Short dissipation times of proto-planetary discs - an artifact of
  selection effects?
  The frequency of discs around young stars, a key parameter for understanding
planet formation, is most readily determined in young stellar clusters where
many relatively coeval stars are located in close proximity. Observational
studies seem to show that the disc frequency decreases rapidly with cluster age
with <10% of cluster stars retaining their discs for longer than 2-6 Myr. Given
that at least half of all stars in the field seem to harbor one or more
planets, this would imply extremely fast disc dispersal and rapid planet
growth. Here we question the validity of this constraint by demonstrating that
the short disc dissipation times inferred to date might have been heavily
underestimated by selection effects. Critically, for ages >3Myr only stars that
originally populated the densest areas of very populous clusters, which are
prone to disc erosion, are actually considered. This tiny sample may not be
representative of the majority of stars. In fact, the higher disc fractions in
co-moving groups indicate that it is likely that over 30% of all field stars
retain their discs well beyond 10 Myr, leaving ample time for planet growth.
Equally our solar system, with a likely formation time > 10 Myr, need no longer
be an exception but in fact typical of planetary systems.
