Jacob Lustig-Yaeger

As part of the James Webb Space Telescope (JWST) Exoplanet Grand Tour Survey, we use panchromatic transmission spectroscopy to connect HAT-P-1 b’s previously studied optical and near-infrared atmosphere to the longer-wavelength molecular bands accessible with JWST. We present JWST Near-infrared Spectrograph G395H transmission spectroscopy of the hot Jupiter HAT-P-1 b over 2.7–5.3 μ m, and combine the new spectrum with archival Hubble Space Telescope Space Telescope Imaging Spectrograph and Wide Field Camera 3 observations for a 0.3–5.3 μ m atmospheric analysis. We independently reduce the JWST data with the Eureka! , FIREFLy , and Tswift pipelines, finding mutually consistent transmission spectra across the G395H bandpass. Atmospheric retrievals yield strong evidence for H ₂ O and CO ₂ with Bayes factors of log 10 B H 2 O = 8.9 and log 10 B CO 2 = 52.3 , while providing tentative evidence for H ₂ S ( log 10 B H 2 S = 1.4 ). The joint H ₂ O and CO ₂ constraints favor an atmosphere near chemical equilibrium, with log 10 M/H = 0.9 9 − 0.14 + 0.19 , corresponding to ∼10× solar or ∼9× relative to the near-solar metallicity host star, and a 3 σ upper limit of C/O < 0.52. Because H ₂ O and CO ₂ provide a metallicity comparatively insensitive to vertical mixing in this temperature regime, their combined detection suggests the composition is dominated by bulk enrichment rather than strong disequilibrium transport. We find no significant evidence for clouds; instead, the persistence of molecular structure across the spectrum argues against strong cloud muting. The tentative H ₂ S signal, if confirmed, would further suggest limited photochemical processing at the pressures probed. Together, the molecular inventory, enriched metallicity, and low C/O ratio point to an oxygen-rich atmosphere and establish HAT-P-1 b as a benchmark for comparative studies of hot Jupiter atmospheric composition.

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.

Detecting and characterizing the atmospheres of rocky exoplanets has proven to be challenging for JWST. Transit spectroscopy of the TRAPPIST-1 planets has been impacted by the effects of spots and faculae on the host star. Secondary eclipses have detected hot rocks, but evidence for atmospheres has been difficult to obtain. However, there is a third option that we call color phase curves. This method will apply to synchronously rotating non-transiting planets as well as transiting planets. A color phase curve uses photometry at a long-IR wavelength where the planetary thermal emission is strong (e.g., 21 microns) divided by photometry at a shorter wavelength where the star dominates (e.g., 12 microns). We avoid wavelengths having potentially strong molecular absorption (e.g., 15 microns) to minimize degeneracies in the color phase curve, and we aim to detect and characterize the planetary atmosphere via its longitudinal heat transfer. The ratio of two wavelengths observed nearly simultaneously is designed to isolate thermal emission from the planet, discriminate against the star, and largely cancel instrumental systematic effects. Moreover, we show that invoking mass-radius relations, and using self-consistent physical models, will permit the longitudinal heat transfer to be measured independent of the orbital inclination. Radial velocity surveys are detecting many new exoplanets, including temperate rocky worlds with Earth-like masses. Most of those planets will not transit, but color phase curves have the potential to detect and characterize their atmospheres.

Stellar activity complicates exoplanet transmission spectra, particularly for smaller planets around M dwarfs with JWST. The transit-light-source (TLS) effect, the imprinting of spectral differences between the average stellar disk and the occulted transit chord onto the transmission spectrum, makes it challenging to directly use the out-of-transit spectrum to correct for stellar contamination. Theory and observations suggest that spots may concentrate toward higher latitudes when the Coriolis force is substantial relative to buoyancy, leaving the equatorial region relatively quiet. Here, we evaluate how the latitudinal distribution of active regions shapes the strength of the TLS effect for planets spanning a range of impact parameters ( b ), using TRAPPIST-1 as a testbed. We first construct a fiducial model to illustrate two distribution regimes. With our model, the moderate- b outer TRAPPIST-1 planets (f, g, h) occult a more typical region of the stellar disk than the inner planets and are thereby less affected by the TLS effect, though their bias may vary more from visit to visit as these active regions evolve with time. More generally, our results imply an impact parameter “sweet spot” for atmospheric characterization, independent of the sign of the active-region temperature contrast, whose location depends on the distribution of active regions. The distribution may be revealed by transit residuals as multiple planets probe different latitudes, while longitudes are sampled in time, such that the variance and frequency of the correlated scatter could constrain active-region filling factors, sizes, and separations.

