H. Jens Hoeijmakers

Ultra-hot Jupiters (UHJs) represent the most extreme class of close-in gas giants, with KELT-9b being the most extreme example known. For this study, we combined 13 transit observations of KELT-9b from six high-resolution spectrographs spanning 316–960 nm to conduct a census of atomic and ionised species in the planet’s atmosphere using the cross-correlation technique. The primary objective is to introduce an agnostic strategy to mitigate velocity-axis correlation bias in parameter uncertainties during cross-correlation analysis. By employing a forward model of the cross-correlation function (CCF) alongside a Monte Carlo bootstrap of flux uncertainties, the individual CCF-derived velocity profiles of each species in the planet’s atmosphere were measured, and potential time dependence in these velocity profiles was assessed. The stellar reference frame was determined, yielding a systemic velocity of V _sys = −17.99 ± 0.04 km s ⁻¹ and a projected rotational velocity of v sin i = 110.54 ± 0.05 km s ⁻¹ , which is consistent across all instruments and observing nights. The combined planetary absorption profiles provide insight into the collective motion of all detectable gas species in the atmosphere. All previously reported species were recovered except for Tb II, and the combined analysis detects 29 species, including additional absorbers C I, Si I, K I, V II, Co I, Zr II, La II, Pr II, and Nd II. All 29 recovered species exceed the conventional 5 σ threshold in the bootstrap Mahalanobis significance test. The collective Doppler shifts display a blueshift of ∆ V _offset = −8.02 ± 0.17 km s ⁻¹ relative to the stellar rest frame, indicating a net day-to-night wind, with no explicit time dependence observed when all species are homogenised. These results constitute a catalogue of detections and retrieved Doppler parameters, forming a foundation for a companion study of KELT-9b that would allow for the most significant detections to be analysed and the differences across individual species to be compared.

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.

Close-in exoplanets span a wide range of orbital architectures and physical properties, consistent with the combined influence of migration processes and formation conditions. Although population synthesis models predict the emergence of distinct planetary populations, establishing a statistically robust connection between observed samples and synthetic populations remains challenging, particularly in high-dimensional parameter spaces dominated by dynamical quantities. Intrinsic population-level organisation within the observed sample of close-in exoplanets is investigated through physically motivated parameters, with the aim of establishing a quantitative link to formation pathways predicted by pebble-accretion models. A two-stage Gaussian mixture model (GMM) was applied to an observed sample of close-in exoplanets, with unsupervised probabilistic clustering performed in a feature space dominated by dynamical descriptors of planet--star interactions. The resulting clusters were mapped onto a pebble-accreted synthetic population within a statistically motivated 3D parameter space. Formation-related quantities, including formation timing, gas fraction, and ice--rock mass ratio, were then employed to establish a probabilistic interpretation and comparison of the mapped clusters. Statistically supported sub-populations were identified without the imposition of predefined classification boundaries, including very-massive gas giants, hot giants, warm-Jupiter-dominated systems, and lower-mass giants. When mapped onto pebble-accreted synthetic populations, these observational clusters exhibited systematic differences in formation timing, gas fraction, and solid growth histories across the dominant populations. Data-driven clustering in a dynamical-parameter space enables a direct and statistically robust comparison between observed exoplanet sub-populations and pebble-accretion synthetic populations. The inferred cluster-level trends point to systematic differences in formation timing and accretion histories among giant populations, with very-massive gas giants preferentially associated with earlier formation epochs than hot-giant and warm-Jupiter-dominated systems. This demonstrates the potential of physically motivated machine-learning approaches to connect observed exoplanets with theoretical formation scenarios at the population level.

Tracing the compositional link between terrestrial super-Earths and their host stars provides clues about their dominant formation pathway. By constraining the stellar abundances of refractory elements, we can predict the core mass fractions (CMFs) of their super-Earths. The level of agreement between this prediction and the planetary CMF derived from their masses and radii can reveal past formation processes, such as mantle stripping and water-rich formation plus sequestration in the planet core. We present the first results from the Near Infrared Planet Searcher (NIRPS) GTO CMF subprogram: an intensive radial velocity campaign to refine masses and compute host stellar abundances of three hot super-Earths around M dwarfs (GJ1132 b, GJ1252 b, and LTT 3780 b). We calculated masses of 1.69 ± 0.15 M _⊕ , 1.54 ± 0.18 M _⊕ , and 2.34 ± 0.10 M _⊕ respectively. We measured the CMFs of these and six further hot super-Earths with masses already available in the literature to a precision of 10–15%. We compared them to CMF predictions made from measuring the Fe, Mg, and Si abundances of their host stars measured from the NIRPS spectra. The CMFs of these planets are smaller than expected from their host stellar abundances to a statistically significant degree. This discrepancy is suggestive of significant reservoirs of water, and while these planets are too hot to harbor surface water, they likely have interior water mass fractions of ~1%.

