Constraining unresolved satellite presence in TNOs combining thermal data and occultations

Discovering satellites around Solar System objects is of significant scientific interest, primarily because their mutual orbits allow us to infer the system's mass [Noll et al. 2008 a]. When size measurements are also available, we can derive the bulk density, offering crucial insight into the internal structure and composition—parameters otherwise inaccessible. Furthermore, characteristics such as size ratio, inclination, and eccentricity provide valuable clues about the system’s formation and evolution [Nesvorný & Vokrouhlický 2019].

However, identifying satellites around trans-Neptunian objects (TNOs) poses a particular challenge due to their vast heliocentric distances. While the widest binaries can be resolved using the Hubble Space Telescope [Grundy et al. 2019; Porter et al. 2024], tighter systems remain beyond the reach of direct imaging techniques.

To address this limitation, we have developed a novel methodology to quantitatively constrain the sizes of potential tight satellites around TNOs by combining stellar occultation data with thermal infrared observations from the TNOs are Cool project [Müller et al. 2010]. Continuing the idea proposed by Ortiz (2020), our approach models the system’s thermal emission using the Near-Earth Asteroid Thermal Model (NEATM, [Harris 1998]), with the area of the primary fixed by the occultation-derived cross-section. We then apply a Bayesian framework to fit the model to the observed thermal fluxes, allowing us to place statistical limits on the diameter of any unresolved companion.

This technique advances previous analyses of TNOs with combined occultation and thermal data. Traditionally, if the sizes derived from the two methods are consistent, no further investigation is made into the possibility of a satellite. Conversely, discrepancies are often attributed to the presence of a companion, but rarely is its size properly quantified.

This methodology has been already applied in Gómez-Limón et al. 2025 for 2007 OC10. In this work, we expand its application to a selected sample of TNOs are Cool targets with published occultation data. In some cases, we are able to confidently rule out the presence of a sizable satellite, setting upper limits on its diameter. In others, our model suggests that a companion is necessary to reconcile the observations, and we provide estimates of the satellite-to-primary size ratio. These findings help motivate targeted searches for the proposed satellites via stellar occultation campaigns or other techniques. To validate our methodology, we also apply it to systems with already known satellites, demonstrating its robustness and potential for broader application.

References:

• Gómez-Limón et al. (2025), A&A, in press, https://arxiv.org/abs/2504.02457 .

• Grundy et al. (2019), Icarus, 334, 62–78.

• Harris (1998), Icarus, 131, 291.

• Müller et al. (2010), A&A, 518, L146.

• Nesvorný & Vokrouhlický (2019), Icarus, 331, 49–61.

• Noll et al. (2008a), The Solar System Beyond Neptune, Univ. of Arizona Press, pp. 345–363.

• Ortiz (2020), 14th Europlanet Science Congress 2020

• Porter et al. (2024), Planet. Sci. J., 5(6), 143.