Determining whether temperate rocky exoplanets orbiting M stars retain atmospheres is currently a central goal of exoplanet astronomy. To this end, the James Webb Space Telescope has begun searching for atmospheres on these worlds with MIRI secondary eclipse spectroscopy and photometry. Here, we develop a novel climate-constrained Bayesian inference framework that yields atmospheric pressure and composition constraints from these datasets, while accounting for planetary, stellar, and model uncertainties. Our approach fits observations with model spectra derived from self-consistent pressure-temperature profiles at radiative–convective equilibrium, thus maximizing the information extracted from the data and providing more robust inferences than retrievals that use parameterized pressure–temperature profiles. We demonstrate the framework on the existing MIRI LRS eclipse spectrum of LTT 1445A b (1.34 R _⊕ and T _eq ≈ 431 K). An atmosphere does not need to be invoked to explain the data, meaning a bare rock model produces an adequate fit. If the planet has an atmosphere, the 2 σ upper limits on surface partial pressures are ≲1 bar for an optically thin gas like O ₂ , N ₂ , or CO, ≲0.1 bar for CO ₂ , ≲10 ⁻³ bar for H ₂ O, and ≲10 ⁻⁴ bar for SO ₂ . Scheduled MIRI F1500W observations could detect one of the thicker atmospheres permitted by the existing data (1 bar O ₂ and 0.01 bar CO ₂ ), if a precision of 20 ppm or better is achieved. This case study demonstrates that climate-constrained Bayesian inversion can turn rocky-planet eclipse spectra into the quantitative constraints necessary to test population-level atmospheric retention hypotheses, like the cosmic shoreline.

NASA’s Habitable Worlds Observatory (HWO) will search for biosignatures on Earth-like exoplanets using reflected light spectroscopy. A critical instrument design parameter is resolving power, which must balance biosignature detectability against exposure time and detector noise constraints. We assess the resolving power needed to detect and characterize key biosignature gases and habitability indicators, including O ₂ , O ₃ , H ₂ O, CH ₄ , CO ₂ , and CO across atmospheres representing the Archean, Proterozoic, and Phanerozoic Earth. We combine analytical detectability calculations spanning spectral resolutions ( λ /Δ λ ) R = 20–5000 with atmospheric retrievals using the rfast radiative transfer model and pyEDITH exposure time calculator for realistic wavelength-dependent noise modeling. In the visible (0.4–1.0 μ m), the nominal resolution R _Vis = 140 is sufficient for detecting O ₂ in Phanerozoic-like atmospheres. Higher resolutions could theoretically reduce exposure times for low-O ₂ Proterozoic atmospheres, but require >10× reductions in dark current and could increase H ₂ O detection exposure times by ∼2 × , penalizing the foundational habitability constraint that anchors downstream biosignature searches. The most efficient path for low-O ₂ atmospheres may instead be indirect inference via O ₃ , whose Hartley–Huggins bands are detectable at R _UV ∼ 7. In the near-infrared (NIR; 1.0–1.7 μ m), R _NIR ≥ 40 is necessary to avoid a degeneracy between CO ₂ and CO that could produce false-positive detections of abundant CO. The nominal R _NIR = 70 is sufficient for characterizing all Earth-through-time cases. These results support HWO’s current baseline resolution choices and provide actionable guidance for finalizing spectrometer requirements while maintaining technological feasibility for the search for life on exoplanets.

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.

As we enter a new era of using JWST to characterize exoplanet atmospheres, a top priority is defining observational needs and parameters in the search for life on an exoplanet. Although some studies have begun to consider Titan-like exoplanet atmospheres, more work is needed to prepare for the potential discovery and characterization of Titan analogues with JWST and future missions. We explore the effectiveness of exoplanet atmospheric retrieval methodologies on the Cassini/VIMS-IR spectrum of Titan assuming agnostic haze properties and tholin hazes, as well as a self-consistent photochemical model. We compare our retrieved chemical abundances and haze profiles against fiducial values and Cassini/ISS-NAC UV observations and the forward modeled truths. We generalize these findings to a Titan-like exoplanet in the TRAPPIST-1 system and find that JWST-like observations have limited potential to identify features beyond methane in a Titan-like atmosphere, but may be able to identify the presence of the haze to a few hundred kilometers. The diagnostic features which would differentiate between different haze model (scattering) properties are shortward of 1 μ m and further demonstrate the important role of nUV—optical observatories in constraining the properties of exoplanet atmospheres. We also find that agnostic priors for the bulk atmosphere can lead to a degeneracy in which a spectrally active gas (CH ₄ ) is erroneously favored as the dominant atmospheric constituent over a spectrally inactive molecule (N ₂ ).

The spectra of exoplanet atmospheres are affected by aerosols (clouds and hazes) of uncertain origin. Proposed aerosol formation mechanisms include gas condensation or photochemical reactions. We measured the transmission spectrum of the tidally locked gas giant exoplanet WASP-94A b and identified asymmetry in its atmosphere. The morning limb is cooler and cloudy, whereas the evening limb is hotter and exhibits gaseous water absorption features. We interpret this difference as being due to the formation of cloud droplets near the morning limb, which evaporate during circulation to the evening limb. The dominant aerosols are clouds cycling between the day and night sides of the atmosphere, not photochemical hazes. The resulting asymmetry can severely bias chemical abundance measurements, unless limb-resolved spectroscopy is available.