External photoevaporation of circumstellar discs constrains the time-scale for planet formation

Planet-forming circumstellar discs are a fundamental part of the star formation process. Since stars form in a hierarchical fashion in groups of up to hundreds or thousands, the UV radiation environment that these discs are exposed to can vary in strength by at least six orders of magnitude. This radiation can limit the masses and sizes of the discs. Diversity in star forming environments can have long lasting effects in disc evolution and in the resulting planetary populations. We perform simulations to explore the evolution of circumstellar discs in young star clusters. We include viscous evolution, as well as the impact of dynamical encounters and external photoevaporation. We find that photoevaporation is an important process in destroying circumstellar discs: in regions of stellar density rho similar to 100 M-circle dot pc(-3) around 80 per cent of discs are destroyed before 2 Myr of cluster evolution. In regions of rho similar to 50 M-circle dot pc(-3) around 50 per cent of discs are destroyed in the same time-scale. Our findings are in agreement with observed disc fractions in young star-forming regions and support previous estimations that planet formation must start in time-scales <0.1-1 Myr.