Young stars are born surrounded in an envelope of dust and gas which in many cases forms a protoplanetary disk. These disks evolve and disperse within a few million years leaving behind newly-formed planetary systems. Nonetheless, formulating a coherent explanation of the evolution of protoplanetary disks remains challenging. The importance and influence of magnetic fields, for instance, continues for the large part to be unclear. Protoplanetary disks are relatively cool and are thought to be only very weakly ionised which provides imperfect coupling to magnetic fields. Understanding the sources of ionisation, such as cosmic rays, present in the disks underpins our overall understanding of how these systems evolve and form planets.
In recent years, ALMA (Atacama Large Millimeter Array) observations have revolutionised the field. ALMA observations have been able to resolve substructure in protoplanetary disks, to constrain the level of turbulence and in some cases to even constrain the ionisation rate. In this context, numerical models are being developed to estimate the ionising effect of cosmic rays originating from the young star itself. These simulations can probe the ionisation rate, as well as the effect on disk chemistry. Recent simulations also suggest that the ability of cosmic rays to penetrate and ionise disks is a disk mass dependent process and can be discussed in comparison to ALMA observations.