X. Luo, V. Bourrier, E. Friden, M. Lafarga, A. Castro-González, M. R. Zapatero Osorio, A. C. Petit, A. C. M. Correia, E. M. Bryant, E. Delgado Mena, S. G. Sousa, V. Zh. Adibekyan, L Sun, J. V. Seidel, S. Gill, J. Fernandez, G. W. Henry, P. J. Wheatley, M. Burleigh, F. Hawthorn, W. Dethier, E. Ahrer, L. Parc, A. Osborn, A. Sozzetti, D. Veras, E. Palle, H. M. Cegla, G. Lacedelli, J. Gonzalez Hernandez, K. Wang, L. Kreidberg, M Godfrey, M. Battley, N Lowson, R. Allart, S. J. Mercier
Abstract
Context. HAT-P-26 b is a well-characterized Neptunian exoplanet (Rp ≃ 6.33 R⊕) orbiting a K1V star. It lies within the Neptunian ridge (Porb ≃ 4.2345 days), following a moderately eccentric orbit (e ≃ 0.12) and hosting a water-rich atmosphere. The possible presence of a stellar companion hinted at by near-infrared (NIR) spectral emission, along with candidate planetary companions inferred from transit timing variations, calls for further investigations of the system architecture. Hence, the HAT-P-26 system is a prominent target for investigating the atmospheric and dynamical evolution of exo-Neptunes and, in particular, via the study carried out by the Ancestry, Traits, and Relations of Exoplanets Inhabiting the Desert Edges and Savanna (ATREIDES) collaboration. Aims. We combined three transit observations of HAT-P-26 b using ESPRESSO on ESO's VLT to precisely characterize its orbital architecture and to investigate whether it has a misaligned orbit, as reported in cases of other evaporating and eccentric planets in the ridge. Methods. We employed the ANTARESS workflow to perform a careful and robust data reduction. We explored the system's orbital architecture using the in-built Rossiter-McLaughlin Revolutions (RMR) technique. Results. HAT-P-26 b exhibits a polar orbit (projected obliquity angle of λ = −78−13+13∘ and a 3D true obliquity angle of ψ = 85−5+5∘). This places it alongside GJ 436 b, HAT-P-11 b, and GJ 3470 b as polar-orbit and eccentric planets lying within the ridge population. It has the lowest density (0.40 ± 0.10 g cm−3) of these four planets, placing it near the density brink where full atmospheric erosion after early disk-driven migration or tidal disruption after high-eccentricity migration has been proposed to occur. Our dynamical analysis suggests that the observed polar orbit might either be a transient state arising from orbital-plane precession or a relic of high-eccentricity migration via Von Zeipel─Lidov─Kozai cycles driven by an inclined stellar companion. We note that in the latter case, the existence of planet c would be strongly disfavored. Conclusions. HAT-P-26 b stands out as a special target in the Neptunian ridge, owing to its low density and polar orbit. The current dynamical analysis is limited by the uncertainties surrounding companion(s) in this system. As better constraints on the companion(s) become available, it will be particularly interesting to further explore the origin of this fluffy ridge Neptune through a joint analysis of migration, evaporation, tidal disruption, and inflation. Our results also demonstrate the importance of homogeneous data reduction when combining multi-epoch transit observations for accurate spin─orbit characterization.
Keywords
techniques: photometric / techniques: radial velocities / planets and satellites: dynamical evolution and stability / planets and satellites: fundamental parameters / planet-star interactions / planets and satellites: individual: HAT-P-26
Astronomy & Astrophysics
Volume 711, Article Number A237, Number of pages 26
2026 July
