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

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.

• The paper establishes a 90% confidence limit on the interstellar number density of asteroids and comets using three contemporary solar system surveys.
• Results show a 90% confidence limit on density of 2.4e-2 au⁻³ for inactive ISOs and 2.4e-4 au⁻³ for potentially active ISOs.
• The findings suggest current ISO spatial density models may be too simplistic and highlight the need for next-generation surveys for improved detection capabilities.

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

The researchers investigate the mysterious scarcity of Interstellar Objects (ISOs) within our solar system, including their failure to detect any substantial asteroidal or cometary interstellar candidates. Their analysis utilizes data derived from three contemporary solar system surveys: Pan-STARRS1, the Catalina Sky Survey, and the Mt. Lemmon Survey. They establish a 90% confidence limit on the ISO number density, $\rho_{IS}^{CL}$, as a function of the slope of the size-frequency distribution and the limiting absolute magnitude. This model incorporates gravitational focusing to simulate a quasi-realistic steady-state population of ISOs.

Methodology

Leveraging the capabilities of the three surveys, the authors simulate detection efficiencies considering different modes of identifying ISOs within the datasets. They established a comprehensive synthetic model of ISOs with distinct orbital parameters, deployed to predict survey detection probabilities. These simulations highlight the various orbital characteristics that affect ISO detectability, such as eccentricities and heliocentric distances.

Results

The results present two distinct confidence limits based on ISO behavior assumptions. For ISOs assumed to be inactive (asteroidal), the confidence limit on their spatial density is $\rho_{IS}^{CL} = 2.4 \times 10^{-2} \, au^{-3}$, a finding consistent with zero detections in the survey data. However, assuming some ISOs demonstrate cometary activity, the limit improves significantly to $\rho_{IS}^{CL} = 2.4 \times 10^{-4} \, au^{-3}$. Factors contributing to these results include hypothetical ISO size-frequency distributions, described by the parameter $\alpha$, with extensive analysis conducted for $\alpha$ within 0.2 to 0.8.

Discussion

The implications of the researchers' findings suggest that existing models of ISO spatial density might oversimplify the dynamics and distribution of such objects beyond our solar system. Notably, their improved confidence limits advise caution against simplistic models premised on the assumption that solars systems similar to ours ubiquitously eject ISOs in similar manners. Furthermore, their findings highlight the scope for surveys like LSST, which could observe greater sky areas, rendering them better equipped to detect ISOs if they do exist within these density estimates.

Theoretical Implications and Future Work

The research poses significant implications for the study of planetary formation and the migration of small bodies. Discrepancies between observed and predicted ISO densities necessitate reconsiderations of solar system-like models regarding the dispersion of planetesimals. Additionally, the paper underscores the severity of the limitation posed by current observational capabilities and the prospective improvements promised by next-generation survey technology. Future directions could involve increasing the depth of such surveys or the discovery of alternative phenomena—such as obscuration effects—that account for lower-than-expected ISO probabilities.

Though refraining from sensational claims, this study represents a rigorous evaluation of the current inability to detect ISOs, paving the way for deeper investigations into the processes and dynamics that govern interstellar space and solar system interactions.