The Kinematic Age of 3I/ATLAS and Its Implications for Early Planet Formation

Abstract The recent discovery of the third interstellar object (3I/ATLAS) expands the known census from two to three and significantly improves statistical inferences regarding the underlying Galactic population. In this Letter, we argue that cometary activity likely significantly contributes to 3I/ATLAS’s brightness since the nuclear size inferred when assuming an asteroidal reflectance implies an untenable interstellar object mass per star. Relative to the Sun, 3I/ATLAS exhibits a high excess velocity of v ∞ = 58 km s −1 , which implies that 3I/ATLAS is relatively old in comparison to previous interstellar objects. Here, we calculate the posterior distribution of ages implied by the kinematics of the interstellar objects and find that 3I/ATLAS is likely ∼3–11 Gyr old, assuming that the interstellar object and stellar age–velocity dispersion relations are equivalent. We also calculate the distribution of host star metallicities and find that 3I/ATLAS has a 12% chance of originating from a star with [Fe/H] ≤ −0.4. These results show that interstellar object formation is likely efficient at low metallicities and early in the history of the Galaxy. Finally, we estimate the interstellar object formation rate throughout Galactic history implied by these three objects. As future interstellar objects are discovered, the framework presented here can be applied to further refine this calculation. Comparison between the interstellar object and stellar formation histories will provide unique insights into the history of stellar system formation in the Galaxy.