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. 
Susanne Pfalzner, Manuel Steinhausen, Karl Menten
astro-ph.GA astro-ph.SR