Orbital Evolution of Distant Comets under Interstellar Gas Drag during the Solar System’s Motion through the Galaxy

As the Solar System moves through the Galaxy, it occasionally passes through regions of diffuse gas, such as the interstellar medium or molecular clouds. While such rarefied gas generally has negligible effects on most Solar System bodies, it may influence the orbits of small, distant comets.

In particular, long-period comets in the Oort cloud, which is thought to host a vast population of icy bodies, are potentially sensitive to weak external forces. Their orbital velocities are extremely small—on the order of a few meters per second—and their bulk densities are typically lower than 1 g/cm³. These characteristics make them susceptible to even modest levels of gas drag.

Some giant molecular clouds (GMCs) possess both high masses and significant gas densities, potentially generating appreciable drag forces. Whereas gravitational perturbations from such clouds are largely independent of comet size, gas drag exhibits a strong size dependence.

In this study, we model the Solar System's motion through a uniform background gas and examine the resulting orbital evolution of distant comets using both analytical methods and numerical simulations. The Epstein drag law is adopted to describe the gas drag in the free molecular regime.

Our preliminary results suggest that two types of orbital evolution are possible: (1) rapid escape from the Solar System, and (2) gradual inward migration toward the inner Solar System. We derive the parameter regimes under which each outcome occurs, based on the comet’s initial size, density, and orbital velocity. These dynamical outcomes may affect the long-term population and size-frequency distribution of comets in the Solar system.

This type of orbital evolution is not limited to the Solar System; it could also occur in cometary clouds surrounding other stars. In future work, we aim to extend our analysis to discuss the size-frequency distribution of bodies ejected into the Galactic environment via this mechanism, as well as their subsequent orbital evolution under the influence of Galactic tides and other large-scale forces. Some of these ejected bodies may eventually enter the Solar System and be detected as interstellar objects, providing a rare observational opportunity to probe the dynamical histories and size distributions of distant cometary populations that would otherwise be unobservable.

This work is supported by the Programme de Planétologie (PNP) of CNRS/INSU, co-funded by CNES.