Melissa J. Hobson

The TRAPPIST-1 system is well known for its seven transiting Earth-sized exoplanets. It has been extensively studied and characterized, notably with transit timing variations (TTVs), to precisely measure the mass of the planets. Using near-infrared spectroscopic observations obtained as part of the SPIRou Legacy Survey and the NIRPS Guaranteed Time Observation programs, we aimed to verify those values through radial velocity (RV) measurements of the system. Our RV analysis reveals that the current data do not have the precision required to individually detect the TRAPPIST-1 planets. However, we confidently detect (Δ ln 𝒵 = 7.53, 1860:1 odds) the combined RV signature of the planets by using the relative masses derived from the TTV analysis, with TRAPPIST-1 b as a proxy for the whole system. For the first time, the RV signal of the TRAPPIST-1 system is recovered: we find a RV semi-amplitude of K _b = 3.65 _−0.83 ^+0.78 m s ⁻¹ . This corresponds to a planetary mass of M _{p , b} = 1.31 ± 0.29 M _⊕ and demonstrates that the RV measurements are consistent with the TTV model ( M _{p , b; TTV} = 1.374 ± 0.069 M _⊕ ). Additionally, the NIRPS RVs constrain the presence of giant planets beyond the snow line, excluding Saturn-mass planets out to 2.7-yr orbits and Neptune-mass objects out to 20 d. Through RV, we determined the stellar activity period to be of 3.22 _−0.20 ^+0.22 d. Its agreement with photometric measurements (K2 and TESS) confirms stellar rotation as the origin of the ~3.3-d periodicity observed for TRAPPIST-1. We further investigated stellar activity with SPIRou polarimetric measurements, and placed an upper limit on the longitudinal field (| B _l | < 40 G, 3 σ ). This limit is compatible with a weak multipolar large-scale magnetic geometry, as observed in some of the later-type rapidly rotating M dwarfs.

Giant planets orbiting low-mass M dwarfs challenge current planet formation theories, which predict that such planets are unlikely to form. Characterizing their atmospheres can provide key insight into their origins, but analysis is complicated by stellar contamination in transmission spectra. We present a JWST/NIRSpec PRISM transmission spectrum of TOI-3235 b, a 604 K, 0.665 M _Jup giant planet orbiting a 0.39 M _⊙ M dwarf. We apply a hierarchical retrieval framework incorporating an atmospheric model, a parametric stellar contamination model, and a Gaussian process (GP) to capture residual structure not explained by these deterministic components. We find that the inferred atmospheric properties are model dependent: retrievals that treat stellar contamination deterministically yield a constrained CH ₄ abundance corresponding to a subsolar metallicity that would challenge standard expectations from both core accretion and gravitational instability, and potentially elevated CO and CO ₂ abundances that would require chemical disequilibrium. However, residual wavelength-correlated structure suggests the deterministic model is incomplete. Including a GP to marginalize over this structure broadens the atmospheric posterior distributions, preventing robust constraints on the atmospheric composition, and highlighting the challenge of characterizing giant planet atmospheres around M dwarfs when physical models are incomplete. We demonstrate that an eclipse observation could clarify these model-dependent ambiguities: detected emission features would constrain the planet’s metallicity and test formation scenarios, while a featureless spectrum would confirm stellar contamination dominates the transmission spectrum, providing an empirical M dwarf contamination spectrum.

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.

