David Ehrenreich

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.

Context . The carbon-to-oxygen (C/O) ratio of a giant planet’s atmosphere has long been theorised to hold compositional information that can be traced back to its formation history. Typically, the C/O ratio of an exoplanetary atmosphere is inferred from abundance measurements of major C- and O-bearing molecules such as CO, CO ₂ , H ₂ O, and OH. However, some exoplanets have such elevated temperatures that molecules can be nearly completely thermally dissociated at the pressure levels that observations probe, making it difficult to measure their C/O ratio via these traditional tracers. Aims . Here, rather than using molecules, we aim to retrieve the C/O ratio of KELT-9b (T _eq = 4000 K) directly from atomic C and O, which are detected in its atmosphere. Methods . We analysed two transits of KELT-9b observed with the MAROON-X high-resolution spectrograph, finding absorption cross-correlation signals from atomic C and O as well as refractory metals. From this, we inferred the vertical temperature structure, the relative proportions of volatile and refractory species, and the C/O ratio of KELT-9b using a 1D local thermodynamic equilibrium atmospheric retrieval framework applied only to spectral regions mostly unaffected by non-local thermodynamic equilibrium effects. Results . We measure the atomic C/O ratio of the atmosphere of KELT-9b to be 0.20 _−0.07 ^+0.13 , which is slightly lower than the stellar value of 0.38 ± 0.15 and significantly below the solar value of 0.59 ± 0.07. We otherwise confirm previous investigations of the terminator region, finding KELT-9b’s atmosphere to be thermally inverted and slightly metal-rich. We measure the volatile-to-refractory ratio, a proxy for the ice-to-rock ratio, to be 1.24 _−0.78 ^+1.93 × solar ([M _vol /M _ref ] = 0.09 _−0.20 ^+0.19 ), which is consistent with both KELT-9 and the Sun. Conclusions . While there are caveats to interpreting observations of such an extreme planet, our results demonstrate that atomic species can be used to measure C/O ratios of exoplanetary atmospheres in high-temperature regimes where molecules are thermally dissociated.

We report the confirmation and characterisation of two moderate-period sub-Jovian planets transiting the bright F-type star HD 148797 ( G = 9.4 mag, T _eff = 6441 ± 51 K). Using photometric time series from TESS and CHEOPS, we determined orbital periods of 42.1 d for HD 148797 b and 68.2 d for HD 148797 c, putting the period ratio very close to the golden mean at 1.619 and therefore near several strong harmonics, and we measured planetary radii of 8.25 ± 0.37 R _E and 8.37 ± 0.38 R _E , respectively. We detected significant anti-correlated transit-timing variations for both planets, which contain enough harmonic information to yield photodynamical masses of 39.3 _−8.5 ⁺¹³ M _E for HD 148797 b and 39.6 ± 9.3 M _E for HD 148797 c. The corresponding bulk densities, 0.39 ± 0.12 and 0.377 ± 0.084 g cm ⁻³ , place both planets among the low-density sub-Jovians of the Neptunian savanna. The architecture of HD 148797 is not unusual within this regime: we find that detected multi-system fractions in the savanna remain at ∼70–90%, and that most savanna multi-planet systems contain at least one adjacent planet pair with P _out / P _in < 3. This pattern suggests that savanna sub-Jovians are commonly found in dynamically cold systems, consistent with smoother migration pathways such as disk-driven migration rather than disruptive high-eccentricity tidal migration. As a bright, co-evolved system hosting two warm savanna sub-Jovians with similar radii and masses, HD 148797 is also a promising target for comparative atmospheric characterisation.

