Luca Borsato

Aims . We aim to constrain the dynamical masses of the four confirmed transiting planets orbiting the infant (20 ± 10 Myr) star V1298 Tau (orbital periods P _b ∼ 24.1 d, P _c ∼ 8.2 d, P _d ∼ 12.4 d, and P _e ∼ 48.7 d), through a new analysis of a homogeneous radial-velocity dataset. This system is a key benchmark for testing formation and early-stage evolutionary models of planets. Methods . We analysed more than 380 high-resolution spectra collected with the HARPS-N spectrograph over a time span of 4.5 yr. To account for dominant stellar activity and transit-timing-derived planet–planet interactions, we analysed radial velocities extracted using two different approaches and combined N -body simulations with multi-dimensional Gaussian-process regression. We tested a set of spectroscopic and photometric activity diagnostics to mitigate the variability due to the stellar magnetic activity in the radial-velocity time series. Results . The mass determinations are sensitive to the choice of the radial-velocity extraction method and activity diagnostics. For some specific models, we measured masses with a significance of 3.5–4 σ for planets b (0.52 _−0.14 ^+0.12 , 0.65 _−0.18 ^+0.21 , 0.67 _−0.19 ^+0.16 , and 0.78 _−0.20 ^+0.19 M _Jup ) and e (0.40 _−0.13 ^+0.14 , 0.66 _−0.18 ^+0.17 , and 0.71 _−0.22 ^+0.18 M _Jup ). These values are compatible with masses of giant planets and are notably higher than those recently derived from transit timing variations alone, which indicate that these are low-density ‘super-puffy’ planets. Injection-and-recovery simulations moderately support this conclusion for planet b. For planets c and d, we primarily provide updated upper mass limits. Conclusions . Our analysis represents an exploratory study to address the complex issue of mass measurement in a multi-planetary system characterised by high levels of stellar activity. While firm conclusions cannot be drawn for V1298 Tau, in a few cases our results leave open the possibility that planet b in particular is a massive giant rather than a sub-Neptune progenitor. This dichotomy may be solved by combining radial velocities and transit time variations in a more complex photo-dynamical modelling, with a larger sample of transits than currently available.

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.

It is inherently challenging to characterize long-period transiting exoplanets because the transit events are rare and last long. These systems provide unique insights into planetary formation, migration, the detection of exomoons, and primordial atmospheres by occupying a sparsely populated region of the exoplanet parameter space, however. The complexity increases further for long-period planets near mean-motion resonances, where transit timing variations (TTVs) can reach amplitudes of several hours to days. We present a coordinated space- and ground-based observing campaign using photometry from NEOSSat, multiple LCOGT sites, MuSCAT, MuSCAT3, Tierras, and NGTS to capture the 19-hour transit of the long-period giant exoplanet HIP 41378 f ( P ≈ 542 d, R ≈ = 9.5 R _⊕ ) on 31 October 2025. Our transit analysis constrains the time of inferior conjunction to T _C = 2460980.888 ± 0.029 BJD _TDB , occurring ∼7 hours earlier than predicted from its linear ephemeris. This significant offset is consistent with the previously reported TTVs of HIP 41378 f, confirming it as the longest-period exoplanet known to exhibit measurable TTVs. By combining this new precise measurement with the transit timings of the two outer planets in the system (HIP 41378 d and HIP 41378 e), we performed a dynamical modeling of the system using the N -body integrator TRADES , refined the ephemeris of HIP 41378 f, and predict future transit events for all three outer transiting planets.

The present-day architecture of planetary systems contains information about their formation and migration histories. The origin of hot Jupiters (HJs; P łesssim 10 d, R_ ̊m p > 8 R_⊕) has long been a matter of debate. While most of them are found to be `lonely', there is a rare population of HJs hosting small inner-orbit companions (eight known as of May 2026). Their peculiar architecture suggests a gentle disc-migration mechanism. In this study, we present the discovery and characterisation of the multi-planet system TOI-1533, comprising an inner sub-Neptune (TOI-1533 b, P_ ̊m orb = 3.63 d, R_ ̊m p = 3.15 R_⊕) and an outer hot giant planet (TOI-1533 c, P_ ̊m orb = 8.06 d, R_ ̊m p > 7.5 R_⊕) with substantial H/He by mass (ρ_ ̊m p < 0.48 g cm -3 $), both transiting an active K-dwarf star (T_ ̊m eff ≈ 5130 K; V (mag) ≈ 11). Our joint modelling of stellar activity and planetary signals from radial velocities (HARPS-N) and transits (TESS) allowed us to detect the planetary Keplerian signals (approximately 10 σ) and isolate the stellar modulation. The inclusion of simultaneous photometry in the multi-dimensional Gaussian processes formalism was a fundamental addition to the spectroscopic activity indicators, enabling the disentanglement of stellar activity from planetary signals. The mass ratio of the two confirmed planets (M_ ̊m b / M_ ̊m c about 0.8), together with the super-Neptune mass of the large outer companion ($M_ ̊m c ≈ 40 M_⊕), makes this system unusual compared to the other known HJs with low-mass inner companions.

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.

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.

The obliquity between a planet’s orbital axis and its host star’s spin axis provides crucial insights into planetary formation and migration. Planets with scaled semi-major axes ( a / R _★ ) large enough to be unaffected by tidal alterations (‘tidally detached’) offer a unique opportunity to study the original obliquity in which the system formed. We therefore observed TOI-1710 b ( a / R _★ ≈ 36) in transit using HARPS-N + GIANO-B, collecting high-precision radial velocities to measure the Rossiter-McLaughlin effect. Spectral analysis of the H α and HeI triple lines was also pursued to evaluate atmospheric photoevaporation. Using our knowledge of the star rotation period (21.5 ± 0.2 d), we estimated a true obliquity of ψ = 149 ⁺¹¹ ₋₁₀ deg, which indicates a retrograde motion, thus placing TOI-1710 b among the most misaligned systems – and making it the only one known to orbit a cool star in retrograde motion. The strong misalignment favours a high-eccentricity migration (HEM) origin for this low-density super-Neptune planet in the savanna region, challenging previous findings that claimed a minor role of HEM in this period-radius(-density) domain. Moreover, the strong misalignment and lack of a detected close stellar companion suggest a purely planetary post-migration misalignment, likely due to planet-planet scattering followed by planet-planet Kozai-Lidov oscillations and tidal circularisation.

The radii of small exoplanets form two populations, super-Earths and sub-Neptunes, separated by a gap known as the radius valley. This feature could be produced by the removal of atmospheres by stellar or internal heating or by the lack of an initial envelope. We used transit photometry and radial velocity measurements to detect and characterize four exoplanets orbiting LHS 1903, a red dwarf star in the Milky Way’s thick disk. These four planets have orbital periods ranging from 2.2 to 29.3 days and span the radius valley within a single planetary system. The derived densities indicate that LHS 1903 b is rocky, whereas LHS 1903 c and LHS 1903 d have extended atmospheres. The most distant planet from the host star, LHS 1903 e, has no gaseous envelope, indicating that it formed from gas-depleted material.