Eccentric Extrasolar Planets from Resonant Saturn Mass Companions

The eccentricity of a system of two planets locked into a 2:1 resonance grows as the planets migrate inwards through a gaseous protoplanetary disk. We show that this can lead to large eccentricities of the inner planet, even when the outer planet has a mass low enough to have eluded detection with current surveys. If most of the observed extrasolar planetary systems initially harboured extra planets at resonance, their mutual interaction could account for the ubiquity of significantly eccentric orbits. This mechanism can be tested by searching for resonant outer partners to known extrasolar planets, which we predict to be of lower mass and with lower eccentricity than the inner planet. Subject headings: planets and satellites: formation — planetary systems — accretion, accretion disks