How Cold Neptunes Shape Planetary Systems

Cold Neptunes appear to be among the most abundant planets in the Galaxy, yet their role in shaping planetary systems remains poorly understood. We study the dynamical evolution of cold Neptune systems born in resonant chains through disk-driven migration and later perturbed by planetesimals leftover from planet formation. Using N-body simulations, we find that planetesimal populations comprising only 1–2% of the total planetary mass are sufficient not only to break the resonance chains but also to trigger a global dynamical instability. The resulting evolution produces a wide range of observable outcomes: some Neptunes are ejected to become free-floating planets, others scatter onto wide orbits, some undergo high-eccentricity tidal migration to become hot Neptunes on nearly polar orbits, while inner resonant chains of super-Earths are often destabilized on ∼100 Myr timescales. I will discuss these outcomes in the context of upcoming microlensing campaigns and ongoing efforts to characterize the orbits of hot Neptunes, highlighting how cold Neptunes may act as key architects of planetary system evolution.