ATREIDES

II. Detection of a polar orbit for HAT-P-26 b, a fluffy planet in the Neptunian ridge

¹ School of Physics and Astronomy, China West Normal University, Nanchong 637009, PR China
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
⁴ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁵ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁶ Centro de Astrobiología, CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁷ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
⁸ CFisUC, Departamento de Física, Universidade de Coimbra, 3004-516 Coimbra, Portugal
⁹ LTE, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, 75014 Paris, France
¹⁰ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹¹ Departamento de Fısica e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹² Yunnan Observatories, Chinese Academy of Sciences, Kunming, China
¹³ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming, China
¹⁴ International Centre of Supernovae, Yunnan Key Laboratory, Kunming, China
¹⁵ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile
¹⁶ Tennessee State University (retired), Nashville, TN 37209 USA
¹⁷ School of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
¹⁸ Rugby School, Lawrence Sheriff St, Rugby, Warwickshire, CV22 5EH, UK
¹⁹ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
²⁰ Centre for Space Domain Awareness, University of Warwick, Coventry CV4 7AL, UK
²¹ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
²² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
²³ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
²⁴ Universidad de La Laguna, Dept. Astrofísica, 38206 La Laguna, Tenerife, Spain
²⁵ Astronomy Unit, Queen Mary University of London, Mile End Road, London E1 4NS, UK
²⁶ Department of Physics and Astronomy, University of Delaware, 217 Sharp Lab, Newark, DE 19716, USA
²⁷ Département de Physique, Institut Trottier de Recherche sur les Exoplanètes, Université de Montréal, Montréal, Québec H3T 1J4, Canada
²⁸ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 22 December 2025
Accepted: 5 May 2026

Abstract

Context. HAT-P-26 b is a well-characterized Neptunian exoplanet (R_p ≃ 6.33 R_⊕) orbiting a K1V star. It lies within the Neptunian ridge (P_orb ≃ 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.