Julio Ignacio Bueno de Camargo

Ring systems have been found around several small bodies in the outer Solar System through stellar occultations. While such observations provide constraints on ring geometry and dynamical interactions, little is known about their origins, lifetimes, or compositions. Here, we report near-infrared stellar-occultation observations obtained with the James Webb Space Telescope (JWST) probing the ring system of the Centaur Chariklo. Our measurements show that Chariklo’s dense inner ring has become significantly more opaque than in previous occultations, suggesting ongoing replenishment or dynamical restructuring. In contrast, the outer ring exhibits a much weaker near-infrared occultation signature than previously observed. This discrepancy may reflect material loss, suggesting that the outer ring could be transient or may arise from wavelength-dependent opacity. These observations demonstrate the power of JWST occultations to probe the evolving structure of rings around small bodies and provide compelling evidence for unexpected changes in Chariklo’s ring system.

We present results from 28 stellar occultations by the large trans-Neptunian object (50000) Quaoar registered between 2018 and 2025. By performing a joint analysis of this occultation dataset, along with nine other published events, we were able to fit an oblate ellipsoid shape, with equatorial semiaxes a and b of 566 . 1 − 2.2 + 2.5 km, and a polar semiaxis, c , of 511 . 2 − 3.7 + 3.6 km. It provides an equivalent volumetric diameter of 1094.4 ± 4.6 km and polar oblateness of 0.097 ± 0.011. Considering an absolute magnitude of H = 2.79 ± 0.35, we derive a geometric albedo of p _V = 0.125 ± 0.038. We have derived new upper limits to the surface pressure of a CH ₄ atmosphere of 0.15 nbar (1 σ ) and 0.65 nbar (3 σ ). We also provide a table with the 36 new astrometric positions for Quaoar. Using the new system mass derived from Weywot’s orbit around Quaoar, we calculated a density of 1.760 ± 0.109 g cm ⁻³ . Moreover, from the derived size and rotation period (8.8394 ± 0.0002 hr, we calculated that, if Quaoar is in Maclaurin hydrostatic equilibrium state, it would have a density of 1.859 ± 0.200 g cm ⁻³ . This result, within the error bars, is compatible with the value we found. Therefore, this work shows that Quaoar can be a Maclaurin object, being eligible as a dwarf planet.

This work presents a methodology for determining the size, three-dimensional shape, and rotational parameters of small solar system objects, with rigorous uncertainty determination, using stellar occultations, rotational and phase-angle light curves. Here we analyze (911) Agamemnon, considered the third-largest Jupiter Trojan, using five rotational light curves, phase-angle data, and 12 occultations (22 positive chords). The methodology uses a genetic algorithm to compare the rotational data with synthetic light curves from the 3D model, obtaining the uncertainties from the χ ² minimization. The model is scaled with the occultation data, to calculate dimensions, and the uncertainties. We obtained the first nonconvex model for Agamemnon using the All-Data Asteroid Modeling, considering new rotational, phase-angle, and occultation data. The methodological process was applied to derive the uncertainties, obtaining the rotational parameters λ = 128 . ° 1 ± 0 . ° 8, β = 2 . ° 3 ± 2 . ° 1, P = 6.581797 ± 0.000003 hr, and D _S = 156.0 ± 7.8 km, with a σ _model of 3 km (indicating the model quality relative to the occultation data). We also used the new data to test the 3D model provided by the Database of Asteroid Models from Inversion Techniques. From the nonconvex shape model rotated to Gaia observation epochs, we derive a geometric albedo of p _G = 0.051 ± 0.005 for Agamemnon. The final results also indicate that (911) Agamemnon may be the second-largest known Jupiter Trojan, being smaller than (624) Hektor and larger than (617) Patroclus.

