The recent and ongoing discovery of extrasolar giant planets has stimulated renewed interest in the theory of planet formation (e.g., Mayor and Queloz, 1995; Marcy et al., 2000; Vogt et al., 2002). In particular the discovery of giant planets close to their central stars has led to the idea that they migrated inward due to gravitational interaction with the gaseous protoplanetary disc out of which they formed.