Alexandre C. M. Correia

Context . The origin of the lower edge of the Neptunian desert remains an open question. The leading hypotheses are tidal disruption of volatile-rich sub-Neptunes following high-eccentricity tidal migration (HEM) and atmospheric escape of their H ₂ /He envelopes. Aims . We aimed to confirm and characterise two TESS candidates near the lower edge of the desert, TOI-426.01 ( P _orb ≈ 1.32 d) and TOI-1839.01 ( P _orb ≈ 1.42 d), orbiting the solar-type stars TOI-426 ( V = 10.1 mag, G2 V) and TOI-1839 ( V = 11.0 mag, G9 V). Methods . We acquired 77 HARPS radial velocities (RVs) of TOI-426 and 70 of TOI-1839, and we performed a joint Bayesian analysis of the RVs and TESS photometry. The RVs of both targets show significant activity signals, which we modelled using shared-timescale and multidimensional Gaussian process frameworks to disentangle stellar variability and derive precise planetary parameters. Results . We confirm TOI-1839 b ( R _p = 2.27 ± 0.10 R _⊕ , M _p = 7.10 ± 0.78 M _⊕ ) and TOI-426 b ( R _p = 2.19 ± 0.09 R _⊕ , M _p = 6.7 ± 1.8 M _⊕ ), and we detect an additional transiting sub-Neptune, TOI-1839 c ( P _orb ≈ 4.02 d, R _p = 2.50 ± 0.13 R _⊕ , M _p = 6.0 ± 1.1 M _⊕ ), and a long-period giant planet, TOI-426 c (assuming a circular orbit, P _orb = 235.8 _−8.5 ^+9.2 d, M _p sin i = 444 ₋₁₇ ⁺¹⁸ M _⊕ ). TOI-426 b ( F ≈ 1700 F _⊕ ) and TOI-1839 b ( F ≈ 1050 F _⊕ ) are among the most highly irradiated sub-Neptunes known to date, but their bulk densities nevertheless require a significant volatile component. For the three transiting planets, our interior-structure and atmospheric-escape models yield short mass-loss timescales of ∼1–100 Myr for H ₂ /He-dominated atmospheres but much longer timescales of ∼100–1000 Gyr for H ₂ O-dominated envelopes, favouring a steam-world scenario for these planets. We also explored the population of short-period low-mass sub-Neptunes with outer long-period giant companions and found a strikingly high incidence (∼35%) at the shortest orbital periods near the desert edge, where successful HEM outcomes are expected to circularise. Conclusions . The survival of highly irradiated volatile-rich sub-Neptunes near the lower edge of the Neptunian desert is difficult to reconcile with a classical picture in which this boundary is set solely by the evaporation of primordial H ₂ /He envelopes. At the same time, the high incidence of long-period giant companions is consistent with an important role for HEM in shaping this region.

We present new CHEOPS transit observations of AU Mic b and AU Mic c obtained between June and September 2024, extending the baseline of transit-timing measurements of this young planetary system. For AU Mic b, the timing signal is well established, with a semi-amplitude (10 ± 3 min) and a characteristic modulation timescale (1168 ± 20 d) consistent with previous determinations. By contrast, the new CHEOPS data show that the large transit-timing deviation of AU Mic c reported previously was not sustained. After the steadily increasing timing trend observed in 2022 and 2023, the 2024 timings returned closer to the zero point of the observed-minus-calculated diagram, indicating a reversal of the previously reported behavior. For AU Mic c, both the transit-timing semi-amplitude (46 ± 26 min) and the characteristic modulation timescale (2150 ± 110 d) remain tentative. These results highlight the importance of continued long-term monitoring of the AU Mic system.

The supermassive black hole at the Galactic centre, Sgr A*, is surrounded by an S cluster of young stars. These stars are predominantly B-type stars, but their origin remains uncertain. Unlike massive stars in the Galactic field, which exhibit a binary fraction of about 70%, those in the S cluster show a significantly lower fraction. It is key to understand this discrepancy for constraining their formation. We show that the binary deficit arises naturally from dynamical interactions between stellar binaries and Sgr A* in an in situ formation scenario. Using K-amplitude data, we derived an observational binary fraction of f _obs = 0.43 ± 0.09 for the S-cluster stars. Our dynamical modelling predicts that interactions with Sgr A* cause 18% of the binaries to merge, 20% to be disrupted, and 62% to survive, yielding a theoretical binary fraction of f _theo = 0.38 ± 0.10. The predicted binary fraction with distance from Sgr A* is also consistent with independent observational constraints. These results show that the decline in binarity towards the Galactic centre is driven by stronger tidal forces, and they support an in situ origin of the S cluster in the same event that produced the Fermi bubbles.

The ν Octantis system is composed of a compact stellar binary and a retrograde S-type planet orbiting near the edge of stability. Here, we investigate the dynamically viable architectures of this system by exploring the planetary orbital elements with a Monte Carlo method. To handle the large number of free parameters, we employed a novel approach in which all relevant orbital parameters were varied simultaneously, which we refer to as corner stability maps. Furthermore, since the osculating orbital period of the planet undergoes large oscillations, we combined these maps with frequency analysis to identify the stable configurations that remain consistent with the observational data. We find that the retrograde planetary orbit most likely lies in the region of influence of the high-order 28/-11 mean-motion resonance, where its long-term stability is enhanced by secular apsidal alignment.