Jupiter Trojans are key remnants of the solar system formation, preserving primordial material and recording the history of planetary migration. We present the first 3D shape model of the Jupiter Trojan (3317) Paris, obtained by combining stellar occultations and rotational light curves. We analyzed 12 occultation events between 2020 and 2022, yielding 27 positive light curves. In addition, we compiled 45 rotational light curves covering ∼8 yr, obtained from observations at the Observatório do Pico dos Dias (OPD-Brazil) and from publicly available databases such as the Asteroid Light Curve Data Exchange Format. Light-curve inversion, combined with occultation constraints, yielded an irregular 3D shape model with an equivalent-volume diameter of 126.2 ± 6.2 km and an equivalent-surface diameter of 131.5 ± 6.5. We refined the rotation period to 7.081013 ± 0.000005 hr, consistent with previous estimates and the spin axis as λ = 100 . ° 5 ± 1 . ° 1 and β = 50 . ° 7 ± 1 . ° 4. Using the surface-equivalent diameter of the shape model, we determined a geometric albedo of p _V = 0.045 ± 0.002 for Paris. This study contributes to improving our understanding of the physical properties of Jupiter Trojans, and the methodologies developed in this work can be applied to the study of other small bodies in the solar system.

We report on the observation and measurement of astrometry, photometry, morphology, and activity of the interstellar object 3I/ATLAS, also designated C/2025 N1 (ATLAS) with the NSF-DOE Vera C. Rubin Observatory. Comet 3I/ATLAS, the third known interstellar object, was discovered on UT 2025 July 1. Rubin Observatory had coincidentally collected images of the object’s region of the sky during routine commissioning. Facilitated by Rubin’s high resolution and large aperture, we successfully recovered object detections from Rubin observations spanning UT 2025 June 21 (10 days before discovery, when 3I/ATLAS was 4.5 au from the Sun) through the date of discovery, and we acquired additional images through UT 2025 July 20 as part of commissioning. We measure on-sky locations of 3I/ATLAS in Rubin ugrizy bands, with a typical precision of ∼70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of ∼3 mas in Rubin images. We measure grizy magnitudes of 3I/ATLAS photometry at ∼0.01 mag precision, detecting no short-term photometric variability above 0.01 mag. We derive an estimated near-nucleus dust-to-nucleus scattering cross-sectional ratio of η ≳ 13 on UT 2025 July 2 based on Rubin photometry and an upper limit nucleus size computed from Hubble Space Telescope observations. We find Rubin colors of g − r = (0.657 ± 0.013) mag, r − i = (0.235 ± 0.018) mag, i − z = (0.147 ± 0.042) mag, and z − y = (0.047 ± 0.052) mag. These data represent the earliest observations of this object by a large (≳8 m class) telescope and illustrate the type of measurements (and discoveries) Rubin’s Legacy Survey of Space and Time will provide after it begins in early 2026.

Context . Trans-Neptunian objects (TNOs) are among the most primitive remnants of the early Solar System. Determining their sizes, shapes, albedos, and surface properties is essential for understanding their origin and evolution. Stellar occultations provide highly accurate size and shape information for TNOs, while photometry constrains their albedo and surface colours. (28978) Ixion is one of the largest TNOs and a prominent Plutino, making it a key target for comparative studies. Aims . The aim of this work is to constrain Ixion’s projected size, shape, absolute magnitude, geometric albedo, and surface colours, and to search for evidence of an atmosphere or circum-object material. Methods . We conducted a series of campaigns targeting stellar occultations by Ixion between 2020 and 2023 as part of the Lucky Star collaboration, gathering 51 observations from eight events, including 30 positive detections. Five multi-chord events were used for a global limb fit, enabling an accurate reconstruction of Ixion’s projected shape. Calibrated photometric data, including new and archival measurements, were analysed to derive its absolute magnitude, phase-curve parameters, and broadband colours. Results . The multi-chord occultations reveal a slightly elongated limb that is well represented by a single projected ellipse with semi-axes a = 363.42 _−3.85 ^+3.53 km and b = 333.98 _−4.96 ^+7.07 km, yielding R _equiv = 348.39 _−4.43 ^+5.37 km ( D _equiv = 696.78 _−8.87 ^+10.75 km), and an apparent oblateness ε ′ = 0.081 _−0.010 ^+0.004 . The geometry is consistent with a moderately flattened, nearly spheroidal body that may show slight departures from axial symmetry. The typical radial residuals (~10 km) support a largely stable shape across the observed epochs, with modest epoch-dependent variations. The phase-curve fit gives H _V = 3.845 ± 0.006, β = 0.1301 ± 0.0078 mag deg ⁻¹ , and p _V = 0.106 _−0.003 ^+0.003 . Multi-band photometry yields B − V = 1.06 ± 0.03, V − R = 0.61 ± 0.02, and R − I = 0.54 ± 0.03, which is consistent with moderately red TNO surfaces. No atmosphere or circum-object material was detected down to our sensitivity limits. The best-sampled event (13 October 2020) also allowed us to measure the angular diameter of the occulted star Gaia DR3 4056440205544338944, θ _⋆ = 0.670 ± 0.010 mas, which corresponds to R _⋆ = 128 ± 10 R _⊙ at the Gaia distance.

