Evidence for Early Stellar Encounters in the Orbital Distribution of Edgeworth-Kuiper Belt Objects

We show that early stellar encounters can explain the high eccentricities and inclinations observed in the outer part (>42 AU) of the Edgeworth-Kuiper Belt (EKB). We consider the proto-Sun as a member of a stellar aggregation that undergoes dissolution on a timescale of ~108 yr, such that the solar nebula experiences a flyby encounter at pericenter distance (q) on the order of 100 AU. Using numerical simulations we show that a stellar encounter pumps up the velocity dispersion in the young solar nebula in the outer parts. In the case of a nearly parabolic encounter with a solar mass companion, the velocity dispersion at a ≳ 0.25q is pumped up to such an extent that collisions between planetesimals would be expected to become highly disruptive, halting further growth of planetesimals. This has the consequence that planet formation is forestalled in that region. We also find that a stellar encounter with pericenter distance q ~ 100-200 AU could have pumped up the velocity dispersion of EKB objects outside 42 AU to the observed magnitude while preserving the velocity dispersion magnitude inside Neptune's 3:2 mean motion resonance (located at 39.5 AU). This allows for the efficient capture of objects by the resonance during a phase of orbital migration by proto-Neptune, which we also test with simulations. We point out that such a stellar encounter generally affects the dynamical and material structure of a protoplanetary disk, and the planetesimal distribution can remain imprinted with this signature over much of the main-sequence lifetime of the star. In particular, our results support the notion that an analogous process has operated in some recently observed extrasolar dust disks.