Aims . We investigate the dynamical evolution of particles in the β Pic system to determine likely formation pathways to the exocomet populations observed in the present day. We aim to relate these results to similar studies recently carried out since the discovery of the inner planet β Pic c. Methods . We simulated the β Pic system using the non-symplectic adaptive N -body integrator IAS15 in REBOUND. We seeded the system with over 100 000 mass-less test particles that were evolved for 25 Myr. We adopted initial conditions and a particle distribution that closely match those used in similar simulations in recent works. Using IAS15, REBOUND resolves close encounters between test particles and the two gas giants in the system, which is crucial for understanding aspects of the dynamical evolution. Results . Planet-disc interactions rapidly clear most of the system within 35 AU, apart from a region within the orbit of β Pic c and a region between 20 and 25 AU. After 10 Myr, exocomets can be sourced continuously from these regions, as well as from the inner edge of the region beyond ∼35 AU, where particles are stable on longer timescales. From the region interior to β Pic c, the exocomets are formed by excitation via mean-motion resonance with β Pic c, obtaining a narrow distribution of radial velocities, consistent with spectroscopic observations. Particles initialised in the outer system might enter onto star-grazing orbits, due to disruption by the two gas giants, causing a wider radial velocity distribution, and we propose that this population corresponds to a second dynamical family previously observed via spectroscopy. These particles typically undergo chaotic dynamical evolution for 10 ² −10 ³ years after passing the water sublimation limit at ∼8 AU, before reaching the sublimation distance of calcium near 0.4 AU, implying that the two families of exocomets could have different volatile contents. This dynamical simulation depends on the assumed initial conditions, including the system parameters and the morphology of the initial disc, precluding accurate predictions of the absolute rates at which particles are generated from within and from the exterior of the orbits of the two gas giants. However, we predict that it is a general property of the system that a fraction of particles that originates from outside of the orbit of β Pic b can be scattered onto star-grazing orbits.

We present an updated characterization of the planetary system orbiting the nearby M2 dwarf GJ 3090 (TOI-177; d =22 pc), based on new high-precision radial velocity (RV) observations from NIRPS and HARPS. With an orbital period of 2.85 d, the transiting sub-Neptune GJ 3090 b has a mass we refine to 4.52 ± 0.47 M _⊕ , which, combined with our derived radius of 2.18 ± 0.06 R _⊕ , yields a density of 2.40 _−0.30 ^+0.33 g∉cm ⁻³ . The combined interior structure and atmospheric constraints indicate that GJ 3090 b is a compelling water-world candidate, with a volatile-rich envelope in which water likely represents a significant fraction. We also confirm the presence of a second planet, GJ 3090 c, a sub-Neptune with a 15.9 d orbit and a minimum mass of 10.0 ± 1.3 M _⊕ , which does not transit. Despite its proximity to the star’s 18 d rotation period, our joint analysis using a multidimensional Gaussian process (GP) model that incorporates TESS photometry and differential stellar temperature measurements distinguishes this planetary signal from activity-induced variability. In addition, we place new constraints on a non-transiting planet candidate with a period of 12.7 d, suggested in earlier RV analyses. This candidate remains a compelling target for future monitoring. These results highlight the crucial role of multidimensional GP modelling in disentangling planetary signals from stellar activity, enabling the detection of a planet near the stellar rotation period that could have remained undetected with traditional approaches.

Ultra-hot gas giants such as WASP-121b provide unique laboratories for exploring atmospheric chemistry and dynamics under extreme irradiation conditions. Uncovering their chemical composition and atmospheric circulation is critical for tracing planet formation pathways. Here, we present a comprehensive atmospheric characterisation of WASP-121b using high-resolution transit spectroscopy across the optical to infrared with HARPS, NIRPS, and CRIRES+ spanning nine transit events. These observations are complemented with five TESS photometric sectors, two EulerCam light curves simultaneous to the HARPS and NIRPS transits, and an extensive radial velocity dataset in order to refine WASP-121b's orbital parameters. A cross-correlation analysis detected iron (Fe), carbon monoxide (CO) and vanadium (V) absorption signals with SNR of 5.8, 5.0, and 4.7, respectively. Our retrieval analysis constrains the water (H ₂ O) abundance to −6.52 _−0.68 ^+0.49 dex, although its absorption signal is effectively muted by the hydride (H ⁻ ) continuum. We constrained the relative abundances of the volatile and refractory elements - which represents a crucial diagnostic of atmospheric chemistry, evolution, and planet formation pathways. The retrieved abundance ratios are broadly consistent with expected values of a solar composition atmosphere in chemical equilibrium, likely indicating minimal disequilibrium chemistry alterations at the probed pressures (∼10 ⁻⁴ −10 ⁻³ bar). We update the orbital parameters of WASP-121b with its largest radial velocity dataset to date. By comparing orbital velocities derived from both the radial velocity analysis and the atmospheric retrieval, we determined a non-zero velocity offset caused by atmospheric circulation, ΔK _p = −15 ± 3 km s ⁻¹ (assuming M _⋆ = 1.38 ± 0.02 M _⊙ ), consistent with predictions from either drag-free or weak-drag 3D global circulation models, while we caution the non-negligible dependence on the assumed stellar mass. These results place new constraints on the thermal structure, dynamics, and chemical inventory of WASP-121b, highlighting the power of multi-wavelength high-resolution spectroscopy to probe exoplanetary atmospheres.

