  We present observational evidence of environmental effects on the formation
and evolution of planetary systems. We combine catalogues of resolved
protoplanetary discs (PPDs) and young stellar objects in the solar
neighbourhood to analyse the PPD size distribution as a function of ambient
stellar density. By running Kolmogorov-Smirnov tests between the PPD radii at
different densities, we find empirical evidence, at the >97% confidence level,
for a change in the PPD radius distribution at ambient stellar densities \Sigma
> 10^{3.5} pc^{-2}. This coincides with a simple theoretical estimate for the
truncation of PPDs or planetary systems by dynamical encounters. If this
agreement is causal, the ongoing disruption of PPDs and planetary systems
limits the possible existence of planets in the habitable zone, with shorter
lifetimes at higher host stellar masses and ambient densities. Therefore,
habitable planets are not likely to be present in long-lived stellar clusters,
and may have been ejected altogether to form a population of unbound,
free-floating planets. We conclude that, while highly suggestive, our results
should be verified through other methods. Our simple model shows that
truncations should lead to a measurable depletion of the PPD mass function that
can be detected with ALMA observations of the densest nearby and young
clusters.
