Planets form in discs of gas and dust around young stars. Within these discs, grains initially smaller than a micron will somehow have to grow into planetary sized objects. This is relatively easy at first: when two grains meet they stick, held together by inter-molecular forces. When particles reach millimeter size, however, this no longer works as well, with collisions often resulting in bouncing rather than sticking. A further complicating issue is that at these sizes, dust particles are rapidly driven towards the central star by friction with the gas. This inward drift can be extremely fast: boulders of 1 meter in size can drift into the star in 100 years, while planet formation typically takes 100,000-1,000,000 years. This 'drift-barrier' has long been a major problem in planet formation.
A promising solution is the 'streaming instability'. As it turns out, in many cases solids streaming through a gas is hydrodynamically unstable. The streaming instability feeds off the drift, creating a turbulent state with strong dust overdensities. These overdensities can subsequently collapse gravitationally into kilometer-sized 'planetesimals'. These objects are large enough to be safe from drift, and are usually strong enough not to be destroyed by collisions. Moreover, the growth time of the instability is shorter than the drift time of the dust, making sure that planetesimals form before all the solids are lost into the central star.
Dust flow in gas discs is usually studies in the simplified case where all particles have the same size. While this greatly simplifies the analysis, in reality there will always be a distribution of sizes. As it turns out, a size distribution can have important dynamical consequences. For example, a wide size distribution can completely disable the streaming instability if the size distribution is not exactly of the right form. If this behavior is generic, this means that the size distribution, which is very difficult to observe directly, can be traced through larger-scale processes in protoplanetary discs. This provides a new window on planet formation, revealing previously unseen processes.
In this project, we are letting the dust size distribution take center stage. What role does the size distribution play in growing planets? How can we perform realistic simulations of a gas-dust mixture with a size distribution? How can we compare simulation outcomes to observations of for example Solar system objects?
