Constraining the size, shape, and albedo of the large trans-Neptunian object (28978) Ixion with multi-chord stellar occultations

¹ Instituto de Astrofísica de Andalucía, IAA-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
² LIRA, CNRS UMR8254, Observatoire de Paris, Meudon, France
³ Federal University of Technology – Paraná (PPGFA/UTFPR), Curitiba, PR, Brazil
⁴ Laboratório Interinstitucional de e-Astronomia (LIneA), Rio de Janeiro, RJ, Brazil
⁵ Observatório Nacional (MCTI), Rio de Janeiro, RJ, Brazil
⁶ LTE, Observatoire de Paris, Université PSL, Sorbonne Université, Université de Lille, LNE, CNRS, Paris, France
⁷ Türkiye National Observatories, TUG, Antalya, Türkiye
⁸ The Scientific and Technological Research Council of Türkiye (TÜBİTAK), Ankara, Türkiye
⁹ naXys, Department of Mathematics, University of Namur, Namur, Belgium
¹⁰ Federal University of Rio de Janeiro – Observatory of Valongo, Rio de Janeiro, Brazil
¹¹ Instituto de Física, Universidade Federal de Uberlândia, Uberlândia, MG, Brazil
¹² UNESP – São Paulo State University, Grupo de Dinâmica Orbital e Planetologia, Guaratinguetá, SP, Brazil
¹³ Institut Polytechnique des Sciences Avancées (IPSA), Ivry-sur-Seine, France
¹⁴ Deutsches Zentrum für Astrophysik (DZA), Görlitz, Germany
¹⁵ Florida Space Institute, University of Central Florida, Orlando, FL, USA
¹⁶ Department of Space Sciences and Technologies, Akdeniz University, Antalya, Türkiye
¹⁷ Trans-Tasman Occultation Alliance (TTOA), Wellington, New Zealand
¹⁸ Institute of Astronomy and Astrophysics, University of Tübingen, Germany
¹⁹ PADC/DIO, Observatoire de Paris, PSL University, France
²⁰ Observatoire des Engarouines, Malemort-du-Comtat, France
²¹ International Occultation Timing Association (IOTA), Fountain Hills, AZ, USA
²² Institute for Astronomy, University of Edinburgh, Edinburgh, UK
²³ Société Astronomique de France (SAF), Paris, France
²⁴ Grupo Alfa Crucis, Brazil
²⁵ Lowell Observatory, Flagstaff, AZ, USA
²⁶ Alma Mater Studiorum – University of Bologna, Department of Physics and Astronomy “A. Righi”, Bologna, Italy
²⁷ KinetX, Inc., Space Navigation and Flight Dynamics Practice, Simi Valley, CA, USA
²⁸ Universidade Estadual de Ponta Grossa (UEPG), Ponta Grossa, Brazil
²⁹ Sapienza Università di Roma, Rome, Italy
³⁰ Campo Catino Astronomical Observatory, Guarcino, Italy
³¹ INFN, Sezione Roma1, Rome, Italy
³² INAF OAC, Via della Scienza, Selargius, Italy
³³ Kenyon Astrophysical Observatory (KAO), Northern California, USA
³⁴ Skynet Robotic Telescope Network, University of North Carolina, Chapel Hill, NC, USA
³⁵ Johnson Space Center Astronomical Society, Houston, TX, USA
³⁶ Independent observer, Santana de Parnaíba, SP, Brazil
³⁷ Independent observer, 77630 Arbonne-la-Forêt, France
³⁸ Department of Physics and Astronomy, Appalachian State University, Boone, NC, USA
³⁹ Centro de Amigos de la Astronomia Reconquista – CAAR, Reconquista, Argentina
⁴⁰ Rand Observatory II – MPC W71, Lake Placid, NY, USA
⁴¹ Pontificia Universidad Católica de Chile, Center for Astro Engineering, Santiago, Chile
⁴² Asociación Argentina Amigos de la Astronomía, Argentina
⁴³ Cruz del Sur Private Observatory, El Peral, San Juan Province, Argentina
⁴⁴ Westport Astronomical Society, Westport, Connecticut, USA
⁴⁵ University of New Haven, West Haven, CT, USA
⁴⁶ SONEAR Observatory – CEAMIG, Caeté, Minas Gerais, Brazil
⁴⁷ Centro de Estudos Astronômicos de Minas Gerais (CEAMIG), Belo Horizonte, Brazil
⁴⁸ Space sciences, Technologies & Astrophysics Research (STAR) Institute, University of Liège, Liège, Belgium
⁴⁹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
⁵⁰ Complejo Astronómico El Leoncito (CASLEO), San Juan, Argentina
⁵¹ Facultad de Ciencias Astronómicas y Geofísicas (UNLP), La Plata, Argentina
⁵² Instituto de Astronomía y Física del Espacio, CONICET–Universidad de Buenos Aires, Argentina
⁵³ Agrupaciones Astronómicas de Madrid (AAM) y Teruel (ACTUEL), Spain
⁵⁴ El Catalejo Observatory (MPC I48), Santa Rosa, La Pampa, Argentina
⁵⁵ Los Cabezones Observatory (MPC X12), Santa Rosa, La Pampa, Argentina
⁵⁶ Federal University of Espírito Santo: Vitória, Espírito Santo, Brazil

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Received: 4 November 2025
Accepted: 12 January 2026

Abstract

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.