have, for the first time, observed a comet factory around a young star. Particles around the young star grow by clumping together, solving a long-standing mystery about how dust particles in discs form comets or planets. The 'dust trap' was found by Nienke van der Marel, a PhD student at Leiden Observatory using the new Atacama Large Millimeter/Submillimeter Array (ALMA).  This artist's impression shows the dust trap in the system Oph-IRS 48. The dust trap provides a safe haven for the tiny rocks in the disc, allowing them to clump together and grow to sizes that allow them to survive on their own Earlier observations using ESO's Very Large Telescope had already shown that the small dust particles also form a similar ring structure. But the ALMA view of where the larger dust particles were found was very different. 'At first, the shape of the dust in the image was a complete surprise for us,' said 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. Suddenly we realised what we had found.'  The 'dust trap' was observed by Nienke van der Marel, a PhD student at Leiden Observatory in the Netherlands using the new Atacama Large Millimeter/submillimeter Array (ALMA). Earlier observations using ESO's Very Large Telescope had already shown that the small dust particles also form a similar ring structure Marel saw an area circled by a ring of gas with a central hole that was probably created by an unseen planet or companion star. 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. 'It's likely that we are looking at a kind of comet factory because the particles can grow in this dust trap up to cometary sizes of a few kilometres,' said Marel.  Artist's impression of the proposed disk structure of Oph IRS 48. The brown spots represent the large and small grains. The larger grains detected by ALMA are concentrated in the dust trap at the bottom of the image. The blue represents the distribution of carbon monoxide gas. The gap in is shown with the proposed planetary body that is sweeping the area clear and providing the conditions necessary to form the dust trap 'The dust is not likely to form full-sized planets at this distance from the star. But in the near future ALMA will be able to observe dust traps closer to their parent stars, where the same mechanisms are at work. 'Such dust traps really would be the cradles for new-born planets.' The dust trap forms as bigger dust particles move in the direction of higher pressure. Separate computer models have shown that such a high pressure region can originate from the motions of the gas at the edge of a gas hole- just like the one found in this disc. 'Around the time that these observations were obtained, we were working on models predicting exactly these kinds of structures: a very lucky coincidence,' said Cornelis Dullemond from the Institute for Theoretical Astrophysics in Heidelberg.  This image from ALMA shows the dust trap in the disc that surrounds the system Oph-IRS 48. The dust trap provides a safe haven for tiny particles in the disc, allowing them to clump together and grow to sizes that allow them to survive on their own The green region shows where the larger particles are located (millimetre-sized) and is the dust trap seen discovered by ALMA. The orange ring shows observations of much finer dust particles (micron-sized) using the VISIR instrument on ESO's Very Large Telescope Share or comment on this article
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