  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 x 44 m x 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.
