Physical Diversity of New Binary Asteroid Candidates from Gaia FPR

Binary asteroids are fundamental probes of the physical and collisional evolution of the Solar System. Their mutual orbits provide one of the few direct ways to estimate asteroid masses, bulk densities, and internal structures. However, the population of known binary systems is small. It remains strongly biased toward small bodies in the near-Earth and inner main-belt populations, while binaries in the intermediate-size regime and in the outer main belt remain poorly characterised. 

In Liberato et al. (2024), we introduced a new method to identify binary asteroid candidates. We searched for astrometric wobble signatures in Gaia Data Release 3, producing the first-ever list of astrometric binary asteroid candidates, several of which have already been independently confirmed. In Liberato et al. (2026), we further refined the method by accounting for specific statistical properties of the Gaia astrometric data and by implementing a selection procedure designed to control the false-detection rate. We then explored the Gaia Focused Product Release (FPR), which contains more than 150,000 objects observed over 66 months. Among them, 47,896 objects had sufficient data for us to investigate the possibility of an undetected companion. Extensive validation tests demonstrated the robustness of the detections: simulations based on noise-only scenarios produced, on average, a number of candidates approximately 80% lower than that obtained from the Gaia FPR data. In addition, astrometric wobble signatures were successfully recovered for nine previously known binary systems, confirming our method's capacity in detecting genuine binary-induced signals. Our analysis identified 343 astrometric binary candidates.

Now, we expand the analysis on the results from Liberato et al. (2026). The FPR candidate population spans a broad range of dynamical classes, with the majority located in the main asteroid belt, but also includes objects associated with the Cybele and Hilda groups, as well as several Jupiter Trojans. The distribution of candidates differs significantly from that of currently known binaries, revealing a larger fraction of objects in the middle and outer main belt. This suggests that our method, associated with the Gaia astrometry precision, is sensitive to binary systems that are difficult to detect with conventional photometric or direct-imaging techniques, reducing the strong observational biases affecting the currently known population.

A substantial fraction of the candidates are associated with asteroid families, indicating that collisional environments may play a major role in binary formation and survival. We identify 95 candidates, as well as one previously known binary, associated with asteroid families. Their distribution in proper element space suggests that binaries may be considerably more common in outer-belt families than currently known. The presence of candidate binaries among family parent bodies, including (369) Aeria, (780) Armenia, and (1303) Luthera, is particularly significant because these systems could provide direct constraints on the masses, densities, and reaccumulation histories of poorly characterised collisional families.

The physical properties of the candidates indicate a diverse population. Approximately 60% of the objects have estimated diameters below ~15 km, consistent with binary formation through rotational fission driven by the YORP effect (Walsh et al. 2008). In contrast, several candidates exceed diameters of ~50 km, a size range where YORP-driven spin-up becomes inefficient and collisional formation mechanisms are expected to dominate. Some intermediate-sized objects, known as Escaping Ejecta Binaries (EEBs; Durda et al. 2004), may form through sub-catastrophic collisions. These EEBs are severely underrepresented in the known binary asteroid population compared with theoretical expectations (Pravec & Harris 2007). Our results show that 27% of our candidates are within the EEB size range, and several are synchronous. This suggests that we may be revealing a significant fraction of a previously hidden population of binary asteroids across the main belt.

The taxonomic distribution of the candidates also differs slightly from that of known binaries. While the fraction of S-type asteroids remain the highest in our sample, the fraction of C-type candidates is significantly enhanced relative to the currently known binary population. This trend is consistent with the larger number of outer main-belt objects recovered by Gaia astrometry. It may suggest that the apparent dominance of S-type binaries in previous surveys (Minker \& Carry 2023) is largely driven by observational bias that favours objects in the inner main belt.

Finally, the availability of multiple Gaia observation windows for several candidates allows independent estimates of orbital parameters and provides an internal consistency check on the astrometric solutions. A significant fraction of the multiple detections show compatible or aliased solutions, reinforcing the physical plausibility of the binary interpretation. Confirmation through stellar occultation observations, or photometric light curves, could substantially increase the number of known binaries in the middle and outer main belt, improve estimates of binary occurrence rates in asteroid families, and provide new constraints on the formation, evolution, and internal structure of small bodies.

These results show the strong potential of Gaia astrometry to reveal a previously hidden population of binary asteroids across the Solar System and to improve the binaries' physical characterization. Some Gaia Data Release 4 results will also be presented to support these findings and prepare the community to the upcoming full release in December 2026.

References:

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6. K. J. Walsh et al. (2008). Nature 454.7201, pp. 188–191.