Most stars form and spend their early life in regions of enhanced stellar density. Therefore the evolution of protoplanetary discs (PPDs) hosted by such stars are subject to the influence of other members of the cluster. Physically, PPDs might be truncated either by photoevaporation due to ultraviolet flux from massive stars, or tidal truncation due to close stellar encounters. Here we aim to compare the two effects in real cluster environments. In this vein we first review the properties of well studied stellar clusters with a focus on stellar number density, which largely dictates the degree of tidal truncation, and far ultraviolet (FUV) flux, which is indicative of the rate of external photoevaporation. We then review the theoretical PPD truncation radius due to an arbitrary encounter, additionally taking into account the role of eccentric encounters that play a role in hot clusters with a 1D velocity dispersion σ v 2 km/s. Our treatment is then applied statistically to varying local environments to establish a canonical threshold for the local stellar density (n c 10 4 pc −3 ) for which encounters can play a significant role in shaping the distribution of PPD radii over a timescale ∼ 3 Myr. By combining theoretical mass loss rates due to FUV flux with viscous spreading in a PPD we establish a similar threshold for which a massive disc is completely destroyed by external photoevaporation. Comparing these thresholds in local clusters we find that if either mechanism has a significant impact on the PPD population then photoevaporation is always the dominating influence.

We present an open access grid of 3930 calculations of externally evaporating protoplanetary discs. This spans a range of disc sizes (1-400 AU), disc masses, UV field strengths (10-10 4 G 0 ) and stellar masses (0.05-1.9 M ). The grid is publicly available for download, and offers a means of cheaply including external photoevaporation in disc evolutionary calculations. It can also be queried using an online tool for quick estimates of instantaneous mass loss rates (e.g for convenient evaluation of real observed systems). fried itself illustrates that for discs around stars ≤ 0.3 M external photoevaporation is effective down to small radii (< 50 AU) down to UV fields at least as weak as 10 G 0 . At the other end of the scale, in a 10 4 G 0 environment photoevaporation is effective down to 1 AU even for stellar masses at least as high as 1.9 M . We also illustrate in which regimes CO survives in the photoevaporative outflow for significant mass loss rates; marking a system a good candidate to detect external photoevaporation in weak-intermediate UV environments through sub-Keplerian rotation. Finally we make illustrative mass loss rate estimates for discs in Taurus based on the Guilloteau et al. (2011) star-disc parameters, finding that around half are expected to have both significant mass loss and retain CO in the photoevaporative outflow.

Planet formation is generally described in terms of a system containing the host star and a protoplanetary disk 1 – 3 , of which the internal properties (for example, mass and metallicity) determine the properties of the resulting planetary system 4 . However, (proto)planetary systems are predicted 5 , 6 and observed 7 , 8 to be affected by the spatially clustered stellar formation environment, through either dynamical star–star interactions or external photoevaporation by nearby massive stars 9 . It is challenging to quantify how the architecture of planetary sysems is affected by these environmental processes, because stellar groups spatially disperse within less than a billion years 10 , well below the ages of most known exoplanets. Here we identify old, co-moving stellar groups around exoplanet host stars in the astrometric data from the Gaia satellite 11 , 12 and demonstrate that the architecture of planetary systems exhibits a strong dependence on local stellar clustering in position-velocity phase space. After controlling for host stellar age, mass, metallicity and distance from the star, we obtain highly significant differences (with p values of 10 −5 to 10 −2 ) in planetary system properties between phase space overdensities (composed of a greater number of co-moving stars than unstructured space) and the field. The median semi-major axis and orbital period of planets in phase space overdensities are 0.087 astronomical units and 9.6 days, respectively, compared to 0.81 astronomical units and 154 days, respectively, for planets around field stars. ‘Hot Jupiters’ (massive, short-period exoplanets) predominantly exist in stellar phase space overdensities, strongly suggesting that their extreme orbits originate from environmental perturbations rather than internal migration 13 , 14 or planet–planet scattering 15 , 16 . Our findings reveal that stellar clustering is a key factor setting the architectures of planetary systems.

The stellar birth environment can significantly shorten protoplanetary disc (PPD) lifetimes due to the influence of stellar feedback mechanisms. The degree to which these mechanisms suppress the time and mass available for planet formation is dependent on the local far-ultraviolet (FUV) field strength, stellar density, and ISM properties. In this work, we present the first theoretical framework quantifying the distribution of PPD dispersal time-scales as a function of parameters that describe the galactic environment. We calculate the probability density function for FUV flux and stellar density in the solar neighbourhood. In agreement with previous studies, we find that external photoevaporation is the dominant environment-related factor influencing local stellar populations after the embedded phase. Applying our general prescription to the Central Molecular Zone of the Milky Way (i.e. the central $\sim 250~\mbox{${\rm pc}$}$), we predict that $90{{\ \rm per\ cent}}$ of PPDs in the region are destroyed within 1 Myr of the dispersal of the parent molecular cloud. Even in such dense environments, we find that external photoevaporation is the dominant disc depletion mechanism over dynamical encounters between stars. PPDs around low-mass stars are particularly sensitive to FUV-induced mass-loss, due to a shallower gravitational potential. For stars of mass ∼1 M⊙, the solar neighbourhood lies at approximately the highest gas surface density for which PPD dispersal is still relatively unaffected by external FUV photons, with a median PPD dispersal time-scale of ∼4 Myr. We highlight the key questions to be addressed to further contextualize the significance of the local galactic environment for planet formation.