Ultra-hot Jupiters (UHJs) in close orbits around early-type stars provide natural laboratories for studying atmospheric escape and star-planet interactions under extreme irradiation and wind conditions. The near-ultraviolet (NUV) regime is particularly sensitive to extended upper atmospheric and magnetospheric structures. We investigate whether star-planet interactions in the WASP-189 system could plausibly account for the early ingress feature suggested by NUV transit fitting models. We analyzed three NUV transits of WASP-189b observed as part of the Colorado Ultraviolet Transit Experiment (CUTE), which employs a 6U CubeSat dedicated to exoplanet spectroscopy. To explore whether the observed transit asymmetry could plausibly arise from a magnetospheric bow shock (MBS), we performed magnetohydrodynamic (MHD) simulations using representative stellar wind velocities and planetary atmospheric densities. During Visit 3, we identified an approximately 31.5-minute phase offset that is consistent with an early ingress. Our MHD simulations indicate that with a wind speed of 573 km s-1 and an upper atmospheric density of about 4.6e-11 kg m-3, a higher-density zone due to compression can form ahead of the planet within five planetary radii where the fast-mode Mach number falls below ~0.56, even without a MBS. Shock cooling and crossing time estimates suggest that such a pileup could produce detectable NUV absorption. Our results indicate that while MBS formation is feasible for WASP-189b, low stellar-wind speeds favor NUV-detectable magnetic pileups over classical bow shocks and enhance the potential detectability of early-ingress signatures.

It is an open question whether small planets around M dwarfs are able to maintain atmospheres. The Hot Rocks Survey aims to address this question by observing nine rocky exoplanets orbiting M dwarfs with MIRI emission photometry to constrain the onset of atmospheres. In this paper, we present two MIRI F1500W (15 μm) eclipses of LTT 3780 b, an ultrashort-period super-Earth (P = 0.768 days, R = 1.325 R _⊕, M = 2.46 M _⊕) that receives 111× Earth’s instellation, the highest in the survey. We find a combined eclipse depth of 312 ± 38 ppm, which is consistent between different data reduction and analysis assumptions, bolstering our confidence in the eclipse detection. This eclipse depth is consistent with the thermal emission from a bare rock surface, with a dayside temperature of T day = 114 3 − 99 + 104 K, 98% ± 9% of the maximum temperature predicted for a zero-albedo, zero heat redistribution blackbody. We are able to confidently rule out CO₂-based atmospheres down to a 0.01 bar surface pressure to greater than 3σ (ruling out an approximately Mars-like atmosphere). We are unable to rule out a pure H₂O 1 bar atmosphere, though we argue that this composition is unlikely on such a highly irradiated planet, nor O₂ atmospheres due to the lack of features in the bandpass, though we can put constraints on CO₂-mixture atmospheres. As a potential bare rock, we consider a variety of surface composition models, but are unable to distinguish between them. However, LTT 3780 b is an excellent target for follow-up JWST observations to determine its surface composition and rule out additional atmospheric compositions.

The intense stellar irradiation of ultra-hot Jupiters results in some of the most extreme atmospheric environments in the planetary regime. On their daysides, temperatures can be sufficiently high for key atmospheric constituents to thermally dissociate into simpler molecular species and atoms. This dissociation drastically changes the atmospheric opacities and, in turn, critically alters the temperature structure, atmospheric dynamics, and day-night heat transport. To date, however, simultaneous detections of the dissociating species and their thermally dissociation products in exoplanet atmospheres have remained rare. In this work we present the simultaneous detections of H₂O and its thermally dissociation product OH on the dayside of the ultra-hot Jupiter WASP-121 b based on high-resolution emission spectroscopy with the recently commissioned Near InfraRed Planet Searcher (NIRPS). We retrieved a photospheric abundance ratio of log₁₀(OH/H₂O) = −0.15 ± 0.20, indicating that there is about as much OH as H₂O at photospheric pressures, which confirms predictions from chemical equilibrium models. We compared the dissociation on WASP-121 b with other ultra-hot Jupiters and show that a trend in agreement with equilibrium models arises. We also discuss an apparent velocity shift of 4.79_−0.97^+0.93 km s⁻¹ in the H₂O signal, which is not reproduced by current global circulation models. Finally, in addition to H₂O and OH, the NIRPS data reveal evidence of Fe and Mg, from which we inferred a Fe/Mg ratio consistent with the solar and host star ratios. Our results demonstrate that NIRPS can be an excellent instrument to obtain simultaneous measurements of refractory and volatile molecular species, thus paving the way for many future studies on the atmospheric composition, chemistry, and the formation history of close-in exoplanets.