Annabella Meech

Observations of highly irradiated gas giant exoplanets have shown helium escaping from their atmospheres. There is limited evidence for atmospheres on rocky exoplanets, perhaps because they have already escaped. We report near-infrared spectroscopic observations of LHS 1140b, a rocky exoplanet that orbits in the habitable zone of a nearby low-mass star. The transit spectra show absorption by helium escaping from the planet’s atmosphere. Helium absorption is detected in 2024 but not in 2025, indicating time-variable atmospheric escape. We interpret these results as indicating an upper atmosphere dominated by helium and depleted in hydrogen, with other volatile species trapped at lower altitudes, which is consistent with atmospheric fractionation models. No helium absorption is detected for LHS 1140c, a smaller and more strongly irradiated exoplanet in the same system.

The search for an atmosphere on the closest rocky M dwarf planet, LTT 1445 A b, has been the subject of intense investigation from both the ground and space. Here, we present the first JWST transmission spectrum of LTT 1445 A b using a single visit spanning 3–5 μ m using NIRSpec/G395H. We conduct two independent reductions of the data using both the Eureka! and ExoTiC-JEDI pipelines. Overall, we measure the NRS1 transit depths to a median precision of ∼23 ppm in 42 spectroscopic channels with uniform widths of 30 pixels (∼0.02 μ m), and the NRS2 transit depths to ∼36 ppm precision in 64 spectroscopic channels, also with uniform widths of 30 pixels. We rule out any statistically significant spectral features at this precision and place limits on atmospheric metallicity using a grid of chemical equilibrium models with grey opaque clouds. Using NIRSpec/G395H alone, we can place limits on the atmospheric metallicity of ≳350 × solar when the opaque pressure level is greater than 0.01 bar. We also conduct a combined analysis of JWST/NIRSpec and Hubble Space Telescope WFC3 transmission data and find our atmospheric limits can be extended ≳500 × solar when considering both datasets. Future analyses both in transit and emission will uncover whether there are detectable atmospheric features.

Context . Young transiting exoplanets provide unique opportunities to probe planetary atmospheres during critical early phases of evolution when atmospheric escape and contraction are most active. HIP 67522 b, a 17 Myr old hot Jupiter with an extraordinarily low bulk density (−3 ), represents an ideal target for high-resolution transmission spectroscopy. Aims . We aim to constrain the mass and characterise the atmospheric composition, thermal structure, and dynamics of HIP 67522 b using ground-based high-resolution near-infrared spectroscopy with VLT/CRIRES+, complementing recent JWST observations. Methods . We obtained 92 high-resolution spectra ( R ≈ 10 ⁵ ) with VLT/CRIRES+ in the K2166 band during a transit on 30 January 2025. We applied cross-correlation techniques and Bayesian nested sampling retrievals to constrain molecular abundances, temperature structure, and atmospheric dynamics. Results . We detected H ₂ O at 20σ and CO at 5σ, confirming the extremely extended atmosphere of this low-mass giant. A velocity offset of −2.9 ± 0.2 km s ⁻¹ indicates day-to-night winds. The rotation velocity has been constrained to −1 at 3σ, consistent with tidal locking. The retrieval analysis suggests a planetary mass of 27.7 _−5.5 ^+5.9 Earth masses and statistically favours a two-temperature atmospheric structure with a discrete change at mbar pressures over an isothermal profile. This mass is twice as high as the mass estimated from JWST atmospheric observations and inconsistent at 3 σ , casting doubt on the actual planetary density of the planet. No matter the choice of atmospheric model, we derived a supersolar C/O ratio that is about 1.5 times solar, along with a supersolar metallicity that might further increase if the atmosphere is cloudy, which is a degeneracy that our data alone cannot resolve. We report a tentative 2 σ detection of HDO with an extreme enrichment factor of ∼1000 relative to the protosolar D/H ratio. If confirmed, this would be the first detection of deuterium in an exoplanet atmosphere and would require an intense escape rate to confirm its presence.

We present a JWST/NIRSpec G395H transmission spectrum of TOI-260 b, a T _eq ∼ 490 K, R _p = 1.76 R _⊕ planet. The transmission spectrum is derived by combining two transit observations, collected as part of the JWST Compositions of Mini-Planet Atmospheres for Statistical Study program. We achieved the same median transit depth precision of 37 ppm in both visits, as well as a median precision of 26 ppm when combining the spectroscopic light curves from the two visits. Implementing a 30 pixel wide ( R ∼ 200) spectroscopic binning scheme, we find that the transmission spectrum is mostly featureless, with a possible feature around 3.17 μ m. We assess the significance of any features in the transmission spectrum with a suite of nonparametric models, which confirm the presence of a potential feature in the NRS1 bandpass and an offset between the NRS1 and NRS2 detectors. To investigate the atmospheric composition of TOI-260 b, we run a series of PLanetary Atmospheric Tool for Observer Noobs retrievals. We do not detect any clear molecular signatures, but the combined data from the two visits are sufficient to constrain the atmospheric metallicity to greater than 200 × solar, assuming no opaque deck ≲2.5 mbar. We also investigate causes of the potential feature near 3.17 μ m; while we find some compatible gaseous species and cannot fully discard the possibility of an astrophysical origin, we suspect a systematics origin owing to the variance in strength and position of the feature. Overall, this look at TOI-260 b adds to the small sample of radius valley planets, which already seem to show a diversity in their atmospheric compositions. Determining the true nature of these enigmatic planets will require a larger telescope time investment.

