Abstract

Protoplanetary discs are the birthplace of planets: knowledge of their evolution mechanism is key to understand how planet formation takes place and to explain the properties of the currently detected exoplanets. Traditionally, planet-forming discs have been thought to evolve viscously: angular momentum redistribution allows for accretion and leads to outward disc spreading. Recently, it was hypothesised instead that accretion is due to angular momentum removal by MHD winds, implying that no disc spreading is expected. We run several 1D gas and dust simulations of viscous and MHD discs with the aim of assessing which evolution mechanism is the dominant one. To do so, we compute disc dust sizes and compare them with ALMA observations. We show that viscous and MHD angular momentum transport determine very different dust disc sizes. In particular, MHD models are compact, as expected from the bulk of the data, while in viscous models dust sizes increase with time. However, current observations lack enough sensitivity to discriminate between the two evolutionary scenarios. Deeper ALMA observation could be helpful to assess the dominant evolution mechanisms of planet-forming discs.