Mass determination of the three long-period Neptune- and sub-Neptune-sized planets transiting TOI-282^★

¹ Dipartimento di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, 10125 Torino, Italy
² European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago de Chile, Chile
³ Space Research Institute, Austrian Academy of Sciences, Schmiedlstraße 6, 8042 Graz, Austria
⁴ Dipartimento di Fisica, Università di Trento, Via Sommarive 14, 38123 Povo, Italy
⁵ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁶ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁸ Osservatorio Astrofisico di Torino, INAF, via Osservatorio 20, Pino Torinese 10025, Italy
⁹ McDonald Observatory and Department of Astronomy, The University of Texas at Austin, USA
¹⁰ Center for Planetary Systems Habitability, The University of Texas at Austin, USA
¹¹ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziądzka 5, 87-100 Toruń, Poland
¹² Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
¹³ Departamento de Astrofísica, Universidad de La Laguna, Astrofísico Francisco Sanchez s/n, 38206 La Laguna, Tenerife, Spain
¹⁴ Astronomy Department and Van Vleck Observatory, Wesleyan University, Middletown, CT 06459, USA
¹⁵ Rheinisches Institut für Umweltforschung an der Universität zu Köln, Aachener Straße 209, 50931 Köln, Germany
¹⁶ Institute of Space Research, German Aerospace Center (DLR), Rutherfordstraße 2, 12489 Berlin, Germany
¹⁷ Astrobiology Center, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁸ National Astronomical Observatory of Japan, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁹ Astronomical Science Program Graduate University for Advanced Studies, SOKENDAI, 2-21-1, Osawa, Mitaka, Tokyo, 181-8588, Japan

^★★ Corresponding author: andrea.barone@astro.su.se

Received: 8 August 2025
Accepted: 23 October 2025

Abstract

TOI-282 is a bright (V = 9.38) F8 main-sequence star known to host three transiting long-period (P_b = 22.9 d, P_c = 56.0 d, and P_d = 84.3 d) small (R_p ≈ 2–4 R_⊕) planets. The orbital period ratio of the two outermost planets, namely TOI-282 c and d, is close to the 3:2 commensurability, suggesting that the planets might be trapped in a mean motion resonance. We combined space-borne photometry from the TESS telescope with high-precision HARPS and ESPRESSO Doppler measurements to refine orbital parameters, measure the planetary masses, and investigate the architecture and evolution of the system. We performed a Markov chain Monte Carlo joint analysis of the transit light curves and radial velocity time series, and carried out a dynamical analysis to model transit timing variations and Doppler measurements along with N-body integration. In agreement with previous results, we found that TOI-282 b, c, and d have radii of R_b = 2.69 ± 0.23 R_⊕, R_c = 4.13_−0.14^+0.16 R_⊕, and R_d = 3.11 ± 0.15 R_⊕, respectively. We measured planetary masses of M_b = 6.2 ± 1.6 M_⊕, M_c = 9.2 ± 2.0 M_⊕, and M_d = 5.8_−1.1^+0.9 M_⊕, which imply mean densities of ρ_b = 1.8_−0.6^+0.7 g cm⁻³, ρ_c = 0.7 ± 0.2 g cm⁻³, and ρ_d = 1.1_−0.2+0.3 g cm⁻³, respectively. The three planets may be water worlds, making TOI-282 an interesting system for future atmospheric follow-up observations with JWST and ELT.