We present the first atmospheric observation and characterisation of the aligned, 1468 K hot Jupiter, NGTS-2b, with one JWST NIRSpec/G395H transit. These observations complete the GO 3838 observing campaign of the BOWIE-ALIGN program, which aims to investigate the link between hot Jupiter atmospheric composition and formation history through the atmospheric analysis of planets orbiting F stars that are aligned and misaligned with the host stellar spin axis. The 2.84–5.18 μm spectrum shows weak absorption features attributed to H₂O and CO₂ absorption, which our free chemistry retrievals fit with posteriors that converge on high mean molecular weight solutions attained through significant H₂O mixing ratios. By comparing our results to interior modelling, we show that some of these solutions exceed the 43.5 × solar upper limit we obtained from our interior structure models. Such solutions are likely due to cloud-metallicity degeneracies and insufficient wavelength coverage to resolve them. We show that, in the case of our observations, the likelihood distribution of H₂O abundances is flat and uninformative, such that our retrievals are biased by the prior. Additionally, our statistically favoured atmospheric solution contains absorption from SO. The chemical abundances retrieved with this model are likely not astrophysically feasible and we demonstrate that the presence of SO is driven by only two data points. Our equilibrium chemistry retrievals hint at a subsolar C/O ratio and supersolar metallicity; however, we find wide posterior distributions that extend to solar values.

We present the 3–5 μ m transmission spectrum of HD 15337 c (TOI-402.02), a sub-Neptune (2.526 R _⊕ , 6.792 M _⊕ , T _eq ∼ 656 K) around a K1V star observed as part of the JWST COMPASS program. We reduce two observations with two pipelines and find consistent transmission spectra. The resulting median precisions in 30 pixel spectroscopic bins for visit 1 are ∼40 ppm and ∼70 ppm and for visit 2 are ∼30 ppm and ∼54 ppm for NRS1 and NRS2, respectively. We attribute the differing precisions to the lack of adequate pre-transit baseline in visit 1 from an early transit arrival caused by previously undetected transit timing variations (TTVs), hinting at a potential exterior companion. Our median JWST timing precision is 10 s, revealing TTVs >20 minutes when combined with previous TESS and CHEOPS data, highlighting JWST’s TTV measurement capabilities. The transmission spectrum of HD 15337 c is featureless and can best be described by a step function with an offset between the NRS1 and NRS2 detectors, likely caused by instrumental systematics. From thermochemical equilibrium retrievals we find that, to >3 σ , the data can rule out atmospheres with metallicities <600 or <310 × solar, depending on the reduction, for opaque pressures greater than a few millibars. HD 15337 c joins other sub-Neptunes with similar masses, radii, and temperatures in possessing a featureless transmission spectrum indicative of high metallicity and/or high-altitude aerosols and adds support to recent studies showing that aerosol opacity reaches a maximum for planets with equilibrium temperatures of 500–700 K.

JWST has already observed near-infrared transmission spectra of over a dozen super-Earths and sub-Neptunes. While some observations have allowed astronomers to characterize sub-Neptunes in unprecedented detail, small feature amplitudes and poorly understood systematics have led to ambiguous results for others. Using the first seven targets from the COMPASS program, which will survey 12 small planets using NIRSpec/G395H, we investigate timeseries systematics. We implement a model that uses the principle components of the normalized pixel fluxes to account for variations in the shape and position of the spectral trace. We find that observations with a smaller number of groups-per-integration benefit most profoundly from the use of this model, and that systematics are particularly strong between 2.8 and 3.5 μ m. Despite these systematics, pandexo is a relatively accurate predictor of the precision of the spectra, with real error bars on average 5% larger in NRS1 and 12% larger in NRS2 than predicted. We compute new limits on metallicity and opaque pressure level for each target and compare these to previous results from the COMPASS program. Next, we coadd spectra from multiple targets to reduce the effective noise in the combined spectra in hopes of detecting transmission features in common between the targets, but this exercise does not yield compelling evidence of any signals. We find that a handful of additional transits are sufficient to break the degeneracy between metallicity and aerosols for the majority of our targets, pointing towards the possibility of unraveling the mysteries of these worlds with future allocations of JWST time.