Nicole L. Wallack

Ultrashort period (USP) rocky planets are expected to be depleted of any substantial gaseous envelope due to the intense irradiation they receive from their host star, making the recent detection of an atmosphere around TOI-561 b particularly surprising. That finding was based on the planet’s bulk density and dayside emission spectrum, but full-orbit phase-curve observations offer a more comprehensive way to constrain the presence and characteristics of a planet’s atmosphere. In this paper, we map TOI-561 b’s 3–5 μ m emission using JWST/NIRSpec multiorbit spectroscopic phase curves, explicitly accounting for the curvature of the out-of-eclipse baseline and possible hotspot offsets. From a simple energy balance argument as well as comparing to general circulation models, we find the phase-curve observations are best explained with high Bond albedo and moderate global heat transfer. Our 37 hr continuous observation provides a long baseline that allows us to model stellar granulation, the planetary phase curve, and dayside emission simultaneously, effectively disentangling these signals and yielding a dayside emission spectrum that is more robust against stellar variability and consistent with previous eclipse-only fits. Using general circulation model outputs, we constrain where clouds can plausibly form and test candidate compositions, finding that silicate clouds, such as SiO ₂ and MgSiO ₃ , can form on the dayside near the terminator and explain the observed albedo. Our findings confirm that TOI-561 b appears to have a global reflective atmosphere, suggesting exchange of volatiles with the interior to maintain it.

AF Lep b is one of the closest analogs to Jupiter in terms of mass (3–4 M _Jup ) and semimajor axis (9 au) amenable to spectroscopic characterization. We present JWST/NIRSpec high-contrast spectroscopy of the planet from 2.85 to 5.3 μ m at R ∼ 3000, which provide detections of CO ₂ , H ₂ S, CH ₄ , ¹² CO (and ¹³ CO), and H ₂ O, as well as complementary JWST/NIRCam imaging that captures the planet’s continuum flux from 4.0 to 4.7 μ m. Combining the JWST observations with spectra from VLTI/GRAVITY and VLT/SPHERE (1.0−2.5 μ m), we carry out atmospheric retrievals that include the effects of clouds and disequilibrium chemistry while allowing the C, O, and S abundances to vary independently. AF Lep b exhibits metal enrichment across C, O, and S with C/H = 2.9 ± 0.5, O/H = 3.7 ± 0.6, and S/H = 4.7 ± 0.7 times solar (and stellar). The planet’s slightly subsolar C/O and C/S are consistent with formation near its observed location, and disfavor formation beyond the CO snowline. The sulfur enrichment in AF Lep b implies significant accretion of disk solids during formation, and we estimate the planet contains 56 ± 7 M _⊕ of solids. The C, O, and S enrichment levels of AF Lep b are similar to those of Jupiter and other super-Jupiters like HR 8799 bcde. We also show that the degree of atmospheric metal enrichment of these imaged planets is similar to the bulk metal enrichment of transiting gas giants with masses greater than ∼1 M _Jup , suggesting that the process and efficiency of metal accretion for gas giants may not be strongly dependent on orbital distance or planet mass.

HATS-6 b is one of several recently discovered giant exoplanets orbiting M dwarf stars (GEMS) and is part of a JWST survey that aims to compare bulk and atmospheric properties of these rare planets against their FGK star counterparts. HATS-6 b is a warm ( T _eq ∼ 700 K), Saturn-mass ( M _p ∼ 0.3 M _Jup ), Jupiter-radius ( R _p ∼ 1 R _Jup ) planet that transits its star every ∼3 days. In this study, we present the transmission spectrum of HATS-6 b obtained with two transits using the PRISM mode of JWST Near Infrared Spectrograph (NIRSpec), spanning a wavelength range of 0.6−5.3 μ m. Analyzing these JWST observations using an iterative approach between forward modeling and free chemistry retrievals, we derive a low-metallicity ( log [ M / H ] = − 1.9 9 − 0.2 + 0.2 ) subsolar-C/O ( log [ C / O ] = − 0.4 6 − 0.2 + 0.2 ) atmosphere, and find strong evidence for H ₂ O, CH ₄ , and NH ₃ at volume mixing ratios (in log [ X ] ) of − 4.8 8 − 0.24 + 0.25 , − 5.3 8 − 0.19 + 0.18 , and − 6.0 3 − 0.19 + 0.18 , respectively. We consistently retrieve a significantly lower T _eq than predicted from the orbital configuration of HATS-6 b, which was impervious to any data reduction and retrieval choices, suggesting a nonzero Bond albedo. Our planetary interior models retrieve bulk metallicities 3 orders of magnitude larger than our retrieved atmospheric metallicity, also suggesting that the atmosphere is not well mixed. We find an excess feature around 3 μ m, and expand on possible explanations for this, such as the presence of HCN or hydrocarbons like C ₂ H ₄ . Yet, due to the degeneracies present for hydrocarbon features in this wavelength region, we do not draw any conclusions about the excess feature and instead encourage further observations and follow-up of this intriguing target.

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.

