Characterising Grains and Composition in Small-Body Ring Systems: A Case Study of Haumea's Ring

During the last decade, occultation measurements have revealed that not only giant planets can host a ring system: Centaur-type objects Chariklo and presumably Chiron, and the dwarf planets Haumea and Quaoar also harbor rings, either single or multiple [1,2,3,4]. While the different dynamical environments and the unique shapes and properties of the main bodies can be vastly different between these systems, similarities such as all the rings being close to the 1:3 spin–orbit resonance are already remarkable. However, both the individual and general formation, evolution and stability of these rings are still under debate.

In the case of Haumea, the canonical ring formation theory states that the ring was formed at the same time and from the same material as Haumea's moons and its dynamical family, which contradicts with the observed indirect estimate of the ring's reflectivity [3,5]. The origin of the ring from giant impact events also raises the question of how the ring could have remained stable for such a long time [6,7].

To test and develop such formation theories, information on the ring's composition are indispensable. Identifying materials helps to better understand the formation of rings around minor and dwarf planets, while the characterisation of the dominant grain sizes is important for stability studies of these systems. However, our current knowledge on these rings is mostly limited to the properties derived from occultation data and brightness estimates of the systems at different observing geometries, while the widely applied simple ring model do not carry any information on the composition [8].

Here, we introduce radiative transfer modelling for small body ring systems to overcome the limitations of the currently used simple ring models such as directly simulate the thermal and visible light emission of rings constructed by using different materials and grain sizes, and taking better account of grain emissivity and anisotropic scattering [9].

In this study, we present a detailed analysis of the Haumea ring system as a case study for other known and forthcoming small body ring systems. We show that carbon-rich or silicate models can be brighter than the thermal emission of the main body itself, and thus can be observed around 10–30 μm. We propose that this mid-infrared excess can be a tracer of smaller (0.1–10 μm) dust grains around any small body system, as it was recently reported around Makemake using JWST [10]. We emphasise the comparison of modelled spectral energy distributions with future multi-wavelength measurements as a diagnostic tool to determine the dominant grain size and characteristic material of a ring, essential inputs to theories of ring formation and evolution.

References:

[1] Braga-Ribas, F., 2014, Nature, 508, 7494, 72-75. [2] Ortiz, J. L., 2015, A&A, 576, A18, 12. [3] Ortiz, J. L., 2017, Nature, 550, 7675, 219-223. [4] Pereira, C. L., 2023, A&A, 673, L4, 14. [5] Noviello, J. L., 2022, PSJ, 3, 9, 225, 19. [6] Sicardy, B., 2025, Philos. Trans. R. Soc. A, 383, 2291, 20240193 [7] Regály, Zs., accepted in A&A, March 2025 (eprint arXiv:2503.17218) [8] Lellouch, E., 2017, A&A, 608, A45, [9] Kalup, Cs., 2024, PASP, 136, 12, 124401, 11. [10] Kiss, Cs., 2024, AJL, 976, 1, L9, 16.