Binary asteroids in mean-motion resonances

The purpose of this study is to investigate the relation between binary asteroids and mean motion resonances (MMRs). For more than 700 asteroids from two catalogues, the Johnston Archive [Johnston, 2024] and the Gaia DR3 VizieR list of binary candidates from Liberato et al. [2024], we applied a resonance identification algorithm, treating all planetary perturbations. Our results showed that the presence of binary asteroids in MMRs largely depends on their dynamical class. The highest percentage, more than 30%, is found in the Trans- Neptunian region, where most of these objects have exhibited resonant librations longer than 10 Myr. For the main-belt asteroid pairs, this percentage is about 10-12%. Contrary to expectations, the more unstable region populated with NEOs, showed a higher percentage of resonant pairs (above 17%), but with temporal resonant captures. These results could indicate that the mean motion resonances, particularly the stronger ones, could play a role in the evolution and formation of binary systems. Finally, we highlight that in the present paper, 82 resonant binary asteroids are newly identified.

💡 Research Summary

The paper presents the first systematic survey of binary asteroids that are trapped in mean‑motion resonances (MMRs) across the Solar System. Using two up‑to‑date catalogs – the Johnston Archive (477 binaries as of October 2024) and a Gaia DR3‑derived list of astrometric binary candidates (357 objects) – the authors assembled a sample of 834 objects spanning Near‑Earth Asteroids (NEAs), Mars‑crossers, main‑belt asteroids, Jupiter Trojans, and Trans‑Neptunian Objects (TNOs).

Methodologically, each object was numerically integrated with the “resonances” Python package (Smirnov 2023). For NEAs and main‑belt bodies the integration time was 100 kyr; for the distant TNOs a longer 10 Myr integration was performed using the Orbit9 integrator. All planetary perturbations (Mercury through Neptune) were included. The algorithm searched for all possible two‑body and three‑body commensurabilities by comparing the asteroid’s semi‑major axis to the resonant values. A resonance was declared present only if (i) the resonant argument σ exhibited libration throughout the integration window, and (ii) the libration frequency of σ matched the dominant frequency of the semi‑major axis oscillation, with amplitudes exceeding a significance threshold (following Gallardo 2006). Short‑period components (< 500 yr) were filtered out to obtain a cleaned argument σ_f, and periodograms of σ and a were inspected to confirm a common dominant period. Objects were classified as “Libration (L)” when σ librated continuously, or “Transient (T)” when libration occurred only in isolated episodes.

The results are summarized in Table 1. In the TNO population, 85 binaries were examined and 26 (30.5 %) were found in MMRs; 19 of these remained in pure libration over the 10 Myr runs, indicating long‑term stability. The majority of resonant TNO binaries occupy the dense 2:3, 3:5, and related Neptune resonances, especially between 44–45 AU, consistent with previous work suggesting that primordial binary formation in a collapsing pebble cloud can be preserved by resonant protection.

In the main belt, 268 binaries were analyzed, with 29 (10.8 %) in resonance. Most of these are in transient states, reflecting the crowded resonance web and higher collisional activity that destabilize binary configurations.

Surprisingly, the dynamically unstable NEA and Mars‑crossing groups show higher resonance fractions than expected: 14 out of 82 NEAs (17.1 %) and 6 out of 34 Mars‑crossers (17.6 %) are resonant, albeit mostly transient. These findings hint that non‑gravitational torques (YORP, BYORP) may facilitate temporary capture into resonances, potentially influencing binary formation pathways for near‑Earth objects.

The survey identified 82 previously unknown resonant binaries, expanding the known sample of resonant systems. Some binaries were found to occupy more than one resonance during the 100 kyr window, and several exhibit resonance hopping, underscoring the complexity of multi‑resonance overlap zones.

The authors acknowledge limitations: the integration timescales (100 kyr–10 Myr) are short compared to the Solar System’s age, preventing definitive statements about Gyr‑scale stability. The study also does not model non‑gravitational forces, collisional evolution, or detailed mass‑ratio effects, which could be crucial for binary survival. Future work should incorporate YORP/BYORP torques, realistic collision cascades, and longer‑term N‑body simulations to assess how resonances protect or disrupt binary systems over billions of years.

Overall, the paper provides a valuable baseline catalog of resonant binary asteroids, demonstrates clear dynamical class dependencies, and opens new avenues for investigating the role of mean‑motion resonances in the formation and long‑term evolution of asteroid binaries.