Context. Ground-based spectrographs operating in the near-infrared (NIR) regime are hampered by various absorption and emission features of Earth’s atmosphere. While considerable attention has been paid to mitigating telluric absorption, correcting telluric emission features remains non-trivial and can significantly affect the observation of faint targets. Aims. We aim to develop and implement automated algorithms for sky background emission correction in the context of high-resolution spectroscopy. These empirical-based algorithms have been officially integrated into both NIRPS data reduction pipelines: NIRPS DRS and APERO DRS. The NIRPS DRS algorithm is additionally functional at the telescope site, enabling quick, real-time telluric-corrected data processing. Designed for flexibility, these techniques can be incorporated into the reduction workflow of any high-resolution spectrograph to improve the radial velocity (RV) performance. Methods. In our approach, a reference sky spectrum is first created by deep-stacking NIRPS sky calibration frames on a common wavelength grid and calculating the weighted median flux per pixel, separately for both the high-accuracy and high-efficiency instrument modes. This process is repeated for all spectral orders and for both the object and sky-calibration fibres: fibre A and fibre B, respectively. In this reference sky spectrum, the sky emission lines can be identified and used to construct a static library. During the data reduction process, the emission features in the library are individually scaled in terms of flux using two distinct techniques, each specific to the two DRS pipelines. Finally, they are locally subtracted from the science observations to minimise their noise contribution to the final spectrum Results. We find that the correction algorithms significantly improve the RV measurements obtained using both the cross-correlation function and line-by-line techniques, enabling NIRPS to achieve submetre-per-second precision in the NIR. The techniques have been successfully validated and demonstrated on both bright (Proxima Centauri) and faint (TOI-406, TOI-3494, TOI-4552) targets, as well as in both high-efficiency and high-accuracy instrument modes. Conclusions. We present the first attempts at empirically subtracting emission line features from high-resolution spectra. These algorithms can be applied independently of the instrumentation and implemented into the data reduction workflow of any high-resolution instrument with minimal modifications required.

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 angle between stellar spin axis and planetary orbital plane can provide key insights into the formation and dynamical evolution of planetary systems. In particular, this measurement in multi-planet systems can be used to further discriminate between different competing migration scenarios. We present six transit observations of the sub-Neptune GJ 3090 b obtained with NIRPS and HARPS. GJ 3090 b is the inner planet of a confirmed multi-planet system orbiting an M dwarf (M2). Using high spectral resolution and high temporal cadence spectroscopic observations, we analyzed the Rossiter-McLaughlin (RM) effect induced by GJ 3090 b to determine its orbital obliquity. Through the RM revolutions technique, we find that the planet is on a retrograde orbit with a derived 3D obliquity of ψ = 136 ⁺²⁴ ₋₁₈ deg. We find no evidence of massive outer planetary or wide stellar binary companions, which disfavors scenarios involving gravitational perturbations from a massive body and instead points toward a primordial misalignment of the protoplanetary disk. Our results establish GJ 3090 b as the first planet on a retrograde orbit discovered around an M dwarf and the first highly misaligned confirmed multi-planet system without a known massive companion. We further propose late secondary disk accretion around GJ 3090, in which the disk is not expected to be aligned with the stellar spin axis, followed by disk-driven migration as the most likely mechanisms to explain the observed architecture. This work also illustrates the capability of the RM revolutions technique when applied to near-infrared data to probe orbital architectures of the smallest planets around M dwarfs that have remained mostly inaccessible.

Context . Condensates are ubiquitous to all Solar System planets with significant ( > 10 mbar) atmospheres. The same is true for most exoplanets with characterised atmospheres, with even ultra-hot exoplanets being able to form clouds on their cooler nightsides. One high-interest condensate is titanium, a highly refractory element that as an oxide (TiO) is a potent optical absorber long believed to be a driver of thermal inversions in highly irradiated exoplanet atmospheres. Observations have shown that some ultra-hot Jupiters have strong thermal inversions despite being significantly Ti-depleted, likely due to cold trapping, raising doubts about whether TiO is the main cause of their inverted temperature structures. Aims . Our aim was to retrieve the titanium-to-iron ratio of the dayside atmosphere of the ultra-hot Jupiter WASP-189b to determine whether the full titanium budget is accounted for in the gas phase. Methods . We analysed thermal emission observations of WASP-189b taken with the HARPS and NIRPS spectrographs using different atmospheric retrieval prescriptions to infer the planet’s atmospheric thermal structure and composition. Results . We observed Fe and Ti signals in cross-correlation and measured a sub-solar Ti/Fe ratio for WASP-189b using both free and chemical equilibrium retrieval approaches. We find the Ti/Fe of the planetary atmosphere to be between 28% and 81% (1- σ bounds) that of the stellar value. Conclusions . The slight underabundance of Ti with respect to Fe on the dayside atmosphere of WASP-189b suggests that some titanium is missing from the gas phase, potentially due to a partial nightside cold trap. In the context of the ultra-hot Jupiter population, the onset of titanium in exoplanetary atmospheres appears to occur progressively, coinciding not with when thermal inversions begin but rather when the vapourisation threshold of titanium is reached on the nightside.

