1I/‘Oumuamua as an N₂ Ice Fragment of an exo-Pluto Surface: I. Size and Compositional Constraints

The origin of the interstellar object 1I/‘Oumuamua has defied explanation. We perform calculations of the non-gravitational acceleration that would be experienced by bodies composed of a range of different ices and demonstrate that a body composed of N₂ ice would satisfy the available constraints on the non-gravitational acceleration, size, and albedo, and lack of detectable emission of CO or CO₂ or dust. We find that ‘Oumuamua was small, with dimensions 45 m × 44 m × 7.5 m at the time of observation at 1.42 au from the Sun, with a high albedo of 0.64. This albedo is consistent with the N₂ surfaces of bodies like Pluto and Triton. We estimate ‘Oumuamua was ejected about 0.4–0.5 Gyr ago from a young stellar system, possibly in the Perseus arm. Objects like ‘Oumuamua may directly probe the surface compositions of a hitherto-unobserved type of exoplanet: “exo-plutos”. In a companion paper (Desch & Jackson, 2021) we demonstrate that dynamical instabilities like the one experienced by the Kuiper belt, in other stellar systems, plausibly could generate and eject large numbers of N₂ ice fragments. ‘Oumuamua may be the first sample of an exoplanet brought to us.