# An Observational Upper Limit on the Interstellar Number Density of Asteroids and Comets

## TL;DR

This study establishes an upper limit on the interstellar object density near the Sun by simulating detection efficiencies across major surveys, challenging assumptions about how other star systems eject planetesimals.

## Contribution

It provides the first observational upper limit on interstellar asteroid and comet density, incorporating realistic population modeling and survey detection efficiencies.

## Key findings

- Upper limit on interstellar object density: 1.4 x 10^{-4} au^{-3}
- Detection efficiencies vary with survey and object properties
- Results suggest interstellar object ejection and distribution may differ from solar system models.

## Abstract

We derived 90% confidence limits (CL) on the interstellar number density ($\rho_{IS}^{CL}$) of interstellar objects (ISO; comets and asteroids) as a function of the slope of their size-frequency distribution and limiting absolute magnitude. To account for gravitational focusing, we first generated a quasi-realistic ISO population to ~750 au from the Sun and propagated it forward in time to generate a steady state population of ISOs with heliocentric distance <50 au. We then simulated the detection of the synthetic ISOs using pointing data for each image and average detection efficiencies for each of three contemporary solar system surveys --- PS1, the Mt. Lemmon Survey, and the Catalina Sky Survey. These simulations allowed us to determine the surveys' combined ISO detection efficiency under several different but realistic modes of identifying ISOs in the survey data. Some of the synthetic detected ISOs had eccentricities as small as 1.01 --- in the range of the largest eccentricities of several known comets. Our best CL of $\rho_{IS}^{CL} = 1.4 \times 10^{-4}$ au$^{-3}$ implies that the expectation that extra-solar systems form like our solar system, eject planetesimals in the same way, and then distribute them throughout the galaxy, is too simplistic, or that the SFD or behavior of ISOs as they pass through our solar system is far from expectations.