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.

Small planets are extremely common in the Galaxy, including planets with masses and radii between those of Earth and Neptune. Characterising the mass of such planets requires ultra-precise radial velocities. The ESPRESSO spectrograph was designed and built for this purpose. We present an overview of the ESPRESSO Guaranteed Time Observations transit follow-up sub-programme, which aimed to confirm and characterise small transiting planet candidates from the K2 and TESS space missions. We analysed the global stellar and planetary properties of the sample of 65 planets in 30 systems characterised by this sub-programme. This includes six systems presented in this paper, for which we either obtain only the upper mass limits or provide updates to previously published parameters. We also placed this sample in the context of the overall population of precisely characterised small planets. After separating the population into insolation regimes, we find a tentative mass threshold at for the rocky to volatile-rich composition transition in the medium-insolation regime, along with a population of likely stripped massive rocky planets in the high-insolation regime. Likewise, we find a correlation between planet mass and stellar metallicity, with more massive planets hosted by more metal-rich stars. We also explored the radius valley, finding that planets below the gap have a tighter mass distribution. We compared planetary masses with typical protoplanetary disk masses, drawing tentative conclusions about likely formation conditions. We also discuss the impact of our observing strategy on our results. The ESPRESSO transit follow-up sub-programme has been highly productive in characterising a diverse population of small planets, which has enabled us to identify population-level features. Likewise, the lessons learned from this sub-programme will be valuable for PLATO follow-up planning.

Context . Detecting and characterizing the atmosphere of rocky planets in the habitable zone is a key objective of exoplanet research in the era of JWST and the upcoming E-ELT. Spectroscopy of a planetary transit is the primary method of obtaining a transmission spectrum of an exoplanetary atmosphere. A lunar eclipse is a natural proxy for an Earth transit, offering a unique opportunity to study Earth’s atmosphere as if it were an exoplanet. Aims . This work aims to derive the empirical, wavelength-dependent effective height of Earth’s atmosphere in the near-infrared, spanning the Y to K bands. Methods . During a lunar eclipse, the penumbra – the region where the Sun is partially occulted by the Earth – contains the signature of the transmission spectrum of the atmosphere along Earth’s limb. The high-resolution (R=70 000) SPIRou spectrograph at CFHT was used to collect spectra during and after the November 19, 2021 eclipse. Two distinct methods, using either one or two penumbral spectra, were employed to derive the effective height of the atmosphere. Each method was evaluated in the context of distinct astrometric and spectrophotometric approaches. Results . The derived effective height spans 964 to 2498 nm and shows the absorption bands of H ₂ O, O ₂ , CH ₄ , and CO ₂ peaking at up to 29.6 km at a spectral resolution of R=70 000, including a 12.7 km offset defining a reference altitude taken from a transit model. The uncertainty assessed from individual penumbra observations and from uncertainties on telluric lines correction and Moon reflectance is found to be 3 σ ≈ 3.6 km. Heights are up to 25.1 km at a convolved resolution of R=1000. Conclusions . The measured heights are in good agreement with theoretical predictions. The high-resolution wavelength-dependent effective heights are made publicly available to the community.