Implications of the interstellar object 1I/'Oumuamua for planetary dynamics and planetesimal formation

TL;DR

The paper explores how 'Oumuamua's properties inform our understanding of interstellar planetesimal ejection, suggesting tidal disruption processes and composition influence its detection and characteristics.

Contribution

It introduces a model where tidal disruption explains 'Oumuamua's size and composition, reconciling observations with planetesimal formation theories.

Findings

0.1-1% of planetesimals undergo tidal disruption before ejection.

'Oumuamua is likely a fragment of a tidally disrupted planetesimal.

Cometary planetesimals are more likely than asteroidal ones to be ejected and detected.

Abstract

'Oumuamua, the first bona-fide interstellar planetesimal, was discovered passing through our Solar System on a hyperbolic orbit. This object was likely dynamically ejected from an extrasolar planetary system after a series of close encounters with gas giant planets. To account for 'Oumuamua's detection, simple arguments suggest that ~1 Earth mass of planetesimals are ejected per Solar mass of Galactic stars. However, that value assumes mono-sized planetesimals. If the planetesimal mass distribution is instead top-heavy the inferred mass in interstellar planetesimals increases to an implausibly high value. The tension between theoretical expectations for the planetesimal mass function and the observation of 'Oumuamua can be relieved if a small fraction (~0.1-1%) of planetesimals are tidally disrupted on the pathway to ejection into 'Oumuamua-sized fragments. Using a large suite of simulations of giant planet dynamics including planetesimals, we confirm that 0.1-1% of planetesimals pass within the tidal disruption radius of a gas giant on their pathway to ejection. 'Oumuamua may thus represent a surviving fragment of a disrupted planetesimal. Finally, we argue that an asteroidal composition is dynamically disfavoured for 'Oumuamua, as asteroidal planetesimals are both less abundant and ejected at a lower efficiency than cometary planetesimals.