The mass density of interstellar objects in the Solar neighbourhood and its Galactic implications

Threemacroscopicinterstellar objects (ISOs) have now been detected onunboundtrajectories through the Solar System, enabling the empirical constraints on their local mass density and Galactic abundance. We infer the local ISO mass density by combining a magnitude-space population model anchored to the Pan-STARRS detection of 1I/‘Oumuamua within the ¯ Otautahi–Oxford frameworkwithaflux-basedPoissonarrival-rate constraint derived from the detection times, discovery distances, and asymptotic velocities of 1I/'Oumuamua, 2I/Borisov, and 3I/ATLAS. Uncertainty in the characteristic mass scale is propagated via a nonparametric bootstrap over published ISO mass estimates, and the two information channels are combined through KDE-weighted importance resampling to obtain a joint posterior for the local ISO mass density, 𝜌local. Assuming that ISOs trace the Galactic stellar or total baryonic mass distribution, we normalise analytic Milky Way density fields at the Solar radius and integrate them over the Galactic volume. The resulting posterior yields 𝜌local = [5.42 × 10−24,2.09 × 10−23,7.61 × 10−23]kg/m3 (95% credible interval), implying total ISO masses 𝑀ISO ∼ 108𝑀⊙, corresponding to ISO mass fractions 𝑓ISO ∼ 10−4–10−3 and a baryon-fraction increase Δ𝑓𝑏 ∼ (4–7) × 10−4. Macroscopic ISOs are therefore a subdominant component of the Milky Way baryon budget, but their total mass is now observationally bounded to within an order of magnitude.