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Most stars are born in clusters and the resulting gravitational interactionsbetween cluster members may significantly affect the evolution of circumstellardiscs and therefore the formation of planets and brown dwarfs. Recent findingssuggest that tidal perturbations of typical circumstellar discs due to closeencounters may inhibit rather than trigger disc fragmentation and so would seemto rule out planet formation by external tidal stimuli. However, the discmodels in these calculations were restricted to disc radii of 40 AU and discmasses below 0.1 M_sun. Here we show that even modest encounters can triggerfragmentation around 100 AU in the sorts of massive (~0.5 M_sun), extended(>=100 AU) discs that are observed around young stars. Tidal perturbation alonecan do this, no disc-disc collision is required. We also show thatvery-low-mass binary systems can form through the interaction of objects in thedisc. In our computations, otherwise non-fragmenting massive discs, onceperturbed, 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 highlyeccentric orbits or are even ejected. While probably not suitable for theformation of Jupiter- or Neptune-type planets, our scenario provides a possibleformation mechanism for brown dwarfs and very massive planets which,interestingly, leads to a mass distribution consistent with the canonicalsubstellar IMF. As a minor outcome, a possible explanation for the origin ofmisaligned extrasolar planetary systems is discussed.

Tidally induced brown dwarf and planet formation in circumstellar discs