The impact of stellar clustering on the observed multiplicity of super-Earth systems: outside-in cascade of orbital misalignments initiated by stellar flybys

Recent studies suggest that the observed multiplicity of super-Earth (SE) systems is correlated with stellar overdensities. This correlation is puzzling as stellar clustering is expected to influence mostly the outer part of planetary systems. In this talk, I will present the possibility that stellar flybys indirectly excite the mutual inclinations of initially coplanar SEs, breaking their co-transiting geometry. We propose that flybys excite the inclinations of exterior substellar companions, which then propagate the perturbation to the inner SEs. We estimated semi-analytically the expected number of ‘effective’ flybys per planetary system, which can be rescaled easily for various system parameters. For a given SE system, there exists an optimal companion architecture that leads to the maximum number of effective flybys; this results from the trade-off between the flyby cross-section and the companion’s impact on the inner system. We expect this mechanism to be efficient in ‘SE + two companions’ systems that were born in dense stellar clusters.