Exoplanet atmospheres at high spectral resolution
Chapter in Handbook of Exoplanets, Springer (2026) 1-38
Abstract:
The spectrum of an exoplanet reveals the physical, chemical, and biological processes that have shaped its history and govern its future. However, observations of exoplanet spectra are complicated by the overwhelming glare of their host stars. Here, we focus on high-resolution spectroscopy (HRS) (R∼5,000−140,000), which helps disentangle and isolate the exoplanet’s spectrum. HRS resolves molecular features into a dense forest of individual lines in a pattern that is unique for a given molecule. For close-in planets, the spectral lines undergo large Doppler shifts during the planet’s orbit, while the host star and Earth’s spectral features remain essentially stationary, enabling a velocity separation of the planet. For slower-moving, wide-orbit planets, HRS, aided by high contrast imaging, instead isolates their spectra using their spatial separation (high contrast spectroscopy; HCS). The planet’s spectral lines are compared with HRS model atmospheric spectra, typically using cross-correlation to sum their signals. It is essentially a form of fingerprinting for exoplanet atmospheres and works for both transiting and non-transiting planets. It measures their orbital velocity, true mass, and simultaneously characterizes their atmosphere. The unique sensitivity of HRS to the depth, shape, and position of the planet’s spectral lines allows it to measure atmospheric composition, structure, clouds, and dynamics, including day-to-night winds and equatorial jets, plus its rotation period and even its magnetic field. These are extracted using statistically robust log-likelihood frameworks and match space-based instruments in their precision. This chapter describes the HRS technique in detail and concludes with future prospects with Extremely Large Telescopes to identify biosignatures on nearby rocky worlds and map features in the atmospheres of giant exoplanets.Atmospheric asymmetries in WASP-121 b revealed by rotational transits detected with JWST
Nature Astronomy (2026)
Abstract:
Close-in exoplanets are tidally locked to their host star and thus exhibit extreme atmospheric temperature gradients. It has been theorized that the fraction of star light absorbed by such planets during transit changes as a function of orbital phase as progressively hotter or colder atmospheric gas rotates into view, but this effect has not been observed so far. Here we show that two transits of the ultrahot Jupiter WASP-121 b, acquired with JWST/NIRSpec and NIRISS, exhibit asymmetric light curves caused by the planet’s rotation during transit. We observe increasing CO absorption and slightly decreasing H
A Clearer View of HAT-P-1 b: JWST NIRSpec G395H Reveals Water, Carbon Dioxide, and Possibly Hydrogen Sulfide
Accepted in AJ
Abstract:
As part of JWST's 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 NIRSpec G395H transmission spectroscopy of the hot Jupiter HAT-P-1 b over 2.7--5.3~μm, and combine the new spectrum with archival HST STIS and WFC3 observations for a 0.3--5.3~μm 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 H2O and CO2 with Bayes factors of log10BH2O=8.9 and log10BCO2=52.3, while providing tentative evidence for H2S (log10BH2S=1.4). The joint H2O and CO2 constraints favor an atmosphere near chemical equilibrium, with log10M/H=0.99+0.19−0.14, corresponding to ∼10× Solar or ∼9× relative to the near-solar metallicity host star, and a 3σ upper limit of C/O <0.52. Because H2O and CO2 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 H2S 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.
A JWST transiting survey of FGK stellar limb darkening: empirical evidence for quadratic laws and atmospheric model comparisons
Accetped in AJ
Abstract:
We present a study of stellar limb-darkening using JWST transit observations of seven exoplanets orbiting FGK host stars, spanning 4200-6800 K. The wide wavelength coverage and high S/N of NIRISS/SOSS and NIRSpec/PRISM enable precise constraints on the wavelength-dependent limb-darkening. Using Bayesian model selection, we find that the quadratic limb-darkening law is statistically preferred over higher-order laws for most FGK stars, recovers consistent intensity profiles for >95% of the stellar disk area, and introduces a minimal bias in the derived transit depths of only ∼14 ppm (1σ) for 6/7 targets. This contrasts with previous studies relying on stellar models. We compare the empirically derived quadratic coefficients to predictions from the PHOENIX, MPS-ATLAS, MURaM, and Stagger stellar atmosphere grids. We introduce a quadratic limb-darkening parameterization in terms of limb intensity (ℓ) and curvature at mid-μ (δ), finding that empirical FGK limb darkening is generally more linear than models predict (δ≲0.1). We identify wavelength-independent offsets between data and model quadratic coefficients, minimized by adopting μmin=0.2 in intensity calculations; we attribute this in part to models overpredicting limb-darkening near the limb where the plane-parallel approximation breaks down. For spherical PHOENIX models, we derive a μ rescaling method using a τ=1 photospheric radius. With these corrections, residual offsets are minimized and all stellar models achieve statistically acceptable fits. We provide recommended limb-darkening offset priors for use in JWST transit analyses, enabling more accurate constraints on exoplanet transmission spectra while accounting for residual stellar model uncertainties.