Can habitable planets form in clustered environments?

TL;DR

This study provides observational evidence that high stellar densities in clusters can truncate protoplanetary disks, potentially hindering habitable planet formation and leading to free-floating planets, with implications for planetary system development.

Contribution

It offers empirical evidence linking stellar density to protoplanetary disk truncation and suggests a causal relationship affecting habitable planet formation in dense environments.

Findings

Significant change in PPD size distribution at stellar densities >10^{3.5} pc^{-2}

Dynamical encounters likely truncate PPDs in dense clusters

Habitable planets less likely to form or survive in high-density environments

Abstract

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.