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Abstract

The first detected interstellar object, ‘Oumuamua, which passed within 0.25 au of the Sun on 2017 September 9, was presumably ejected from a stellar system. We use its newly determined non-Keplerian trajectory together with the reconstructed Galactic orbits of 7 million stars from Gaia DR2 to identify past close encounters. Such an “encounter” could reveal the home system from which ‘Oumuamua was ejected. The closest encounter, at 0.60 pc (0.53-0.67 pc, 90% confidence interval), was with the M2.5 dwarf HIP 3757 at a relative velocity of 24.7 {km} {{{s}}}^-1, 1 Myr ago. A more distant encounter (1.6 pc) but with a lower encounter (ejection) velocity of 10.7 {km} {{{s}}}^-1 was with the G5 dwarf HD 292249, 3.8 Myr ago. Two more stars have encounter distances and velocities intermediate to these. The encounter parameters are similar across six different non- gravitational trajectories for ‘Oumuamua. The ejection of ‘Oumuamua by scattering from a giant planet in one of the systems is plausible, but requires a rather unlikely configuration to achieve the high velocities found. A binary star system is more likely to produce the observed velocities. None of the four home candidates have published exoplanets or are known to be binaries. Given that the 7 million stars in Gaia DR2 with 6D phase space information is just a small fraction of all stars for which we can eventually reconstruct orbits, it is a priori unlikely that our current search would find ‘Oumuamua’s home star system. As ‘Oumuamua is expected to pass within 1 pc of about 20 stars and brown dwarfs every Myr, the plausibility of a home system also depends on an appropriate (low) encounter velocity.