External photoevaporation of circumstellar disks constrains the
  timescale for planet formation
  Planet-forming circumstellar disks 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 disks are
exposed to can vary in strength by at least six orders of magnitude. This
radiation can limit the masses and sizes of the disks. Diversity in star
forming environments can have long lasting effects in disk evolution and in the
resulting planetary populations. We perform simulations to explore the
evolution of circumstellar disks 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 disks: in regions of stellar density $\rho \sim 100
\mathrm{\ M}_\odot \mathrm{\ pc}^{-3}\mathrm{\ }$ around 80% of disks are
destroyed before 2 Myr of cluster evolution. Our findings are in agreement with
observed disk fractions in young star forming regions and support previous
estimations that planet formation must start in timescales < 0.1 - 1 Myr.
