Planets form in proto-planetary discs, gas and dust structures that orbit young stars. It has long been known that these discs are accretion discs, i.e. that matter moves radially inwards in them and eventually falls onto the central star. The reason why this happens however is a long-standing problem. The traditional paradigm is to invoke turbulence. Indeed, high enough turbulence, which acts as an effective viscosity, can explain accretion. However, it remains difficult to explain why discs should be turbulent. In parallel, proving in observations that discs are indeed turbulent has turned out to be incredibly elusive, and most results of the last few years have shown that, if present, turbulence is actually low in proto-planetary discs, questioning whether it is strong enough to drive accretion.
The alternative is that discs instead evolve through a very different process, called magneto-hydrodynamics (MHD) winds. These winds, launched by the magnetic field, can remove angular momentum from the disc, causing the material left in the disc to spiral inwards and eventually accrete onto the star.
The two different mechanisms predict very different ways in which the disc will evolve and eventually disperse, leaving behind a fully formed planetary system like our own Solar System.
Understanding whether discs are turbulent or evolve because of winds is of fundamental importance not only for the consequences on disc evolution, but also because the levels of turbulence affect nearly every process in planet formation: for example the accretion of gas and solids on young, forming planets, as well as the way they migrate through the disc. Solving the problem of why discs accrete is therefore one of the major questions in planet formation and it is the main question that the DiscEvol project wishes to tackle.
