  The 2:1 orbital resonances of the GJ 876 system can be easily established by
the differential planet migration due to planet-nebula interaction. Significant
eccentricity damping is required to produce the observed orbital
eccentricities. The geometry of the GJ 876 resonance configuration differs from
that of the Io-Europa pair, and this difference is due to the magnitudes of the
eccentricities involved. We show that a large variation in the configuration of
2:1 and 3:1 resonances and, in particular, asymmetric librations can be
expected among future discoveries.
