ATREIDES II. Detection of a polar orbit for HAT-P-26 b, a fluffy planet in the Neptunian ridge
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.
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BibTeX
@article{luo2026atreides,
title = {{ATREIDES II. Detection of a polar orbit for HAT-P-26 b, a fluffy planet in the Neptunian ridge}},
author = {X. Luo and V. Bourrier and E. Fridén and M. Lafarga and A. Castro-González and M R Zapatero Osorio and A C Petit and A C M Correia and E M Bryant and E. Delgado Mena and S G Sousa and V. Adibekyan and L. Sun and J V Seidel and S. Gill and J. Fernández and G W Henry and P J Wheatley and M R Burleigh and F. Hawthorn and W. Dethier and E.-M. Ahrer and L. Parc and A. Osborn and A. Sozzetti and D. Veras and E. Pallé and H M Cegla and G. Lacedelli and J I González Hernández and K. Wang and L. Kreidberg and M. Godfrey and M P Battley and N. Lowson and R. Allart and S J Mercier},
journal = {Astronomy \& Astrophysics - A\&A},
year = {2026},
month = jul,
doi = {10.1051/0004-6361/202558747},
url = {https://hal.science/hal-05760286v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - ATREIDES II. Detection of a polar orbit for HAT-P-26 b, a fluffy planet in the Neptunian ridge
AU - X. Luo
AU - V. Bourrier
AU - E. Fridén
AU - M. Lafarga
AU - A. Castro-González
AU - M R Zapatero Osorio
AU - A C Petit
AU - A C M Correia
AU - E M Bryant
AU - E. Delgado Mena
AU - S G Sousa
AU - V. Adibekyan
AU - L. Sun
AU - J V Seidel
AU - S. Gill
AU - J. Fernández
AU - G W Henry
AU - P J Wheatley
AU - M R Burleigh
AU - F. Hawthorn
AU - W. Dethier
AU - E.-M. Ahrer
AU - L. Parc
AU - A. Osborn
AU - A. Sozzetti
AU - D. Veras
AU - E. Pallé
AU - H M Cegla
AU - G. Lacedelli
AU - J I González Hernández
AU - K. Wang
AU - L. Kreidberg
AU - M. Godfrey
AU - M P Battley
AU - N. Lowson
AU - R. Allart
AU - S J Mercier
PY - 2026
DA - 2026/07/17
JO - Astronomy & Astrophysics - A&A
DO - 10.1051/0004-6361/202558747
UR - https://hal.science/hal-05760286v1
AB - 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.
SN - 0004-6361
SN - 1432-0746
ER -