Induced planet formation in stellar clusters: a parameter study of star–disc encounters

We present a parameter study of the possibility of tidally triggered disc instability. Using a restricted N ‐body model that allows for a survey of an extended parameter space, we show that a passing dwarf star with a mass between 0.1 and 1 M ⊙ can probably induce gravitational instabilities (GIs) in the pre‐planetary solar disc for prograde passages with minimum separations below 80–170 au for isothermal or adiabatic discs. Inclined and retrograde encounters lead to similar results but require slightly closer passages. Such encounter distances are quite likely in young moderately massive star clusters. The induced GIs may lead to enhanced planetesimal formation in the outer regions of the protoplanetary disc and could therefore be relevant for the existence of Uranus and Neptune, whose formation time‐scale of about 100 Myr is inconsistent with the disc lifetimes of about a few Myr according to observational data by Haisch, Lada & Lada. The relatively small gas/solid ratio in Uranus and Neptune can be matched if the perturbing fly‐by occurred after early gas depletion of the solar system, i.e. when the solar system was older than about 5 Myr. We also confirm earlier results by Heller that the observed 7° tilt of the solar equatorial plane relative to the ecliptic plane could be the consequence of such a close encounter.