Vol. 538
In section 10. Planets and planetary systems

Trapping dust particles in the outer regions of protoplanetary disks

The presence of millimeter-size dust particles in the outer regions of protoplanetary disks, confirmed by radio telescopes, has been a puzzle for more than a decade. While the growth of these particles from micron size has been shown to be easy even in tenuous parts of disks, the particles would migrate inward very quickly on timescales shorter than a million years and be lost, in contradiction to the observations. Pinilla et al. propose and test a new explanation: pressure bumps in the gas disk (such as results from magneto-rotational instabilities) can in principle slow the radial migration of millimeter-size grains, simply because the gas drag is proportional to the pressure gradient. They show that, indeed, the amplitude of pressure bumps in disks predicted by MHD simulations are sufficient to explain the retention of milliter-size grains, thereby explaining the observations. The work is not over, however, because the bumps are assumed to be static and the time evolution of these structures is key to determining whether the grains are retained. But the article shows that ALMA will be able to directly test this scenario by imaging these disks and looking for ring structures up to the distance of Taurus.