Interstellar Comets and the New Insights to Planet Formation They Provide

Minor bodies have opened windows into our understanding of the solar system planets’ formation and evolution. The Kirkwood gaps in the asteroid belt led to the discovery that seemingly immutable solar system bodies were chaotic, and the discovery of the Kuiper belt led to the realization that the giant planets underwent a transient period of violent instability. The discoveries of the first two interstellar minor bodies, 1I/`Oumuamua and 2I/Borisov, provided a chance to obtain new constraints on the formation location of ejected planetesimals and the occurrence rate and dynamical evolution of extrasolar planets responsible for their ejection. The forthcoming Rubin Observatory Legacy Survey of Space and Time (LSST) should detect ≥3 interstellar comets every year. We advocate for future measurements of the carbon to oxygen ratio in these comets, which traces formation location within the original protostellar disk. We review similar measurements for solar system comets, which indicate formation interior to the CO snowline. By comparing cometary C/O ratios calculated with various molecular species, we argue that measurements of H₂O, CO₂ and CO should be prioritized for interstellar comets. We quantify the extent to which these measurements probe the primordial composition of an interstellar comet as a function of age and trajectory, by calculating the relative change in diameter via erosion in the interstellar medium and the Solar System. We estimate that ~60% of ‘Oumuamua-like and ~10% of Borisov-like interstellar objects detected by the LSST will exhibit cometary activity representative of the primordial composition. By accounting for uncertainties and scatter in previously measured cometary and stellar C/O ratios, we estimate that these measurements for ≥5 objects will reveal a formation location interior or exterior to the CO snowline of ejected comets. Based on the anomalously high C/O ratio of 2I/Borisov and C/2016 R2 with respect to typical solar system comets, we argue that comets ejected via giant planet scattering largely form exterior to the CO snowline, while the remnant and bound H₂O dominated population formed interior to the CO snowline. We provide an overview of the currently operational and forthcoming facilities capable of producing these measurements which will test this hypothesis and provide key insights into the efficiency of and mechanisms for cometary ejection in exoplanetary systems.