Anna Gagnebin

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.

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 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.

We present JWST NIRSpec/G395H transmission spectroscopy observations of GJ 357 b, a warm (T _eq ≈ 525 K) super-Earth (1.2 R _⊕, 1.84 M _⊕) orbiting a nearby M3-type star, with a median precision of 18 ppm and 27 ppm in NRS1 and NRS2, respectively. These precisions are obtained by binning the spectrum into 53 spectroscopic channels with a resolution of 60 pixels (around 0.02 μm) each. Our analysis of the transmission spectrum reveals no detectable atmospheric spectral features. By comparing the observed spectrum with 1D forward models, we rule out atmospheres with mean molecular weights (MMWs) lower than 8 g mol⁻¹ to 3σ, as well as atmospheres with metallicities less than 300 × solar. The lack of a low-MMW primary atmosphere is consistent with a primordial H₂-rich atmosphere having escaped, given the planet’s ≳5 Gyr age, relatively low surface gravity (log(g) = 3.09), and its likely history of substantial incident extreme UV radiation. We conclude that GJ 357 b most likely possesses either a high-MMW secondary atmosphere, is perhaps rich in oxidized gases like CO₂, or is a bare rock with no atmosphere. Upcoming scheduled JWST thermal emission observations could help distinguish between these scenarios by detecting signatures indicative of atmospheric heat redistribution or molecular absorption.

We confirm the planetary nature of a pair of transiting sub-Neptune exoplanets orbiting the bright F-type subgiant star TOI-6054 (V = 8.02, K = 6.673) as a part of the OrCAS radial velocity survey using WIYN/NEID observations. We find that TOI-6054b and TOI-6054c have radii of 2.64 ± 0.15R _⊕ and 2.82 ± 0.17R _⊕, respectively, and masses of 12.5 ± 1.7M _⊕ and 9.3 ± 1.9M _⊕. The planets have zero-albedo equilibrium temperatures of 1360 ± 32 K and 1143 ± 28 K. The host star has expanded and will evolve off of the main sequence within the next ∼500 Myr, and the resulting increase in stellar luminosity has more than doubled the stellar flux the two planets receive compared to the start of the host star’s main-sequence phase. Consequently, TOI-6054b may be losing some of its primordial hydrogen/helium atmosphere—if it has one. Based on dynamical simulations performed using the orbital parameters of the two planets, TOI-6054b, and TOI-6054c are very likely in a 5:3 mean motion resonance. The TOI-6054 system thus has the potential to be an excellent candidate for future atmospheric follow-up observations, with two similarly sized sub-Neptunes around a bright star. We also estimate that if TOI-6054b is currently losing its H/He atmosphere, this should be observable from space and from the ground.

Sub-Neptunes, the most common planet type, remain poorly understood. Their atmospheres are expected to be diverse, but their compositions are challenging to determine, even with JWST. Here, we present the first JWST spectroscopic study of the warm sub-Neptune GJ 3090 b (2.13 R _⊕, T _{eq,A = 0.3} ∼ 700 K), which orbits an M2V star, making it a favorable target for atmosphere characterization. We observed four transits of GJ 3090 b: two each using JWST NIRISS/SOSS and NIRSpec/G395H, yielding wavelength coverage from 0.6 to 5.2 μm. We detect the signature of the 10833 Å metastable helium triplet at a statistical significance of 5.5σ with an amplitude of 434 ± 79 ppm, marking the first such detection in a sub-Neptune with JWST. This amplitude is significantly smaller than predicted by solar-metallicity forward models, suggesting a metal-enriched atmosphere that decreases the mass-loss rate and attenuates the helium feature amplitude. Moreover, we find that stellar contamination, in the form of the transit light source effect, dominates the NIRISS transmission spectra, with unocculted spot and faculae properties varying across the two visits separated in time by approximately 6 months. Free retrieval analyses on the NIRSpec/G395H spectrum find tentative evidence for highly muted features and a lack of CH₄. These findings are best explained by a high-metallicity atmosphere (>100× solar at 3σ confidence for clouds at ∼μbar pressures) using chemically consistent retrievals and self-consistent model grids. Further observations of GJ 3090 b are needed for tighter constraints on the atmospheric abundances and to gain a deeper understanding of the processes that led to its potential metal enrichment.

The atmospheres of planets between the size of Earth and Neptune at short orbital periods have been under intense scrutiny. Of the approximately dozen planets in this regime with atmospheres studied so far, a few appear to have prominent molecular features, while others appear relatively void of detectable atmospheres. Further work is therefore needed to understand the atmospheres of these planets, starting with observing a larger sample. To this end, we present the 3–5 μm transmission spectrum of TOI-776 c, a warm (T _eq ∼ 420 K), ∼2 R _⊕, ∼7 M _⊕ planet orbiting an M1V star, measured with JWST NIRSpec/G395H. By combining two visits, we measure a median transit precision of ∼18 and ∼32 ppm in the NRS1 and NRS2 detectors, respectively. We compare the transmission spectrum to both nonphysical and physical models and find no strong evidence for molecular features. For cloud-top pressures larger than 10⁻³ bars, we rule out atmospheric metallicities less than 180–240× solar (depending on the reduction and modeling technique), which corresponds to a mean molecular weight of ∼6–8 g mol⁻¹. However, we find that simple atmosphere mixture models (H₂O + H₂/He or CO₂ + H₂/He) give more pessimistic constraints, and we caution that mean molecular weight inferences are model dependent. We compare TOI-776 c to the similar planet TOI-270 d and discuss possible options for further constraining TOI-776 c’s atmospheric composition. Overall, we suggest that these TOI-776 c observations may represent a combination of planetary and stellar parameters that fall just below the threshold of detectable features in small-planet spectra; finding this boundary is one of the main goals of the COMPASS program.

The atmospheres of planets between the size of Earth and Neptune at short orbital periods have been under intense scrutiny. Of the ~dozen planets in this regime with atmospheres studied so far, a few appear to have prominent molecular features while others appear relatively void of detectable atmospheres. Further work is therefore needed to understand the atmospheres of these planets, starting with observing a larger sample. To this end, we present the 3-5 micron transmission spectrum of TOI-776 c, a warm (Teq ~420 K), ~2 Rearth, ~7 Mearth planet orbiting an M1V star, measured with JWST NIRSpec/G395H. By combining two visits, we measure a median transit precision of ~18 ppm and ~32 ppm in the NRS1 and NRS2 detectors, respectively. We compare the transmission spectrum to both non-physical and physical models, and find no strong evidence for molecular features. For cloud-top pressures larger than 10^-3 bar, we rule out atmospheric metallicities less than 180-240x solar (depending on the reduction and modeling technique), which corresponds to a mean molecular weight of ~6-8 g/mol. However, we find simple atmosphere mixture models (H2O+H2/He or CO2+H2/He) give more pessimistic constraints, and caution that mean molecular weight inferences are model dependent. We compare TOI-776 c to the similar planet TOI-270 d, and discuss possible options for further constraining TOI-776 c's atmospheric composition. Overall, we suggest these TOI-776 c observations may represent a combination of planetary and stellar parameters that fall just below the threshold of detectable features in small planet spectra; finding this boundary is one of the main goals of the COMPASS program.