2026ApJ..1000L..39A

Catastrophic planetesimal disruptions offer a unique opportunity to study and characterize large planetesimal populations in exoplanetary systems that are not currently detectable by modern observatories. The unexpected discovery of a second collision event in the Fomalhaut system raises important questions about the planetesimal population and dynamical state inside the Fomalhaut main belt that led to two collisions in 20 yr. We present a statistical model developed and applied to the archetypal Fomalhaut system to provide new constraints on the bulk properties of the planetesimals in Fomalhaut's main belt. Utilizing the constraints provided by the spatially resolved Fomalhaut cs1 and cs2 collision events, we retrieve the belt parameters that best reproduce the observed collision rate while remaining consistent with the system's age and dust mass. Our best-fit model suggests a total main-belt mass of 200-360 M_⊕, with the transition from a collisionally evolved to a primordial planetesimal population occurring at a radius of $11{5}_{-10}^{+30}$ km and a maximum planetesimal radius of $38{0}_{-202}^{+643}$ km. We estimate a catastrophic collision rate of $0.08{6}_{-0.048}^{+0.067}$ collision events per year for planetesimals with radii ≥100 km in the region interior to the main belt. Our findings show that further observable collisions are likely, motivating continued monitoring of Fomalhaut and other nearby debris disks.