Context . Trans-Neptunian objects (TNOs) are thought to be some of the most ancient and primitive bodies in our Solar System. Understanding their basic physical properties is crucial to unraveling their origin and the evolution of the outer Solar System beyond Neptune. Stellar occultations are a highly effective and sensitive method of studying these distant and faint objects, allowing us to gather essential information about their physical characteristics. (24835) 1995 SM ₅₅ is one of the few members of the Haumea orbital cluster and, therefore, is an especially relevant body to study within the TNO population. Aims . The main objectives of the present work are to determine the projected size, absolute magnitude, and geometric albedo of 1995 SM ₅₅ and to analyze the results compared to Haumea. Methods . We predicted a stellar occultation by this TNO for February, 25, 2024, carried out a specific campaign to observe the occultation, and derived the projected size and shape from the occultation observations using an elliptical fit to the occultation chords. We also analyzed a large set of photometric observations of (24835) 1995 SM ₅₅ to obtain the absolute magnitude and the rotational period. Finally, we combined these results to derive the geometric albedo of this TNO. Results . The occultation was successfully detected from seven instruments located at five different sites; 33 other sites reported negative detections. Using an elliptical fit to the occultation chords, we determined the size and shape of the limb of (24835) 1995 SM ₅₅ during the occultation: an ellipse with semi-axes (104.3 ± 0.4) × (83.5 ± 0.5) km. The area-equivalent diameter ( D _eq,A ) for this ellipse is 186.7 ± 1.8 km. This is consistent with the upper limit of 250 km estimated from Herschel Observatory thermal data. From our photometric observations, we derived an absolute magnitude ( H _V ) of 4.55 ± 0.03, a phase slope parameter of 0.04 ± 0.02 mag/deg, and a V − R value of 0.37 ± 0.05. The rotational variability has a maximum peak-to-valley amplitude (∆ m ) of 0.05 mag, but we could not derive an unambiguous rotational period. Combining the projected size from the occultation with the absolute photometry, we obtained a geometric albedo in the V band ( p _V ) of 0.80 ± 0.04 for 1995 SM ₅₅ . This value is remarkably high for a TNO and somewhat higher than that of Haumea, but consistent with the concept that 1995 SM ₅₅ is a member of the orbital cluster of Haumea.

The centaur (2060) Chiron has long been a candidate for hosting material in orbit, based on occultation and photometric and spectroscopic data. Here, we present a multichord stellar occultation observed on 2023 September 10 UT that reveals new and complex structures surrounding Chiron. High-cadence light curves show multiple secondary events that are best explained (when compared with a multishell interpretation) with a system of three confined rings located at average radii of 273, 325, and 438 km, the outermost of which lies beyond Chiron’s Roche limit. The rings appear coplanar, with a mean pole orientation of λ = 151^∘ ± 4^∘ and β = 20^∘ ± 6^∘. A broader, disklike structure extends from about 200 to 800 km, and a newly detected faint feature is observed at ∼1380 km. Chiron thus appears as the fourth small solar system body known for hosting a ring system. Comparisons with previous occultation events that have occurred since 1994 show that these features are not permanent. With these observations, we may witness for the first time the ongoing formation and evolution of a ring system.