Long-period transiting planets are essential edge-cases for understanding exoplanet formation and evolution. Their detection is challenging due to the infrequent nature of their transits, making every observation of these objects very valuable. Our goal is to confirm and further characterise the long-period planetary candidate TOI-4409,b (initially identified by TESS) through additional observations using the Antarctica-based ASTEP telescope, supplemented with CHAT, OMES, LCOGT, and PEST light-curve data and radial-velocity (RV) measurements collected with the HARPS, FEROS, and PFS instruments. We jointly analysed the photometric data from eight TESS, four ASTEP, one CHAT, two OMES, four LCOGT, and one PEST light curve(s), and RV data from the HARPS, FEROS, and PFS instruments, covering observations from 2018 to 2024. TOI-4409,b is a bona fide puffy warm super-Neptune exoplanet with a planet radius of 7.44 ± 0.19,R_⊕, and a mass of 0.047 ± J ,(3σ upper limit <0.095,M_ J) orbiting a low-mass main-sequence star (effective temperature of 4928 ± 48, and a stellar radius of 0.720 ± 0.018, _⊙) with a period of 92.49179 ± 0.00010 days. The integration of various observational methods and consistent transit signals across multiple instruments confirms the planetary nature of TOI-4409,b. The combined transit, imaging, chromaticity, and RV evidence supports the confirmation of the planet. K R With a radius of 7.44 ± 0.19,R_⊕ and a low bulk density of 0.20 ± 0.07 -3 $, TOI-4409,b occupies a sparsely populated region of parameter space among warm, puffy super-Neptunes on long-period orbits. The detection of candidate transit timing variations with an ∼10-minute semi-amplitude hints at the presence of an additional body in the system. The favourable host-star brightness and extended transit duration ($∼7 hours) make TOI-4409,b a promising target for atmospheric characterisation with JWST and Ariel.

We present the first results from the POSEIDON survey, aimed at constraining the dynamical origins of transiting Neptunes through stellar obliquity measurements. We report Rossiter–McLaughlin observations of two Neptunes, TOI-181 b and TOI-883 b, obtained with high-resolution spectroscopy from Magellan/Planet Finder Spectrograph and WIYN/NEID. TOI-181 b is on a 4.5-day orbit with a sky-projected spin–orbit misalignment λ = 32 . ° 0 − 6. ° 5 + 6. ° 3 and a low eccentricity ( e < 0.12 with 2 σ confidence). TOI-883 b has a longer orbital period of 10 days with λ = 2 2 ° − 14 ° + 15 ° and eccentricity e = 0.16 ± 0.03. The significant misalignment of TOI-181 b and the significant eccentricity of TOI-883 b are suggestive of high-eccentricity migration for both systems. After adding these and other new measurements to the sample, we analyze the obliquity distribution of the host stars of transiting Neptunes. Earlier studies had suggested that the obliquity distribution is bimodal, with peaks corresponding to aligned orbits and polar orbits; the addition of more measurements has weakened the evidence for bimodality. The current sample appears to be consistent with a population of well-aligned systems and a smaller population with nearly random obliquities. This distribution resembles that observed for more massive planets, suggesting that transiting Jupiters and Neptunes originate from similar dynamical processes.

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.

We present the TESS-based discoveries of planetary systems orbiting the late G dwarfs TOI-2494 and TOI-5143, each of which harbors a hot giant planet and a smaller interior planet. TOI-2494 hosts a transiting mini-Neptune ( P = 2.41 days, R P = 2.3 5 − 0.15 + 0.16 R _⊕ ) and a hot Saturn ( P = 8.38 days) with grazing transits, while TOI-5143 hosts a transiting mini-Neptune ( P = 2.38 days, R P = 2.7 3 − 0.17 + 0.19 R _⊕ ) and a hot Jupiter ( P = 5.21 days) with grazing transits. We measure the masses of TOI-2494 c (80 ± 19 M _⊕ ) and TOI-5143 c (208 ± 14 M _⊕ ), place upper limits on the masses of the smaller planets, and explore the architectures of the planetary systems. TOI-2494 c and TOI-5143 c join a small but growing number of short-period giant planets known to be flanked by smaller companions. While the absence of close neighbors to most hot Jupiters would be consistent with disruptive dynamical evolution, the presence of nearby small planets in some hot Jupiter systems points to a separate, dynamically quiet formation pathway. In support of this conclusion, we present preliminary evidence that hot giant planets with small nearby companions tend to have low mutual orbital inclinations, consistent with the mutual inclination distribution of the dynamically quiet population of compact systems of multiple super-Earths and mini-Neptunes.