The extreme 98º axial tilt of Uranus remains one of the most intriguing puzzles in planetary science, commonly attributed to a series of giant impacts during the late stages of planetary formation. This scenario argues that an impactor of 1–3 Earth masses at a grazing angle could deliver the angular momentum required to reach the current orientation and also generate a debris disk for satellite formation and potentially explain Uranus’ anomalously low internal heat flux [1, 2]. The specific impact parameters required for this hypothesis are tightly constrained and, therefore, while giant impacts remain a plausible contributor, they do not provide a fully self-consistent explanation for Uranus' present configuration.Recent dynamical studies suggest that secular spin-orbit resonances, driven by the tidal migration of ancient satellites, could offer a compelling alternative explanation [3]. This mechanism has already been successfully invoked to explain the obliquity of Saturn, driven by tidal migration of Titan [4], and the future tilting of Jupiter. In this context, Uranus would represent the final stage of this dynamical progression: a planet that has completed its tilting phase.Following the work of Saillenfest et al. (2022) [5], it has been demonstrated that the outward tidal migration of a single ancient satellite with a mass higher than 4e-4 Uranus masses could successfully tilt Uranus from a small initial obliquity towards 90º.This study aims to numerically revisit and extend these findings using the self-consistent open-source N-body code TIDYMESS [6], by simulating the coupled tidal and rotational dynamics of a proto-Uranian system. We explore the tilting scenario of Uranus by analyzing the dynamical evolution of its spin-axis as a function of satellite distance and identifying the spin-orbit resonances capable of driving large-scale obliquity evolution. Our numerical approach accounts for gravitational field to the quadrupole order and integrates the equations of motion for orbital, spin, and deformation dynamics. Building on the scenarios proposed by Saillenfest [5], we investigate and broaden the parameter space of a hypothetical ancient satellite, specifically its mass and migration rate, to determine the conditions under which Uranus can be tilted from a low initial obliquity to its current state. Preliminary results focus on identifying the stability limits of such satellites at high obliquities and the potential for chaotic spin-axis evolution. By exploring different satellite configurations and migration timescales, this work aims to refine our understanding of the dynamical history of the ice giants and the role of tidal-migration mechanisms in planetary dynamics.References[1] Kegerreis, J. A., et al. (2018). The Astrophysical Journal, 861(1):52.[2] Reinhardt, C., et al. (2020). Monthly Notices of the Royal Astronomical Society, 492:5336-5353.[3] Gomes, R. (2026). Icarus, 453:117055.[4] Saillenfest, M., et al. (2021). Astronomy & Astrophysics, 647:A92.[5] Saillenfest, M., et al. (2022). Astronomy & Astrophysics, 668:A108.[6] Boekholt, T. C. N. & Correia, A. C. M. (2023). Monthly Notices of the Royal Astronomical Society, 522(2):2885-2900.

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.

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. 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. 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. 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,R^0.84, which is less dense than the non-resonant population, for which we found M=2.78,R^1.32. 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.

Spin--orbit misalignment is increasingly observed in binary stars, but its origin remains uncertain. We study the secular spin dynamics of stellar binaries and derive the associated Cassini states. We isolate a spin--spin resonance that arises when the two stellar spin-precession frequencies become commensurate. This resonance can excite the obliquity of the secondary to high values even when the primary is only weakly tilted. Magnetic braking favors the formation of high-obliquity states, whereas tidal dissipation may suppress them. Spin--spin coupling may thus help explain the observed diversity of obliquities in young stellar binaries.

Context . Following the 2022 alert of a TESS object of interest transiting TOI-5624 (a G7 V star ∼ 100 pc away), a CHEOPS campaign in 2023 detected four planetary signals at P _b ≈ 3.4, P _c ≈ 7.9, P _d ≈ 13.7, and P _e ≈ 21.5 days. These signals were later confirmed by additional TESS and CHEOPS photometry in 2024-2025. Aims . By using TESS and CHEOPS photometry, along with HARPS-N and SOPHIE high-resolution spectra, we determined the planet properties and performed a dynamical analysis of the system. Methods . After analysing the photometric data, we extracted and modelled the radial velocity (RV) time series using two independent methodologies, both within a Markov chain Monte Carlo framework. We further integrated the N -body equations of motion, while simultaneously fitting the transit times and the detrended RVs, to dynamically characterise the system. Results . We present the discovery of four transiting sub-Neptunes with radii of R _b = 2.314 ± 0.035 R _⊕ , R _c = 2.474 ± 0.042 R _⊕ , R _d = 3.584 _−0.050 ^+0.051 R _⊕ , and R _e = 3.247 _−0.043 ^+0.042 R _⊕ , along with masses of M _b = 9.4 ± 1.4 M _⊕ , M _c = 4.8 ± 1.9 M _⊕ , Md = 4.9 ± 2.2 M _⊕ , and M _e = 8.9 _−3.0 ^+2.9 M _⊕ . Our photometric analysis revealed that the outermost transiting planet TOI-5624e shows significant transit-timing variations (TTVs). Indeed, we found a robust Keplerian signal in the RV time series close to the 2:1 period commensurability with TOI-5624 e, which explains the TTV pattern exhibited by TOI-5624 e according to our dynamical analysis. We labelled this non-transiting planet as TOI-5624 f and determined its minimum mass to be M _f sin i _f = 13.0 ± 3.7 M _⊕ . Conclusions . Among the known systems hosting more than four planets, the remarkable precision with which the radii have been measured (3σ) of the mass for at least three planets has previously been reached only for TRAPPIST-1. Additional photometric observations will enable a better sample of the TTV modulation and a more robust dynamical determination of the masses.