Capture of Interstellar Planetesimals in Protoplanetary Discs in Binary Star Systems

Planet formation requires growth over 12 orders of magnitude in size, from micron-sized dust grains to fully formed planets. The formation of km sized planetesimals is the most challenging stage, where radial drift and fragmentation rapidly remove meter- sized solids from the protoplanetary disc. Overcoming this barrier requires favourable conditions for streaming instability to concentrate solids. These problems are exacerbated in binary star systems, where gravitational perturbations and disc truncation reduce the efficiency of planetesimal formation pathways. Planetesimal capture provides an alternative mechanism where unbound planetesimals are captured by the protoplanetary disc through gas-drag, potentially bypassing the most restrictive growth stages. We investigate this process in circumstellar and circumbinary discs using over 30 million numerical experiments across a range of binary separations. For circumstellar discs, capture of large planetesimals is suppressed below 50 au, with the strongest reduction at around 10 au, broadly consistent with the observed decrease in planet occurrence in binaries. Tight binaries can still capture O(1) km-sized planetesimals, increasing to O(10) km-sized in wider systems during the birth cluster's lifetime. Circumbinary discs are much less efficient, with capture rates dropping to around 1 percent of the single-star case for binaries above 1 au and becoming negligible for large planetesimals at wider separations. Overall, planetesimal capture provides a supplementary channel for delivering solids to binary systems and mitigating inefficiencies in in-situ planetesimal formation.