Capture of interstellar objects II: by the Solar system

TL;DR

This paper analyzes how interstellar objects are captured by the Solar system, finding that capture rates depend on local phase-space density and are not influenced by the Sun's galactic environment, with most captured objects having short residence times.

Contribution

It provides a detailed analysis of ISO capture mechanisms, rates, and phase-space distribution, challenging previous ideas about environmental effects on capture frequency.

Findings

Capture dominated by slow ISOs with $v_4$ km/s

Estimated 2 ISOs captured per 1000 years within 5 au

Capture and ejection rates are approximately equal

Abstract

Capture of interstellar objects (ISOs) into the Solar system is dominated by ISOs with asymptotic incoming speeds $v_\infty<4\,$km\,s$^{-1}$. The capture rate is proportional to the ISO phase-space density in the Solar vicinity and does not vary along the Sun's Galactic orbit, i.e.\ is not enhanced during a passage through a cloud of ISOs (in contrast to previous suggestions). Most bound orbits crossing those of Jupiter and Saturn are fully mixed with unbound phase space, implying that they hold the same ISO phase-space density. Assuming an interstellar number density $n_{iso}\sim0.1\,$au$^{-3}$, we estimate that in 1000 years the planets capture $\sim2$ ISOs (while $\sim17$ fall into the Sun), resulting in a population of $\sim8$ captured ISOs within 5\,au of the Sun at any time, less than the number of visiting ISOs passing through the same volume on hyperbolic orbits. In terms of phase-space volume, capture onto and ejection from the Solar system are equal, such that on average ISOs will not remain captive at $a\lesssim2000\,$au for extensive periods.