Tidal fragmentation as the origin of 1I/2017 U1 (‘Oumuamua)

Abstract

The first discovered interstellar object (ISO), ‘Oumuamua (1I/2017 U1) shows a dry and rocky surface, an unusually elongated shape, with short-to-long axis ratio c∕a ≲ 1∕6, a low velocity relative to the local standard of rest (~10 km s−1), non-gravitational accelerations and tumbles on a timescale of a few hours1–9. The inferred number density (~3.5 × 1013−2 × 1015 pc−3) for a population of asteroidal ISOs10,11 outnumbers cometary ISOs12 by ≥103, in contrast to the much lower ratio (≲10−2) of rocky/icy Kuiper belt objects13. Although some scenarios can cause the ejection of asteroidal ISOs14,15, a unified formation theory has yet to comprehensively link all ‘Oumuamua’s puzzling characteristics and to account for the population. Here we show by numerical simulations that ‘Oumuamua-like ISOs can be prolifically produced through extensive tidal fragmentation and ejected during close encounters of their volatile-rich parent bodies with their host stars. Material strength enhanced by the intensive heating during periastron passages enables the emergence of extremely elongated triaxial ISOs with shape c∕a ≲ 1∕10, sizes a ≈ 100 m and rocky surfaces. Although volatiles with low sublimation temperature (such as CO) are concurrently depleted, H2O buried under surfaces is preserved in these ISOs, providing an outgassing source without measurable cometary activities for ‘Oumuamua’s non-gravitational accelerations during its passage through the inner Solar System. We infer that the progenitors of ‘Oumuamua-like ISOs may be kilometre-sized long-period comets from Oort clouds, kilometre-sized residual planetesimals from debris disks or planet-sized bodies at a few astronomical units, orbiting around low-mass main-sequence stars or white dwarfs. These provide abundant reservoirs to account for ‘Oumuamua’s occurrence rate.