Triggered planet formation in young stellar clusters

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Understanding the origin of the planets is one of the oldest astrophysical research fields, and there are still unanswered principal questions. One of them is how Uranus and Neptune formed within the limited available time during the existence of the protoplanetary disk, from the reservoir of material out of which the planets formed. According to observational and theoretical data the lifetime of such a disk has been estimated to be less than about ten million years. On the other hand, the widely accepted model of planet formation - collisions of planetesimals followed by gravitational accretion of surrounding material - would take more than 100 million years especially for the outermost giant planet Neptune and even longer for the large trans-Neptunian objects (TNOs) like Sedna or the recently discovered 2003 UB313 (often referred to as Xena in the media). Another question concerns the orbits of the discovered extrasolar planets (exoplanets), which have high eccentricities and inclinations in contrast to the nearly circular orbits in the Solar System. The project outlined here addresses both these fundamental questions. The central idea postulates a close encounter between the very young Sun and a (most likely) low-mass star within the native stellar cluster of our Solar System. Such an encounter could cause local gravitational instabilities that fragment and collapse, forming protoplanetary cores of sufficient mass for a quick following accretion process. The aim is to investigate this scenario by hydrodynamical computations of the overall physics within the disk down to the process of gravitational collapse and planet core formation, and subsequent accretion.