We present new Keck/KPIC high-resolution spectroscopic detections of three ultrahot Jupiters (UHJs) in the K band: WASP-189 b (signal-to-noise ratio (SNR) = 7.2), MASCARA-1 b (SNR = 8.6), and TOI-1518 b (SNR = 7.1), as well as a tentative detection of KELT-9 b (SNR = 5.0). We perform a uniform set of atmospheric retrieval analysis on these objects, as well as previously reported KPIC observations of WASP-33 b (SNR = 11.2) and KELT-20 b (SNR = 10.5), We perform atmospheric retrievals for the pressure–temperature ( P − T ) profile, orbital velocity parameters, v sin i , and abundances of CO, H ₂ O, OH, and Fe, with parameterized mixing profiles to account for the expected vertical abundance variations of H ₂ O and OH. We also perform a set of retrievals assuming chemical equilibrium, which are generally in good agreement with the free retrievals. Except for KELT-9 b, the retrieved spectra are dominated by CO emission features, with additional weak H ₂ O or OH features consistent with thermal dissociation of H ₂ O. KELT-9 b, which is significantly hotter, appears to have very weak molecular features. Dissociation limits our ability to reliably constrain H ₂ O or OH abundances from K -band data alone, resulting in poor constraints on the C/O ratio. For all objects, the atmospheric abundances from detected carbon and oxygen species are 1–10 × solar. These results highlight the importance of wide spectral coverage for high-resolution retrievals. Additional observations to expand phase and wavelength coverage are needed to better constrain oxygen species and possible spatial inhomogeneities from dissociation.

The growing sample of giant exoplanets around M dwarf stars (GEMS) helps probe the extremes of giant planet formation. Comparing the properties of this sample with their FGK counterparts can help us understand how planet formation and migration depend on stellar mass. We initiated a large Cycle 2 JWST transmission spectroscopy survey of seven GEMS. Here we present the atmospheric characterization using two JWST transits of TOI-5293 A b, a 0.5 M _Jup planet orbiting an early M dwarf with a period of ∼3 days. The two NIRSpec/PRISM transits indicate the planet is eclipsing a rapidly changing (heterogeneous) stellar photosphere. We see that Visit 1 had heterogeneity crossings across the entire transit chord, rendering inferences from it to be unreliable. The Visit 1 spectrum exhibits a downward slope at μ m suggestive of stellar contamination from faculae. In contrast, for Visit 2 we are able to model the heterogeneity crossings and obtain a transmission spectrum free from stellar contamination. We therefore limit our conclusions to a detailed analysis of Visit 2, and using Bayesian free chemistry retrievals, we find a low atmospheric metallicity ( log [ M / H ] = − 1.0 3 − 0.44 + 0.53 × solar) and supersolar C/O ratio ( 1.2 3 − 0.75 + 2.94 ). The retrievals yield Bayes factors that indicate strong evidence for CH ₄ as well as low-significance detections of CO ₂ , H ₂ O, and NH ₃ . Finally, using thermal evolution models we find that the radius of TOI-5293 A b is inflated above theoretical expectations (∼1.07 R _Jup ), despite it having an temperature of ∼700 K, and hence we were unable to constrain its bulk composition.

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.

Intense high-energy irradiation of close-in gaseous exoplanets drives the rapid escape of their atmospheres, fundamentally shaping planetary demographics. While atmospheric loss is routinely observed via transit absorption in atomic hydrogen, helium, and metal ions, the underlying physical properties, specifically the thermal structure, outflow dynamics, and mass-loss rate, remain poorly constrained due to inherent degeneracies in the transmission geometry. Here we report the first detection of atomic hydrogen emission from the escaping atmosphere of a gas giant. Using high-resolution spectroscopy of the ultrahot Jupiter KELT-9 b, we detect a hydrogen Balmer line (H α 6564.6 Å) emission signature originating from the planetary dayside. The emission line profile features a distinctive double-peaked shape with 0.1%–0.15% peak amplitudes at ±30 km s ⁻¹ and central self-absorption. This profile breaks transmission degeneracies, providing direct observational constraints on the vertical thermal structure, excited-state hydrogen populations, and wind dynamics in the upper atmosphere of KELT-9 b. Initial modeling reveals a vigorous outflow with a mass-loss rate above 10 ¹³ g s ⁻¹ , among the highest measured to date for gaseous exoplanets. Our results establish hydrogen airglow emission as a powerful diagnostic of atmospheric escape, opening a new observational window into the evolution of worlds in extreme radiation environments.

HATS-75 b is one of the recently discovered Giant Exoplanets Around M-dwarf Stars (GEMS) with a transmission spectrum shaped by both its atmosphere and the active stellar surface it transits. As part of a JWST program studying seven GEMS, we observed three transits of HATS-75 b with the NIRSpec PRISM instrument (0.6–5.3 μ m). The planet’s spectra exhibit a slightly larger transit depth at shorter wavelengths, indicative of hazes or stellar contamination due to stellar heterogeneities outside the transit chord, i.e., the transit light source (TLS) effect. While both a hazy atmospheric model or TLS model can replicate the transmission spectrum, independent evidence (e.g., stellar rotation, spot-crossing events) favors a model that includes contamination from unocculted starspots and faculae. Within this stellar heterogeneity/TLS-based framework, atmospheric retrievals yield remarkably low atmospheric metallicity ( log [ M / H ] = − 1.7 4 − 0.76 + 0.92 ) and supersolar carbon-to-oxygen ( C / O = 1.0 4 − 0.09 + 0.40 ), which paired with a best-fit interior model with bulk metallicity of Z _p = 0.20 ± 0.04 implies poor vertical mixing within the planet. Retrievals also detect robust absorption signatures of CH ₄ , CO, and CO ₂ . We obtain only an upper limit for H ₂ O, consistent with its atmospheric spectral features being masked by stellar contamination. These results underscore the importance of accounting for stellar heterogeneity when interpreting exoplanet transmission spectra and highlight HATS-75 b as a significant asset to our understanding of giant exoplanets around M dwarfs with JWST.

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.