Star-disc encounters and their role in planet formation

It is sometimes surmised that star-disc encounters may be a sufficient - or even necessary - condition for self-gravitating discs to fragment and form planets via gravitational instability. Our recent simulations support neither of these hypotheses. We find that discs whose thermal timescale is sufficiently long that they do not fragment in isolation are also stable against fragmentation during an encounter. We moreover find that if the disc enters a regime of relatively short cooling timescale, fragmentation does not require that this regime is entered rapidly. Consequently, discs will fragment if they enter the regime of rapid cooling by secular evolution, and impulsive changes - such as those resulting from an encounter - are not necessary. We thus conclude that planet formation is not promoted by a cluster environment