  Most stars are born in clusters and the resulting gravitational interactions
between cluster members may significantly affect the evolution of circumstellar
discs and therefore the formation of planets and brown dwarfs. Recent findings
suggest that tidal perturbations of typical circumstellar discs due to close
encounters may inhibit rather than trigger disc fragmentation and so would seem
to rule out planet formation by external tidal stimuli. However, the disc
models in these calculations were restricted to disc radii of 40 AU and disc
masses below 0.1 M_sun. Here we show that even modest encounters can trigger
fragmentation around 100 AU in the sorts of massive (~0.5 M_sun), extended
(>=100 AU) discs that are observed around young stars. Tidal perturbation alone
can do this, no disc-disc collision is required. We also show that
very-low-mass binary systems can form through the interaction of objects in the
disc. In our computations, otherwise non-fragmenting massive discs, once
perturbed, fragment into several objects between about 0.01 and 0.1 M_sun,
i.e., over the whole brown dwarf mass range. Typically these orbit on highly
eccentric orbits or are even ejected. While probably not suitable for the
formation of Jupiter- or Neptune-type planets, our scenario provides a possible
formation mechanism for brown dwarfs and very massive planets which,
interestingly, leads to a mass distribution consistent with the canonical
substellar IMF. As a minor outcome, a possible explanation for the origin of
misaligned extrasolar planetary systems is discussed.
