Migration of giant planets in planetesimal discs

Planets orbiting a planetesimal circumstellar disc can migrate inward from their initial positions because of dynamical friction between planets and p lanetesimals. The migration rate depends on the disc mass and on its time e volution. Planets that are embedded in long-lived planetesimal discs, havin g total mass of 10(-4)-0.01 M-circle dot, can migrate inward a large distan ce and can survive only if the inner disc is truncated or as a result of ti dal interaction with the star. In this case the sernimajor axis, a, of the planetary orbit is less than 0.1 an. Orbits with larger a are obtained for smaller values of the disc mass or for a rapid evolution (depletion) of the disc. This model may explain not only several of the orbital features of t he giant planets that have been discovered in recent years orbiting nearby stars, but also the metallicity enhancement found in several stars associat ed with short-period planets.