A. Leleu, A. Alnajjarine, J.-B. Delisle, A. Brandeker, A. C. M. Correia, S. G. Sousa, T. G. Wilson, N. Billot, C. H. Broeg, M Rosenqvist, J. Korth, A. Bonfanti, C. Pezzotti, B. Akinsanmi, Y. Alibert, R. Alonso, T. Barczy, D. Barrado, S. C. C. Barros, W. Baumjohann, W. Benz, M. Bergomi, L. Borsato, A. Castro-González, A. Collier Cameron, C. Corral Van Damme, Sz. Csizmadia, P. E. Cubillos, M. B. Davies, M. Deleuil, A. Deline, O. Demangeon, B.-O. Demory, A. Derekas, B. Edwards, D. Ehrenreich, A. Erikson, A. Fortier, L. Fossati, M. Fridlund, D. Gandolfi, K. Gazeas, M. Gillon, M. Gudel, M. Gunther, A. Heitzmann, C. Helling, K. G Isaak, T. Keller, L. L. Kiss, D. Kitzmann, E. Kopp, K. W. F. Lam, J. Laskar, A. Lecavelier Des Etangs, M. Lendl, A. Luntzer, D. Magrin, P. F. L. Maxted, B. Merin, C. Mordasini, V. Nascimbeni, G. Olofsson, H. P. Osborn, R. Ottensamer, I. Pagano, E. Palle, G. Peter, D. Piazza, G. Piotto, D. Pollacco, D. Queloz, R. Ragazzoni, N. Rando, H. Rauer, I. Ribas, N. C. Santos, G. Scandariato, D. Segransan, A. E. Simon, A. M. S. Smith, M. Stalport, S. Sulis, G. Szabo, S. Udry, S. Ulmer-Moll, V. Van Grootel, J. Venturini, E. Villaver, N. A. Walton
Abstract
Context. The K2-138 system consists of a K-dwarf transited by six planets in the sub-Neptune regime, with radii ranging from 1.5 to 3.4 R⊕ and orbital periods of between 2.3 and 42 days. The five innermost planets form a chain of zero-order three-body resonances, while the outermost planet is suspected to be linked to the chain through a first-order three-body resonances. The transit timing variations induced by the resonant configuration enable a precise determination of the system's architecture. The fine-tuning and fragility of K2-138 and other resonant chains ensure that no significant scattering or collision event has taken place since the formation and migration of the planets in the protoplanetary disc, and hence provide important anchors for planet formation models. Aims. We aim to improve the characterisation of the architecture of this system, in particular the planetary masses and its resonant configuration. In addition, we place this system in context with other resonant chains, and expand on the recent finding that resonant sub-Neptunes are puffier than non-resonant ones by deriving a mass-radius relationship for each population. Methods. We performed a global analysis of all available photometry and radial velocity data for K2-138 using photo-dynamical modelling of the light curve. We also tried different sets of priors on the masses and eccentricity, as well as different stellar activity models, to study their effects on the masses estimated by transit timing variations and radial velocities. Results. We show that our joint photo-dynamical and radial velocity analysis resulted in a robust mass determination for planets b to f, with a precision of ∼15% for the mass of planet f, and better than 10% for planets b to e. We also show that K2-138 is indeed currently locked in the resonant configuration, the five inner planets librating around an equilibrium of the chain; however, existing data are not sufficient to verify if the outer triplet is librating in a first-order three-body resonance. Finally, we demonstrate that the mass-radius relationship of resonant sub-Neptunes is well described by M = 2.54 R0.84, which is less dense than the non-resonant population, for which we found M = 2.78 R1.32. We also found that the resonant population has a smaller spread around its mass-radius relationship than the non-resonant population, pointing towards an inter-system similarity in the composition of the resonant sub-Neptunes.
Keywords
methods: data analysis / celestial mechanics / planets and satellites: detection
Astronomy & Astrophysics
Volume 711, Article Number A221, Number of pages 19
2026 July
