Formation mechanism of spontaneous dust traps(red) in a protoplanetary disk after the formation of a spontaneous dust trap, visible as a bright dust ring. JF Gonzalez©
One of the major questions in astronomy today is how do planets form? Until recently, no theory has been able to provide a complete answer. However, an international team of astrophysicists led by researchers from the Centre de Recherche Astrophysique de Lyon (Université Claude Bernard Lyon 1, CNRS, ENS Lyon) in France has discovered the spontaneous formation of “dust traps”, finally allowing astronomers to link the different stages of planet formation.
Astronomers know that planets – both in our Solar System and the exoplanet found around other stars – formed in the dusty disks that surround young stars. These disks comprises of gas and dust grains and somehow the small dust grains need to grow into larger bodies that are the building blocks of planets. With over 3,500 exoplanets detected to date in our Galaxy, their formation mechanism must be both efficient and universal.
There are several steps in planet formation, and some are better understood that others. The processes that convert tiny micron-sized grains into small pebble-sized aggregates (1 to 10 cm in size), and the conversion of kilometre-sized ‘planetesimals’ into planetary core (100s of km in size) are both relatively well understood. The intermediate stage – the growth from pebbles to planetesimals – remains problematic.
There are two main barriers to planet formation that need to be overcome if pebbles are to become planetesimals. Firstly, it has been known for decades that the aerodynamic drag of gas on dust grains in these disks make grains drift rapidly towards the disk centre. This is a major problem for planet formation: as dust grains migrate towards the central star, they are removed from the disk, leaving no material to form planetary bodies. Secondly, high-velocity collisions between growing dust grains can fragment them into a large number of smaller pieces, which is another major problem for planet formation.
The only locations in planet forming disks where these problems can be overcome are called “dust traps”. In these high-pressure regions, the drift motion slows, allowing dust grains to accumulate. With their reduced velocity, the grains can also avoid fragmentation when they collide.
Until now, astronomers thought that such traps could only exist in very specific environments. However, an international team of astrophysicists led by researchers from the Centre de Recherche Astrophysique de Lyon (Université Claude Bernard Lyon 1, CNRS, ENS Lyon) in France has discovered a mechanism which can explain the growth of solid particles from pebbles to planetesimals.
Using numerical simulations and analytical calculations, the team have shown that dust traps form spontaneously in a wide range of disks, and that dust traps are therefore much more frequent than previously assumed. The researchers have highlighted the key role of the drag of dust on the gas in this mechanism. In most astronomical situations, the gas tells the dust how to move, but sometimes – when there is a lot of dust – the dust can tell the gas how to move. “This effect, known as aerodynamic drag back-reaction, was generally ignored because it is negligible in most situations. However, its effects become important in regions of strong dust concentrations, like those found in the planet formation process,” explains Jean-François Gonzalez, associate professor of the Université Claude Bernard at the Centre de Recherche Astrophysique de Lyon (CRAL).
The effect of the back-reaction is to slow the inward drift of the grains, which gives them time to grow in size. Once large enough, the grains are their own master and the gas can no longer govern their motion. The gas, under the influence of this back-reaction, will be pushed outwards and form a high-pressure region: the dust trap. These spontaneous traps can then efficiently concentrate the grains coming from the outer disk regions, creating a very dense ring of solids. These dust traps are favourable environments for planetesimal formation.
This discovery of the spontaneous formation of dust traps provides a simple and robust solution to the long-standing problem in the understanding of planet formation.

References: Self-induced dust traps: overcoming planet formation barriers, Monthly Notices of the Royal Astronomical Society, Oxford University Press, january 2017 on arXiv