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

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Abstract
HAT-P-26,b is a well-characterized Neptunian exoplanet ̊m p R_⊕) orbiting a K1V star. It lies within the Neptunian ridge (P_ ̊m orb days), following a moderately eccentric orbit (e 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. 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. We employed the 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. ANTARESS HAT-P-26,b exhibits a polar orbit (projected obliquity angle of łambda = -78_ -13 ^ +13 ^ ̧irc and a 3D true obliquity angle of ψ = 85_ -5 ^ +5 ^̧irc). 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 ) 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. g cm^ -3 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.
Funding Information
  • The European Union (101052347)
  • FCT – Fundação para a Ciência e a Tecnologia, I.P. (UID/04434/2025)
  • FCT – Fundação para a Ciência e a Tecnologia, I.P. (UIDB/04434/2020)
  • FCT – Fundação para a Ciência e a Tecnologia, I.P. (UIDP/04434/2020)
  • the FCT Scientific Employment Stimulus program (2023.06055.CEECIND/CP2839/CT0005)
  • National Natural Science Foundation of China (12003063)
  • International Centre of Supernovae, Yunnan Key Laboratory (202302AN360001)
  • Science and Technology Facilities Council (ST/M001962/1)
  • Science and Technology Facilities Council (ST/S002642/1)
  • Science and Technology Facilities Council (ST/W003163/1)
  • European Research Council (947634)
  • the Swiss National Science Foundation (51NF40_182901)
  • the Swiss National Science Foundation (51NF40_205606)
  • the Natural Science Foundation Youth Program of Sichuan Province (2024NSFSC1363)
  • the Doctoral Initiation Fund of China West Normal University (22kE036)
  • the China Scholarship Council and the German Academic Exchange Service (57678375)
  • FCT – Fundação para a Ciência e a Tecnologia, I.P. (UID/04564/2025)
  • UK Research and Innovation (ST/X001121/1)
  • European Regional Development Fund
  • Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (P500PT_222212)
  • Massachusetts Institute of Technology (Desmond Fellowship)
  • Agencia Estatal de Investigación (PID2024-158486OB-C32)
  • the Spanish Ministry of Science, Innovation and Universities (MICIU) project (PID2023-149982NB-I00)
  • UK Research and Innovation (EP/Z000890/1)
  • University of Delaware – Annie Jump Cannon Fund (PHYS462112)
  • Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (NCCR PlanetS)
  • UK Research and Innovation (MR/S035214/1)
  • UK Research and Innovation (MR/Y011759/1)

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